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[ NETINFO:X25.DOC ]
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ACKNOWLEDGMENTS
This specification was prepared by BBN Communications Corporation
under contract to the Defense Data Network Program Management
Office of the Defense Communications Agency.
The specification has been reviewed by the Defense Communications
Engineering Center for accuracy and completeness. The draft of
this specification has been disseminated to industry by the
National Bureau of Standards for review and comments which have
been incorporated in the final specification. This specification
has been approved for use on the Defense Data Network by the DoD
Protocol Standards Steering Group.
Comments on this specification should be directed to the Defense
Communications Agency, ATTN: Defense Data Network Program Managment
Office, Code B610, Washington, D.C. 20305
Table of Contents
1 INTRODUCTION.......................................... 1
1.1 Background.......................................... 1
1.1.1 X.25 and FIPS 100/Federal Standard 1041........... 1
1.1.2 X.25-to-X.25 and X.25-to-1822
Interoperability................................ 2
1.2 Compliance.......................................... 4
1.2.1 Compliance With CCITT X.25 and FIPS
100/Fed. Std. 1041.............................. 4
1.2.2 DTE Compliance With This Specification............ 4
2 INTERFACE SPECIFICATION............................... 6
2'1 Call Establishment Conventions...................... 6
2.1.1 Addressing........................................ 6
2.1.1.1 Address Formats and Fields...................... 6
2.1.1.1.1 Reserved...................................... 7
2.1.1.1.2 Flag.......................................... 7
2.1.1.1.3 DDN host Identifier........................... 7
2.1.1.1.4 Sub-Address................................... 7
2.1.1.2 Supplying Missing Address Information........... 7
2.1.2 DDN-Specific Facilities........................... 8
2.1.2.1 Type of Service Selection....................... 8
2.1.2.2 Call Precedence................................. 9
2.1.3 Protocol Identification.......................... 10
2.1.4 Logical Channel Assignment....................... 10
2.2 Packet Level Procedures............................ 11
2.3 Link Level Procedures.............................. 12
2.3.1 Link Level Parameters and Options................ 12
2.3.2 Timer T1 and Parameter T2........................ 12
2.3.3 Maximum I Frame Size............................. 13
2.4 Physical Level Specifications...................... 14
3 BIBLIOGRAPHY......................................... 16
APPENDIX A: DDN X.25 Implementation Details............ A-1
A-1 Introduction...................................... A-1
A-2 Operational Features of DDN X.25 DCE Releases..... A-1
A-2.1 Initial Feature Support......................... A-1
A-2.2 Exception-Handling Procedures................... A-2
A-2.2.1 Non-Octet-Aligned Data........................ A-2
A-2.2.2 RESTART REQUEST Packet........................ A-2
A-2.2.3 RESET REQUEST Packet.......................... A-2
A-2.2.9 CLEAR REQUEST Packet.......................... A-3
A-2.3 Virtual Circuit Resource Availability........... A-3
A-3 Detailed Features and Facilities
Specifications.................................. A-3
A-3.1 Additional Diagnostic Codes..................... A-3
A-3.2 X.25 IP Interoperability Considerations......... A-6
A-3.3 The DDN Logical Addressing Facility............. A-7
A-3.3.1 Logical Addresses............................. A-7
A-3.3.2 Enabling and Disabling Logical Addresses...... A-7
A-4 Limitations of DDN Basic X.25 Service............. A-8
A-5 Derivation of DDN X.25 Addresses.................. A-9
APPENDIX Q: DDN Synchronous Level 1 Specification...... B-1
B-1 Introduction...................................... B-1
B-2 Supported Interfaces.............................. B-1
APPENDIX C: Federal Information Processing Standard
Publication 100...................................... C-1
TABLES
DDN X.25 Address Fields................................... 7
"Derivation of Maximum I Frame Size".................... 14
DDN X.25 Physical Signaling Rates and Interfaces......... 15
Additional Packet Level Diagnostic Codes................ A-4
IP Precedence to X.25 Precedence Mapping................ A-6
EIA and CCITT Interchange Circuits...................... B-3
Signal Selection by CCITT Interchange Circuit
Number................................................ B-4
Typical Level 1 Connection Schemes...................... B-5
Interface Type by Service Speed......................... B-7
RS-232-C Interface...................................... B-8
MIL-188-114 Interface (and equivalents)................. B-9
V.35 Interface......................................... B-10
FIGURES
Typical Level 1 Connection Schemes...................... B-4
INTRODUCTION
This report specifies the attachment of an X.25 host to the
Defense Data Network (DDN). In particular, this report describes
specific options and features of CCITT Recommendation X.25 (1980)
and Federal Information Processing Standard (FIPS) 100/Federal
Standard (Fed. Std.) 1041 (July 1983) required of a host X.25
implementation to enable that host to communicate with a DDN X.25
Interface Message Processor ("IMP", the DDN packet switching
node). This report, in conjunction with FIPS 100/Fed. Std.
1041, should enable DDN host site managers and others planning to
attach a host by means of X.25, rather than the 1822 interface,
to determine, first, whether or not the X.25 implementation of
the host in question is adequate for operation with DDN, and,
second, what options, parameter settings, etc. must or may be
selected for operation with DDN.
This report assumes that the reader is familiar with CCITT
Recommendation X.25 and FIPS 100/Fed. Std. 1041. A copy of FIPS
100/Fed. Std. 1041 is attached as Appendix C of this report.
In this document, the term "Administration" refers to the
Defense Communications Agency (DCA Code B610, Washington, D. C.
20305).
1.1 Background
1.1.1 X.25 and FIPS 100/Federal Standard 1041
The CCITT Recommendation X.25 describes the interface
between host computers (data terminal equipment, or DTEs) and
data circuit-terminating equipment (DCEs, which effect
communication with remote hosts over computer networks) for hosts
operating in the packet mode on public data networks. The X.25
interface standard is defined as three independent architectural
levels, following the Open Systems Interconnection (OSI)
Reference Model. The three levels are:
Level 1: The PHYSICAL level of the connection. The
physical, electrical, functional, and
procedural characteristics to activate,
_____________
* As used in this report, "1822 interface" refers to the
interface specified in Bolt Beranek and Newman Inc. (BBN) Report
No. 1822, "Specification for the Interconnection of a Host and an
IMP," revision of December 1981.
-1-
maintain, and deactivate the physical link
between the DTE and the DCE.
Level 2: The LINK level of the connection. The link
access procedure for data interchange across
the link between the DTE and the DCE.
Level 3: The PACKET level of the connection. The
packet format and control procedures for the
exchange of packets containing control
information and user data between the DTE and
the DCE, and between the DTE and a remote
DTE.
CCITT Recommendation X.25 contains many options and
implementation choices. FIPS 100/Fed. Std. 1041, which specifies
the general use of X.25 for the Federal Government, defines some
of the choices left open in X.25. This document describes the
X.25 interface to a particular network, DDN. Thus in several
areas where X.25 allows a choice, a single choice appropriate for
DDN is specified; in areas which X.25 leaves unspecified,
addressing in particular, conventions are specified that are
consistent with the overall architecture of DDN and the
interoperability goals described below. The effect of this
approach is to make DDN service available to hosts in a way that
requires no changes to a host DTE implementation that is
compliant with FIPS 100/Fed. Std. 1041 and CCITT Recommendation
X.25. By implementing extensions described in this
specification, a host will be able to take advantage of
additional DDN features required in military networks, such as
precedence and logical addressing.
The reader is referred to CCITT Recommendation X.25 and to
FIPS 100/Fed. Std. 1041 for detailed information not provided in
the body of this document.
1.1.2 X.25-to-X.25 and X.25-to-1822 Interoperability
A key goal of the DDN X.25 implementation is
interoperability among all DDN subscribers. That is, effective
communication should be possible, not only between subscribers
attached to the DDN using identical vendor-supplied X.25
implementations, but between subscribers using different X.25
implementations, and between a subscriber using an X.25 interface
to the DDN and a subscriber using an 1822 interface to the DDN.
Achieving this goal of interoperability requires that all DDN
-2-
X.25 subscribers conform to this interface specification and
implement the DoD standard higher level protocols. True
interoperability among DDN hosts requires, in particular,
implementation of the DoD standard protocols TCP (Transmission
Control Protocol) and IP (Internet Protocol), as well as the
higher-level protocols which implement DDN standard services,
" when such services are provided by the host: the Telnet
Protocol
for character-oriented terminal support, the File Transfer
Protocol (FTP) for file movement between hosts, and the Simple
Mail Transfer Protocol (SMTP) for communication between
electronic mail service hosts.
The DDN X.25 DCE offers two types of service to X.25 DTEs:
1. DDN Standard X.25 Service, which, when used in
conjunction with DoD standard protocols, provides
interoperable communication between an X.25 DTE
and other DDN hosts that also implement the DoD
standard protocols, whether they are connected to
DDN via the 1822 interface or via the X.25
interface;
and
2. DDN Basic X.25 Service, which provides
communication only between an X.25 DTE and other
DDN X.25 DTEs implementing compatible higher-level
protocols.
Section 2.1.2.1 of this report describes the conventions to
be used by a DTE to specify the type of service desired for each
X.25 virtual call. All DDN X.25 DTEs will be required to develop
and initiate a plan to use the DoD standard protocol architecture
and DDN standard X.25 service.
Use of DDN basic X.25 service imposes some restrictions on
the nature of the network communications service that a host can
obtain. These restrictions are discussed in Appendix A, Section
A-4.
-3-
1.2 Compliance
1.2.1 Compliance With CCITT X.25 and FIPS 100/Fed. Std. 1041
The DDN X.25 Interface Specification is compliant with CCITT
Recommendation X.25 and FIPS 100/Fed. Std. 1041. The DDN X.25
DCE supports all facilities specified as E (essential) by FIPS
100/Fed. Std. 1041, and no facilities specified as A
(additional). The additional facilities not supported are:
(i) datagrams and associated facilities,
and
(ii) bilateral closed user groups.
In that FIPS 100/Fed. Std. 1041 describes features for a
DCE, DDN X.25 DTEs may support any or all facilities specified as
either E or A by FIPS 100/Fed Std. 1041. However, DDN X.25 DTEs
must not use the facilities identified above that are not
supported by the DDN X.25 DCE.
1.2.2 DTE Compliance With This Specification
This document specifies several areas in which the DDN X.25
DCE is capable of operating in several modes. For example,
Section 2.4 lists a number of signaling rates supported by the
DCE. In such cases, a DDN X.25 DTE must implement at least one
of the options listed (or the set of options required of a DTE by
FIPS 100/Fed. Std. 1041) but need not implement all of the
options listed (unless required by FIPS 100/Fed. Std. 1041).
