1、ITU-T RECMN*Q.920 93 m 4b259L 0584550 492 = INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU DIGITAL SUBSCRIBER SIGNALLING SYSTEM No. 1 (DSSI) Q.920 (03/93) DIGITAL SUBSCRIBER SIGNALLING SYSTEM NO. 1 USER-NETWORK INTERFACE DATA LINK LAYER - GENERAL ASPECTS
2、(DSSI) - ISDN ITU-T Recommendation Q.920 (Previously “CCITT Recommendation”) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECHN*Q-920 93 48b2591 058Y55L 329 m FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is
3、 a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Con
4、ference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recommendation 4.920 was revised by the ITU-T Study Group XI (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1
5、993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITU), the CCITT ceased to exist as of 28 February 1993. In its place, the ITU Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the C
6、CIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recommendation, no change has been made in the text to references containing the acronyms “CCITT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Fut
7、ure editions of this Recommendation will contain the proper terminology related to the new ITU structure. 2 telecommunication administration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a O ITU 1994 All rights res
8、erved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Han
9、dling ServicesITU-T RECMN*Q*920 93 m 4862593 0584552 2b5 m CONTENTS Page 1 General . 1 2 3 4 Concepts and terminology Overview description of LAPD functions and procedures 3.1 General 3.2 Unacknowledged operation 3.3 Acknowledged operation 3.4 Establishment of information transfer modes . Service ch
10、aracteristics 4.1 General 4.2 Services provided to layer 3 . 4.3 4.4 Administrative services . 4.5 Model of the data link service . 4.6 Services required from the physical layer . Services provided to layer management . 2 5 5 6 6 8 11 11 11 12 12 13 14 5 Data link layer . Management structure . 16 5
11、.1 Data link procedure . 17 5.2 Multiplex procedure 17 5.3 Structure of the data link procedure 17 5.4 Overall data link layer functional diagram . 18 References 20 l Recommendation Q.920 (03/93) i I COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handl
12、ing ServicesRecommendation 4.920 DIGITAL SUBSCRIBER SIGNALLING SYSTEM No. 1 (DSS1) ISDN USER-NETWORK INTERFACE DATA LINK LAYER - GENERAL ASPECTS (Mlaga-Torremolinos, 1984; modijed at Helsinki, 1993) 1 General NOTE - This Recommendation was originally written to describe the general aspects of the pr
13、otocol and procedures specified in Recommendation 4.921 i. With the introduction of Recommendation Q.922 2, it is necessary for 4.920 to evolve to a general introduction to the data link layer protocols and procedures specified in the Q.920-Series of Recommendations. It is for further study to place
14、 into proper perspective those aspects of Q.920 which apply uniquely to Q.921 in the current text. This Recommendation describes in general terms the link access procedure on the D-channel, LAPD. The application of this protocol to other channel types is for further study. Details are provided in Re
15、commendation 4.921 i. The purpose of LAPD is to convey information between layer 3 entities across the ISDN user-network interface using the D-channel. The definition of LAPD takes into consideration the principles and terminology of: - Recommendations X.200 3 and X.210 4 - Reference model and layer
16、 service conventions for Open Systems Interconnection (OSI); Recommendation X.25 5 - LAPB user-network interface for packet mode terminals; and IS0 3309 6 and IS0 4335 7 - High-level Data Link Control (HDLC) standards for frame structure and elements of procedures. - - In order that LAPD can be appl
17、icable for communication in a direct user-to-user situation (e.g. PBX-to-PBX communication over a leased circuit), or in a mixed publidprivate network situation, the user side and network side must be designated to the adjacent data link entities. In a direct user-to-user situation, a static assignm
18、ent (outside of the LAPD protocol) may be used to establish the user-network relationship. In a mixed public/private network situation, the public network assumes the network side, and the private network assumes the user side. LAPD is a protocol that operates at the data link layer of the OS1 archi
19、tecture. The relationship between the data link layer and other protocol layers is defined in Recommendation 1.320 SI. NOTES 1 The physical layer is currently defined in Recommendations 1.430 9 and 1.431 lo and layer 3 is defined in Recommendations Q.930 li, Q.931 i21 and X.25 5. References should b
20、e made to these Recommendations for the complete definition of the protocols and procedures across the ISDN user-network interface. 2 The term “data link layer” is used in the main text of this Recommendation. However, mainly in figures and tables, the terms “layer 2” and “L2” are used as abbreviati
21、ons. Furthermore, in accordance with Recommendations 4.930 li and 4.931 121, the term “layer 3” is used to indicate the layer above the data link layer. LAPD is independent of transmission bit rate. It requires a duplex, bit transparent D-channel. The characteristics of the D-channel are defined in
22、Recommendation 1.412 13. Clause 2 describes basic concepts used in this Recommendation and Recommendation 4.921. Clause 3 gives an overview description of LAPD functions and procedures. Clause 4 summarizes the services that the data link layer provides to layer 3 and the services that the data link
23、layer requires from the physical layer. Clause 5 provides an overview of the data link layer structure. Recommendation Q.920 (03/93) I COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*Q*920 93 m 4862593 0584554 038 m 2 Concep
24、ts and terminology The basic structuring technique in the OS1 reference model is layering. According to this technique, communication among application processes is viewed as being logically partitioned into an ordered set of layers represented in a vertical sequence as shown in Figure 1. IF I LM ma
25、nagement (see Figm 10) FIGURE 114.920 Layeriiig A data link layer service access point (SAP) is the point at which the data link layer provides services to layer 3. Associated with each data link layer SAP is one or more data link connection endpoint(s). See Figure 2. A data link connection endpoint
26、 is identified by a data link connection endpoint identifier as seen from layer 3 and by a data link connection identifier (DEI) as seen from the data link layer. Data link layer seMce Data link COMBction endpoint Data link Data link iayer T1155480-93/d2 FIGURE UQ.920 Entities, service access points
27、 and endpoints 2 Recommendation 4.920 (03/93) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesITU-T RECMN*Q=920 93 m 4862593 0584555 T74 m Entities exist in each layer. Entities in the same layer, but in different systems which must exc
28、hange information to achieve a common objective are called “peer entities”. Entities in adjacent layers interact through their common boundary. The services provided by the data link layer are the combination of the services and functions provided by both the data link layer and the physical layer.
