1、BRITISH STANDARD BS 7267:1990 ISO/IEC TR 10029:1989 Open Systems Interconnection: Guide to operation of an X.25 interworking unitBS7267:1990 This British Standard, having been prepared under the directionof the Information Systems Technology StandardsPolicy Committee, waspublished under the authorit
2、yof the Board of BSI andcomes into effecton 31July1990 BSI 03-2000 The following BSI references relate to the work on this standard: Committee reference IST/6 Draft for comment 89/62161 DC ISBN 0 580 18290 8 Committees responsible for this British Standard The preparation of this British Standard wa
3、s entrusted by the Information Systems Technology Standards Policy Committee (IST/-) to Technical Committee IST/6, upon which the following bodies were represented: Association for Payment Clearing Services British Computer Society British Telecommunications plc Business Equipment and Information Te
4、chnology Association Department of Trade and Industry (Information Technology Standards Unit) Department of Trade and Industry (National Physical Laboratory) Electricity Supply Industry in England and Wales Electronic Engineering Association HM Treasury (Central Computer and Telecommunications Agenc
5、y) Information Technology Users Standards Association Institute of Data Processing Management Inter-Universities Computing Committee Joint Network Team National Computing Centre Ltd. National Health Service OFTEL (Office of Telecommunications) Post Office Telecommunication Engineering and Manufactur
6、ing Association Telecommunications Managers Association Amendments issued since publication Amd. No. Date CommentsBS7267:1990 BSI 03-2000 i Contents Page Committees responsible Inside front cover National foreword ii 1 Scope 1 2 Normative references 1 3 Abbreviations 1 4 General considerations 1 5 P
7、rocedures for restart 2 6 Procedures for virtual call setup and clearing 2 7 Procedures for data and interrupt transfer 3 8 Procedures for flow control 3 9 Procedures for reset 3 10 Optional user facilities 4 Figure 1 Operation of an X.25 to X.25 IWU 2 Table 1 Categorization of optional user facilit
8、ies 5 Publications referred to Inside back coverBS7267:1990 ii BSI 03-2000 National foreword This British Standard has been prepared under the direction of the Information Systems Technology Standards Policy Committee. It is identical with Technical Report10029:1989 “Information technology Telecommu
9、nications and information exchange between systems Operation of an X.25 interworking unit”, published jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). The UK has actively participated, through BSI Technical Committee IST/6,
10、in the development of the Technical Report. Up-to-date information on existing and projected British Standards publications for Open Systems Interconnection is included in the “Basic List” published as part of the BSI Information Technology Service (BITS) initiative. For details contact: Marketing D
11、epartment, BSI, Linford Wood, Milton Keynes, MK146LE. There is no British Standard corresponding to ISO8881:1989 1) . The Technical Committee has reviewed the provisions of ISO8881:1989, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this
12、 standard. ISO 8880-2 is currently at the stage of draft International Standard and is expected to be issued as a British Standard when the International Standard has been published. The texts of draft International Standards are available to members of the BITS (BSI Information Technology Service)
13、scheme. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document co
14、mprises a front cover, an inside front cover, pages i and ii, pages1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Cross-references In
15、ternational Standard Corresponding British Standard ISO 8208:1987 BS 7219:1989 Specification for X.25 packet level protocol for data terminal equipment for information processing systems (Identical) 1) It is intended that an identical British Standard will be published in due course.BS7267:1990 BSI
16、03-2000 1 1 Scope This Technical Report describes the function of the IWU in terms of the mapping between Protocol Data Units (PDUs) that it receives from one interface, and PDUs that it then transmits, which could be on either interface of the IWU. Additionally, this Technical Report describes the
17、procedures that an IWU can initiate on each interface independently. Each interface corresponds to one instance of a DTE/DXE connection, and both interfaces of the Interworking Unit operate as a DTE as specified by ISO8208. NOTEThe term “DXE” is used as defined in ISO8208 meaning“.those contexts whe
18、re it would not matter whether a DTE or a DCE was being referred to.” 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of this Technical Report. At the time of publication, the editions indicated were valid. All standards