Determining the adequacy of the options supported by a DTE vendor
for meeting a DDN subscriber's requirements is the responsibility
of the subscriber.
In addition to the CCITT X.25 and FIPS 100/Fed. Std. 1041
requirements described in Section 1.2.1 above, DDN X.25 DTEs may
wish to take advantage of additional DDN-specific features that
are compatible extensions to the public standards.
Implementation of a DDN-specific feature by a host is required
only if the host wishes to take advantage of the service or
information provided by the feature. For example, a host that
wishes to establish calls only at the default precedence level
assigned to it need not implement the precedence facility
described in Section 2.1.2.2. However, a host that wishes to
have flexibility in the precedence of the calls it establishes
must implement this facility.
-4-
Any deficiencies with respect to this specification in a
vendor-supplied X.25 DTE implementation contemplated for use with
the DDN X.25 DCE should be rectified so as to attain compliance
with this specification. Proper operation with DDN of an X.25
DTE that is not compliant with this specification cannot be
guaranteed and should not be attempted. To this end, a test
program is available through the Administration.
2 INTERFACE SPECIFICATION
2.1 Call Establishment Conventions
This section specifies DDN X.25 call establishment
conventions.
2.1.1 Addressing
DDN addresses are assigned to subscriber DTEs by the
Administration. Two basic forms of address are provided:
physical addresses, which correspond to the node number and DCE
port number of the node to which the DTE is connected, and
logical addresses, which are mapped transparently by DCE software
into a corresponding physical network address. Each DTE is
assigned one physical address, and may be assigned one or more
logical addresses. All DDN addresses are either twelve or
fourteen BCD (binary-coded decimal) digits in length. A calling
DTE need not determine whether a given address is a physical or
logical address, in order to establish a call to that address.
.2.1.1.1 Address Formats and Fields
DDN addresses have the following format:
ZZZZ F DDDDDDD (SS)
The various fields of the address are presented in Table 2.1 and
are explained below.
Length
Field Meaning (BCD digits)
ZZZZ Reserved (must be zero) 4
F Flag 1
DDDDDDD DDN Host Identifier 7
(SS) Sub-address (optional) 0 or 2
TOTAL 12 or 14
Table 2.1 DDN X.25 Address Fields
-6-
2.1.1.1.1 Reserved
The Reserved field corresponds to the DNIC field generally
used in public data networks. Pending assignment of a DDN DNIC,
this field must be zero.
2.1.1.1.2 Flag
The Flag field is used to differentiate physical and logical
addressing. The value zero indicates physical addressing, while
the value one indicates logical addressing. A value of nine is
used in the setup of calls to enable and disable logical
addresses; see Appendix A, Section A-3.3.1.
2.1.1.1.3 DDN Host Identifier
The DDN Host Identifier is a seven-digit address, either
logical or physical, assigned to a subscriber DTE by the DDN
Administration.
2.1.1.1.4 Sub-Address
The Sub-Address may be used by a DTE for any.purpose. It is
carried across the network without modification. Its presence is
optional.
2.1.1.2 Supplying Missing Address Information
The DDN X.25 DCE incorporates a mechanism to supply
"missing" address information in CALL REQUEST and CALL ACCEPTED
packets received from an attached DTE. This mechanism is useful
in DTE software testing and physical address determination.
If a DTE sends a CALL REQUEST packet with no calling address
field, the local DCE will insert the physical calling DDN Host
Identifier with no subaddress field. If a DTE sends a CALL
REQUEST or CALL ACCEPTED packet with either or both calling or
called addresses that contain F = zero and DDDDDDD = zero, the
local DCE will replace the DDN Host Identifier field (DDDDDDD)
with the physical address of the DTE.
-7-
DTE implementors are cautioned that use of this mechanism in
accepting calls to a DTE's logical address (See Appendix A,
Section A-3.3) can result in confusion on the part of the calling
DTE and is not advised.
2.1.2 DDN-Specific Facilities
Two DDN-specific features are requested by means of
"private" or non-CCITT facilities in CALL REQUEST and CALL
ACCEPTED packets. If either or both of these facilities are
requested in a CALL REQUEST or CALL ACCEPTED packet, they must
follow all CCITT X.25 facilities and must be preceded by a single
facility marker, two octets of zero.
2.1.2.1 Type of Service Selection
The DDN X.25 provides two types of service, DDN basic X.25
service and DDN standard X.25 service. DDN standard X.25 service
provides only local DTE to local DCE support of the X.25
connection. Data is carried via the network to its destination
(using protocols internal to the network), where it is delivered
using the access protocol of the destination host (i.e., either
1822 or DDN standard X.25 service). This access method is
oriented towards DDN X.25 hosts using the DoD standard TCP/IP
higher level protocols. No X.25 procedures change when using DDN
standard X.25 service; however, the significance of the
procedures changes (see Appendix A, Section A-3.2). There is no
end-to-end X.25-level acknowledgment or guarantee of delivery of
data packets with DDN standard X.25 service; reliability of DDN
standard X.25 service is provided instead by the use of a
reliable transport protocol.
DDN basic X.25 service provides end-to-end call management
with significance as described in CCITT Recommendation X.25 and
FIPS 100/Fed. Std. 1041. This access method is oriented towards
hosts that have existing higher level protocol implementations
that require reliable packet delivery at the network level.
Selection of DDN standard or DDN basic X.25 service must be
made on a call-by-call basis by the DDN X.25 DTE at the time of
call setup. To specify DDN standard X.25 service, a DTE must
include in the CALL REQUEST packet a facility two octets long,
coded as follows:
00000100 00000001
-8-
If this facility is not specified, DDN basic X.25 service will be
provided.
2.1.2.2 Call Precedence
The precedence of a call is negotiated by an X.25 DTE by
means of a facility two octets long, coded as:
00001000 000000XX
where XX is the precedence, from 0 (lowest precedence) to 3
(highest precedence). If this facility is not used, the call
will be established at the subscriber's default precedence.
A DTE is not permitted to establish a call at a precedence
level higher than that authorized for that DTE by the
Administration. An attempt to do so will result in the DDN X.25
DCE returning to the DTE a CLEAR INDICATION packet with clearing
cause 00001001, "Out of order," with diagnostic code 194,
"Requested precedence too high".
Calls of a lower precedence may be cleared by a DCE if DCE
or other network resources are required, or if access to the
local or remote DTE is required (for a call of higher
precedence). In this event, a CLEAR INDICATION packet will be
sent with the clearing cause 00000101, "Network congestion," and
with a diagnostic code specifying the reason for the preemption.
The diagnostic codes employed for this purpose are 192, "Cleared
due to higher precedence call at local DCE," and 193, "Cleared
due to higher precedence call at remote DCE". Similarly, an
attempt to establish a call may be unsuccessful if network
resources are engaged in calls of higher priority than that
requested. In this case, a CLEAR INDICATION packet will be sent
with the clearing cause 00001001, "Out of order," and with either
diagnostic code 192 or 193, as appropriate.
The diagnostic codes described in the preceding paragraphs
are DDN-specific diagnostic codes; additional information about
these codes may be found in Appendix A, Section A-3.1.
-9-
2.1.3 Protocol Identification
X.25 DTEs employing the DoD standard TCP/IP protocol
architecture must indicate this by means of the call user data
field of the CALL REQUEST packet. The first octet of this field
must be set to 11001100 to identify the DoD standard protocol
architecture.
Indication of the use of the DoD standard protocol
architecture is independent of the selection of DDN standard or
DDN basic X.25 service by means of the facility specified in
Section 2.1.2.1 above. Therefore, a host employing the DoD
standard protocol architecture and using DDN standard X.25
service must include both the DDN standard X.25 service facility
and the call user data DoD standard protocol identification in
its CALL REQUEST packet.
A DTE using a protocol architecture other than the standard
DoD protocol architecture is free to use any call user data
protocol identification recognized by the DTEs with which it
wishes to communicate. Identification of protocol architectures
other than the DoD standard architecture is not standardized or
enforced by the Administration. Subscribers are cautioned,
therefore, that conflicts among various vendor-assigned protocol
identifications may arise.
2.1.4 Logical Channel Assignment
The assignment of logical channels by the DDN X.25 DCE
follows the requirements and guidelines of FIPS 100/Fed. Std.
1041 and Annex A of CCITT X.25. Within the guidelines of CCITT
X.25 Annex A, the range of logical channel numbers assigned to
permanent virtual circuits, incoming, two-way, and outgoing
virtual calls for DDN DCEs is configured for each DTE attached to
a DCE by the Administration.
DDN X.25 DTEs must follow the logical channel selection
requirements of FIPS 100/Fed. Std. 1041.
The number of logical channels available to a DTE is
dependent upon the configuration of the DCE to which the DTE is
attached, and upon the dynamic requirements placed upon other
DCEs that share the same DDN packet switching node.
-10-
2.2 Packet Level Procedures
DDN X.25 packet level procedures are as specified by FIPS
100/Fed. Std. 1041 and CCITT X.25. The following additional
information is provided:
1. The maximum window size that may be negotiated is
seven.
2. Modulo 128 packet level sequence numbering is not
supported.
3. Maximum packet sizes of 16, 32, 64, 128, 256, 512,
and 1024 octets may be negotiated.
4. The DDN X.25 DCE uses additional packet level
diagnostic codes, specified in Appendix A, Table
A-1. DDN X.25 DTEs may, but are not required to,
make use of the information conveyed by these
codes.
5. The Qualifier bit (Q-bit) is passed transparently
by the DDN X.25 DCE in DDN basic X.25 service.
DTEs using DDN basic X.25 service may use the Q-
bit in any way that is consistent with FIPS
100/Fed. Std. 1041.
6. The DDN X.25 DCE implements the diagnostic packet.
It is sent under conditions specified in Annex D
of CCITT X.25. The DTE is not required to act on
the information provided in diagnostic packets.
7. DTEs using DDN standard X.25 service must restrict
the maximum number of data bits in a complete
packet sequence to be no more than 8056. This
ensures that the data from a packet sequence
transmitted by an X.25 host will fit within the
maximum 1822 message length limit upon delivery to
an 1822 host. This restriction is necessary as
existing 1822 host implementations are not re-
quired to accept messages longer than 8063 bits. *
________________
* DTEs using DDN standard X.25 service will generally be
transmitting Internet Protocol datagrams, the length of which, by
convention, does not approach this limit. Therefore, unless a
protocol other than the Internet Protocol is used with DDN
standard X.25 service, this is a technical restriction that will
have no practical impact upon the design of DTE software. See
Appendix A, Section A-3.2.
-11-
DDN X.25 DTEs connecting to DDN through an X.25
Internet Private Line Interface (IPLI) must reduce
the maximum complete packet sequence length by an
additional 256 bits to allow for IPLI overhead.
