1、BRITISH STANDARD BS5397-6: 1990 ISO7478:1987 (incorporating corrigendum) High-level data link control (HDLC) procedures Part 6: Specification for multilink procedures UDC 681.327.8:621.391BS5397-6:1990 This British Standard, having been prepared under the directionof the Information Systems Technolo
2、gy StandardsPolicy Committee, waspublished under the authorityof the Board of BSIandcomes into effecton 31May1990 BSI12-1999 First published November1985 The following BSI references relate to the work on this standard: Committee reference IST/6 Draft for comment89/62721 DC ISBN 0 580 18041 7 Commit
3、tees responsible for this British Standard The preparation of this British Standard was 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
4、 Computer Society British Telecommunications Plc Business Equipment and Information Technology 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 Electro
5、nic Engineering Association HM Treasury (Central Computer and Telecommunications Agency) 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 Telecommu
6、nication Engineering and Manufacturing Association Telecommunications Managers Association Amendments issued since publication Amd. No. Date of issue CommentsBS5397-6:1990 BSI 12-1999 i Contents Page Committees responsible Inside front cover National foreword ii 0 Introduction 1 1 Scope and field of
7、 application 1 2 Definitions and parameters 1 3 Multilink frame format 3 4 Transmitter operation 4 5 Receiver operation 6 6 Initializing the MLP 8 7 Resetting the MLP 8 Annex Examples of multilink resetting procedures 11 Figure 1 Multilink functional organization 2 Figure 2 Multilink frame formats 5
8、 Figure 3 Transmitter flow control 6 Figure 4 Reception of multilink frames 7 Figure 5 Detecting lost multilink frames 9 Publications referred to Inside back coverBS5397-6:1990 ii BSI 12-1999 National foreword This Part of BS5397 has been prepared under the direction of the Information Systems Techn
9、ology Standards Policy Committee. It is identical with ISO7478:1987, “Information processing systems Data communication Multilink procedures”, published by the International Organization for Standardization (ISO) and incorporates Technical Corrigendum1:1989 (ISO7478:1987/Cor.1:1989). It is a revisio
10、n of BS5397-6:1985 to align the text with ISO7478 and Technical Corrigendum1 which were published subsequently. The UK has actively participated, through BSI Technical Committee IST/6, in the development of the International Standard. A number of International Standards dealing with various aspects
11、of High-level Data Link Control (HDLC) procedures have been published by ISO and these have been issued as separate parts of BS5397. With the exception of Part9 and Part1, all parts are currently being revised to ensure alignment of the BritishStandard with the latest ISO text. Other parts are as fo
12、llows: Part1: Specification for frame structure; Part2: Specification for consolidation of elements of procedures; Part5: Specification for consolidation of classes of procedures; Part7: Specification forX.25 LAPB-compatible DTE data link procedures; Part8: Specification for frame level address assi
13、gnment; Part9: Specification for general purpose XID frame information field content and format. 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 Servi
14、ce (BITS) initiative. For details contact: Marketing Department, BSI, Linford Wood, Milton Keynes, MK146LE. 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 S
15、tandard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to12, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorpor
16、ated. This will be indicated in the amendment table on the inside front cover.BS5397-6:1990 BSI 12-1999 1 0 Introduction Multilink procedures reference the layers of the ISO Open Systems Interconnection (OSI) reference model; specifically the physical, data link, and network layers. The multilink pr
17、ocedures (MLP) reside in the data link layer. Multilink procedures provide the means for accepting data units from the network layer, scheduling data units for transmission and retransmission over a group of parallel data links, and reordering the received data units prior to delivering them to the
18、network layer. Multilink procedures provide the following general features: a) achieve economy and reliability of service by providing multiple connections between data stations; b) permit addition and deletion of connections without interrupting the service provided by the multiple connections; c)
19、optimize bandwidth utilization of a group of connections through load sharing; d) achieve graceful degradation of service when a connection(s) fails; e) provide each multiple connection group with a single logical data link appearance to the network layer; and f) provide, when required, resequencing
