[ netinfo/x25.doc ]



                        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


                              APPENDICES




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 B: 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.



































                                 -5-






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















                                 -15-






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.

























                                 -16-







             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


    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-3






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-4






(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-5








 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


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-6








                              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-7






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-8








                              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.