2.3 Link Level Procedures
DDN X.25 link level procedures are as specified by FIPS
100/Fed. Std. 1041 and CCITT X.25. This section presents
additional information.
2.3.1 Link Level Parameters and Options
1. The default value of K, the maximum number of
sequentially numbered I frames that the DCE will
have outstanding (unacknowledged) at any given
time, is seven. A DDN X.25 DCE may be configured
on a per-DTE basis to provide optional values of K
from one to six.
2. The default value of N2, the maximum number of
transmissions and retransmissions of a frame
following the expiration of the T1 timer, is
twenty. This value can be changed to any value
from one to 200 as a DCE configuration parameter
on a per-DTE basis.
3. The optional 32-bit FCS is not supported.
2.3.2 Timer T1 and Parameter T2
The period of the timer T1 used by the DDN X.25 DCE reflects
assumptions about the processing speed of the DTE. The DCE
assumes that parameter T2, the response latency of the DTE to a
frame from the DCE, is no greater than 1/2 second. Likewise, the
DCE guarantees that its parameter T2, the latency in responding
to frames from the DTE, is 1/2 second for signaling rates of
19.2 Kb/s or slower, and 1/4 second for faster links.
A lower bound for timer T1 may be computed to be 4X + T2,
based on the assumptions that:
* the link propagation time is negligible,
-12-
* the worst-case frame transmission time is X,
* timer T1 is started when a frame is scheduled for
output,
* each frame is scheduled just as transmission of
the previous frame starts,
* frames are not aborted, and
* each frame and its predecessor are of maximum
length Nl = 8248 bits (see Section 2.3.3 below).
As an example, for a signaling rate of 9.6 Kb/s, this
yields X = .86 sec. If T2 is .5 sec., the total time for the DTE
to respond in the worst case should be 3.9 seconds. In fact, the
DCE uses a T1 timer value of 4 seconds for a link speed of 9.6
Kb/s.
In no case does the DCE use a value for T1 smaller than 3
seconds. This means that, for faster links, the DTE's T2
parameter may be lengthened because the X term in the above
formula is smaller. For links of 19.2 Kb/s or faster, DTEs are
expected to satisfy latency requirements that allow the DCE to
use the formula 4X + T2 (DTE) < 3 seconds = T1 (DCE).
The DTE may choose any value for T1 that is compatible with
the DCE's T2 parameter values. The value of T1 used by the DTE
may always be set longer than the formula indicates, with the
result that recovery from certain types of link errors will be
slower. However, the DCE's parameter T2 cannot be reduced, so
the formula should be viewed as yielding a lower bound on the
DTE's T1 timer.
2.3.3 Maximum I Frame Size
The maximum number Nl of bits in an I Frame is 8248,
accommodating a data packet with up to 1024 data octets. The
derivation of this number is shown in Table 2.2.
DTEs using DDN standard X.25 service must observe the
restriction on the number of data bits in a complete packet
sequence given in Section 2.2 above.
-13-
X.25 No. of
Field Name Level Bits
Address 2 8
Control 2 8
General Format Identifier 3 4
Logical Channel Number 3 12
Packet Type 3 8
User Data 3 8192 (max)
Frame Check Sequence 2 16
TOTAL 8248 (max)
Table 2.2 Derivation of Maximum I Frame Size
2.4 Physical Level Specifications
The DDN X.25 physical level specification is in conformance
with FIPS 100/Fed. Std. 1041 and CCITT X.25. This section
presents additional information.
A DDN X.25 DTE may either be collocated with its DCE or may
be connected to it via an access line. In all cases the DTE
presents a physical DTE interface; the DDN will supply the
matching DCE interface. DDN X.25 service offers four physical
level interfaces: RS-232-C (CCITT V.28), RS-449, both balanced
and unbalanced (CCITT V.ll and V.10, respectively; also MIL-188-
114 balanced and unbalanced), and CCITT V.35. Appendix B of this
document describes in detail the choices of physical interface
available to the DDN subscriber and the specifications for each
type of interface. Table 2.3, below, summarizes the physical
interfaces available at each data rate supported by the DDN X.25
DCE, and indicates which interfaces are recommended at each
signaling rate.
A DDN X.25 DTE may implement any or all of the signaling
rates shown. At each signaling rate implemented, the DTE must
offer at least one of the physical interface options listed as
"R" (recommended) or "A" (available) for that rate in Table 2.3.
Implementors are encouraged to offer the widest variety of
signaling rates and physical interfaces practical to maximize
the ease of use of their equipment in DDN.
-14-
Physical Signaling Rate in Kb/s
Interface 1.2 2.4 4.8 9.6 14.4 48 50 56 64 100
RS-232-C R R R R R - - - - -
RS-449 unbal. A A A A - - - - - -
(and equiv.)
RS-449 balanced A A A A A A A A A R
(and equiv.)
CCITT V.35 - - - - - R A R R A
Legend
R = Recommended
A = Available
- = Not available
(Taken from Appendix B, Table B-4
Table 2.3 DDN X.25 Physical Signaling Rates and Interfaces
3 BIBLIOGRAPHY
1. "Specification for the Interconnection of a Host and an IMP".
Report No. 1822, Bolt Beranek and Newman Inc" Cambridge,
MA, revision of December 1981.
2. CCITT Recommendation X.25, "Interface Between Data Terminal
Equipment (DTE) and Data Circuit Terminating Equipment (DCE)
for Terminals Operating in the Packet Mode on Public Data
Networks," International Telegraph and Telephone
Consultative Committee Yellow food, Vol. VIII.2, Geneva,
1981.
3. "Defense Data Network Subscriber Interface Guide," Defense
Communications Agency, Washington, DC, July 1983.
4. "Internet Protocol Transition Workbook," SRI International,
Menlo Park, CA, March 1982.
5. "Internet Protocol Implementation Guide," SRI International,
Menlo Park, CA, August 1982.
APPENDIX A: DDN X.25 Implementation Details
A-1 Introduction
This Appendix serves three purposes. First, it provides
information concerning the planned evolution of DDN X.25
capabilities. Second, it provides information on the use of
certain DDN X.25 features and facilities at a greater level of
detail than is appropriate for inclusion in the body of the DDN
X.25 Interface Specification. Specifications for the use of DDN
X.25 features and facilities given in this Appendix are mandatory
on the part of DDN X.25 DTEs that wish to make use of these
features and facilities. Finally, this Appendix presents a
discussion of the limitations on the use of DDN services that
will be encountered by hosts using only DDN basic X.25 service.
A-2 Operational Features of DDN X.25 DCE Releases
The capabilities of the DDN X.25 DCE will evolve over time
from an initial set of capabilities to the full capabilities of
this DDN X.25 Interface Specification. This section describes
release-dependent features of the DDN X.25 DCE. Implementors
should note that not all optional facilities of the specification
will initially be available for use by DTEs.
Releases of new DCE capabilities will be compatible with DTE
hardware and software implementations that meet the full DDN X.25
Interface Specification.
A-2.1 Initial Feature Support
The initial release of the DDN X.25 DCE will support flow
control parameter negotiation and fast select. In addition, the
DDN X.25 DCE may be configured by the DDN Administration to
provide non-standard default window and packet sizes as described
in CCITT X.25 Sections 7.1.2 and 7.2.1. The call precedence and
type of service selection facilities will be accepted, but not
acted upon, by the network. Only DDN basic X.25 service will be
supported. Planned future DCE releases will support all
facilities specified in FIPS 100/Federal Standard 1041 with the
exception of those "additional" facilities that are listed in
Section 1.2.1 of this document.
A-1
A detailed schedule of DDN X.25 DCE releases and the
capabilities of each release will be supplied in a separate
document.
A-2.2 Exception-Handling Procedures
Certain of the exception- or error-handling procedures of
the initial release of the DDN X.25 DCE differ in detail from the
procedures specified in FIPS 100/Federal Standard 1041. These
differences are described below. A later release of the DDN X.25
DCE will bring these procedures into conformance. In the
interim, the variances in these procedures will not preclude
satisfactory operation between the DCE and a DTE, provided the
DTE operates in accordance with FIPS 100/Federal Standard 1041.
A-2.2.1 Non-Octet-Aligned Data
Data packets received by the DDN X.25 DCE that are not
aligned on an octet boundary are discarded at the link level.
They are not passed to the DCE packet level, and no packet level
diagnostic code is returned to the DTE.
A-2.2.2 RESTART REQUEST Packet
The DDN X.25 DCE will not discard, but will instead act
upon, a RESTART REQUEST packet that
(i) is too long (unless it exceeds the maximum frame
size for the link level),
or
(ii) contains a non-zero cause field.
A-2.2.3 RESET REQUEST Packet
The DDN X.25 DCE will not discard, but will instead act
upon, a RESET REQUEST packet that contains a non-zero reset cause
field.
A-2
A-2.2.4 CLEAR REQUEST Packet
The DDN X.25 DCE will not discard, but will instead act
upon, a CLEAR REQUEST packet that contains a non-zero clearing
cause field.
A-2.3 Virtual Circuit Resource Availability
In its current implementation, the DDN X.25 packet switching
node is capable of supporting a minimum of one hundred
simultaneous virtual circuits. As was discussed in Section
2.1.4, resources of the node are shared dynamically among the
DCEs attached to the node. Therefore, no explicit guarantees are
made of the number of simultaneous virtual circuits that can be
made by a single DTE. Depending upon the configuration of the
node, the number of simultaneous circuits supported by the node
can be significantly greater than one hundred.
A-3 Detailed Features and Facilities Specifications
This section provides detailed specifications and
descriptions of use for certain DDN X.25 features and facilities.
A-3.1 Additional Diagnostic Codes
The DDN X.25 DCE is capable of providing additional
information to DTEs in RESTART, RESET, CLEAR INDICATION, and
DIAGNOSTIC packets by means of diagnostic codes that are
extensions to the set of diagnostic codes given in Annex E of
CCITT Recommendation X.25. These codes are taken from the set of
codes "reserved for network specific diagnostic information," and
are thus not in conflict with code assignments made in Annex E.
The values of these codes, and their meanings, are given in Table
A-1 below.
A-3
Code
Value Meaning
128 IMP is unavailable. The packet-forwarding
mechanisms of the network are unavailable to the
DCE. Sent in RESET, CLEAR and RESTART packets.
130 Link level came up. Sent in RESTART and RESET
packets.
131 Link level went down at remote DTE. Sent in CLEAR
and RESET packets.
132 Remote DTE restarted. Sent in CLEAR and RESET
packets.
133 Local resources not available for call
establishment. The local DCE has too few
resources to establish another call. Sent in
CLEAR and DIAGNOSTIC packets.
134 Remote resources not available for call
establishment. The remote DCE has too few
resources to establish another call. Sent in
CLEAR packets.