20、 of the received data units prior to delivering them to the network layer. 1 Scope and field of application This International Standard specifies multilink procedures where a multiplicity of parallel data links at the data link layer are used to provide a variable bandwidth data link between network
21、 layer entities. The multilink procedures (MLP) exist as a new upper sublayer of the data link layer, operating between the network layer and a multiplicity of single data link protocol functions (SLPs) in the data link layer (see Figure 1). This International Standard does not specify the way in wh
22、ich the SLPs indicate to the MLP that the transmission of a multilink frame has successfully been completed. These multilink procedures do not preclude the use of different single link procedures, each with differing delay characteristics and/or line speeds to form one multilink group. When the proc
23、edures defined by this InternationalStandard are to be used on one or more parallel data links, both ends of the data link must know that these procedures are to be used before the first multilink frame is sent. This could be achieved by a prior agreement that all communications on this data link wi
24、ll use these procedures, or by one of the SLPs negotiating the use of these procedures, or by some other means. The method by which both ends achieve a common understanding as to the use or non-use of these multilink procedures is not defined in this International Standard. 2 Definitions and paramet
25、ers 2.1 MLP reset confirmation bit (C) the MLP reset confirmation bit is used in reply to the R bit set to1 to confirm that all of the MLP state variables have been reset. The C bit set to0 is used in normal communication; i.e.,no reset request has been activated. The C bit set to1 is used to reply
26、to the multilink frame with the R bit set to1, and indicates that the MLP state variable reset process has been completed. In this C=1 case, the multilink frame is used without a data unit field 2.2 multilink procedures (MLP) the protocols employed to transfer data over one or more SLPs which form a
27、 group 2.3 multilink send sequence number MN(S) value of the multilink sequence number assigned to a multilink frame NOTEThe number assigned lies in the range of0 to4095 inclusive and is used to resequence, when resequencing is required, and to detect missing or duplicate multilink frames at the rec
28、eiving multilink procedure data station (MLP) before data units are delivered to the network layer. 2.4 lost multilink frame timer(MT1) during low traffic periods, the expiration of this timer provides the means to determine that the multilink frame with MN(S) equal to MV(R) is lost 2.5 group busy t
29、imer(MT2) optional. At the receiving MLP, the expiration of this timer provides the means to determine that a “blocked” condition exists because buffers were exhausted before resequencing could be accomplishedBS5397-6:1990 2 BSI 12-1999 2.6 MLP reset confirmation timer(MT3) timer MT3 is used to prov
30、ide a means of identifying that the multilink frame with the C bit set to1 that is expected following the transmission of the multilink frame with the R bit set to1 has not been received 2.7 multilink receive state variable MV(R) denotes the MN(S) of the next in-sequence multilink frame to be transf
31、erred to the network layer NOTEThe MV(R) identifies the lower edge of the receive window. 2.8 multilink send state variable MV(S) denotes the next in-sequence MN(S) to be assigned to a multilink frame to be sent over this multilink group 2.9 multilink frame acknowledgement state variable MV(T) denot
32、es the MN(S) of the oldest multilink frame awaiting an indication of acknowledgement from the local SLP NOTEThe MV(T) identifies the lower edge of the transmit window. 2.10 multilink window size (MW) for a given direction of transmission, both the transmitting MLP and the receiving MLP shall use the
33、 same value of MW NOTEThe transmit window contains the sequence numbers MV(T) to MV(T)+MW1 inclusive. The receive window contains the sequence numbers MV(R) to MV(R)+MW1 inclusive. 1) Figure 1 Multilink functional organization 1) MW is a system parameter which can never exceed4095MX. Factors which w
34、ill affect the value of parameter MW include, but are not limited to, link transmission and propagation delays, the number of links, the range of multilink frame lengths, and single link parameters (N retransmission attempts, response times, and outstanding number of unacknowledged I frames).BS5397-
35、6:1990 BSI 12-1999 3 2.11 receive MLP window guard (MX) the range of sequence numbers of fixed size, of higher value than those in the receive window, that, if received as MN(S), indicates that any multilink frames in the range MV(R) to MN(S)-received MW that have not been received have been lost 2.
