
[ NETINFO:X25.DOC ]                                              
[ 1/85, REF ]



                        ACKNOWLEDGMENTS



This specification was prepared by BBN Communications Corporation
under contract to the Defense Data Network Program Management
Office of the Defense Communications Agency.

The specification has been reviewed by the Defense Communications
Engineering Center for accuracy and completeness.  The draft of
this specification has been disseminated to industry by the
National Bureau of Standards for review and comments which have
been incorporated in the final specification.  This specification
has been approved for use on the Defense Data Network by the DoD
Protocol Standards Steering Group.






























Comments on this specification should be directed to the Defense 
Communications Agency, ATTN: Defense Data Network Program Managment
Office, Code B610, Washington, D.C.   20305



                     Table of Contents




1  INTRODUCTION.......................................... 1
1.1  Background.......................................... 1
1.1.1  X.25 and FIPS 100/Federal Standard 1041........... 1
1.1.2   X.25-to-X.25 and X.25-to-1822
         Interoperability................................ 2
1.2  Compliance.......................................... 4
1.2.1   Compliance With CCITT X.25 and FIPS
         100/Fed. Std. 1041.............................. 4
1.2.2  DTE Compliance With This Specification............ 4

2  INTERFACE SPECIFICATION............................... 6
2'1  Call Establishment Conventions...................... 6
2.1.1  Addressing........................................ 6
2.1.1.1  Address Formats and Fields...................... 6
2.1.1.1.1  Reserved...................................... 7
2.1.1.1.2  Flag.......................................... 7
2.1.1.1.3  DDN host Identifier........................... 7
2.1.1.1.4  Sub-Address................................... 7
2.1.1.2  Supplying Missing Address Information........... 7
2.1.2  DDN-Specific Facilities........................... 8
2.1.2.1  Type of Service Selection....................... 8
2.1.2.2  Call Precedence................................. 9
2.1.3  Protocol Identification.......................... 10
2.1.4  Logical Channel Assignment....................... 10
2.2  Packet Level Procedures............................ 11
2.3  Link Level Procedures.............................. 12
2.3.1  Link Level Parameters and Options................ 12
2.3.2  Timer T1 and Parameter T2........................ 12
2.3.3  Maximum I Frame Size............................. 13
2.4  Physical Level Specifications...................... 14

3  BIBLIOGRAPHY......................................... 16

APPENDIX A:  DDN X.25 Implementation Details............ A-1

A-1  Introduction...................................... A-1
A-2  Operational Features of DDN X.25 DCE Releases..... A-1
A-2.1  Initial Feature Support......................... A-1
A-2.2  Exception-Handling Procedures................... A-2
A-2.2.1  Non-Octet-Aligned Data........................ A-2
A-2.2.2  RESTART REQUEST Packet........................ A-2
A-2.2.3  RESET REQUEST Packet.......................... A-2
A-2.2.9  CLEAR REQUEST Packet.......................... A-3
A-2.3  Virtual Circuit Resource Availability........... A-3
A-3   Detailed Features and Facilities
       Specifications.................................. A-3
A-3.1  Additional Diagnostic Codes..................... A-3
A-3.2  X.25 IP Interoperability Considerations......... A-6
A-3.3  The DDN Logical Addressing Facility............. A-7
A-3.3.1  Logical Addresses............................. A-7
A-3.3.2  Enabling and Disabling Logical Addresses...... A-7
A-4  Limitations of DDN Basic X.25 Service............. A-8
A-5  Derivation of DDN X.25 Addresses.................. A-9

APPENDIX Q: DDN Synchronous Level 1 Specification...... B-1

B-1  Introduction...................................... B-1
B-2  Supported Interfaces.............................. B-1

APPENDIX C:  Federal Information Processing Standard
  Publication 100...................................... C-1




                          TABLES




DDN X.25 Address Fields................................... 7
 "Derivation of Maximum I Frame Size".................... 14
DDN X.25 Physical Signaling Rates and Interfaces......... 15
Additional Packet Level Diagnostic Codes................ A-4
IP Precedence to X.25 Precedence Mapping................ A-6
EIA and CCITT Interchange Circuits...................... B-3
Signal Selection by CCITT Interchange Circuit
  Number................................................ B-4
Typical Level 1 Connection Schemes...................... B-5
Interface Type by Service Speed......................... B-7
RS-232-C Interface...................................... B-8
MIL-188-114 Interface (and equivalents)................. B-9
V.35 Interface......................................... B-10
                          FIGURES




Typical Level 1 Connection Schemes...................... B-4
INTRODUCTION

      This report specifies the attachment of an X.25 host to the
Defense Data Network (DDN).  In particular, this report describes
specific options and features of CCITT Recommendation X.25 (1980)
and Federal Information Processing Standard (FIPS) 100/Federal
Standard (Fed.  Std.) 1041 (July 1983) required of a host X.25
implementation to enable that host to communicate with a DDN X.25
Interface Message Processor ("IMP", the DDN packet switching
node).  This report, in conjunction with FIPS 100/Fed. Std.
1041, should enable DDN host site managers and others planning to
attach a host by means of X.25, rather than the 1822 interface,
to determine, first, whether or not the X.25 implementation of
the host in question is adequate for operation with DDN, and,
second, what options, parameter settings, etc. must or may be
selected for operation with DDN.

      This report assumes that the reader is familiar with CCITT
Recommendation X.25 and FIPS 100/Fed. Std. 1041.  A copy of FIPS
100/Fed. Std. 1041 is attached as Appendix C of this report.

      In this document, the term "Administration" refers to the
Defense Communications Agency (DCA Code B610, Washington, D. C.
20305).



1.1   Background

1.1.1 X.25 and FIPS 100/Federal Standard 1041

      The CCITT Recommendation X.25 describes the interface
between host computers (data terminal equipment, or DTEs) and
data   circuit-terminating   equipment   (DCEs,   which    effect
communication with remote hosts over computer networks) for hosts
operating in the packet mode on public data networks.  The X.25
interface standard is defined as three independent architectural
levels, following the Open Systems Interconnection (OSI)
Reference Model.  The three levels are:

      Level 1:  The PHYSICAL level of the connection.  The
                physical,    electrical,    functional,   and
                procedural characteristics to activate,
_____________
* As used in this report, "1822 interface" refers to the
interface specified in Bolt Beranek and Newman Inc. (BBN) Report
No. 1822, "Specification for the Interconnection of a Host and an
IMP," revision of December 1981.




                               -1-
                maintain, and deactivate the physical link
                between the DTE and the DCE.

      Level 2:  The LINK level of the connection.  The link
                access procedure for data interchange across
                the link between the DTE and the DCE.

      Level 3:  The PACKET level of the connection.  The
                packet format and control procedures for the
                exchange of packets containing control
                information and user data between the DTE and
                the DCE, and between the DTE and a remote
                DTE.


      CCITT Recommendation X.25 contains many options and
implementation choices.  FIPS 100/Fed. Std. 1041, which specifies
the general use of X.25 for the Federal Government, defines some
of the choices left open in X.25.  This document describes the
X.25 interface to a particular network, DDN.  Thus in several
areas where X.25 allows a choice, a single choice appropriate for
DDN is specified; in areas which X.25 leaves unspecified,
addressing in particular, conventions are specified that are
consistent with the overall architecture of DDN and the
interoperability goals described below.  The effect of this
approach is to make DDN service available to hosts in a way that
requires no changes to a host DTE implementation that is
compliant with FIPS 100/Fed. Std. 1041 and CCITT Recommendation
X.25.      By   implementing   extensions  described   in   this
specification, a host will be able to take advantage of
additional DDN features required in military networks, such as
precedence and logical addressing.

      The reader is referred to CCITT Recommendation X.25 and to
FIPS 100/Fed. Std.  1041 for detailed information not provided in
the body of this document.



1.1.2 X.25-to-X.25 and X.25-to-1822 Interoperability

      A   key   goal   of   the   DDN   X.25   implementation   is
interoperability among all DDN subscribers.  That is, effective
communication should be possible, not only between subscribers
attached   to   the  DDN  using  identical  vendor-supplied  X.25
implementations, but between subscribers using different X.25
implementations, and between a subscriber using an X.25 interface
to the DDN and a subscriber using an 1822 interface to the DDN.
Achieving this goal of interoperability requires that all DDN



                               -2-
X.25 subscribers conform to this interface specification and
implement the DoD standard higher level protocols.  True
interoperability among DDN hosts requires, in particular,
implementation of the DoD standard protocols TCP (Transmission
Control Protocol) and IP (Internet Protocol), as well as the
higher-level protocols which implement DDN standard services,
 " when such services are provided by the host:  the Telnet
Protocol
for character-oriented terminal support, the File Transfer
Protocol (FTP) for file movement between hosts, and the Simple
Mail Transfer Protocol (SMTP) for communication between
electronic mail service hosts.

   The DDN X.25 DCE offers two types of service to X.25 DTEs:

      1.   DDN Standard X.25 Service, which, when used in
           conjunction with DoD standard protocols, provides
           interoperable communication between an X.25 DTE
           and other DDN hosts that also implement the DoD
           standard protocols, whether they are connected to
           DDN via the 1822 interface or via the X.25
           interface;

           and

      2.   DDN  Basic   X.25   Service,    which    provides
           communication only between an X.25 DTE and other
           DDN X.25 DTEs implementing compatible higher-level
           protocols.

     Section 2.1.2.1 of this report describes the conventions to
be used by a DTE to specify the type of service desired for each
X.25 virtual call.  All DDN X.25 DTEs will be required to develop
and initiate a plan to use the DoD standard protocol architecture
and DDN standard X.25 service.

     Use of DDN basic X.25 service imposes some restrictions on
the nature of the network communications service that a host can
obtain.  These restrictions are discussed in Appendix A, Section
A-4.













                               -3-
1.2   Compliance

1.2.1 Compliance With CCITT X.25 and FIPS 100/Fed. Std. 1041

      The DDN X.25 Interface Specification is compliant with CCITT
Recommendation X.25 and FIPS 100/Fed. Std.  1041.  The DDN X.25
DCE supports all facilities specified as E (essential) by FIPS
100/Fed.   Std.    1041,  and no  facilities  specified  as  A
(additional).  The additional facilities not supported are:

      (i) datagrams and associated facilities,
            and
      (ii) bilateral closed user groups.

      In that FIPS 100/Fed. Std. 1041 describes features for a
DCE, DDN X.25 DTEs may support any or all facilities specified as
either E or A by FIPS 100/Fed Std. 1041.  However, DDN X.25 DTEs
must not use the facilities identified above that are not
supported by the DDN X.25 DCE.



1.2.2 DTE Compliance With This Specification

      This document specifies several areas in which the DDN X.25
DCE is capable of operating in several modes.  For example,
Section 2.4 lists a number of signaling rates supported by the
DCE.  In such cases, a DDN X.25 DTE must implement at least one
of the options listed (or the set of options required of a DTE by
FIPS 100/Fed.  Std.  1041) but need not implement all of the
options listed (unless required by FIPS 100/Fed.  Std.  1041).
Determining the adequacy of the options supported by a DTE vendor
for meeting a DDN subscriber's requirements is the responsibility
of the subscriber.

      In addition to the CCITT X.25 and FIPS 100/Fed.  Std.  1041
requirements described in Section 1.2.1 above, DDN X.25 DTEs may
wish to take advantage of additional DDN-specific features that
are     compatible   extensions     to   the   public   standards.
Implementation of a DDN-specific feature by a host is required
only if the host wishes to take advantage of the service or
information provided by the feature.  For example, a host that
wishes to establish calls only at the default precedence level
assigned to it need not implement the precedence facility
described in Section 2.1.2.2.  However, a host that wishes to
have flexibility in the precedence of the calls it establishes
must implement this facility.