136 Remote host dead. The link to the remote DTE is
down. Sent in CLEAR and RESET packets.
137 Remote IMP dead. The IMP to which the remote DTE
is attached is down. Sent in CLEAR and RESET
packets.
138 Logical subnetwork access barred. The remote DTE
cannot be reached because of a communities-of-
interest prohibition. Sent in CLEAR and RESET
packets.
139 Connection lost. An internal error has occurred
at either the remote or the local DCE which has
made their virtual circuit data structures
inconsistent. Sent in CLEAR and RESET packets.
140 Response lost. A response from the remote DCE
failed to arrive within a reasonable time. Sent
in CLEAR and RESET packets.
A-4
141 Calling logical address not enabled or not
authorized. Sent in CLEAR packets.
142 Calling logical name incorrect for this DTE. Sent
in CLEAR packets.
143 Called logical name not authorized. Sent in CLEAR
packets.
144 Called logical name not enabled. Sent in CLEAR
packets.
145 Called logical name has no enabled DTEs. Sent in
CLEAR packets.
146 Use of logical addresses invalid in this network.
Sent in CLEAR packets.
147 Declared logical name now in effect. Sent in
CLEAR packets.
148 Declared logical name was already in effect. Sent
in CLEAR packets.
149 Declared logical name is now disabled. Sent in
CLEAR packets.
150 Declared logical name was already disabled. Sent
in CLEAR packets.
151 Incoming calls barred. Sent in CLEAR packets.
152 Outgoing calls barred. Sent in CLEAR packets.
192 Cleared due to higher precedence call at local
DCE. Sent in CLEAR packets.
193 Cleared due to higher precedence call at remote
DCE. Sent in CLEAR packets.
194 Requested precedence too high. The DTE is not
authorized to establish a call at the requested
precedence level. Sent in CLEAR packets.
Table A-1. Additional Packet Level Diagnostic Codes
A-5
A-3.2 X.25 IP Interoperability Considerations
When DDN standard X.25 service is requested at call
establishment (as described in Section 2.1.2.1), the call is in
effect established between the DTE and a local X.25 entity. This
entity subsequently extracts the IP datagrams from the X.25 data
packets for transmission through the DDN Internet. This approach
requires that certain conventions be followed:
1. IP datagrams are to be sent as X.25 complete
packet sequences. That is, datagrams begin on
packet boundaries and the M ("more data") bit is
used for datagrams that are larger than one
packet. Only one IP datagram is to be sent per
X.25 complete packet sequence.
2. By convention, the maximum IP datagram size is 576
octets. This packet size can most efficiently be
accommodated by negotiating an X.25 maximum packet
size of 1024; alternatively, a DTE may use an X.25
complete packet sequence to transmit an IP
datagram.
3. Because the X.25 connection is in effect
terminated locally, the D and Q bits have no
significance and should be set to zero.
4. The precedence bits of the IP type-of-service
field are to be mapped into X.25 precedence bits
(see Section 2.1.2.2) as specified in Table A-2.
IP Precedence X.25 Precedence
000 00
001 01
010 10
011 - 111 11
Table A-2. IP Precedence to X.25 Precedence Mapping
A-6
A-3.3 The DDN Logical Addressing Facility
The DDN logical addressing facility allows references to
hosts by either their physical network address or by one or more
location-independent logical addresses, and allows hosts to
exercise partial control over the logical address(es) by which
they can be referenced. Implementation of DDN logical addressing
by a host is optional.
The DDN Administration will assign seven-digit logical
addresses, and will maintain a logical addressing data base. The
host is then responsible for notifying the network ("enabling")
of the "names" (logical addresses), if any, by which it wishes to
be known. It cannot receive calls addressed to a name or
originate calls under that name unless it has enabled that name.
It also cannot enable a name that is not authorized for that
physical address. Names can also be enabled automatically by the
network, under the control of the Administration.
A-3.3.1 Logical Addresses
Logical addressing is invoked when a called address is
supplied to the IMP with the flag digit F = one. The logical
address consists of seven BCD digits. This name is mapped by the
logical addressing facility into a DDN physical network address.
The logical name need not be unique for the physical address, nor
is the physical address necessarily unique for the name.
A-3.3.2 Enabling and Disabling Logical Addresses
To enable and disable logical addresses, the DDN X.25 host
must send declarative CALL REQUEST packets to the DCE using a
called address with the format:
ZZZZ F DDDDDDD (SS)
where the address fields are as described in Section 2.1.1. The
Flag F must be set to nine, the DDN Host Identifier field
specifies the logical address under consideration, and the
subaddress field, which must be present, specifies the type of
transaction. Declarative calls are cleared immediately by the
local DCE.
A-7
If SS is zero, the logical name is enabled in normal mode,;
that is, that physical port will accept incoming calls to that
name, and allow outgoing calls from that name. If SS is one, the
logical name is disabled. If SS is two, the logical address is
enabled in reverse translation mode; in this mode, the called
address field of incoming call packets will be translated into a
physical address (i.e., an address containing a flag F = 0), if
it was given by the calling DTE (X.25 host), as a logical address
(i.e., containing a flag F = 1).
Whenever a DTE comes up, or restarts, the logical names for
that DTE are returned to their default state, which may be either
enabled or disabled, as configured by the DDN Administration.
A-4 Limitations of DDN Basic X.25 Service
The Defense Data Network is an Internetwork environment.
That is, DDN as a whole is made up of a number of constituent
packet switching networks that are interconnected via gateways.
Communication across gateways requires the use of the Internet
Protocol, which, for a host accessing DDN using X.25, requires
that the host implement the DoD standard protocol architecture
and employ DDN standard X.25 service. In addition, a classified
host is attached to a DDN constituent network of lower
classification by means of an Internet Private Line Interface
(IPLI). IPLIs, which themselves contain gateways, also require
the use of the Internet Protocol; moreover, they do not, as
currently designed, offer an X.25 host interface. These
attributes of the DDN Internet have two implications for users of
DDN basic X.25 service:
1. DDN hosts that do not implement IP and higher-
level DDN protocols, and which use only DDN basic
X.25 service, cannot communicate across gateways.
Their network communication is therefore
restricted to a single DDN constituent network.
2. X.25 hosts cannot be provided classified service
on a constituent network of lower classification.
Should X.25 host access be developed for the IPLI
in the future, classified network access will be
made available to hosts using DDN standard X.25
service only.
A-8
A-5 Derivation of DDN X.25 Addresses
All DDN hosts are assigned addresses by the Administration.
The address of a DDN host may be obtained from the Network
Information Center (NIC), represented as an ASCII text string in
what is called "host table format". This section describes the
process by which DDN X.25 addresses in the format described in
Section 2.1.1 may be derived from addresses in NIC host table
format.
A NIC host table address consists of the ASCII text string
representations of four decimal numbers separated by periods,
corresponding to the four octets of a thirty-two bit Internet
address. The four decimal numbers are referred to in this
section as "n", "h", "l", and "i." Thus, a host table address
may be represented as "n.h.l.i" Each of these four numbers will
have either one, two, or three decimal digits and will never have
a value greater than 255. For example, in the host table address
"10.2.0.124", n=10, h=2, l=0, and i=124. To convert a host table
address to a DDN X.25 address:
1. If h < 64, the host table address corresponds to
the DDN X.25 physical address
ZZZZ F IIIHHZZ (SS)
where:
ZZZZ = 0000
as required in Section 2.1.1.1.1;
F = 0 because the address is a physical
address;
III is a three decimal digit
representation of "i", right-adjusted
and padded with leading zeros if
required;
HH is a two decimal digit representation
of "h", right-adjusted and padded
with leading zeros if required;,
ZZ = 00
and
(SS) is optional, as described in Section
2.1.1.1.4.
A-9
In the example given above, the host table address
10.2.0.124 corresponds to the DDN X.25 physical
address 000001240200.
2. If h > 64 or h = 64, the host table address
corresponds to the DDN X.25 logical address
ZZZZ F RRRRRZZ (SS)
where:
ZZZZ = 0000
as required in Section 2.1.1.1.1;
F = 1 because the address is a logical
address;
RRRRR is a five decimal digit
representation of the result "r" of
the calculation
r = h * 256 + i
(note that the decimal representation
of "r" will always require five
digits);
ZZ = 00
and
(SS) is optional, as described in Section
2.1.1.1.4.
Thus, the host table address 10.83.0.207
corresponds to the DDN X.25 logical address
000012145500.
In both cases, the "n" and "l" fields of the host table
address are not used.
A-10
APPENDIX B: DDN Synchronous Level 1 Specification
B-1 Introduction
A host may connect to the Defense Data Network at the link
level using the asynchronous bit serial protocol described in BBN
Report No. 1822 as either a local host (LH) or a distant host
(DH). A host may also connect to the DDN by means of a
synchronous bit serial protocol at the link level, using either
the method described in BBN Report No. 1822, HDH, or the DDN X.25
interface. Neither LH nor DH is recommended for new
implementations.
This section describes the functional, electrical, and
mechanical connection (the level 1 connection) that is required
when either an HDH or an X.25 host is connected to the DDN.
Hosts connecting to the DDN via HDH or X.25 require a synchronous
modem connection or the equivalent, which will be supplied as
part of the DDN service. The host will present the DTE interface
while the DDN-provided equipment will present the DCE interface.
A long-term goal of the DDN is for all level 1 connections
to be accomplished with the MIL-188-114 balanced interface. Its
general equivalents are EIA RS-449/422, CCITT V.ll, and Fed. Std.
1031/1020. The DDN cannot implement this at present due to the
limited availability of commercial vendor hardware. In order to
facilitate future DDN compatibility, all new system acquisitions
should specify MIL-188-114 balanced as a required interface, in
addition to an alternate interface. The selection of an
alternate interface should not preclude utilization of the MIL-
188-114 balanced interface when it becomes supportable.
B-2 Supported Interfaces
DDN presently supports four synchronous level 1 interfaces.
They are:
1. EIA RS-232-C, CCITT V.28 & V.24;
2. MIL-188-114 balanced, EIA RS-449&422, CCITT V.ll,
Fed. Std. 1031/1020;
3. MIL-188-114 unbalanced, EIA RS-449&423, CCITT
V.10, Fed. Std. 1031/1030; and
B-1
4. CCITT V.35.
Table B-1 is a dictionary of terms that relates the CCITT
signal ID to the EIA signal ID and to the more common
abbreviations. Table B-2 identifies signals as either required,
optional, or not used.
Figure B-1 and Table B-3 identify typical DTE connections to
the DDN. The required subscriber services will dictate which
scheme is selected for a particular DTE.
Table B-4 relates required speed of service to interface
type.