19、are subject to revision, and parties to agreements based on this Technical Report are encouraged to investigate the possibility of applying the most recent editions of the standards listed below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 8208:1987, Inf
20、ormation processing systems Data communications X. 25 Packet Level Protocol for Data Terminal Equipment. ISO 8880-2:, Information processing systems Protocol combinations to provide and support the OSI network service Part 2: Provision and support of the connection-mode network service 2) . ISO 8881
21、:, Information processing systems Data communications Use of the X.25 Packet Level Protocol in Local Area Networks 2) . 3 Abbreviations 4 General considerations 4.1 Logical channels The logical channel identifiers used on one interface of the IWU will not, in general, correspond to those used on the
22、 other interface of the IWU. The IWU performs the mapping function between logical channel identifiers on the two interfaces. This mapping is one-to-one and is established during Virtual Call setup. 4.2 Interface types and references The operation of the IWU is described in terms of its two interfac
23、es. For ease of reference only, the two interfaces are referred to as “interface 1” and “interface 2”. Refer toFigure 1. The procedures described in this Technical Report are applicable to either interface. In this Technical Report external events, are described in terms of one direction only; in so
24、me cases a corresponding event occurs on interface2. For events initiated by the IWU the procedures at both interfaces are described. 4.3 Mapping between dissimilar protocol configurations Mapping between PDUs received on interface1 might not be on a one-to-one basis to those transmitted on interfac
25、e2 due to the differences which are possible within ISO8208. 2) To be published. DCE Data Circuit-terminating Equipment DTE Data Terminal Equipment IWU Interworking unit LAN Local Area Network OSI Open Systems Interconnection PDU Protocol Data Unit PLP Packet Level Protocol WAN Wide Area NetworkBS72
26、67:1990 2 BSI 03-2000 5 Procedures for restart 5.1 Interface restart not initiated by the IWU When receiving a RESTART INDICATION packet on interface1, the IWU confirms this restart by sending a RESTART CONFIRMATION packet across interface1 according to the procedures in clause4 of ISO8208. For each
27、 Virtual Call, if any, that had existed on interface1, the IWU initiates clearing procedures on interface2. The Clearing Cause Field is set to either “00000000” or “10000000” as indicated in Table5 of ISO8208. 5.2 Interface restart initiated by the IWU To restart interface 1, the IWU transmits a RES
28、TART REQUEST packet as specified in4.1 of ISO8208. The restarting cause code to be used is given in Table7 of ISO8208. For each Virtual Call, if any, that had existed on interface1, the IWU initiates clearing procedures on interface2. 6 Procedures for virtual call setup and clearing 6.1 Virtual call
29、 setup 6.1.1 Virtual call initiation When the IWU receives an INCOMING CALL packet on interface 1 it a) determines the remote interface (interface 2); b) matches window sizes and packet sizes for interface1 (implied by default or explicitly requested in the INCOMING CALL packet) to those available o
30、n interface 2 (available by default or through negotiation if allowed on the interface); NOTEIf the IWU does not have segmentation and reassembly capabilities, the IWU should ensure that the maximum DATA packet size on each interface is identical. If the packet sizes on interface1 and interface2 are
31、 dissimilar, the IWU should clear the call. If the IWU does have segmentation and reassembly capabilities, then the relationship of packet sizes on the two interfaces is a result of IWU action during virtual call setup. c) matches optional user facilities requested from interface 1 to those availabl
32、e on interface 2 (see also clause14 of ISO8208); d) selects a free logical channel on interface 2; and e) transmits a CALL REQUEST packet on interface 2. If any anomalies result from the above procedures (for example, no logical channel allowing outgoing access available on interface 2, or requested
33、 facilities not being available on interface 2) the IWU clears the call on interface1. A CALL REQUEST packet is not transmitted on interface2 in this case. NOTEWhere the data link layer is not already active at an interface, the IWU should establish a data link connection using the procedures approp
34、riate to that interface, for example according to ISO7776 or ISO8802-2. Figure 1 Operation of an X.25 to X.25 IWUBS7267:1990 BSI 03-2000 3 6.1.2 Virtual call response 6.1.2.1 Successful call response If the IWU receives a CALL CONNECTED packet as a result of having transmitted a CALL REQUEST packet