                               -4-
      Any deficiencies with respect to this specification in a
vendor-supplied X.25 DTE implementation contemplated for use with
the DDN X.25 DCE should be rectified so as to attain compliance
with this specification.  Proper operation with DDN of an X.25
DTE that is not compliant with this specification cannot be
guaranteed and should not be attempted.  To this end, a test
program is available through the Administration.
2 INTERFACE SPECIFICATION

2.1 Call Establishment Conventions

      This section specifies DDN X.25 call establishment
conventions.



2.1.1 Addressing

     DDN addresses are assigned to subscriber DTEs by the
Administration.     Two  basic  forms  of  address  are  provided:
physical addresses, which correspond to the node number and DCE
port number of the node to which the DTE is connected, and
logical addresses, which are mapped transparently by DCE software
into a corresponding physical network address.  Each DTE is
assigned one physical address, and may be assigned one or more
logical addresses.  All DDN addresses are either twelve or
fourteen BCD (binary-coded decimal) digits in length.  A calling
DTE need not determine whether a given address is a physical or
logical address, in order to establish a call to that address.



.2.1.1.1 Address Formats and Fields

     DDN addresses have the following format:

                       ZZZZ F DDDDDDD (SS)

The various fields of the address are presented in Table 2.1 and
are explained below.
                                              Length
           Field     Meaning                (BCD digits)

            ZZZZ     Reserved (must be zero)    4

             F       Flag                       1

          DDDDDDD    DDN Host Identifier        7

            (SS)     Sub-address (optional)   0 or 2

                               TOTAL         12 or 14


                Table 2.1 DDN X.25 Address Fields




                               -6-
2.1.1.1.1 Reserved

      The Reserved field corresponds to the DNIC field generally
used in public data networks.  Pending assignment of a DDN DNIC,
this field must be zero.



2.1.1.1.2 Flag

      The Flag field is used to differentiate physical and logical
addressing.  The value zero indicates physical addressing, while
the value one indicates logical addressing.  A value of nine is
used in the setup of calls to enable and disable logical
addresses; see Appendix A, Section A-3.3.1.



2.1.1.1.3 DDN Host Identifier

      The DDN Host Identifier is a seven-digit address, either
logical or physical, assigned to a subscriber DTE by the DDN
Administration.



2.1.1.1.4 Sub-Address

      The Sub-Address may be used by a DTE for any.purpose.  It is
carried across the network without modification.  Its presence is
optional.



2.1.1.2 Supplying Missing Address Information

      The DDN X.25 DCE incorporates a mechanism to supply
"missing" address information in CALL REQUEST and CALL ACCEPTED
packets received from an attached DTE.  This mechanism is useful
in DTE software testing and physical address determination.

      If a DTE sends a CALL REQUEST packet with no calling address
field, the local DCE will insert the physical calling DDN Host
Identifier with no subaddress field.  If a DTE sends a CALL
REQUEST or CALL ACCEPTED packet with either or both calling or
called addresses that contain F = zero and DDDDDDD = zero, the
local DCE will replace the DDN Host Identifier field (DDDDDDD)
with the physical address of the DTE.




                               -7-
      DTE implementors are cautioned that use of this mechanism in
accepting calls to a DTE's logical address (See Appendix A,
Section A-3.3) can result in confusion on the part of the calling
DTE and is not advised.



2.1.2 DDN-Specific Facilities

      Two DDN-specific features are requested by means of
"private" or non-CCITT facilities in CALL REQUEST and CALL
ACCEPTED packets.  If either or both of these facilities are
requested in a CALL REQUEST or CALL ACCEPTED packet, they must
follow all CCITT X.25 facilities and must be preceded by a single
facility marker, two octets of zero.



2.1.2.1 Type of Service Selection

      The DDN X.25 provides two types of service, DDN basic X.25
service and DDN standard X.25 service.  DDN standard X.25 service
provides only local DTE to local DCE support of the X.25
connection.  Data is carried via the network to its destination
(using protocols internal to the network), where it is delivered
using the access protocol of the destination host (i.e., either
1822 or DDN standard X.25 service).  This access method is
oriented towards DDN X.25 hosts using the DoD standard TCP/IP
higher level protocols.  No X.25 procedures change when using DDN
standard X.25 service; however, the significance of the
procedures changes (see Appendix A, Section A-3.2).  There is no
end-to-end X.25-level acknowledgment or guarantee of delivery of
data packets with DDN standard X.25 service; reliability of DDN
standard X.25 service is provided instead by the use of a
reliable transport protocol.

      DDN basic X.25 service provides end-to-end call management
with significance as described in CCITT Recommendation X.25 and
FIPS 100/Fed. Std.  1041.  This access method is oriented towards
hosts that have existing higher level protocol implementations
that require reliable packet delivery at the network level.

      Selection of DDN standard or DDN basic X.25 service must be
made on a call-by-call basis by the DDN X.25 DTE at the time of
call setup.  To specify DDN standard X.25 service, a DTE must
include in the CALL REQUEST packet a facility two octets long,
coded as follows:

                        00000100 00000001



                               -8-
If this facility is not specified, DDN basic X.25 service will be
provided.



2.1.2.2 Call Precedence

      The precedence of a call is negotiated by an X.25 DTE by
means of a facility two octets long, coded as:

                        00001000 000000XX

where XX is the precedence, from 0 (lowest precedence) to 3
(highest precedence).  If this facility is not used, the call
will be established at the subscriber's default precedence.

      A DTE is not permitted to establish a call at a precedence
level higher than that authorized for that DTE by the
Administration.  An attempt to do so will result in the DDN X.25
DCE returning to the DTE a CLEAR INDICATION packet with clearing
cause 00001001, "Out of order," with diagnostic code 194,
"Requested precedence too high".

      Calls of a lower precedence may be cleared by a DCE if DCE
or other network resources are required, or if access to the
local or remote DTE is required (for a call of higher
precedence).  In this event, a CLEAR INDICATION packet will be
sent with the clearing cause 00000101, "Network congestion," and
with a diagnostic code specifying the reason for the preemption.
The diagnostic codes employed for this purpose are 192, "Cleared
due to higher precedence call at local DCE," and 193, "Cleared
due to higher precedence call at remote DCE".  Similarly, an
attempt to establish a call may be unsuccessful if network
resources are engaged in calls of higher priority than that
requested.  In this case, a CLEAR INDICATION packet will be sent
with the clearing cause 00001001, "Out of order," and with either
diagnostic code 192 or 193, as appropriate.

      The diagnostic codes described in the preceding paragraphs
are DDN-specific diagnostic codes; additional information about
these codes may be found in Appendix A, Section A-3.1.











                               -9-
2.1.3 Protocol Identification

      X.25 DTEs employing the DoD standard TCP/IP protocol
architecture must indicate this by means of the call user data
field of the CALL REQUEST packet.  The first octet of this field
must be set to 11001100 to identify the DoD standard protocol
architecture.

      Indication of the use of the DoD standard protocol
architecture is independent of the selection of DDN standard or
DDN basic X.25 service by means of the facility specified in
Section 2.1.2.1 above.  Therefore, a host employing the DoD
standard protocol architecture and using DDN standard X.25
service must include both the DDN standard X.25 service facility
and the call user data DoD standard protocol identification in
its CALL REQUEST packet.

      A DTE using a protocol architecture other than the standard
DoD protocol architecture is free to use any call user data
protocol identification recognized by the DTEs with which it
wishes to communicate.  Identification of protocol architectures
other than the DoD standard architecture is not standardized or
enforced by the Administration.  Subscribers are cautioned,
therefore, that conflicts among various vendor-assigned protocol
identifications may arise.



2.1.4 Logical Channel Assignment

      The assignment of logical channels by the DDN X.25 DCE
follows the requirements and guidelines of FIPS 100/Fed. Std.
1041 and Annex A of CCITT X.25.  Within the guidelines of CCITT
X.25 Annex A, the range of logical channel numbers assigned to
permanent virtual circuits, incoming, two-way, and outgoing
virtual calls for DDN DCEs is configured for each DTE attached to
a DCE by the Administration.

      DDN X.25 DTEs must follow the logical channel selection
requirements of FIPS 100/Fed. Std. 1041.

      The number of logical channels available to a DTE is
dependent upon the configuration of the DCE to which the DTE is
attached, and upon the dynamic requirements placed upon other
DCEs that share the same DDN packet switching node.







                              -10-
2.2 Packet Level Procedures

       DDN X.25 packet level procedures are as specified by FIPS
100/Fed.  Std.  1041 and CCITT X.25.  The following additional
information is provided:

       1. The maximum window size that may be negotiated is
          seven.

       2. Modulo 128 packet level sequence numbering is not
          supported.

       3. Maximum packet sizes of 16, 32, 64, 128, 256, 512,
          and 1024 octets may be negotiated.

       4. The DDN X.25 DCE uses additional packet level
          diagnostic codes, specified in Appendix A, Table
          A-1.  DDN X.25 DTEs may, but are not required to,
          make use of the information conveyed by these
          codes.

       5. The Qualifier bit (Q-bit) is passed transparently
          by the DDN X.25 DCE in DDN basic X.25 service.
          DTEs using DDN basic X.25 service may use the Q-
          bit in any way that is consistent with FIPS
          100/Fed. Std. 1041.

       6. The DDN X.25 DCE implements the diagnostic packet.
          It is sent under conditions specified in Annex D
          of CCITT X.25.  The DTE is not required to act on
          the information provided in diagnostic packets.

       7. DTEs using DDN standard X.25 service must restrict
          the maximum number of data bits in a complete
          packet sequence to be no more than 8056.  This
          ensures that the data from a packet sequence
          transmitted by an X.25 host will fit within the
          maximum 1822 message length limit upon delivery to
          an 1822 host.  This restriction is necessary as
          existing 1822 host implementations are not re-
          quired to accept messages longer than 8063 bits. *
________________
* DTEs using DDN standard X.25 service will generally be
transmitting Internet Protocol datagrams, the length of which, by
convention, does not approach this limit.  Therefore, unless a
protocol other than the Internet Protocol is used with DDN
standard X.25 service, this is a technical restriction that will
have no practical impact upon the design of DTE software.  See
Appendix A, Section A-3.2.




                              -11-
           DDN X.25 DTEs connecting to DDN through an X.25
           Internet Private Line Interface (IPLI) must reduce
           the maximum complete packet sequence length by an
           additional 256 bits to allow for IPLI overhead.



2.3   Link Level Procedures

      DDN X.25 link level procedures are as specified by FIPS
100/Fed.  Std.  1041 and CCITT X.25.  This section presents
additional information.



2.3.1 Link Level Parameters and Options

      1.  The default value of K, the maximum number of
          sequentially numbered I frames that the DCE will
          have outstanding (unacknowledged) at any given
          time, is seven.  A DDN X.25 DCE may be configured
          on a per-DTE basis to provide optional values of K
          from one to six.

      2.  The default value of N2, the maximum number of
          transmissions   and  retransmissions  of  a  frame
          following the expiration of the T1 timer, is
          twenty.  This value can be changed to any value
          from one to 200 as a DCE configuration parameter
          on a per-DTE basis.