Together, these tables and figures serve as a guide to level
1 interface selection. From these, most systems will be able to
identify the most appropriate interface. However, this
information is not all-inclusive. Other interface arrangements
may be possible; contact your DDN representative for assistance
as required.
Demarcation Point
(mating connectors)
DTE DCE
|------------] [------(1) Modem RS-232-C
|
| |---------] [------(2) Modem V.35
|---|--|----|
| |----] [------(3) LDM RS-232-C, MIL-188-119
| |
| |----] [------(4) Null Modem Cable
| HOST |
| |----] [------(5) SME Cable plus clock source
| |
| |----] [------(6) DCS MIL-188-114
|--|--|--|--|
| | |-------] [------(7) DES RS-232-C, RS-449, V.35
| |
| |----------] [------(8) KG MIL-188-114 balanced
|
|-------------] [------(9) IPLI MIL-188-114 balanced
Figure B-1. Typical Level 1 Connection Schemes
B-2
EIA CCITT ABBRM NAME
ID ID NAME
--- ----- ------ ---------------------------------
AA 101 FG Frame (Chassis/Protective) Ground
AB 102 SG Signal/Supply Common
SC 102a -- RS-449 DTE Common
RC 102b -- RS-949 DCE Common
BA 103 TD Transmit Data
BB 104 RD Receive Data
CA 105 RTS Request to Send
CB 106 CTS Clear to Send
CC 107 DSR Data Set Ready
CD 108.2 DTR Data Terminal Ready
CF 109 DCD Data Carrier Detect
CG 110 SQ Signal Quality
CH 111 -- Signal Rate Selector to DCE
CI 112 -- Signal Rate Selector to DTE
DA 113 ETC External Transmit Clock
DB 114 TC Transmit Clock
DD 115 RC Receive Clock
-- 116 -- Select Standby
-- 117 -- Standby Indicator
SBA 118 STD Secondary Transmit Data
SBB 119 SRD Secondary Receive Data
SCA 120 SRS Secondary Request to Send
SCB 121 SCS Secondary Clear to Send
SCF 122 SCD Secondary Carrier Detect
SCG 123 SSQ Secondary Signal Quality
-- 124 -- Select Frequency Group
CE 125 RI Ringing Indicator
-- 126 -- Select Transmit Frequency
-- 127 -- Select Receive Frequency
-- 128 -- External Receive Clock
-- 129 RR Request to Receive
-- 130 -- Secondary Transmit Tone
-- 131 -- Receive Character Timing
-- 132 -- Return to Non-Data Mode
-- 133 RTR Ready to Receive .
-- 134 -- Received Data Present
-- 136 -- New Signal
-- 140 RL Remote loopback
-- 141 LL Local loopback
-- 142 TM Test Status Monitor
-- 191 -- Transmit Voice Answer
192 -- Receive Voice Answer
Table B-1. EIA and CCITT Interchange Circuits
B-3
Required: 101, 102, 103, 104, 105, 106, 107, 108.2,
109, 113, 114, and 115
Optional: 110, 125, 140, 141, and 142
(These may be required IAW future DDN
developments; it is strongly recommended
that these at least be available for
implementation upon requirement)
Not used: 111, 112, 116, 117, 118, 119, 120, 121, 122,
123, 124, 126, 127, 128, 129, 130, 131, 132,
133, 134, 136, 191, and 192
Table B-2. Signal Selection by CCITT Interchange Circuit Number
B-4
Scheme (From
Fig. B-1) Explanation
(1) Modem RS-232 at spe eds of 1200, 2400, 4800, 9600 or
14400 b/s over long haul leased voice grade
telephone facilities
(2) Modem CCITT V.35 at speeds of 48, 50, 56, 64 Kb/s over
leased group (37KHz) grade facilities or in CONUS
the Digital Data Service facilities.
(3) Limited Distance Modem
LDM generally available at 9600 b/s and below in
an RS-232 version. Other types are available for
all speeds.
(4) Null modem A Null Modem is a length of cable with the signal
leads crossed so as to present a DCE interface.
To be used in local connection schemes where
either the DTE or the DCE has a clocking source
capability. All four supported level 1
interfaces are available. If DTE clock and DCE
clock are both available, DTE clock will be
preferred.
(5) Synchronous Modem Eliminator
SME is a length of cable with a hardware device
interjected. The device allows convenient
crossing of signals so as to present a DCE
interface. The device also provides clocking
when neither the DTE nor the DCE has such
capability. All four supported level 1
interfaces are available.
(6) DCS Microwave
DCS is generally a military microwave system
which provides the MIL-188-114 balanced or
unbalanced interfaces. It implies a speed of 50
Kbps and is usually found O-CONUS. Selection of
this scheme requires selection of (4) or (5).
(7) Data Encryption Standard
DES is a commercial encryption device used by the
DoD as a privacy device. DES is available with
either RS-232, V.35, or RS-449/422.
B-5
(8) KG KG devices are U. S. Government encryption
devices under strict NSA control. The
requirement for security and KG devices requires
the selection of the MIL-188-114 balanced
interface.
(9) Internet Private Line Interface
IPLI devices are security level community of
interest isolation devices. The requirement for
IPLI service requires the selection of the MIL-
188-114 balanced interface.
Notes and Considerations
1. Interface (2), Modem, 48Kb/s is generally only
available O-CONUS.
2. MIL-188-114 balanced is deemed equivalent to RS-449
with RS-422, the difference being that MIL-188-114 is
more tolerant of noise on signal common and more
tolerant of common mode noise.
3. MIL-188-114 unbalanced is deemed equivalent to RS-449
with RS-423. In most cases where MIL-188-114 balanced
is specified, MIL-188-114 unbalanced is also available,
but it is not recommended.
4. There are system enhancements under long term
development for use in the DDN which may request
additional control leads beyond those listed as
required. The implementation of these enhancements
will not limit operational capabilities but may impact
the ability of the Network Monitoring Center to assist
with host and host access line diagnosis. These
enhancements may request signals from the optional
category.
Table B-3. Typical Level 1 Connection Schemes
B-6
Signaling Rate in Kb/s
Physical
Interface 1.2 2.4 4.8 9.6 14.4 48 50 56 64 100
RS-232-C R R R R R* - - - - -
MIL-188-114 A A A A - - - - - -
unbal. (& equiv.)
MIL-188-114 A A A A A* A A A A R**
bal. (& equiv.)
CCITT V.35 - - - - - R A R R A
Legend
R = Recommended
A = Available
- = Not available
* = Only available using modems
** - Only available using a local cable
connection
Table B-4. Interface Type by Service
Speed
B-7
Signal Name Abbrev Pin No. EIA ID Signal Source
----------- ------ ------- ------ -------------
Frame Ground FG 1 AA DTE/DCE
Transmitted Data TD 2 BA DTE
Received Data RD 3 BB DCE
Request to Send RTS 4 CA DTE
Clear to Send CTS 5 CB DCE
Data Set Ready DSR 6 CC DCE
Signal Ground SG 7 AB DTE/DCE
Data Carrier Detect DCD 8 CF DCE
Transmit Clock TC 15 DB DCE
Receive Clock RC 17 DD DCE
Data Terminal Ready DTR 20 CD DTE
Ext. Transmit Clock ETC 24 DA DTE
Wired Spare -- 18 -- ---
Wired Spare -- 22 -- ---
Wired Spare -- 25 -- ---
Required pins: 1, 2, 3, 4, 5, 6, 7, 8, 15, 17, 20, 24
Optional pins: 9, 10, 18, 22, 25
Notes
1. The DTE will present a CANNON DB-25P male connector
with pinouts as above or equivalent hardware with
identical pinouts.
2. The DCE will present a CANNON DB-2SS female
connector or equivalent.
Table B-5. RS-232-C Interface
B-8
Signal Name Abbrev Pin Nos EIA ID Signal Source
----------- ------ ------- ------ -------------
Send D ta SD 4,22 BA DTE
Send Timing ST 5,23 DB DCE
Receive Data RD 6,24 BB DCE
Request to Send RTS 7,25 CA DTE
Receive Timing RT 8,26 DD DCE
Clear to Send CTS 9,27 CB DCE
Local 100pback LL 10 -- DTE
Data Mode DM 11,29 CC DCE
Terminal Ready TR 12,30 CD DTE
Receiver Ready RR 13,31 CF DCE
Remote 100pback RL 14 -- DTE
Terminal Timing TT 17,35 DA DTE
Test Mode TM 18 -- DCE
Signal Ground SG 19 AB DTE/DCE
Receive Common RC 20 RC DCE
Send Common SC 37 SC DTE
Wired Spare -- 1 -- ---
Wired Spare -- 3,21 -- ---
Required pins: 4,22; 5,23; 6,24; 7,25; 8,26; 9,27,;
11,29; 12,30; 13,31; 17,35; 19; 20; 37
Optional pins: 10; 14; 18; 1; 3,21
Notes:
1. The DTE will present a CANNON DC-37P male connector
with pinouts as above or equivalent hardware with
identical pinout.
2. The DCE will present a CANNON DC-37S female
connector or equivalent.
Table B-6. MIL-188-114 Interface (and equivalents)
B-9
Signal Name Abbrev Pin Nos. EIA ID Signal Source
----------- ------ -------- ------
-------------
Frame Ground FG A AA DTE/DCE
Signal Ground SG B AB DTE/DCE
Transmit Data TD P/S BA DTE
Receive Data RD R/T BB DCE
Request to Send RTS C CA DTE
Clear to Send CTS D CB DCE
Data Set Ready DSR E CC DCE
Data Carrier Detect DCD F CF DCE
Local 100pback LL K -- DTE
Ext. Transmit Clock ETC U/W DA DTE
Transmit Clock TC Y/aa DB DCE
Receive Clock RC V/X DD DCE
Required Pins: A; B; P/S; R/T; C; D; E; F; U/W; Y/aa;
V/X
Optional Pins: K
Notes:
1. The DTE will present a Winchester MRA(C)-34D-JTCH-H8
male connector with pinout as above or equivalent
hardware with the identical pinout.
2. The DCE will present a mating female connector.
Table B-7. V.35 Interface
B-10
APPENDIX C
FEDERAL INFORMATION
PROCESSING STANDARDS PUBLICATION 100
FEDERAL STANDARD 1041
1983 JULY 6
ANNOUNCING THE JOINT STANDARD FOR
INTERFACE BETWEEN DATA TERMINAL EQUIPMENT (DTE)
AND DATA CIRCUIT-TERMINAL EQUIPMENT (DCE)
FOR OPERATION WITH PACKET-SWITCHED DATA
COMMUNICATIONS NETWORKS
Federal Information Processing Standards Publication are developed
and issued
by the National Bureau of Standards pursuant to section
111(f)(2) of the
Federal Property and Administrative Services Act of 1949, as
amended, Public
Law 89-306 (79 Stat.1127), Executive order 11717 (38 FR 12315
dated May 11,
1973), and Part 6 of Title 15 Code of Federal Regulations (CFR).