29、Cooperation between data link layer entities is governed by a peer-to-peer protocol specific to the layer. In order for information to be exchanged between two or more layer 3 entities, an association must be established between the layer 3 entities in the data link layer using a data link layer pro
30、tocol. This association is called a data link connection. Data link connections are provided by the data link layer between two or more SAPS (see Figure 3). Data link layer service I I Data link connection endpoint I FIGURE 3/4.920 Peer-tpeer relationship Data link layer protocol-data-units (PDUs) a
31、re conveyed between data link layer entities by means of a physical connection, making use of physical service-data-units (SDUs). Layer 3 requests services from the data link layer via service primitives. The same applies for the interaction between the data link layer and the physical layer. The pr
32、imitives represent, in an abstract way, the logical exchange of information and control between the data link layer and adjacent layers. They do not specify or constrain implementation. The primitives that are exchanged between the data link layer and adjacent layers are of the following four types
33、(see also Figure 4): a) request; b) indication; c) response; and d) confirm. The request primitive type is used when a higher layer is requesting a service from the next lower layer. The indication primitive type is used by a layer providing a service to notify the next higher layer of any specific
34、activity which is service related. The indication primitive may be the result of an activity of the lower layer related to the primitive type request at the peer entity. Recommendation Q.920 (03/93) 3 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Hand
35、ling ServicesLayer 3 Confirm Data link Response T1155500-93d4 Data link layer protocol NOTE - The same principle applies for data link layer-physical interactions. FIGURE 4Q.920 Primitive action sequence The response primitive type is used by a layer to acknowledge receipt, from a lower layer, of th
36、e primitive type indication. The confirm primitive type is used by the layer providing the requested service to confirm that the activity has been completed. Layer-to-layer interactions are specified in Recommendation 4.921 i. Information is transferred, in various types of data units, between peer
37、entities and between entities in adjacent layers that are attached to a specific SAP, see 5.6Bc.200 3. The data units are of two types: - protocol-data-units (PDUs) of a peer-to-peer protocol; and - interface-data-units (IDUs). These may contain information relevant to the peer entities at the ends
38、of the connection or information of local significance such as layer-to-layer information concerning status and specialized service requests. The PDUs of the layer 3 peer-to-peer protocol are carried by the data link connection in the form of service data units (SDUs). The contents of IDUs containin
39、g layer-to-layer information concerning status and specialized service requests, which are of local significance, are never conveyed over a data link connection or a physical connection. This Recommendation specifies (see also Figure 5): a) the peer-to-peer protocol for the transfer of information a
40、nd control between any pair of data link layer service access points; and b) the interactions between the data link layer and layer 3, and between the data link layer and the physical layer. 4 Recommendation Q.920 (03193) COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicense
41、d by Information Handling ServicesITU-T RECMN*Q.920 93 m 4862573 0584557 847 m Data link layer entity Data link layer peer-tepeer-protocol Data link layer entity I Ptn5ical connection I Physical law entity FIGURE 5lQ.920 Data link layer reference dei Physical layer entity 3 Overview description of L
42、APD functions and procedures 3.1 General The purpose of LAPD is to convey information between layer 3 entities across the ISDN user-network interface using the D-channel. Specifically LAPD will support: - multiple terminal installations at the user-network interface; - multiple layer 3 entities. All
43、 data link layer PDUs are transmitted in frames which are delimited by flags. (A flag is a unique bit pattern.) The frame structure is defined in Recommendation 4.921 i. Recommendation Q.920 (03193) 5 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Hand
44、ling ServicesITU-T RECMNUQ.920 93 W 4862591 0584558 783 W LAPD includes functions for: the provision of one or more data link connections on a D-channel. Discrimination between the data link connections is by means of a data link connection identifier (DLCI) contained in each frame; frame delimiting
45、, alignment and transparency, allowing recognition of a sequence of bits transmitted over a D-channel as a frame; sequence control, to maintain the sequential order of frames across a data link connection; detection of transmission, format and operational errors on a data link connection; recovery f
46、rom detected transmission, format, and operational errors; notification to the management entity of unrecoverable errors; and flow control. Data link layer functions provide the means for information transfer between multiple combinations of data link connection endpoints. The information transfer m
47、ay be via point-to-point data link connections or via broadcast data link connections. In the case of point-to-point information transfer, a frame is directed to a single endpoint, while in the case of broadcast information transfer, a frame is directed to one or more endpoints. Figure 6 shows three
48、 examples of point-to-point information transfer. Figure 7 shows an example of broadcast infor- mation transfer. Two types of operation of the data link layer are defined for layer 3 information transfer: unacknowledged and acknowledged. They may coexist on a single D-channel. 3.2 Unacknowledged ope
49、ration With this type of operation, layer 3 information is transmitted in unnumbered information (UI) frames. At the data link layer the U1 frames are not acknowledged. Even if transmission and format errors are detected, no error recovery mechanism is defined. Flow control mechanisms are not defined. Unacknowledged operation is applicable for point-to-point and broadcast information transfer; that is, a U1 frame may be sent to a specific endpoint or broadcast to multiple endpoints associated with a specific service access point identifier (SAPI).
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