      3.  The optional 32-bit FCS is not supported.



2.3.2 Timer T1 and Parameter T2

      The period of the timer T1 used by the DDN X.25 DCE reflects
assumptions about the processing speed of the DTE.  The DCE
assumes that parameter T2, the response latency of the DTE to a
frame from the DCE, is no greater than 1/2 second.  Likewise, the
DCE guarantees that its parameter T2, the latency in responding
to frames from the DTE, is 1/2 second for signaling rates of
19.2 Kb/s or slower, and 1/4 second for faster links.

      A lower bound for timer T1 may be computed to be 4X +  T2,
based on the assumptions that:

        * the link propagation time is negligible,



                              -12-
        * the worst-case frame transmission time is X,

        * timer T1 is started when a frame is scheduled for
           output,

        * each frame is scheduled just as transmission of
           the previous frame starts,

        * frames are not aborted, and

        * each frame and its predecessor are of maximum
        length Nl = 8248 bits (see Section 2.3.3 below).

      As an example, for a signaling rate of 9.6 Kb/s, this
yields X = .86 sec.  If T2 is .5 sec., the total time for the DTE
to respond in the worst case should be 3.9 seconds.  In fact, the
DCE uses a T1 timer value of 4 seconds for a link speed of 9.6
Kb/s.

      In no case does the DCE use a value for T1 smaller than 3
seconds.  This means that, for faster links, the DTE's T2
parameter may be lengthened because the X term in the above
formula is smaller.  For links of 19.2 Kb/s or faster, DTEs are
expected to satisfy latency requirements that allow the DCE to
use the formula 4X + T2 (DTE) < 3 seconds = T1 (DCE).

      The DTE may choose any value for T1 that is compatible with
the DCE's T2 parameter values.  The value of T1 used by the DTE
may always be set longer than the formula indicates, with the
result  that  recovery  from certain types of link errors will be
slower.  However, the DCE's parameter T2 cannot be reduced, so
the formula should be viewed as yielding a lower bound on the
DTE's T1 timer.



2.3.3 Maximum I Frame Size

      The maximum number Nl of bits in an I Frame is 8248,
accommodating a data packet with up to 1024 data octets.  The
derivation of this number is shown in Table 2.2.

      DTEs using DDN standard X.25 service must observe the
restriction on the number of data bits in a complete packet
sequence given in Section 2.2 above.







                              -13-
                                      X.25 No. of
            Field  Name              Level   Bits

            Address                     2       8
            Control                     2       8
            General Format Identifier   3       4
            Logical Channel Number      3      12
            Packet Type                 3       8
            User Data                   3    8192 (max)
            Frame Check Sequence        2      16

                                TOTAL        8248 (max)


          Table 2.2 Derivation of Maximum I Frame Size


2.4 Physical Level Specifications

     The DDN X.25 physical level specification is in conformance
with FIPS 100/Fed.  Std.  1041 and CCITT X.25.  This section
presents additional information.

     A DDN X.25 DTE may either be collocated with its DCE or may
be connected to it via an access line.  In all cases the DTE
presents a physical DTE interface; the DDN will supply the
matching DCE interface.  DDN X.25 service offers four physical
level interfaces:  RS-232-C (CCITT V.28), RS-449, both balanced
and unbalanced (CCITT V.ll and V.10, respectively; also MIL-188-
114 balanced and unbalanced), and CCITT V.35.  Appendix B of this
document describes in detail the choices of physical interface
available to the DDN subscriber and the specifications for each
type of interface.  Table 2.3, below, summarizes the physical
interfaces available at each data rate supported by the DDN X.25
DCE, and indicates which interfaces are recommended at each
signaling rate.

     A DDN X.25 DTE may implement any or all of the signaling
rates shown.  At each signaling rate implemented, the DTE must
offer at least one of the physical interface options listed as
"R" (recommended) or "A" (available) for that rate in Table 2.3.
Implementors are encouraged to offer the widest variety of
signaling rates and physical interfaces practical to maximize
the ease of use of their equipment in DDN.






                              -14-
Physical                    Signaling Rate in Kb/s
Interface         1.2  2.4 4.8  9.6 14.4  48   50   56   64   100

RS-232-C          R    R    R    R   R    -     -   -     -   -

RS-449 unbal.     A    A    A    A   -    -     -   -     -   -
(and equiv.)

RS-449 balanced   A    A    A    A   A    A    A    A    A    R
(and equiv.)

CCITT V.35        -    -    -    -   -     R    A   R    R    A

                  Legend

                  R = Recommended
                  A = Available
                  - = Not available


                (Taken from Appendix B, Table B-4

   Table 2.3 DDN X.25 Physical Signaling Rates and Interfaces
3 BIBLIOGRAPHY

1.  "Specification for the Interconnection of a Host and an IMP".
    Report No.  1822, Bolt Beranek and Newman Inc" Cambridge,
    MA, revision of December 1981.

2.  CCITT Recommendation X.25, "Interface Between Data Terminal
    Equipment (DTE) and Data Circuit Terminating Equipment (DCE)
    for Terminals Operating in the Packet Mode on Public Data
    Networks,"     International     Telegraph    and    Telephone
    Consultative Committee Yellow food, Vol.  VIII.2, Geneva,
    1981.

3.  "Defense Data Network Subscriber  Interface  Guide,"  Defense
    Communications Agency, Washington, DC, July 1983.

4.  "Internet Protocol Transition Workbook,"  SRI  International,
    Menlo Park, CA, March 1982.

5.  "Internet Protocol Implementation Guide," SRI  International,
    Menlo Park, CA, August 1982.
          APPENDIX A:  DDN X.25 Implementation Details




A-1  Introduction

     This Appendix serves three purposes.  First, it provides
information concerning the planned evolution of DDN X.25
capabilities.  Second, it provides information on the use of
certain DDN X.25 features and facilities at a greater level of
detail than is appropriate for inclusion in the body of the DDN
X.25 Interface Specification.  Specifications for the use of DDN
X.25 features and facilities given in this Appendix are mandatory
on the part of DDN X.25 DTEs that wish to make use of these
features and facilities.  Finally, this Appendix presents a
discussion of the limitations on the use of DDN services that
will be encountered by hosts using only DDN basic X.25 service.



A-2  Operational Features of DDN X.25 DCE Releases

     The capabilities of the DDN X.25 DCE will evolve over time
from an initial set of capabilities to the full capabilities of
this DDN X.25 Interface Specification.  This section describes
release-dependent features of the DDN X.25 DCE.  Implementors
should note that not all optional facilities of the specification
will initially be available for use by DTEs.

     Releases of new DCE capabilities will be compatible with DTE
hardware and software implementations that meet the full DDN X.25
Interface Specification.



A-2.1 Initial Feature Support

     The initial release of the DDN X.25 DCE will support flow
control parameter negotiation and fast select.  In addition, the
DDN X.25 DCE may be configured by the DDN Administration to
provide non-standard default window and packet sizes as described
in CCITT X.25 Sections 7.1.2 and 7.2.1.  The call precedence and
type of service selection facilities will be accepted, but not
acted upon, by the network.  Only DDN basic X.25 service will be
supported.    Planned   future DCE  releases  will  support  all
facilities specified in FIPS 100/Federal Standard 1041 with the
exception of those "additional" facilities that are listed in
Section 1.2.1 of this document.




                               A-1
     A detailed schedule of DDN X.25 DCE releases and the
capabilities of each release will be supplied in a separate
document.



A-2.2 Exception-Handling Procedures

     Certain of the exception- or error-handling procedures of
the initial release of the DDN X.25 DCE differ in detail from the
procedures specified in FIPS 100/Federal Standard 1041.  These
differences are described below.  A later release of the DDN X.25
DCE will bring these procedures into conformance.  In the
interim, the variances in these procedures will not preclude
satisfactory operation between the DCE and a DTE, provided the
DTE operates in accordance with FIPS 100/Federal Standard 1041.



A-2.2.1 Non-Octet-Aligned Data

     Data packets received by the DDN X.25 DCE that are not
aligned on an octet boundary are discarded at the link level.
They are not passed to the DCE packet level, and no packet level
diagnostic code is returned to the DTE.



A-2.2.2 RESTART REQUEST Packet

     The DDN X.25 DCE will not discard, but will instead act
upon, a RESTART REQUEST packet that

       (i) is too long (unless it exceeds the maximum frame
           size for the link level),

           or

     (ii) contains a non-zero cause field.



A-2.2.3 RESET REQUEST Packet

     The DDN X.25 DCE will not discard, but will instead act
upon, a RESET REQUEST packet that contains a non-zero reset cause
field.





                               A-2
A-2.2.4 CLEAR REQUEST Packet

     The DDN X.25 DCE will not discard, but will instead act
upon, a CLEAR REQUEST packet that contains a non-zero clearing
cause field.



A-2.3 Virtual Circuit Resource Availability

     In its current implementation, the DDN X.25 packet switching
node is capable of supporting a minimum of one hundred
simultaneous virtual circuits.  As was discussed in Section
2.1.4, resources of the node are shared dynamically among the
DCEs attached to the node.  Therefore, no explicit guarantees are
made of the number of simultaneous virtual circuits that can be
made by a single DTE.  Depending upon the configuration of the
node, the number of simultaneous circuits supported by the node
can be significantly greater than one hundred.



A-3  Detailed Features and Facilities Specifications

     This  section   provides   detailed   specifications    and
descriptions of use for certain DDN X.25 features and facilities.



A-3.1 Additional Diagnostic Codes

     The DDN X.25 DCE is capable of providing additional
information to DTEs in RESTART, RESET, CLEAR INDICATION, and
DIAGNOSTIC packets by means of diagnostic codes that are
extensions to the set of diagnostic codes given in Annex E of
CCITT Recommendation X.25.  These codes are taken from the set of
codes "reserved for network specific diagnostic information," and
are thus not in conflict with code assignments made in Annex E.
The values of these codes, and their meanings, are given in Table
A-1 below.












                               A-3
Code
Value Meaning

  128  IMP   is   unavailable.    The   packet-forwarding
      mechanisms of the network are unavailable to the
      DCE.  Sent in RESET, CLEAR and RESTART packets.

  130 Link level came up.  Sent in RESTART and RESET
      packets.

  131 Link level went down at remote DTE.  Sent in CLEAR
      and RESET packets.

132   Remote DTE restarted.  Sent in CLEAR and RESET
      packets.

  133 Local   resources   not   available    for    call
      establishment.     The   local   DCE  has  too  few
      resources to establish another call.  Sent in
      CLEAR and DIAGNOSTIC packets.

134   Remote   resources   not   available   for    call
      establishment.     The   remote  DCE  has  too  few
      resources to establish another call.  Sent in
      CLEAR packets.

136   Remote host dead.  The link to the remote DTE is
      down.  Sent in CLEAR and RESET packets.

137   Remote IMP dead.  The IMP to which the remote DTE
      is attached is down.  Sent in CLEAR and RESET
      packets.

138   Logical subnetwork access barred.  The remote DTE
      cannot be reached because of a communities-of-
      interest prohibition.  Sent in CLEAR and RESET
      packets.

139   Connection lost.  An internal error has occurred
      at either the remote or the local DCE which has
      made their virtual circuit data structures
      inconsistent.  Sent in CLEAR and RESET packets.

140   Response lost.  A response from the remote DCE
      failed to arrive within a reasonable time.  Sent
      in CLEAR and RESET packets.





                          A-4
141 Calling  logical  address  not  enabled   or   not
    authorized.  Sent in CLEAR packets.