Federal Standards in the "telecommunication" series are
developed by the
Office of the Manager, National Communication System. These Federal
Standards
are issued by the General Services Administration pursuant to the
Federal
Property and Administrative Services Act of 1949, as amended.
Name of Standard: Interface Between Data Terminal Equipment (DTE)
and Data
Circuit-Terminating Equipment (DCE) for Operation with
Packet-Switched Data
Communications Networks.
Category of Standard: Hardware, Data Transmission.
Explanation: Federal automated data processing equipment,
services, and
telecommunication equipment using public packet-switched data
communications
networks (PSDCN) based on the family of CCITT Recommendations
derived from
X.l and X.2 shall employ the interface and protocols specified in
this joint
standard. In addition, designers of these internally operated and
maintained
Federal networks employing packet-switched technology should
consider the use
of this interface as appropriate. The joint standard provides:
- A family of physical layer interfaces, from which a
particular
interface may be selected; and
- A single data link layer control procedure; and
- Packet level procedures for virtual calls and
permanent virtual
circuits, and an optional datagram operation.
The mandatory interface attributes of this joint standard are
summarized as
follows:
PHYSICAL LEVEL
Transmission rates: 2.4, 4.8, 9.6 Kbits/s
Interface: one or more of the following: RS-232-C,
X.2l, RS-449
LINK LEVEL:
Procedure: LAPB
Parameter K: 7
Smallest N l: l64 Octets
PACKET LEVEL:
Services: Virtual call and permanent virtual
circuit
Packet types: All basic plus Diagnostic packets.
Packet Reject
shall not be used.
User data field Octet-aligned
length:
Packet sequence Modulo 8
numbering:
D bit procedure: Supported by all DCEs; DTE need not
employ the
D bit when sending to
the DCE, but no DTE shall reject
incoming packet
with the D bit set to l or 0 as having
this bit
in error unless it is known by receiver
that the
sender has no D bit capability.
X.25 diagnostic Use standard codes whenever they apply;
non-std
codes: codes may be used for events not listed
in X.25
within a period of 24 months after the
effective
date of this standard.
Fast Select: DCEs shall implement fast select; DTE
need not
employ fast select when sending to DCE,
but all
DTEs with higher level functionality
which
allows response to fast select must be
able to
accept incoming fast select packet.
Interrupt packet: Receipt of a DTE interrupt packet
before a
previous DTE interrupt packet has been
confirmed
is an error condition.
Duplicated facility The last appearing facility code should
be
codes: treated by the DTE as if it were the
only
appearance of that code.
Non-zero cause field Discarded
of restart request
packet:
Restart request too Discarded
long in state r1:
This joint standard is intended to enhance interoperability by
specifying
certain subsets and other constraints on Federal use of CCITT
Recommendation
X.25.
The Government's intent in employing this joint standard is to
reduce the
cost of acquiring and using Federal automated data processing
equipment,
services, and telecommunication equipment with PSDCN. The joint
standard is
also intended to reduce the cost of acquiring and using
Government-owned or
leased PSDCN. These goals will be achieved by:
- increasing the available alternative sources of supply;
- Increasing the reutilization of Government resources; and,
- Assuring the required interoperability.
Approving Authority: Secretary of Commerce (Federal Information
Processing
Standards). Administrator, General Services Administration
(Federal
Standards).
Maintenance Agency: The National Bureau of Standards and the
Office of the
Manager, National Communications System will jointly maintain this
standard
coordinating as necessary with the General Services Administration
(GSA).
Cross Index: The following are related standards upon which this
FIPS PUB is
based. The inclusion of a particular standard on this list
does not
necessarily mean that the standard is applicable in all cases to
which this
FIPS PUB applies.
(a) International Standard 2110-1980: Data Communication-25 pin
DTE/DCE
Interface Connector and Pin Assignments.
(b) International Telegraph and Telephone Consultative Committee
(CCITT)
recommendations V.24 (1980): List of Definitions for Interchange
Circuits
Between Data Terminal Equipment and Data Circuit Terminating
Equipment.
(c) CCITT Recommendation V.28 (1980) Electrical Characteristics
for
Unbalanced Double-Current Interchange Circuits.
(d) Electronics Industries Association (EIA) RS-232-C (1969
August):
Interface Between Data Terminal Equipment and Data Communication
Equipment
Employing Serial Binary Data Interchange.
(e) International Standard 4902-1980: Data Communication-37-Pin
and 9-Pin
DTE/DCE Interface Connectors and Pin Assignments.
(f) CCITT recommendation V.11(X.27) (1980): electrical
Characteristics for
Balanced Double-Current Interchange Circuits for General Use with
Integrated
Circuit Equipment in the Field of Data Communications.
(g) EIA RS-422-A (1978 June): Electrical Characteristics of
Balanced
Voltage Digital Interface Circuits.
(h) Federal Standard 1020A (1980 January): Telecommunications:
Electrical
Characteristics of Balanced Voltage Digital Interface Circuits.
(i) CCITT Recommendation V.10 (X26) (1980): Electrical
Characteristics for
Unbalanced Double-Current Interchange Circuits for General Use with
Integrated Circuit Equipment in the Field of Data Communications.
(k) Federal Standard 1030A (1980 January): Telecommunications:
Electrical
characteristics of Unbalanced Voltage Digital Interface Circuits.
(l) CCITT Recommendation X.21bis (1980): Use on Public Data
Networks of
Data Terminal Equipment which are Designed for Interfacing to
Synchronous
V-series Modems.
(m) CCITT Recommendation V.54 (1980): Loop Test Devices for
Modems.
(n) EIA RS-449 (1977 November): general Purpose 37-Position
Interface
Between Data Terminal Equipment and Data Circuit-Terminating
Equipment.
(o) Federal Standard 1031 (1980 June): Telecommunications
General Purpose
37-position and 9-position Interface Between Data Terminal
Equipment and Data
Circuit Terminating Equipment (implementing instructions in the
form of a
Federal Property Management Regulation have not yet been issued.
the General
Services Administration is considering canceling FED-STD 1031.
Furthermore,
a Federal Information Processing Standard for ADP applications
corresponding
to Federal Standard 1031 has not been adopted by the National
Bureau of
Standards.)
(p) International Standard 4903-1980: Data Communication-15-pin
DTE/DCE
Interface Connector and Pin Assignments.
(q) EIA Industrial Electronics Bulletin No. 12 (1977 November):
Application Notes on Interconnection Between Interface Circuits
Using RS-449
and RS-232-C.
(r) Draft International Standard 2593 (1980): Data
Communication-34-pin
DTE/DCE Interface Connector and Pin Assignments.
(s) CCITT Recommendation V.35 (1980): Data Transmission at 48
Kilobits per
second Using 60-108 kHz Group Band Circuits.
(t) CCITT Recommendation X.21 (1980): general Purpose Interface
Between
Data Terminal Equipment and Data Circuit-Terminating Equipment for
Synchronous Operation on Public Data Networks.
(u) CCITT recommendation V.5 (1980): Standardization of
Data-Signalling
Rates for Synchronous Data Transmission in the General Switched
Telephone
networks.
(v) CCITT Recommendation V.6 (1980): Standardization of
Data-Signalling
Rates for Synchronous Data Transmission on Leased Telephone-Type
Circuits.
(w) American National Standard X3.1-1976: Synchronous Signalling
Rates for
Data Transmission.
(x) Federal Information Processing Standard Publication 22-1
(1977
September): Synchronous Signaling Rates Between Data Terminal and
Data
Communication Equipment. (FIPS PUB 22-1 is identified also as
FED-STD 1013.)
(y) Federal Standard 1013 (1977 August): Telecommunications:
Synchronous
Signaling Rates Between Data Terminal Equipment and Data
Circuit-Terminating
Equipment utilizing 4 kHz Circuits (FED-STD 1013) is identified
also as FIPS
PUB 22-1.)
(z) American National Standard X3.36-1975: Synchronous
High-Speed Data
Signaling Rates Between Data Terminal Equipment and Data
Communication
Equipment.
(aa) Federal Information Processing Standards Publication 37
(1975 June):
Synchronous High Speed Data Signaling Rates Between Data Terminal
Equipment
and Data Communication Equipment. (FIPS PUB 37 is identified also
as
FED-STD 1001.)
(ab) Federal Standard 1001 (1975 June): Telecommunications:
Synchronous
High-Speed Data Signaling Rates Between Data Terminal Equipment and
Data
Communications Equipment. (FED-STD 1001 is identified also as FIPS
PUB 37.)
(ac) EIA RS-269-B (1976 January): Synchronous Signaling Rates
for Data
transmission.
(ad) International Standard 3309-1979: Data Communication-High
Level Data
Link control Procedures-Frame Structure.
(ae) International Standard 4335-1979: Data Communication-High
Level Data
Link control Procedures-Elements of Procedures.
(af) Addendum 1 to International Standard 4335-1979: Data
Communication-High Level Data Link control Procedures-Elements of
Procedures.
(ag) Addendum 2 to International Standard 4335-1979: Data
Communication-High Level Data Link Control Procedures-Elements of
procedures.
(ah) International Standard 6256-1980: Data Communication-High
-Level Data
Link Control Procedures-Balanced Class of Procedures.
(ai) American National Standard X3.66-1979: Advanced Data
Communication
Control procedures (ADCCP).
(aj) Federal Information Processing Standards Publication 71
(1980 May) as
revised by the Federal Register notice 47 FR 23798, dated June 1,
1982 and
corrected by the notice 47 FR 25397 dated June 11, 1982: Advanced
Data
Communication Control Procedures (ADCCP). (FIPS PUB 71 is
technically
consistent with FED-STD 1003A.)
(ak) Federal Information Processing Standards Publication 78
(1980
September): Guideline for Implementing Advanced Data Communication
Control
Procedures (ADCCP).
(al) Federal Standard 1003A (1981 August): Telecommunications:
Synchronous
bit-Oriented Data Link Control Procedures (FED-STD 1003A is
technically
consistent with FIPS PUB 71.)
(am) CCITT Recommendation X.25 (1980): Interface Between Data
Terminal
Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for
Terminals
Operating in the Packet Mode on Public Data Networks.
(an) Draft Proposed International Standard 7498: Data
Processing-Open
Systems Interconnection-Basic Reference Model.
(ao) CCITT Recommendation X.1 (1980): International User Classes
of
Service in Public Data Networks.
(ap) CCITT Recommendation X.2 (1980): International User
Facilities in
Public Data Networks.
(aq) CCITT Recommendation X.96 (1980): Call Progress Signals in
Public
Data Networks.