36、12 range of abnormal multilink frames (MZ) the range of sequence numbers in which MN(S) should not be received during normal operation NOTEMultilink frames received in range MZ should be discarded. 2.13 number of SLP retransmission attempts (N) the number of times that a single link procedure data s
37、tation (SLP) attempts retransmission of a multilink frame before notifying the MLP of the situation NOTESubsequent SLP action should be dependent upon SLP design. Subsequent MLP action should involve assigning the multilink frame to the same or one or more other SLPs for transmission. 2.14 MLP reset
38、 request bit (R) the MLP reset request bit is used to request a reset of the MLP state variables. The R bit set to0 is used in normal communication; i.e.,no request for a multilink reset. The R bit set to1 is used to request the reset of the receiving MLP state variables in the R=1 case, the multili
39、nk frame data unit field does not contain higher layer information, but may contain an optional Cause Field that incorporates the reason for the reset NOTEFor the DTE-DCEX.25 interface application 2) , a Cause Field of8 bits is specified. The encoding of that8-bit field is a subject for further stud
40、y. 2.15 sequence check option bit (S) the S bit is only significant when V=1 (indicating that sequencing of received multilink frames shall not be required). S=1 shall mean no MN(S) number has been assigned. S=0 shall mean an MN(S) number has been assigned so, although sequencing shall not be requir
41、ed, a duplicate multilink frame check may be made, as well as a missing multilink frame identified 2.16 single link procedures (SLP) the data link protocols employed to establish, maintain, transfer data, and terminate a logical data link over a single data circuit 2.17 void sequencing bit (V) the V
42、 bit indicates if a received multilink frame shall be subjected to sequencing constraints. V=1 means sequencing shall not be required. V=0 means sequencing shall be required 3 Multilink frame format To permit resequencing of data units (forexample,packets), a multilink control (MLC) field shall be p
43、rovided. This field shall be transmitted as the first two octets in the information field of an SLP transmission unit. Figure 2 shows the MLC field and its relationship to the data unit and the SLP header/trailer. NOTESome format modification may be required if code dependent procedures are used by
44、the SLP. The two octet MLC field shall immediately follow the SLP header and shall extend the data link layer envelope used for transporting data units between network layer entities. The single link and multilink control fields shall be generated and used at the data link layer only and shall not b
45、e forwarded to the network layer. The multilink sequence number MN(S) shall consist of twelve bits, providing a modulo count of4096. MN(S) shall be split into two fields, MNH(S) and MNL(S), as shown in Figure 2. Four control bits shall be provided in the MLC field for use in providing additional mul
46、tilink control features. The void sequencing bit V shall be used to indicate when resequencing of received data units into the sequence received from the network layer shall not be required prior to release to the network layer. When V=1, the data unit shall not have to be resequenced and the receiv
47、ing MLP shall immediately deliver the data unit to the network layer, regardless of its order in the received sequence. When V=0, the data units shall be kept in sequence prior to release to the network layer. V=0 shall indicate an MN(S) number is present. 2) See ISO7776, Information processing syst
48、ems Data communication High-level data link control procedures Description of theX.25LAPB-compatible DTE data link procedures.BS5397-6:1990 4 BSI 12-1999 The sequence check option bit S shall have significance only when V=1 (indicating that resequencing of multilink frames is not required). The S bi
49、t shall have no significance when V=0. When V=1 and S=1, the transmitting MLP shall not have assigned an MN(S) value to the multilink frame. Consequently, the data unit contained in that multilink frame shall be forwarded to the network layer without checking for sequence integrity or duplicates. When V=1 and S=0, the transmitting MLP shall have assigned an MN(S) value to the multilink frame to facilitate checking for duplicate or missing multilink frames, even though sequence integrity is not observed. Duplicate multilink frames sh
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