142 Calling logical name incorrect for this DTE.  Sent
    in CLEAR packets.

143 Called logical name not authorized.  Sent in CLEAR
    packets.

144 Called logical name not enabled.   Sent   in  CLEAR
    packets.

145 Called logical name has no enabled DTEs.  Sent   in
    CLEAR packets.

146 Use of logical addresses invalid in this network.
    Sent in CLEAR packets.

147 Declared logical name  now  in  effect.   Sent   in
    CLEAR packets.

148 Declared logical name was already in effect.  Sent
    in CLEAR packets.

149 Declared logical name is now  disabled.   Sent   in
    CLEAR packets.

150 Declared logical name was already disabled.  Sent
    in CLEAR packets.

151 Incoming calls barred.  Sent in CLEAR packets.

152 Outgoing calls barred.  Sent in CLEAR packets.

192 Cleared due to higher  precedence  call  at   local
    DCE.  Sent in CLEAR packets.

193 Cleared due to higher precedence call at remote
    DCE.  Sent in CLEAR packets.

194 Requested precedence too high.  The DTE is not
    authorized to establish a call at the requested
    precedence level.  Sent in CLEAR packets.


Table A-1. Additional Packet Level Diagnostic Codes






                          A-5
A-3.2 X.25 IP Interoperability Considerations

      When DDN standard X.25 service is requested at call
establishment (as described in Section 2.1.2.1), the call is in
effect established between the DTE and a local X.25 entity.  This
entity subsequently extracts the IP datagrams from the X.25 data
packets for transmission through the DDN Internet.  This approach
requires that certain conventions be followed:

      1. IP datagrams are  to  be  sent  as  X.25  complete
         packet sequences.  That is, datagrams begin on
         packet boundaries and the M ("more data") bit is
         used for datagrams that are larger than one
         packet.  Only one IP datagram is to be sent per
         X.25 complete packet sequence.

      2. By convention, the maximum IP datagram size is 576
         octets.  This packet size can most efficiently be
         accommodated by negotiating an X.25 maximum packet
         size of 1024; alternatively, a DTE may use an X.25
         complete packet sequence to transmit an IP
         datagram.

      3. Because the X.25 connection is in effect
         terminated locally, the D and Q bits have no
         significance and should be set to zero.

      4. The precedence bits of the IP type-of-service
         field are to be mapped into X.25 precedence bits
         (see Section 2.1.2.2) as specified in Table A-2.


               IP Precedence     X.25 Precedence

                   000                  00
                   001                  01
                   010                  10
                 011 - 111              11


      Table A-2. IP Precedence to X.25 Precedence Mapping











                              A-6
A-3.3 The DDN Logical Addressing Facility

     The DDN logical addressing facility allows references to
hosts by either their physical network address or by one or more
location-independent logical addresses, and allows hosts to
exercise partial control over the logical address(es) by which
they can be referenced.  Implementation of DDN logical addressing
by a host is optional.

     The DDN Administration will assign seven-digit logical
addresses, and will maintain a logical addressing data base.  The
host is then responsible for notifying the network ("enabling")
of the "names" (logical addresses), if any, by which it wishes to
be known.  It cannot receive calls addressed to a name or
originate calls under that name unless it has enabled that name.
It also cannot enable a name that is not authorized for that
physical address.  Names can also be enabled automatically by the
network, under the control of the Administration.



A-3.3.1 Logical Addresses

     Logical addressing is invoked when a called address is
supplied to the IMP with the flag digit F = one.  The logical
address consists of seven BCD digits.  This name is mapped by the
logical addressing facility into a DDN physical network address.
The logical name need not be unique for the physical address, nor
is the physical address necessarily unique for the name.



A-3.3.2 Enabling and Disabling Logical Addresses

     To enable and disable logical addresses, the DDN X.25 host
must send declarative CALL REQUEST packets to the DCE using a
called address with the format:

                      ZZZZ F DDDDDDD (SS)

where the address fields are as described in Section 2.1.1.  The
Flag F must be set to nine, the DDN Host Identifier field
specifies the logical address under consideration, and the
subaddress field, which must be present, specifies the type of
transaction.  Declarative calls are cleared immediately by the
local DCE.







                              A-7
      If SS is zero, the logical name is enabled in normal mode,;
that  is,  that  physical port will accept incoming calls to that
name, and allow outgoing calls from that name.  If SS is one, the
logical name is disabled.  If SS is two, the logical address is
enabled in reverse translation mode; in this mode, the called
address field of incoming call packets will be translated into a
physical address (i.e., an address containing a flag F = 0), if
it was given by the calling DTE (X.25 host), as a logical address
(i.e., containing a flag F = 1).

      Whenever a DTE comes up, or restarts, the logical names for
that DTE are returned to their default state, which may be either
enabled or disabled, as configured by the DDN Administration.



A-4   Limitations of DDN Basic X.25 Service

      The Defense Data Network is an Internetwork environment.
That  is,  DDN  as  a whole is made up of a number of constituent
packet switching networks that are interconnected via gateways.
Communication across gateways requires the use of the Internet
Protocol, which, for a host accessing DDN using X.25, requires
that  the  host  implement the DoD standard protocol architecture
and employ DDN standard X.25 service.  In addition, a classified
host is attached to a DDN constituent network of lower
classification by means of an Internet Private Line Interface
(IPLI).   IPLIs,  which themselves contain gateways, also require
the use of the Internet Protocol; moreover, they do not, as
currently designed, offer an X.25 host interface.  These
attributes of the DDN Internet have two implications for users of
DDN basic X.25 service:

      1.  DDN hosts that do not implement IP and higher-
          level DDN protocols, and which use only DDN basic
          X.25 service, cannot communicate across gateways.
          Their    network    communication   is   therefore
          restricted to a single DDN constituent network.

      2.  X.25 hosts cannot be provided classified service
          on a constituent network of lower classification.
          Should X.25 host access be developed for the IPLI
          in the future, classified network access will be
          made available to hosts using DDN standard X.25
          service only.







                              A-8
  A-5 Derivation of DDN  X.25 Addresses

        All DDN hosts are assigned addresses by the Administration.
  The address of a DDN host may be obtained from the Network
  Information Center (NIC), represented as an ASCII text string in
  what  is  called "host table format".  This section describes the
  process by which DDN X.25 addresses in the format described in
  Section 2.1.1 may be derived from addresses in NIC host table
  format.

        A NIC host table address consists of the ASCII text string
  representations of four decimal numbers separated by periods,
  corresponding to the four octets of a thirty-two bit Internet
  address.  The four decimal numbers are referred to in this
  section as "n", "h", "l", and "i."  Thus, a host table address
  may be represented as "n.h.l.i"  Each of these four numbers will
  have either one, two, or three decimal digits and will never have
  a value greater than 255.  For example, in the host table address
  "10.2.0.124", n=10, h=2, l=0, and i=124.  To convert a host table
  address to a DDN X.25 address:

        1.  If h < 64, the host table address corresponds to
            the DDN X.25 physical address

                           ZZZZ F IIIHHZZ (SS)

            where:

                 ZZZZ = 0000
                         as required in Section 2.1.1.1.1;

                 F = 0   because the address is a physical
                         address;

                 III     is a three decimal digit
                         representation of "i", right-adjusted
                         and padded with leading zeros if
                         required;

                 HH      is a two decimal digit representation
                         of "h", right-adjusted and padded
                         with leading zeros if required;,

                 ZZ = 00
                         and

                 (SS)    is optional, as described in Section
                         2.1.1.1.4.




                              A-9
          In the example given above, the host table address
          10.2.0.124 corresponds to the DDN X.25 physical
          address 000001240200.

      2.  If h > 64 or  h  =  64,   the  host   table  address
          corresponds to the DDN X.25 logical address

                      ZZZZ F RRRRRZZ (SS)

          where:

               ZZZZ = 0000
                      as required in Section 2.1.1.1.1;

               F = 1  because the address is a logical
                      address;

               RRRRR  is a five decimal digit
                      representation of the result "r" of
                      the calculation

                                 r = h * 256 + i

                      (note that the decimal representation
                      of "r" will always require five
                      digits);

               ZZ = 00
                      and

               (SS)   is optional, as described in Section
                      2.1.1.1.4.

          Thus,    the host    table   address    10.83.0.207
          corresponds   to  the  DDN  X.25  logical  address
          000012145500.

     In both cases, the "n" and "l"  fields  of  the  host   table
address are not used.













                              A-10
       APPENDIX B:  DDN Synchronous Level 1 Specification



B-1  Introduction

     A host may connect to the Defense Data Network at the link
level using the asynchronous bit serial protocol described in BBN
Report No. 1822 as either a local host (LH) or a distant host
(DH).  A host may also connect to the DDN by means of a
synchronous bit serial protocol at the link level, using either
the method described in BBN Report No. 1822, HDH, or the DDN X.25
interface.  Neither LH nor DH is recommended for new
implementations.

     This section describes the functional, electrical, and
mechanical connection (the level 1 connection) that is required
when either an HDH or an X.25 host is connected to the DDN.
Hosts connecting to the DDN via HDH or X.25 require a synchronous
modem connection or the equivalent, which will be supplied as
part of the DDN service.  The host will present the DTE interface
while the DDN-provided equipment will present the DCE interface.

     A long-term goal of the DDN is for all level 1 connections
to be accomplished with the MIL-188-114 balanced interface.  Its
general equivalents are EIA RS-449/422, CCITT V.ll, and Fed. Std.
1031/1020.  The DDN cannot implement this at present due to the
limited availability of commercial vendor hardware.  In order to
facilitate future DDN compatibility, all new system acquisitions
should specify MIL-188-114 balanced as a required interface, in
addition to an alternate interface.  The selection of an
alternate interface should not preclude utilization of the MIL-
188-114 balanced interface when it becomes supportable.



B-2  Supported Interfaces

     DDN presently supports four synchronous level 1 interfaces.
They are:

     1.  EIA RS-232-C, CCITT V.28 & V.24;

     2.  MIL-188-114 balanced, EIA RS-449&422, CCITT V.ll,
          Fed. Std. 1031/1020;

     3.  MIL-188-114 unbalanced, EIA RS-449&423, CCITT
         V.10, Fed. Std. 1031/1030; and




                              B-1
     4.  CCITT V.35.

     Table B-1 is a dictionary of terms that relates the CCITT
signal ID to the EIA signal ID and to the more common
abbreviations.  Table B-2 identifies signals as either required,
optional, or not used.

     Figure B-1 and Table B-3 identify typical DTE connections to
the DDN.  The required subscriber services will dictate which
scheme is selected for a particular DTE.

     Table B-4 relates required speed of service to interface
type.

     Together, these tables and figures serve as a guide to level
1 interface selection.  From these, most systems will be able to
identify  the  most   appropriate   interface.    However,   this
information is not all-inclusive.  Other interface arrangements
may be possible; contact your DDN representative for assistance
as required.