Applicability: The technical specifications of this joint standard
shall be
employed in the acquisition, design, and development of all federal
automated
data processing equipment, services, and telecommunication
equipment and
PSDCN whenever an interface based on CCITT Recommendation
X.25 (1980),
Interface Between Data Terminal Equipment (DTE) and
Data
Circuit-Terminating Equiment (DCE) for Terminals Operating in the
Packet
Mode on Public Networks>1, is required. Referred to below
as CCITT
Recommendation X.25, Recommendation X.25, or X.25.
Implementation: The provisions of this joint standard are
effective July 6,
1983. Any applicable equipment or service ordered on or after the
effective
date, or procurement action for which solicitation documents have
not been
issued by that date, must conform to the provisions of this
standard unless a
waiver has been granted in accordance with the procedures described
below.
This joint standard shall be reviewed by the Institute for Computer
Sciences
and Technology, National Bureau of Standards and the Office of the
Manager,
National Communications System, within five years after its
effective date.
This review shall take into account technological trends and other
factors to
determine if the joint standard should be affirmed, revised, or
withdrawn.
Specifications: This joint standard adopts a subset, identified
below, of the
International Telegraph and Telephone Consultative Committee's
Recommendation
X.25.
(a) At the physical level, the provisions of Section
1 of CCITT
Recommendation X.25 shall be used. As a minimum, networks shall
support
dedicated circuit access; other types of access (e.g., through
the general
switched telephone network) may also be offered.
CClTT Recommendation X.1 standardizes data signalling
rates of
2.4, 4.8, 9.6, and 48 kbits/s for packet mode interfaces. At
a minimum,
networks shall support the synchronous data signalling rates of
2.4, 4.8, and
9.6 kbits/s full duplex; other speeds (e.g., 19.2 kbits/s) may
also be
offered. The 48 kbits/s rate need not be supported in those
locations where
it is not available; 56 kbits/s is recommended in its place (see
American
National Standard X3.36-1975 and related documents referenced
above). The
term "user class of service" used in X.25 refers to the data
signalling rate
of DTE/DCE interface.
In accordance with CCITT Recommendation X.25,
networks shall
provide one or more of the following interface options:
i. CCITT Recommendation X.21;
ii. EIA RS-232-C, which is essentially equivalent to
one of the
options in CCITT Recommendation X.21bis;
iii. CCITT Recommendation X.21bis option that is
equivalent to
RS-449 using only the EIA RS-423A unbalanced electrical
characteristics.
Interworking between EIA RS-232-C on one side of the
interface and
RS-449 on the other side is permitted in accordance with EIA
Industrial
Electronics Bulletin Number 12. Where interworking with RS-232-C
equipment is
not required, the provisions described below employing RS-449
with the
RS-422A electrical characteristics may optionally be employed at
signalling
rates below 48 kbit/s.
Networks which support 48 or 56 kbits/s data
signalling rates
shall provide one or more of the following interface options:
i. CCITT Recommendation X.21;
ii. CCITT Recommendation X.21bis option that
specifies CCITT
Recommendation V.35; or
iii. CCITT Recommendation X.21bis option that
specifies CCITT
Recommendation V.36 which is equivalent to EIA RS-449.
NOTE: Current study in national and international standards
groups may
result in the development of additional physical interfaces.
Each such
physical interface will be evaluated for inclusion in this joint
standard. If
there are significant savings, one physical interface may be
selected as the
future mandatory physical interface.
NOTE: DTE purchasers and designers should determine which
physical
interface(s) is provided by the associated DCE(s).
(b) Only the LAPB link level procedures shall be used.
NOTE: These procedures are a subset of those described in FIPS
PUB 71 and
Federal Standard 1003A and correspond to FIPS PUB 78 recommended
class B.
This subset is identified as follows:
i. Link configuration: two combined stations on a
point-to-point
link.
ii. Class of procedures: balanced asynchronous (BA)
with options
two and eight. The RSET command shall not be used. (RSET is found
in option
11 of the Fips PUB 71. RSET is part of the basic repertoire
in Federal
Standard 1003A; option 11 of federal Standard 1003A deletes the
RSET command.
Note that RSET is not part of CCITT Recommendation X.25.)
iii. Two-way simultaneous operation shall be employed.
iv. The smallest N1, (the maximum number of
bits in an
information frame excluding flags and zero bit insertion for
transparency),
which shall be supported shall be 164 octets (the maximum length
of) fast
select caIl setup packet). If a DTE neither transmits, nor
receives for
processing by higher level functionality fast select packets, an
N1 as
small as 135 octets may be supported by the DTE.
v. The address of the combined station provided by the
network
shall be 10000000; the address of the other combined station
shall be
11000000; where the left-hand bit is the least significant bit (bit
number
1) and shall be transmitted first. This convention is consistent
with the
provisions of FIPS 71 and Federal Standard 1003A.
vi. The FCS shall be a 16-bit sequence as indicated
in Section
2.2.7. DTE/DCE may also employ the 32-bit FCS as indicated in
FIPS PUB 71
(revised) and FED-STD 1003A. DTE/DCE equipment using the 32-bit FCS
shall be
able to also operate with the 16-bit FCS. The smallest N1 shall be
166 octets
when the 32-bit FCS is used. If a DTE neither transmits, nor
receives for
processing by higher level functionality fast select packets, an
Nl as small
as 137 octets may be supported by the DTE when the 32-bit FCS is
used.
NOTE: FIPS PUB 78 provides a detailed discussion of the
relative merits
of the 16-bit and 32-bit FCS.
vii. The frame reject information field shall be
padded with 4
zero bits in bit positions 21 through 24 of the information field
to provide
a length of three octets.
viii. It is required that all implementations be
capable of
operating with K=7; optionally, values of 1 to 6 are permissible
with modulo
8 operation and values 1 to 127 are permissible with modulo 128
operation.
NOTE: DTE purchasers and designers should determine that
values of k
other than 7 are supported by the associated DCE(s).
(c) The user data field of packets shall be an integral number
of octets.
If a packet is received which shows a user data field not
equal to an
integral number of octets, the receiving DTE/DCE shall follow
the packet
level procedures for processing a packet type which is too
long. A new
diagnostic code "non-octet aligned packet," consistent with
the Data
Communications-X.25 Packet Layer Specification for Terminal
Equiment,
ISO DP 8208, November 8, 1982, is recommended as #82.
(d) The reject packet shall not be used.
(e) All DCE restart confirmation, DCE reset confirmation, and
DCE clear
confirmation packets shall be interpreted by the DTE as
having local
significance only.
(f) The D-bit shall be implemented by all networks. DTE's need
not employ
the D-bit procedures when transmitting to the network, but no
DTE shall
reject incoming packets with the D-bit set to 1 or 0 as having
this bit in
error unless the receiving DTE knows the remote DTE has not
implemented the
D-bit procedure; in this case, the receipt of a D-bit set to 1 may
be treated
by the receiving DTE as an error condition.
(g) The selection of logical channel number for new virtual
calls shall
follow the procedures suggested in Section 4.1.2 Note 2, Annex A
Note 5, and
Annex A Note 6, of the CCITT Recommendation X.25.
(h) It is required that all implementations be capable of
operating with
packet sequence numbering modulo 8; optionally, implementations
of packet
sequence numbering modulo 128 are also permitted.
NOTE: DTE purchasers and designers should determine if the
associated
DCE(s) support packet sequence numbering modulo 128.
(i) All DTE's and DCE's shall follow the flow control
principles outlined
in the first two sentences of the first paragraph of Section
4.4. 1.3 of
CCITT Recommendation X.25.
(j) The alternative procedure for passing packets containing
a P(S) that
is out of sequence but within the window as described in the third
paragraph
of Section 4.4.1.3 of CCITT Recommendation X.25 shall not be used.
(k) The second sentence of Section 4.4.1.4 Note 2 shall not
apply. This
sentence permits networks to defer updating the window for data
packets with
D =0, and sent within the window but before a data packet with
D= 1, until
the network receives a corresponding P(R) for the packet with D=
1.
(1) The resetting cause field of a reset request packet shall
be set to
zero. If a reset request is received with a non-zero resetting
cause field,
the packet shall be discarded. The network shall then initiate the
resetting
procedure with the resetting cause field indicating local/remote
procedure
error.
(m) The clearing cause field of a clear request packet shall
be set to
zero. If a clear request packet is received with a non-zero
clearing cause
field, the packet shall be discarded. The network shall then
initiate the
clearing procedure with the clearing cause field indicating
local/remote
procedure error.
(n) The restarting cause field of a restart request packet
shall be set
to zero. If a restart request packet is received with a
non-zero restart
cause field, the restart request packet shall be discarded
without further
action. Optionally, the DCE may generate a diagnostic
packet with a
recommended diagnostic code #81 (improper cause code from DTE),
which is
consistent with the <1Data Communication-X.25 Packet Layer>1
<1Specification
for Data Terminal Equiment,>1 ISO DP 8208, November 8, 1982.
(o) A diagnostic code shall be provided in all clear
request, reset
request, and restart request packets in accordance with the codes
listed in
Annex E of CCITT Recommendation X.25 whenever they apply;
non-assigned
codings in X.25 may be used for events not listed in X.25 within
the period
of 24 months after the effective date of this standard. Prior to
the end of
this 24 month period, this standard will be reviewed by NBS to
determine
whether the standard should be revised to incorporate a
different table.
After this revision, codes not specifically listed shall not be
used.
(p) A generic diagnostic code shall not be used when a more
specific
diagnostic code is known to be applicable.
(q) The network diagnostic codes shall be used in accordance
with the
codes listed in Annex E of CCITT Recommendation X.25 whenever
they apply;
non-assigned codings in X.25 may be used for events not listed in
X.25 within
the period of 24 months after the effective date of this standard.
Prior to
the end of this 24 month period, this standard will be reviewed
by NBS to
determine whether the standard should be revised to incorporate
a different
table. After this revision, network diagnostic codes not
specifically listed
shall not be used.
(r) The network shall consider the receipt of a DTE interrupt
packet
before a previous DTE interrupt packet has been confirmed as an
error, and
shall execute the error procedure described in Annex C, Table
C-4/X.25 and
the corresponding note 2.
(s) The timeouts and time limits specified in Annex D shall
be observed
by all DTE and DCE equipment. T21 shall not be less than the value
given in
table D-2/X.25. The preferred actions listed in table D-2/X.25
shall be
followed.
(t) When the link level procedures enter the logically
disconnected
state, the associated packet level procedures shall clear all
virtual calls
and reset all permanent virtual circuits and datagram logical
channels. When
the link level procedures reenter the information transfer
state, the
associated packet level procedures shall execute the restart
procedure. The
terms "logically disconnected state" and "information transfer
state" are
used as defined in American National Standard X3.66-1979
(referenced above).