35、on interface2, it transmits a CALL ACCEPTED packet on the logical channel corresponding to the original INCOMING CALL packet on interface1. 6.1.2.2 Unsuccessful call response If the IWU receives a CLEAR INDICATION packet as a result of having transmitted a CALL REQUEST packet on interface 2, it a) f
36、ollows the procedures of 5.5.2 of ISO8208 for responding to a CLEAR INDICATION packet on interface 2; and b) initiates a clearing procedure on the logical channel corresponding to the original INCOMING CALL packet on interface1. Details of the clearing cause and diagnostic codes to be transmitted on
37、 interface1 are given in Annex A of ISO8208. 6.2 Virtual call clearing 6.2.1 Clearing not initiated by the IWU When the IWU receives a CLEAR INDICATION packet on an interface, it a) follows the procedures of 5.5.2 of ISO 8208 for responding to a CLEAR INDICATION packet; and b) initiates clearing pro
38、cedures on the corresponding interface if the logical channel indicated in the CLEAR INDICATION packet is associated with a logical channel on another interface. Details of the clearing cause and diagnostic codes to be transmitted on interface1 are given in Annex A of ISO 8208. 6.2.2 Clearing initia
39、ted by the IWU To clear a Virtual Call on interface 1, the IWU transmits a CLEAR REQUEST packet as specified in 5.5.1 of ISO8208. The clearing cause codes defined in Table5 of ISO8208 (with bit8 set to1) are used. The clearing diagnostic codes defined in Figure14 of ISO8208 are used. If the Virtual
40、Call is associated with a logical channel on interface2, the IWU initiates clearing procedures on interface 2. 7 Procedures for data and interrupt transfer 7.1 Procedures for data transfer The aspects of data transfer, as defined in clause6 of ISO8208, that are relevant to IWU operation are a) the m
41、aximum User Data field size of the two interfaces and the need to perform segmentation and reassembly using the M-bit (see6.2 and6.4 of ISO8208); and b) the integrity of complete packet sequences, including the settings of the D-bit and Q-bit (see6.3, 6.5, and6.6 of ISO8208) in each DATA packet. 7.2
42、 Procedures for interrupt When the IWU receives an INTERRUPT packet on interface1, it follows the procedures indicated in6.8.2 of ISO8208. The Interrupt User Data carried in the INTERRUPT packet received on interface1 is transmitted in an INTERRUPT packet on interface2 following the procedures for i
43、nterrupt transmission specified in6.8.1 of ISO8208. When the IWU receives an INTERRUPT CONFIRMATION packet on interface2 having previously transmitted an Interrupt packet on that interface, the IWU transmits an INTERRUPT CONFIRMATION packet on interface1 as specified in6.8.3 of ISO8208. NOTEThe proc
44、edures for error conditions associated with interrupt are defined in6.8.2 of ISO8208. 8 Procedures for flow control The aspects of flow control, as defined in clause7 of ISO8208, that are relevant to IWU operation are a) that the IWU may use flow control procedures independently on each interface; a
45、nd b) that the receipt of a DATA packet with the D-bit set to1 should not result in window rotation on interface1 until rotation of the window on interface2 for all the user data in the originally received DATA packet. 9 Procedures for reset 9.1 Resetting not initiated by the IWU If the IWU receives
46、 a RESET INDICATION packet on interface 1 it a) follows the procedures of8.2 of ISO8208 for responding to a RESET INDICATION packet; and b) initiates a resetting procedure for the corresponding logical channel on interface2 as defined in8.1 of ISO8208.BS7267:1990 4 BSI 03-2000 9.2 Resetting initiate
47、d by the IWU If the IWU needs to reset a Virtual Call other than as a result of9.1 b) it follows the procedures of8.1 of ISO8208 for originating a RESET REQUEST on interface 1 and the corresponding logical channel on interface 2. The resetting cause codes defined in Table6 of ISO8208 (with bit8 set
48、to1) are used. The resetting diagnostic codes defined in Figure14 of ISO8208 are used. The IWU buffers subsequent user data received after the resetting of interface1 is completed but before the resetting on interface2 is completed. 10 Optional user facilities 10.1 Categorization of facilities The I
49、WU is required to administer the X.25 optional user facilities. It is necessary for the IWU to have knowledge of the administrative arrangements of the subnetworks which it interconnects. In this regard the IWU is asymmetrical with respect to the two interfaces (as distinct from the general procedures described earlier). The IWU gains this knowledge by local administrative arrangements, or by use of the On-line Facility Registration facility. The optional user facilities, as specified in clause13 and clause14 of ISO8208, are categorized as follows. NOT