           Demarcation Point
          (mating connectors)

               DTE DCE

    |------------] [------(1) Modem    RS-232-C
    |
    |  |---------] [------(2) Modem    V.35
|---|--|----|
|           |----] [------(3) LDM      RS-232-C, MIL-188-119
|           |
|           |----] [------(4) Null Modem  Cable
|    HOST   |
|           |----] [------(5) SME      Cable plus clock source
|           |
|           |----] [------(6) DCS      MIL-188-114
|--|--|--|--|
   |  |  |-------] [------(7) DES      RS-232-C, RS-449, V.35
   |  |
   |  |----------] [------(8) KG       MIL-188-114 balanced
   |
   |-------------] [------(9) IPLI     MIL-188-114 balanced


         Figure B-1. Typical Level 1 Connection Schemes





                              B-2
EIA      CCITT      ABBRM      NAME
ID        ID         NAME
---      -----       ------    ---------------------------------
AA        101        FG        Frame (Chassis/Protective) Ground
AB        102        SG        Signal/Supply Common
SC        102a       --        RS-449 DTE Common
RC        102b       --        RS-949 DCE Common
BA        103        TD        Transmit Data
BB        104        RD        Receive Data
CA        105        RTS       Request to Send
CB        106        CTS       Clear to Send
CC        107        DSR       Data Set Ready
CD        108.2      DTR       Data Terminal Ready
CF        109        DCD       Data Carrier Detect
CG        110        SQ        Signal Quality
CH        111        --        Signal Rate Selector to DCE
CI        112        --        Signal Rate Selector to DTE
DA        113        ETC       External Transmit Clock
DB        114        TC        Transmit Clock
DD        115        RC        Receive Clock
--        116        --        Select Standby
--        117        --        Standby Indicator
SBA       118        STD       Secondary Transmit Data
SBB       119        SRD       Secondary Receive Data
SCA       120        SRS       Secondary Request to Send
SCB       121        SCS       Secondary Clear to Send
SCF       122        SCD       Secondary Carrier Detect
SCG       123        SSQ       Secondary Signal Quality
--        124        --        Select Frequency Group
CE        125        RI        Ringing Indicator
--        126        --        Select Transmit Frequency
--        127        --        Select Receive Frequency
--        128        --        External Receive Clock
--        129        RR        Request to Receive
--        130        --        Secondary Transmit Tone
--        131        --        Receive Character Timing
--        132        --        Return to Non-Data Mode
--        133        RTR       Ready to Receive .
--        134        --        Received Data Present
--        136        --        New Signal
--        140        RL        Remote loopback
--        141        LL        Local loopback
--        142        TM        Test Status Monitor
--        191        --        Transmit Voice Answer
          192        --        Receive Voice Answer


          Table B-1. EIA and CCITT Interchange Circuits



                               B-3
    Required: 101, 102, 103, 104, 105, 106, 107, 108.2,
               109, 113, 114, and 115

    Optional:  110, 125, 140, 141, and 142
               (These may be required IAW future DDN
               developments; it is strongly recommended
               that these at least be available for
               implementation upon requirement)

    Not used: 111, 112, 116, 117, 118, 119, 120, 121, 122,
              123, 124, 126, 127, 128, 129, 130, 131, 132,
              133, 134, 136, 191, and 192












Table B-2. Signal Selection by CCITT Interchange Circuit Number











                              B-4
Scheme (From
  Fig. B-1)     Explanation

(1) Modem       RS-232 at spe eds of 1200, 2400, 4800, 9600 or
                14400 b/s over long haul leased voice grade
                telephone facilities

(2) Modem       CCITT V.35 at speeds of 48, 50, 56, 64 Kb/s over
                leased group (37KHz) grade facilities or in CONUS
                the Digital Data Service facilities.

(3) Limited Distance Modem
                LDM generally available at 9600 b/s and below in
                an RS-232 version.  Other types are available for
                all speeds.

(4) Null modem  A Null Modem is a length of cable with the signal
                leads crossed so as to present a DCE interface.
                To be used in local connection schemes where
                either the DTE or the DCE has a clocking source
                capability.  All four supported level 1
                interfaces are available.  If DTE clock and DCE
                clock are both available, DTE clock will be
                preferred.

(5) Synchronous Modem Eliminator
                SME is a length of cable with a hardware device
                interjected.  The device allows convenient
                crossing of signals so as to present a DCE
                interface.  The device also provides clocking
                when neither the DTE nor the DCE has such
                capability.  All four supported level 1
                interfaces are available.

(6) DCS Microwave
                DCS is generally a military microwave system
                which provides the MIL-188-114 balanced or
                unbalanced interfaces.  It implies a speed of 50
                Kbps and is usually found O-CONUS.  Selection of
                this scheme requires selection of (4) or (5).

(7) Data Encryption Standard
                DES is a commercial encryption device used by the
                DoD as a privacy device.  DES is available with
                either RS-232, V.35, or RS-449/422.




                              B-5
(8) KG          KG devices are U. S. Government encryption
                devices under strict NSA control.  The
                requirement for security and KG devices requires
                the selection of the MIL-188-114 balanced
                interface.

(9) Internet Private Line Interface
                IPLI devices are security level community of
                interest isolation devices.  The requirement for
                IPLI service requires the selection of the MIL-
                188-114 balanced interface.


    Notes and Considerations

1.  Interface (2), Modem, 48Kb/s is generally only
    available O-CONUS.

2.  MIL-188-114 balanced is deemed equivalent to RS-449
    with RS-422, the difference being that MIL-188-114 is
    more tolerant of noise on signal common and more
    tolerant of common mode noise.

3.  MIL-188-114 unbalanced is deemed equivalent to RS-449
    with RS-423.  In most cases where MIL-188-114 balanced
    is specified, MIL-188-114 unbalanced is also available,
    but it is not recommended.

4.  There are system enhancements under long term
    development for use in the DDN which may request
    additional control leads beyond those listed as
    required.  The implementation of these enhancements
    will not limit operational capabilities but may impact
    the ability of the Network Monitoring Center to assist
    with host and host access line diagnosis.  These
    enhancements may request signals from the optional
    category.










        Table B-3. Typical Level 1 Connection Schemes




                             B-6

                             Signaling Rate in Kb/s
Physical
Interface        1.2  2.4  4.8  9.6 14.4  48   50   56   64   100

RS-232-C         R    R    R    R    R*    -   -    -    -    -

MIL-188-114      A    A    A    A    -     -   -    -    -    -
unbal. (& equiv.)

MIL-188-114      A    A    A    A    A*   A    A    A    A    R**
bal. (& equiv.)

CCITT V.35       -    -    -    -    -     R   A    R    R    A

                      Legend

                 R = Recommended
                 A = Available
                 - = Not available
                 * = Only available using modems
                 ** - Only available using a local cable
                     connection












                          Table B-4. Interface Type by Service
Speed








                              B-7

Signal Name          Abbrev   Pin No.   EIA ID     Signal Source
-----------          ------   -------   ------     -------------
Frame Ground          FG         1        AA          DTE/DCE
Transmitted Data      TD         2        BA            DTE
Received Data         RD         3        BB            DCE
Request to Send       RTS        4        CA            DTE
Clear to Send         CTS        5        CB            DCE
Data Set Ready        DSR        6        CC            DCE
Signal Ground         SG         7        AB          DTE/DCE
Data Carrier Detect   DCD        8        CF            DCE
Transmit Clock        TC        15        DB            DCE
Receive Clock         RC        17        DD            DCE
Data Terminal Ready   DTR       20        CD            DTE
Ext. Transmit Clock   ETC       24        DA            DTE
Wired Spare           --        18        --            ---
Wired Spare           --        22        --            ---
Wired Spare           --        25        --            ---

     Required pins: 1, 2, 3, 4, 5, 6, 7, 8, 15, 17, 20, 24
     Optional pins: 9, 10, 18, 22, 25

         Notes

     1.  The DTE will present a CANNON DB-25P male connector
       with pinouts as above or equivalent hardware with
         identical pinouts.

     2.  The DCE will present a CANNON DB-2SS female
         connector or equivalent.







                 Table B-5. RS-232-C Interface







                              B-8
Signal Name       Abbrev      Pin Nos    EIA ID    Signal Source
-----------       ------      -------    ------    -------------
Send D ta           SD         4,22        BA           DTE
Send Timing         ST         5,23        DB           DCE
Receive Data        RD         6,24        BB           DCE
Request to Send     RTS        7,25        CA           DTE
Receive Timing      RT         8,26        DD           DCE
Clear to Send       CTS        9,27        CB           DCE
Local 100pback      LL         10          --           DTE
Data Mode           DM         11,29       CC           DCE
Terminal Ready      TR         12,30       CD           DTE
Receiver Ready      RR         13,31       CF           DCE
Remote 100pback     RL         14          --           DTE
Terminal Timing     TT         17,35       DA           DTE
Test Mode           TM         18          --           DCE
Signal Ground       SG         19          AB         DTE/DCE
Receive Common      RC         20          RC           DCE
Send Common         SC         37          SC           DTE
Wired Spare         --          1          --           ---
Wired Spare         --         3,21        --           ---

     Required pins:  4,22; 5,23; 6,24; 7,25; 8,26; 9,27,;
                    11,29; 12,30; 13,31; 17,35; 19; 20; 37
     Optional pins:  10; 14; 18; 1; 3,21

         Notes:

     1.  The DTE will present a CANNON DC-37P male connector
         with pinouts as above or equivalent hardware with
         identical pinout.

     2.  The DCE will present a CANNON DC-37S female
         connector or equivalent.






      Table B-6. MIL-188-114 Interface (and equivalents)







                              B-9
Signal Name         Abbrev    Pin Nos.     EIA ID    Signal Source
-----------         ------     --------     ------    
-------------

Frame Ground          FG        A          AA         DTE/DCE
Signal Ground         SG        B          AB         DTE/DCE
Transmit Data         TD        P/S        BA           DTE
Receive Data          RD        R/T        BB           DCE
Request to Send       RTS       C          CA           DTE
Clear to Send         CTS       D          CB           DCE
Data Set Ready        DSR       E          CC           DCE
Data Carrier Detect   DCD       F          CF           DCE
Local 100pback        LL        K          --           DTE
Ext. Transmit Clock   ETC       U/W        DA           DTE
Transmit Clock        TC        Y/aa       DB           DCE
Receive Clock         RC        V/X        DD           DCE

    Required Pins: A; B; P/S; R/T; C; D; E; F; U/W; Y/aa;
                    V/X
    Optional Pins:  K

        Notes:

      1.  The DTE will present a Winchester MRA(C)-34D-JTCH-H8
          male connector with pinout as above or equivalent
          hardware with the identical pinout.

      2.  The DCE will present a mating female connector.









                    Table B-7. V.35 Interface







                             B-10








    APPENDIX C

  FEDERAL INFORMATION
           PROCESSING STANDARDS PUBLICATION 100

                         FEDERAL STANDARD 1041

                           1983 JULY 6

                  ANNOUNCING THE JOINT STANDARD FOR
            INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE)
              AND DATA CIRCUIT-TERMINAL EQUIPMENT (DCE)
               FOR OPERATION WITH PACKET-SWITCHED DATA
                        COMMUNICATIONS NETWORKS

Federal Information Processing Standards Publication are developed
and issued
by the National  Bureau of  Standards pursuant  to section 
111(f)(2) of  the
Federal Property and Administrative Services Act of 1949, as
amended,  Public
Law 89-306 (79 Stat.1127), Executive order  11717 (38 FR 12315
dated May  11,
1973), and Part 6 of Title 15 Code of Federal Regulations (CFR).

Federal Standards  in the  "telecommunication" series  are
developed  by  the
Office of the Manager, National Communication System. These Federal
Standards
are issued by the General Services Administration pursuant to the
Federal
Property and Administrative Services Act of 1949, as amended.

Name of Standard: Interface  Between Data Terminal  Equipment (DTE)
and  Data
Circuit-Terminating Equipment (DCE) for  Operation with
Packet-Switched  Data
Communications Networks.

Category of Standard:  Hardware, Data Transmission.