Link level procedures enter the logically disconnected state
when a DISC
command is sent and a UA response is received, for example. The
link level
procedure shall also be considered to be in the logically
disconnected state
after N2 (re)transmissions of SABM or DISC, where N2 is as defined
in CClTT
Recommendation X.25. The logically disconnected state is not
assumed after N2
(re)transmissions of other types of frames.
(u) lf a restart request packet is received in state rl which
exceeds the
maximum permitted length, the DCE shall discard the restart
request packet
without further action. Optionally, the DCE may generate a
diagnostic packet
with diagnostic code #39 (packet too long).
(v) In the event that a facility code appears more than
once in a
facility field, the receiving DTE detecting this condition should
treat the
last appearance of the particular code as if it were the only
appearance of
that code.
(w) All networks shall supply diagnostic packets when
their use is
suggested in CClTT Recommendation X.25. No DTE shall rejcct
diagnostic
packets as errors.
(x) ln Section 6.1.1, the second paragraph, the last phrase,
"and is set
to 0 in all other packets", shall be interpreted that the
Qualifier bit is
set to 0 in all other packets except data packets. For the
case of data
packets, the Qualifier bit is set to 0 or 1 as indicated in Section
4.3.6 of
CClTT Recommendation X.25.
(y) The list of user facilities for packet-switched data
networks,
extracted from CCITT Recommendation X.2, is given below. These
facilities
are described in Section 7 of CCITT Recommendation X.25. The
following
further constraints apply:
i. Networks shall provide the facilities designated as
essential
"E" below.
ii. Networks shall also implement the Fast Select and
Fast Select
Acceptance facilities to facilitate more efficient operation in
conveying
higher layer protocol information or user data during call
establishment.
DTE's need not employ fast select packets when transmitting to the
network,
but all DTE's associated with the higher level functionality
which allows
response to a fast select packet must be able to accept incoming
fast select
packets.
iii. The packet retransmission facility shall not be
used.
iv. All DTE's which employ any of the facilities
labelled as
additional "A" below (except Fast Select and Fast Select
Acceptance) shall
also be capable of operating without employing any A facilities
(except Fast
Select and Fast Select Acceptance).
v. The throughput class value of 48,000 bits/s may be
interpreted
as 56,000 bits/s in those locations where 56,000 bits/s access is
used.
Facilities of packet-switched data networks:
User Facility VC PVC
DG*
Optional user facilities assigned
for an agreed contractual period:
Extended packet sequence numbering
(modulo) A A
A*
Non-standard default window sizes A A
A*
Non-standard default packet sizes
16, 32, 64, 256, 512, 1024 A A
-
Default throughput class assignment A A
A*
Flow control parameter negotiation E -
-
Throughput class negotation E -
-
Packet retransmission A*** A***
A***
Incoming calls barred E -
E*
Outgoing calls barred E -
E*
One-way logical channel outgoing E -
A*
One-way logical channel incoming A -
A*
Closed user group E -
E*
Closed user group with outgoing
access A -
A*
Closed user group with incoming
access A -
A*
Incoming calls barred within a
closed user group A -
A*
Outgoing calls barred within a
closed user group A -
A*
Bilateral closed user group A -
A*
Bilateral closed user group with
outgoing access A -
A*
Reverse charging acceptance A -
A*
Fast select acceptance A** -
-
Datagram queue length selection* - -
A*
Datagram service signal logical
channel* - -
A*
Datagram non-delivery indication* - -
E*
Datagram delivery confirmation* - -
E*
D-bit modification A A
-
Optional user facilities requested
by the DTE on a per call basis
Closed user group selection E -
E*
Bilateral closed user group selection A -
A*
Reverse charging A -
A*
RPOA selection A -
A*
Flow control parameter negotiation E -
-
Fast select A** -
-
Throughput class negotiation E -
-
Abbreviated address calling FS -
A*
Datagram non-delivery indication - -
E*
Datagram delivery confirmation - -
E*
NOTE: Detailed explanations of these facilities are provided
in CCITT
Recommendation X.25.
LEGEND:
E = An essential user facility to be offered by all
networks.
A = An additional user facility which may be offered
by certain
networks.
FS = Further study is required. This standard will be
modified when this
study is complete.
- = Not applicable.
DG = Applicable when the datagram service is being used.*
VC = Applicable when the virtual call service is being used.
PVC = Applicable when the permanent virtual circuit service
is being used.
* - The datagram service and its related facilities may
be used
only when:
- there is to be a one-way transfer of information which
does not
require recovery at the network layer; and,
- a response to this transfer of information is not
required at the
network layer.
NOTES: 1. At the present time, the transfer of datagram
packets across
international borders through public packet-switching networks
is not
permitted 2. DCE's are not required to provide datagram service.
DTE's are
not required to generate or accept datagrams and datagram-related
packets.
** - Fast select shall be provided by all DCE's. All DTE's
associated with
the higher level functionality which allows response to a fast
select packet
must be capable of accepting incoming fast select packets, but
need not
generate fast select packets.
*** The packet retransmission facilities shall not be used.
(z) The list of the applicable call progress signals,
extracted from
CCITT Recommendation X.96, is given below. These signal definitions
apply to
the cause codes specified in CCITT Recommendation X.25.
The related
circumstances giving rise to each call progress signal is also
defined in
table 1 below. The significance of categories indicates broadly
the type of
action expected of the DTE receiving the signal:
Category Significance
A Requested action confirmed by network.
B Call cleared because the procedure is
complete.
C1 and C2 Call cleared. The calling DTE should call again
soon: the
next attempt may be successful. However, after
a number of
unsuccessful call attempts with the same
response, the
cause could be assumed to be in Category D1 or
D2. The
interval between successive attempts and the
number of
maximum attempts will depend on a number of
circumstances
including:
- nature of the call progress signal
- users' traffic pattern
- tariffs
- possible regulations by the network provider.
OR
Reset. The DTE may continue to transmit data
recognizing that data loss may have occurred.
D1 and D2 Call cleared. The calling DTE should take other
action to
clarify when the call attempt might be
successful.
OR
Reset (for permanent virtual circuit only).
The DTE should cease data transmission and take
other action
as appropriate.
C1 and D1 Due to subscriber condition.
C2 and D2 Due to network condition.
The sequence of call progress signals in table 1 implies, for
Categories C
and D, the order of call set-up processing by the network. ln
general, the
DTE can assume, on receiving a call progress signal, that no
condition higher
up in the table is present. Network congestion is an exception
to this
general rule. The actual coding of call progress signals does not
necessarily
reflect this sequence.
Users and DTE manufacturers are warned to make due allowance
to possible
later extensions to this table by providing appropriate fallback
routines for
unexpected signals.
-----------------------------------------------------------------
--------------
-----------------------------------------------------------------
--------------
Call Progress Definition
Category
Signal
-----------------------------------------------------------------
--------------
Delivery The datagram has been
A
confirmation accepted by the destination DTE.
Local procedure A procedure error caused by the DTE
C1
error is detected by the DCE at the local
DTE/DCE interface.
Network A condition exists in the network
C2
congestion such as:
1) temporary network congestion
2) temporary fault condition within
the network, including procedure error
within a network or an international
link.
Invalid A facility requested by the calling
D1 or D2
facility DTE is detected as invalid by the DCE
request at the local DTE/DCE interface.
Possible reasons include:
- request for a facility which has not
been subscribed to by the DTE;
- request for a facility which is not
available in the local network:
- request for a facility which has not
been recognized as valid by the local
DCE.
RPOA out The RPOA nominated by the calling DTE is
D2
of order unable to forward the call.
Not The called DTE address is
D1
obtainable out of the numbering plan or not
assigned to any DTE.
Access barred The calling DTE is not permitted
D1
the connection to the called DTE.
Possible reasons include:
- unauthorized access between the
calling
DTE and thc called DTE.
- incompatible closed user group.
Reverse charging The called DTE has not subscribed
D1
acceptance not to the reverse charging acceptance
subscribed facility.
Fast select The called DTE has not subscribed
D1
acceptance not to the fast select acceptance
subscribed facility.
Incompatible The remote DTE/DCE interface or the
D1
destination or the transit network does not support
a function or facility requested (eg.the
datagram service).
Out of Order The remote number is out of order. D1 or
D2
Possible reasons include:
- DTE is Uncontrolled Not Ready:
- DCE Power off:
- Network fault in the local loop:
- X.25 Level 1 not functioning:
- X.25 Level 2 not in operation.
Number busy The called DTE is detected by the DCE C1
as engaged on other call(s), and
therefore as not being able to accept
the incoming call. (In the case of the
datagram service..the queue at the
destination DCE is full.)
Remote A procedure error caused by the D1
procedure remote DTE is detected by the DCE
error at the remote DTE/DCE interface.
Network Network is ready to resume normal C1
operational operation after a temporary failure
or congestion.
Remote DTE Remote DTE/DCE interface is ready C1 or D1
operational to resume normal operation after a
temporary failure or out of order
condition (e.g., restart at the remote
DTE/DCE interface. Loss of data may
have occurred.
DTE originated The remote DTE has intiated B or D1
a clear, reset, or restart procedure.
Waivers: Waiver of this standard is required when an interface
based on CCITT
Recommendation X.25 (1980) is to be employed and has either
one of the
following conditions: 1) The interface has options that are not
permitted by
this standard; 2) The interface does not implement all options
mandated by
this standard.
Heads of agencies desiring a waiver from the requirements
stated in this
standard, so as to acquire applicable equipment or service not
conforming to
this standard, shall submit a request for waiver to the
Administrator,
General Services Administration, for review and approval.
Approval will be
granted if, in the judgment of the Administrator after consultation
with the
Assistant Secretary of Commerce for Productivity, Technology and
Innovation,
based on all available information including that provided in
the waiver
requests, a major adverse economic or operational impact would
occur through
conformance with this standard.
A request for waiver shall include a justification for the waiver,
including
a description and discussion of the adverse economic or
operational impact
that would result from conforming to this standard as
compared to the
alternative for which the waiver is requested. ICST and NCS
will provide
technical assistance, as required, to GSA.
Where to Obtain Copies: Copies of this publication are for
sale by the
National Technical Information Service, U.S. Department of
Commerce,
Springfield, VA 22161. When ordering, refer to Federal
Information
Processing Standards Publication 100 (FIPS-PUB- l00)/Federal
Standard 1041
(FED-STD 1041), and title. When microfiche is desired, this
should be
specified. Payment may be made by check, money order, purchase
order, credit
card, or deposit account.
The CCITT X.25 specifications upon which this publication is based
may also
be obtained from NTIS. Specify PB82-187766; the cost is $50;
telephone (703)
487-4650.
re>

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