Explanation: Federal  automated  data  processing  equipment, 
services,  and
telecommunication equipment using public packet-switched data 
communications
networks (PSDCN) based on  the family of  CCITT Recommendations
derived  from
X.l and X.2 shall employ the interface and protocols specified in
this  joint
standard. In addition, designers of these internally operated and 
maintained
Federal networks employing packet-switched technology should
consider the use
of this interface as appropriate. The joint standard provides:

    - A  family  of  physical  layer  interfaces,  from  which  a 
particular
      interface may be selected; and
    -  A single data link layer control procedure; and
    - Packet  level  procedures  for  virtual  calls  and 
permanent  virtual
      circuits, and an optional datagram  operation.
       

The mandatory interface attributes of  this joint standard are
summarized  as
follows:

PHYSICAL LEVEL

   Transmission rates:      2.4, 4.8, 9.6 Kbits/s

   Interface:              one or more of the following: RS-232-C,
X.2l, RS-449

LINK LEVEL:

   Procedure:               LAPB

   Parameter K:             7

   Smallest N l:            l64 Octets


PACKET LEVEL:

    Services:               Virtual call and permanent virtual
circuit

    Packet types:           All basic plus Diagnostic packets.
Packet Reject
                            shall  not be used.


    User data field         Octet-aligned
       length:


    Packet sequence         Modulo 8
      numbering:

    D bit procedure:        Supported  by all DCEs;  DTE need not 
employ the
                            D  bit when sending to
                            the DCE, but no DTE shall reject
incoming packet
                            with the D bit set to l or  0 as having
this bit
                            in error unless it is known by receiver
that the
                            sender  has no D bit capability.
                            
    X.25 diagnostic         Use standard codes whenever they apply;
non-std
      codes:                codes may be used for events not listed
in X.25
                            within a period of 24 months after the
effective
                            date of this standard.
     
    Fast Select:            DCEs shall implement fast select; DTE
need not
                            employ fast select when sending to DCE,
but all 
                            DTEs with higher level functionality
which
                            allows response to fast select must be
able to
                            accept incoming fast  select packet.

    Interrupt packet:       Receipt of a DTE interrupt packet
before a
                            previous DTE interrupt  packet has been
confirmed
                            is an error condition.
                           
    Duplicated facility     The last appearing facility code should
be
       codes:               treated by the DTE as if it  were the
only 
                            appearance of that code.

    Non-zero cause field    Discarded
      of restart request
      packet:

    Restart request too     Discarded
      long in state r1:

This joint standard  is intended  to enhance  interoperability by 
specifying
certain subsets and other constraints on Federal use of CCITT 
Recommendation
X.25.

The Government's intent  in employing this  joint standard is  to
reduce  the
cost of  acquiring and  using Federal  automated data  processing 
equipment,
services, and telecommunication equipment with PSDCN.  The joint
standard  is
also intended to reduce the cost  of acquiring and using
Government-owned  or
leased PSDCN. These goals will be achieved by:
    -  increasing the available alternative sources of supply;
    -  Increasing the reutilization of Government resources; and,
    -  Assuring the required interoperability.

Approving Authority: Secretary  of Commerce  (Federal Information 
Processing
Standards).   Administrator,   General   Services   Administration 
 (Federal
Standards).

Maintenance Agency: The National  Bureau of Standards and  the
Office of  the
Manager, National Communications System  will jointly maintain this 
standard
coordinating as necessary with the General Services Administration
(GSA).

Cross Index: The following are related standards upon which this
FIPS PUB  is
based. The  inclusion  of  a  particular  standard  on  this  list 
does  not
necessarily mean that the standard is  applicable in all cases to
which  this
FIPS PUB applies.

   (a) International Standard 2110-1980: Data Communication-25 pin
DTE/DCE
Interface Connector and Pin Assignments.
   (b) International Telegraph and Telephone Consultative Committee
(CCITT)
recommendations V.24 (1980): List of Definitions for Interchange
Circuits
Between Data Terminal Equipment and Data Circuit Terminating
Equipment.
   (c) CCITT Recommendation V.28 (1980) Electrical Characteristics
for
Unbalanced Double-Current Interchange Circuits.
   (d) Electronics Industries Association (EIA) RS-232-C (1969
August):
Interface Between Data Terminal Equipment and Data Communication
Equipment
Employing Serial Binary Data Interchange.
   (e) International Standard 4902-1980: Data Communication-37-Pin
and 9-Pin
DTE/DCE Interface Connectors and Pin Assignments.
   (f) CCITT recommendation V.11(X.27) (1980): electrical
Characteristics for
Balanced Double-Current Interchange Circuits for General Use with
Integrated
Circuit Equipment in the Field of Data Communications.
   (g) EIA RS-422-A (1978 June): Electrical Characteristics of
Balanced
Voltage Digital Interface Circuits.
   (h) Federal Standard 1020A (1980 January): Telecommunications:
Electrical
Characteristics of Balanced Voltage Digital Interface Circuits.
   (i) CCITT Recommendation V.10 (X26) (1980): Electrical
Characteristics for
Unbalanced Double-Current Interchange Circuits for General Use with
Integrated Circuit Equipment in the Field of Data Communications.
   (k) Federal Standard 1030A (1980 January): Telecommunications:
Electrical
characteristics of Unbalanced Voltage Digital Interface Circuits.
   (l) CCITT Recommendation X.21bis (1980): Use on Public Data
Networks of
Data Terminal Equipment which are Designed for Interfacing to
Synchronous
V-series Modems.
   (m) CCITT Recommendation V.54 (1980): Loop Test Devices for
Modems.
   (n) EIA RS-449 (1977 November): general Purpose 37-Position
Interface
Between Data Terminal Equipment and Data Circuit-Terminating
Equipment.
   (o) Federal Standard 1031 (1980 June): Telecommunications
General Purpose
37-position and 9-position Interface Between Data Terminal
Equipment and Data
Circuit Terminating Equipment (implementing instructions in the
form of a
Federal Property Management Regulation have not yet been issued.
the General
Services Administration is considering canceling FED-STD 1031.
Furthermore,
a Federal Information Processing Standard for ADP applications
corresponding
to Federal Standard 1031 has not been adopted by the National
Bureau of
Standards.)
   (p) International Standard 4903-1980: Data Communication-15-pin
DTE/DCE
Interface Connector and Pin Assignments.
   (q) EIA Industrial Electronics Bulletin No. 12 (1977 November):
Application Notes on Interconnection Between Interface Circuits
Using RS-449
and RS-232-C.
   (r) Draft International Standard 2593 (1980): Data
Communication-34-pin
DTE/DCE Interface Connector and Pin Assignments.
   (s) CCITT Recommendation V.35 (1980): Data Transmission at 48
Kilobits per
second Using 60-108 kHz Group Band Circuits.
   (t) CCITT Recommendation X.21 (1980): general Purpose Interface
Between
Data Terminal Equipment and Data Circuit-Terminating Equipment for
Synchronous Operation on Public Data Networks.
   (u) CCITT recommendation V.5 (1980): Standardization of
Data-Signalling
Rates for Synchronous Data Transmission in the General Switched
Telephone
networks.
   (v) CCITT Recommendation V.6 (1980): Standardization of
Data-Signalling
Rates for Synchronous Data Transmission on Leased Telephone-Type
Circuits.
   (w) American National Standard X3.1-1976: Synchronous Signalling
Rates for
Data Transmission.
   (x) Federal Information Processing Standard Publication 22-1
(1977
September): Synchronous Signaling Rates Between Data Terminal and
Data
Communication Equipment. (FIPS PUB 22-1 is identified also as
FED-STD 1013.)
   (y) Federal Standard 1013 (1977 August): Telecommunications: 
Synchronous
Signaling Rates Between Data Terminal Equipment and Data
Circuit-Terminating
Equipment utilizing 4 kHz Circuits (FED-STD 1013) is identified
also as FIPS
PUB 22-1.)
   (z) American National Standard X3.36-1975: Synchronous
High-Speed Data
Signaling Rates Between Data Terminal Equipment and Data
Communication
Equipment.
  (aa) Federal Information Processing Standards Publication 37
(1975 June):
Synchronous High Speed Data Signaling Rates Between Data Terminal
Equipment
and Data Communication Equipment. (FIPS PUB 37 is identified also
as
FED-STD 1001.)
  (ab) Federal Standard 1001 (1975 June): Telecommunications:
Synchronous
High-Speed Data Signaling Rates Between Data Terminal Equipment and
Data
Communications Equipment. (FED-STD 1001 is identified also as FIPS
PUB 37.)
   (ac) EIA RS-269-B (1976 January): Synchronous Signaling Rates
for Data
transmission.
   (ad) International Standard 3309-1979: Data Communication-High
Level Data
Link control Procedures-Frame Structure.
   (ae) International Standard 4335-1979: Data Communication-High
Level Data
Link control Procedures-Elements of Procedures.
   (af) Addendum 1 to International Standard 4335-1979: Data
Communication-High Level Data Link control Procedures-Elements of
Procedures.
   (ag) Addendum 2 to International Standard 4335-1979: Data
Communication-High Level Data Link Control Procedures-Elements of
procedures.
   (ah) International Standard 6256-1980: Data Communication-High
-Level Data
Link Control Procedures-Balanced Class of Procedures.
   (ai) American National Standard X3.66-1979: Advanced Data
Communication
Control procedures (ADCCP).
   (aj) Federal Information Processing Standards Publication 71
(1980 May) as
revised by the Federal Register notice 47 FR 23798, dated June 1,
1982 and
corrected by the notice 47 FR 25397 dated June 11, 1982: Advanced
Data
Communication Control Procedures (ADCCP). (FIPS PUB 71 is
technically
consistent with FED-STD 1003A.)
   (ak) Federal Information Processing Standards Publication 78
(1980
September): Guideline for Implementing Advanced Data Communication
Control
Procedures (ADCCP).
   (al) Federal Standard 1003A (1981 August): Telecommunications:
Synchronous
bit-Oriented Data Link Control Procedures (FED-STD 1003A is
technically
consistent with FIPS PUB 71.)
   (am) CCITT Recommendation X.25 (1980): Interface Between Data
Terminal
Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for
Terminals
Operating in the Packet Mode on Public Data Networks.
   (an) Draft Proposed International Standard 7498: Data
Processing-Open
Systems Interconnection-Basic Reference Model.
   (ao) CCITT Recommendation X.1 (1980): International User Classes
of
Service in Public Data Networks.
   (ap) CCITT Recommendation X.2 (1980): International User
Facilities in
Public Data Networks.
   (aq) CCITT Recommendation X.96 (1980): Call Progress Signals in
Public
Data Networks.

Applicability: The technical specifications of  this joint standard
shall  be
employed in the acquisition, design, and development of all federal
automated
data processing  equipment,  services, and  telecommunication 
equipment  and
PSDCN whenever  an  interface  based on  CCITT  Recommendation 
X.25  (1980),
Interface   Between    Data    Terminal   Equipment    (DTE)    and 
  Data
Circuit-Terminating Equiment (DCE) for Terminals Operating in the 
Packet
Mode  on  Public  Networks>1,  is  required.   Referred  to  below 
as  CCITT
Recommendation X.25, Recommendation X.25, or X.25.

Implementation: The provisions of this  joint standard are
effective July  6,
1983. Any applicable equipment or service  ordered on or after the 
effective
date, or procurement action  for which solicitation  documents have
not  been
issued by that date, must conform to the provisions of this
standard unless a
waiver has been granted in accordance with the procedures described
below.

This joint standard shall be reviewed by the Institute for Computer 
Sciences
and Technology, National Bureau of Standards  and the Office of the 
Manager,
National Communications System, within five  years after its
effective  date.
This review shall take into account technological trends and other
factors to
determine if the joint standard should be affirmed, revised, or
withdrawn.

Specifications: This joint standard adopts a subset, identified
below, of the
International Telegraph and Telephone Consultative Committee's
Recommendation
X.25.

    (a) At  the  physical  level,  the  provisions  of  Section 
1  of  CCITT
Recommendation X.25  shall be  used.  As  a minimum, networks shall 
support
dedicated circuit access; other  types of access  (e.g., through
the  general
switched telephone network) may also be offered.

           CClTT Recommendation  X.1 standardizes  data signalling 
rates  of
2.4, 4.8,  9.6, and  48 kbits/s  for packet  mode interfaces.  At
a  minimum,
networks shall support the synchronous data signalling rates of
2.4, 4.8, and
9.6 kbits/s  full duplex;  other  speeds (e.g.,  19.2  kbits/s) may 
also  be
offered. The 48 kbits/s rate need  not be supported in those
locations  where
it is not  available; 56 kbits/s  is recommended in  its place (see 
American
National Standard  X3.36-1975 and  related documents  referenced
above).  The
term "user class of service" used in X.25 refers to the data
signalling  rate
of DTE/DCE interface.
           In accordance  with  CCITT  Recommendation  X.25, 
networks  shall
provide one or more of the following interface options:

           i.  CCITT Recommendation X.21;
           ii.  EIA RS-232-C, which is  essentially equivalent to
one of  the
options in CCITT Recommendation X.21bis;
           iii.  CCITT Recommendation  X.21bis option that  is
equivalent  to
RS-449 using only the EIA RS-423A unbalanced electrical
characteristics.

           Interworking between EIA RS-232-C on one side of the
interface and
RS-449 on  the other  side is  permitted in  accordance with  EIA 
Industrial
Electronics Bulletin Number 12. Where interworking with RS-232-C
equipment is
not required,  the  provisions  described below  employing  RS-449 
with  the
RS-422A electrical characteristics may  optionally be employed at 
signalling
rates below 48 kbit/s.
           Networks which  support 48  or 56  kbits/s data 
signalling  rates
shall provide one or more of the following interface options:

           i.  CCITT Recommendation X.21;
           ii.  CCITT  Recommendation  X.21bis option  that 
specifies  CCITT
Recommendation V.35; or
           iii.  CCITT  Recommendation X.21bis  option that 
specifies  CCITT
Recommendation V.36 which is equivalent to EIA RS-449.

    NOTE: Current study  in national and  international standards
groups  may
result in  the  development  of additional  physical  interfaces. 
Each  such
physical interface will be evaluated for inclusion in this joint
standard. If
there are significant savings, one physical interface may be
selected as  the
future mandatory physical interface.
    NOTE: DTE  purchasers and  designers  should determine  which 
physical
interface(s) is provided by the associated DCE(s).

    (b) Only the LAPB link level procedures shall be used.

    NOTE: These procedures are a subset of those described in FIPS
PUB 71 and
Federal Standard 1003A  and correspond to  FIPS PUB 78  recommended
class  B.
This subset is identified as follows:

           i.  Link configuration: two combined stations on a 
point-to-point
link.
           ii.  Class of procedures: balanced asynchronous (BA)
with  options
two and eight. The RSET command shall  not be used. (RSET is found
in  option
11 of  the Fips  PUB 71.  RSET is  part of  the basic  repertoire
in  Federal
Standard 1003A; option 11 of federal Standard 1003A deletes the
RSET command.
Note that RSET is not part of CCITT Recommendation X.25.)
           iii.  Two-way simultaneous operation shall be employed.
            iv.   The  smallest  N1,  (the  maximum  number  of 
bits  in  an
information frame excluding flags and  zero bit insertion for 
transparency),
which shall be supported shall be 164 octets (the maximum length
of) fast
select caIl setup  packet). If  a DTE  neither transmits,  nor
receives  for
processing by higher level  functionality fast select  packets, an
N1 as
small as 135 octets may be supported by the DTE.
           v.  The address of the combined station provided by the 
network
shall be  10000000; the  address of  the other  combined station 
shall  be
11000000; where the left-hand bit is the least significant bit (bit 
number
1) and shall be transmitted first.  This convention is consistent
with  the
provisions of FIPS 71 and Federal Standard 1003A.
           vi.  The FCS shall  be a 16-bit sequence  as indicated
in  Section
2.2.7. DTE/DCE may also  employ the 32-bit  FCS as indicated  in
FIPS PUB  71
(revised) and FED-STD 1003A. DTE/DCE equipment using the 32-bit FCS
shall  be
able to also operate with the 16-bit FCS. The smallest N1 shall be
166 octets
when the 32-bit FCS  is used. If  a DTE neither  transmits, nor
receives  for
processing by higher level functionality fast select packets, an
Nl as  small
as 137 octets may be supported by the DTE when the 32-bit FCS is
used.

    NOTE: FIPS PUB 78 provides a  detailed discussion of the
relative  merits
of the 16-bit and 32-bit FCS.

           vii.  The frame reject  information field shall  be
padded with  4
zero bits in bit positions 21 through 24 of the information field
to  provide
a length of three octets.
           viii. It  is  required  that all  implementations  be 
capable  of
operating with K=7; optionally, values of 1 to 6 are permissible
with  modulo
8 operation and values 1 to 127 are permissible with modulo 128
operation.

    NOTE: DTE  purchasers and  designers should  determine that 
values of  k
other than 7 are supported by the associated DCE(s).

    (c) The user data field of packets shall be an integral number
of octets.
If a  packet is  received which  shows  a user  data field  not
equal  to  an
integral number  of octets,  the receiving  DTE/DCE shall  follow
the  packet
level procedures  for processing  a packet  type  which is  too
long.  A  new
diagnostic code  "non-octet  aligned  packet,"  consistent  with 
the  Data
Communications-X.25 Packet Layer  Specification for Terminal 
Equiment,
ISO DP 8208, November 8, 1982, is recommended as #82.
    (d) The reject packet shall not be used.
    (e) All DCE restart confirmation,  DCE reset confirmation, and
DCE  clear
confirmation packets  shall  be  interpreted  by  the  DTE  as 
having  local
significance only.
    (f) The D-bit shall be implemented by all networks. DTE's need
not employ
the D-bit  procedures when  transmitting to  the network,  but no 
DTE  shall
reject incoming packets with the  D-bit set to 1 or  0 as having
this bit  in
error unless the receiving DTE knows  the remote DTE has not
implemented  the
D-bit procedure; in this case, the receipt of a D-bit set to 1 may
be treated
by the receiving DTE as an error condition.
    (g) The selection of logical channel  number for new virtual
calls  shall
follow the procedures suggested in Section 4.1.2 Note 2, Annex A
Note 5,  and
Annex A Note 6, of the CCITT Recommendation X.25.
    (h) It is required that all implementations be capable of
operating  with
packet sequence  numbering modulo  8; optionally,  implementations
of  packet
sequence numbering modulo 128 are also permitted.

    NOTE: DTE purchasers  and designers  should determine  if the 
associated
DCE(s) support packet sequence numbering modulo 128.

    (i) All DTE's and DCE's shall follow the flow control
principles outlined
in the first  two sentences of  the first  paragraph of Section 
4.4. 1.3  of
CCITT Recommendation X.25.
    (j) The alternative procedure for passing packets containing
a P(S)  that
is out of sequence but within the window as described in the third 
paragraph
of Section 4.4.1.3 of CCITT Recommendation X.25 shall not be used.
    (k) The second sentence of Section  4.4.1.4 Note 2 shall not
apply.  This
sentence permits networks to defer updating the window for data
packets  with
D =0, and sent within  the window but before a  data packet with
D= 1,  until
the network receives a corresponding P(R) for the packet with D=
1.
    (1) The resetting cause field of a  reset request packet shall
be set  to
zero. If a reset request is  received with a non-zero resetting
cause  field,
the packet shall be discarded. The network shall then initiate the 
resetting
procedure with the  resetting cause field  indicating local/remote 
procedure
error.
    (m) The clearing cause field  of a clear request  packet shall
be set  to
zero. If a clear  request packet is received  with a non-zero
clearing  cause
field, the packet  shall be discarded.  The network shall  then
initiate  the
clearing procedure  with the  clearing  cause field  indicating 
local/remote
procedure error.
    (n) The restarting cause field of  a restart request packet
shall be  set
to zero. If  a restart  request packet is  received with  a
non-zero  restart
cause field, the restart  request packet shall  be discarded
without  further
action.  Optionally,  the  DCE  may  generate  a  diagnostic 
packet  with  a
recommended diagnostic  code #81  (improper cause  code from  DTE),
which  is
consistent with the <1Data Communication-X.25 Packet Layer>1 
<1Specification
for Data Terminal Equiment,>1 ISO DP 8208, November 8, 1982.
    (o) A  diagnostic code  shall be  provided in  all clear 
request,  reset
request, and restart request packets in  accordance with the codes
listed  in
Annex E  of  CCITT  Recommendation X.25  whenever  they  apply; 
non-assigned
codings in X.25 may be used for  events not listed in X.25 within
the  period
of 24 months after the effective date  of this standard. Prior to
the end  of
this 24 month  period, this  standard will be  reviewed by  NBS to 
determine
whether the  standard should  be revised  to incorporate  a
different  table.
After this revision, codes not specifically listed shall not be
used.
    (p) A generic  diagnostic code  shall not be  used when  a more 
specific
diagnostic code is known to be applicable.
    (q) The network  diagnostic codes shall  be used in  accordance
with  the
codes listed in  Annex E of  CCITT Recommendation X.25  whenever
they  apply;
non-assigned codings in X.25 may be used for events not listed in
X.25 within
the period of 24 months after the  effective date of this standard.
Prior  to
the end of this  24 month period,  this standard will be  reviewed
by NBS  to
determine whether the standard should  be revised to incorporate
a  different
table. After this revision, network diagnostic codes not
specifically  listed
shall not be used.
    (r) The network shall  consider the receipt of  a DTE interrupt 
packet
before a previous DTE interrupt packet has been confirmed as an
error,  and
shall execute the error procedure described in Annex C, Table
C-4/X.25  and
the corresponding note 2.
    (s) The timeouts and time limits  specified in Annex D shall
be  observed
by all DTE and DCE equipment. T21 shall  not be less than the value
given  in
table D-2/X.25.  The preferred  actions  listed in  table D-2/X.25 
shall  be
followed.
    (t) When  the  link level  procedures  enter the  logically 
disconnected
state, the associated packet level  procedures shall clear all
virtual  calls
and reset all permanent virtual circuits and datagram logical
channels.  When
the link  level  procedures  reenter  the  information  transfer 
state,  the
associated packet level procedures shall  execute the restart
procedure.  The
terms "logically  disconnected state"  and "information  transfer
state"  are
used as defined in American National Standard X3.66-1979
(referenced  above).
Link level  procedures enter  the logically  disconnected state 
when a  DISC
command is sent and a  UA response is received,  for example. The
link  level
procedure shall also be considered to be in the logically
disconnected  state
after N2 (re)transmissions of SABM or DISC,  where N2 is as defined
in  CClTT
Recommendation X.25. The logically disconnected state is not
assumed after N2
(re)transmissions of other types of frames.
    (u) lf a restart request packet is received in state rl which
exceeds the
maximum permitted length, the  DCE shall discard  the restart
request  packet
without further action. Optionally, the DCE may generate a
diagnostic  packet
with diagnostic code #39 (packet too long).
    (v) In  the event  that  a facility  code appears  more  than
once  in  a
facility field, the receiving DTE  detecting this condition should
treat  the
last appearance of the particular code as  if it were the only
appearance  of
that code.
    (w) All  networks  shall supply  diagnostic  packets when 
their  use  is
suggested in  CClTT  Recommendation  X.25. No  DTE  shall  rejcct 
diagnostic
packets as errors.
    (x) ln Section 6.1.1, the second paragraph, the last phrase,
"and is  set
to 0 in all other  packets", shall be interpreted  that the
Qualifier bit  is
set to 0  in all  other packets  except data packets.  For the 
case of  data
packets, the Qualifier bit is set to 0 or 1 as indicated in Section
4.3.6  of
CClTT Recommendation X.25.
    (y) The  list of  user facilities  for packet-switched  data 
networks,
extracted from CCITT Recommendation X.2, is given below.  These 
facilities
are described  in Section  7 of  CCITT Recommendation  X.25. The 
following
further constraints apply:
           i.  Networks shall provide the facilities designated as 
essential
"E" below.
           ii.  Networks shall also implement the Fast Select and
Fast Select
Acceptance facilities  to facilitate  more efficient  operation in 
conveying
higher layer protocol  information or  user data  during call 
establishment.
DTE's need not employ fast select  packets when transmitting to the 
network,
but all DTE's  associated with  the higher level  functionality
which  allows
response to a fast select packet must be able to accept incoming
fast  select
packets.
           iii.  The packet retransmission facility shall not be
used.
           iv.  All  DTE's which  employ any  of the  facilities
labelled  as
additional "A" below (except Fast  Select and Fast Select
Acceptance)  shall
also be capable of operating without employing any A facilities
(except  Fast
Select and Fast Select Acceptance).
           v.  The throughput class value of 48,000 bits/s may be
interpreted
as 56,000 bits/s in those locations where 56,000 bits/s access is
used.

              Facilities of packet-switched data networks:

      User Facility                             VC       PVC    
DG*

Optional user facilities assigned
for an agreed contractual period:

Extended packet sequence numbering
(modulo)                                        A        A      
A*
Non-standard default window sizes               A        A      
A*
Non-standard default packet sizes
  16, 32, 64, 256, 512, 1024                    A        A      
-
Default throughput class assignment             A        A      
A*
Flow control parameter negotiation              E        -      
-
Throughput class negotation                     E        -      
-
Packet retransmission                           A***     A***   
A***
Incoming calls barred                           E        -      
E*
Outgoing calls barred                           E        -      
E*
One-way logical channel outgoing                E        -      
A*
One-way logical channel incoming                A        -      
A*
Closed user group                               E        -      
E*
Closed user group with outgoing
access                                          A        -      
A*
Closed user group with incoming
access                                          A        -      
A*
Incoming calls barred within a
closed user group                               A        -      
A*
Outgoing calls barred within a
closed user group                               A        -      
A*
Bilateral closed user group                     A        -      
A*
Bilateral closed user group with
outgoing access                                 A        -      
A*
Reverse charging acceptance                     A        -      
A*
Fast select acceptance                          A**      -      
-
Datagram queue length selection*                -        -      
A*
Datagram service signal logical
channel*                                        -        -      
A*
Datagram non-delivery indication*               -        -      
E*
Datagram delivery confirmation*                 -        -      
E*
D-bit modification                              A        A      
-

Optional user facilities requested
by the DTE on a per call basis

Closed user group selection                     E        -      
E*
Bilateral closed user group selection           A        -      
A*
Reverse charging                                A        -      
A*
RPOA selection                                  A        -      
A*
Flow control parameter negotiation              E        -      
-
Fast select                                     A**      -      
-
Throughput class negotiation                    E        -      
-
Abbreviated address calling                     FS       -      
A*
Datagram non-delivery indication                -        -      
E*
Datagram delivery confirmation                  -        -      
E*

NOTE: Detailed  explanations  of  these  facilities  are  provided 
in  CCITT
Recommendation X.25.

LEGEND:
     E   =  An essential user facility to be offered by all
networks.
     A   =  An  additional  user  facility which  may  be  offered 
by  certain
networks.
     FS  =  Further study is required. This standard will be
modified when this
study is complete.
     -   = Not applicable.
     DG  = Applicable when the datagram service is being used.*
     VC  = Applicable when the virtual call service is being used.
     PVC = Applicable when the permanent virtual circuit service
is being used.

     * - The  datagram  service and  its  related facilities  may 
be  used
only when:
       - there is  to be  a one-way transfer  of information  which
does  not
require recovery at the network layer; and,
       - a response to  this transfer of information  is not
required at  the
network layer.
     NOTES: 1. At the present time,  the transfer of datagram
packets  across
international  borders  through  public  packet-switching  networks 
is   not
permitted 2. DCE's are  not required to provide  datagram service.
DTE's  are
not required to generate or accept datagrams and datagram-related
packets.

** - Fast select shall  be provided by all  DCE's. All DTE's
associated  with
the higher level functionality which allows response to a fast
select  packet
must be  capable of  accepting incoming  fast select  packets, but 
need  not
generate fast select packets.

***  The packet retransmission facilities shall not be used.

     (z) The list  of the  applicable call progress  signals,
extracted  from
CCITT Recommendation X.96, is given below. These signal definitions
apply  to
the  cause  codes  specified  in  CCITT  Recommendation  X.25.  
The  related
circumstances giving rise  to each call  progress signal is  also
defined  in
table 1 below. The significance of  categories indicates broadly
the type  of
action expected of the DTE receiving the signal:

Category           Significance

A                  Requested  action  confirmed  by  network.   
B                  Call   cleared  because  the procedure is
complete.
C1 and C2          Call cleared. The calling DTE should  call again
soon: the
                   next attempt may be successful. However, after
a number of
                   unsuccessful call attempts with the same 
response, the
                   cause could be assumed to be in Category D1 or
D2. The
                   interval between  successive attempts and the
number of
                   maximum attempts will depend on a number of
circumstances
                   including:

                   - nature of the call progress signal
                   - users' traffic pattern
                   - tariffs
                   - possible regulations by the network provider.
                   OR
                   Reset. The DTE may continue to transmit data
                   recognizing that data loss may have occurred.
D1 and D2          Call cleared. The calling DTE should take other
action to
                   clarify when the call  attempt might be
successful.
                   OR
                   Reset (for permanent virtual circuit only).
                   The DTE should cease data transmission and take
other action
                   as appropriate.
C1 and D1          Due to subscriber condition.
C2 and D2          Due to network condition.

The sequence of call  progress signals in table  1 implies, for
Categories  C
and D, the order of  call set-up processing by  the network. ln
general,  the
DTE can assume, on receiving a call progress signal, that no
condition higher
up in  the table  is present.  Network  congestion is  an exception 
to  this
general rule. The actual coding of call progress signals does not
necessarily
reflect this sequence.

Users and DTE  manufacturers are  warned to  make due  allowance
to  possible
later extensions to this table by providing appropriate fallback
routines for
unexpected signals.


-----------------------------------------------------------------
--------------
-----------------------------------------------------------------
--------------
  Call Progress               Definition                         
   Category
     Signal
-----------------------------------------------------------------
--------------
   Delivery               The datagram has been                  
   A
   confirmation           accepted by the destination DTE.

   Local procedure        A procedure error caused by the DTE    
   C1
   error                  is detected by the DCE at the local
                          DTE/DCE interface.

   Network                A condition exists in the network      
   C2
   congestion             such as:
                          1) temporary network congestion
                          2) temporary fault condition within
                          the network, including procedure error
                          within a network or an international
link.

  Invalid                 A facility requested by the calling    
   D1 or D2
  facility                DTE is detected as invalid by the DCE
  request                 at the local DTE/DCE interface.
                          Possible reasons include:
                          -  request for a facility which has not
                          been subscribed to by the DTE;
                          -  request for a facility which is not
                          available in the local network:
                          -  request for a facility which has not
                          been recognized as valid by the local
DCE.

  RPOA out                The RPOA nominated by the calling DTE is 
 D2
  of order                unable to forward the call.

  Not                     The called DTE address is              
   D1
  obtainable              out of the numbering plan or not 
                          assigned to any DTE.

  Access barred           The calling DTE is not permitted       
   D1
                          the connection to the called DTE.
                          Possible reasons include:
                          -  unauthorized access between the
calling
                          DTE and thc called DTE.
                          -  incompatible closed user group.

  Reverse charging        The called DTE has not subscribed      
   D1
  acceptance not          to the reverse charging acceptance
  subscribed              facility.

  Fast select             The called DTE has not subscribed      
   D1
  acceptance not          to the fast select acceptance
  subscribed              facility.

  Incompatible            The remote DTE/DCE interface or the    
   D1
  destination             or the transit network does not support
                          a function or facility requested (eg.the
  datagram service).

  Out of Order  The remote number is out of order.         D1 or
D2
  Possible reasons include:
- DTE is Uncontrolled Not Ready:
- DCE Power off:
- Network fault in the local loop:
- X.25 Level 1 not functioning:
- X.25 Level 2 not in operation.

  Number busy  The called DTE is detected by the DCE     C1
  as engaged on other call(s), and
  therefore as not being able to accept
  the incoming call. (In the case of the
  datagram service..the queue at the
  destination DCE is full.)

  Remote  A procedure error caused by the             D1
  procedure  remote DTE is detected by the DCE
  error  at the remote DTE/DCE interface.

  Network  Network is ready to resume normal     C1
  operational  operation after a temporary failure
          or congestion.

  Remote DTE  Remote DTE/DCE interface is ready     C1 or D1
  operational             to resume normal operation after a
  temporary failure or out of order
  condition (e.g., restart at the remote
  DTE/DCE interface. Loss of data may 
                          have occurred.

  DTE originated  The remote DTE has intiated     B or D1
                          a clear, reset, or restart procedure.

Waivers: Waiver of this standard is required when an interface
based on CCITT
Recommendation X.25  (1980) is  to be  employed  and has  either
one  of  the
following conditions: 1) The interface has options that are not
permitted  by
this standard; 2) The  interface does not implement  all options
mandated  by
this standard.

Heads of agencies  desiring a  waiver from  the requirements 
stated in  this
standard, so as to acquire applicable equipment or service not
conforming  to
this standard,  shall  submit a  request  for waiver  to  the 
Administrator,
General Services Administration,  for review and  approval.
Approval will  be
granted if, in the judgment of the Administrator after consultation
with  the
Assistant Secretary of Commerce for Productivity, Technology and 
Innovation,
based on  all available  information including  that provided  in
the  waiver
requests, a major adverse economic or operational impact would
occur  through
conformance with this standard.

A request for waiver shall include a justification for the waiver, 
including
a description and discussion  of the adverse  economic or
operational  impact
that would  result  from conforming  to  this  standard as 
compared  to  the
alternative for which  the waiver  is requested.  ICST and  NCS
will  provide
technical assistance, as required, to GSA.

Where to  Obtain Copies:  Copies of  this  publication are  for
sale  by  the
National  Technical  Information  Service,  U.S.   Department  of 
 Commerce,
Springfield,  VA  22161.   When   ordering,  refer  to  Federal  
Information
Processing Standards Publication  100 (FIPS-PUB-  l00)/Federal
Standard  1041
(FED-STD 1041),  and  title.  When  microfiche  is desired,  this 
should  be
specified. Payment may be made by check, money order, purchase
order,  credit
card, or deposit account.

The CCITT X.25 specifications upon which  this publication is based
may  also
be obtained from NTIS.  Specify PB82-187766; the cost is $50;
telephone (703)
487-4650.


re> 
