1、CEPT T/S*4b-20*E 232b4L4 0008348 2 Page 1 (Old reference T/CS 46-20 E) Recommendation T/S 46-20 (T/GSI 04-02/2 E) (Nice 1985, revised by correspondence May 1987) ISDN USER-NETWORK INTERFACE DATA LINK LAYER SPECIFICATION APPLICATION OF CCITT RECOMMENDATIONS Q.920/1.440 AND Q.921/1.441 Recommendation
2、proposed by Working Group T/WG 1 1 “Signalling, Protocols and Switching” (SPS) Text of the Recommendation adopted by the “Telecommunications” Commission: “The European Conference of Postal and Telecommunications Administrations, considering - that CCITT Recommendations Q.920/1.440 and Q.921/1.441 de
3、fine the ISDN user-network interface data link layer, - that there are a number of options and points requiring further specification in Recommendations Q.920/1.440 and 4.92 1 p.441, - that the harmonization of the ISDN user-network interface is an important requirement for European Adminis- tration
4、s, O recommends that European Administrations who wish to provide ISDN services apply the CCITT Recommendations Q.920/1.440 and Q.921/1.441 in accordance with the specification defined below.” INTRODUCTION This Recommendation is a revised version of CEPT Recommendation T/CS 46-20 (Nice 1985). It is
5、based on the CCITT Recommendations Q.920/1.440 and Q.921/1.441 versions proposed by CCITT Study Group XI for the accelerated procedure for the provisional approval of Recommendations. It is intended to reduce the number of options which exist within CCITT Recommendations Q.920/1.440 and Q.921/1.441
6、and to provide the additional specification text necessary for clarification as well as to ensure harmonization of the ISDN user-network interface within Europe. All procedures at the ISDN user-network interface apply to both the S and T reference points. CCITT Recommendations Q.920/1.440 and Q.921/
7、1.441 apply with the modifications specified below. The modifica- tions are presented based on the CCITT Recommendations as appearing in a document for the accelerated procedure for the provisional approval of Recommendations Q.920/1.440 and Q.921/1.441. The following editorial conventions have been
8、 applied: - the layout of this Recommendation is aligned with CCITT Recommendations Q.920/1.440 and Q.921/1.441 for the accelerated procedure, except CEPT modifications which require additional sections or figures and tables; - modifications made for the sake of consistency and clarification are ind
9、icated with asterisks in the left margin; - modifications to select options for CEPT networks are indicated with asterisks in the left margin or encircled by - editorial modifications are indicated with characters “E” in the left margin. O asterisks; Edition of December 15, 1987 - CEPT T/S*4b-20*E D
10、 232b414 0008349 4 D TIS 46-20 E Page 2 4 i Recommendation Q.920 (1.440) ISDN USER-NETWORK INTERFACE DATA LINK LAYER - GENERAL ASPECTS 1 General This Recommendation describes in general terms the Link Access Procedure on the Dchannel, LAPD. The application of this protocol to other channel types is
11、for further study. Details are provided in Recommendation (2.921 (1.441) l. 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 consideration4he principles and terminology of: - Recornmend
12、ations X.200 2 and X.210 3 - the reference model and layer service conventions for Open Systems Interconnection (OSI); Recommendation X.25 4 - LAPB user-network interface for packet mode terminals; and IS0 3309 5 and IS0 4335 6 - High-level Data Link Control (HDLC) standards for frame structure and
13、elements of procedures. - - LAPD is a protocol that operates at the data link layer of the OS1 architecture. The relationship between the data link layer and other protocol layers is defined in Recommendation 1.320 m. Note I - The physical layer is currently defined in Recommendations 1.430 8 and 1.
14、431 9 and layer 3 is defined in Recommendations Q.930 (1.450) lo, 0.931 (1.451) il, and X.25 4. References should be made to these Recommendations for the complete definition of the protocols and procedures across the ISDN user-network interface. Note 2 - The term “data link layer“ is used in the ma
15、in text of this Recommendation. However, mainly in figures and tables, the terms “layer 2“ and “U“ are used as abbreviations. Furthermore, in accordance with Recommendations 0.930 (1.450) lo and Q.931 (1.451) ll, the term “layer 3“ is used to indicate the layer above the data link layer. LAPD is ind
16、ependent of transmission bit rate. It requires a duplex, ba transparent Dchannel. The characteristics of the Dchannel are defined in Recommendation 1.412 12. 9 2 below describes basic concepts used in this Recommendation and Recommendation (2.921. i l 0 3 gives an overview description of LAPD functi
17、ons and procedures. 0 4 summarizes the services that the data link layer provides to layer 3 and the services that the 4 data link layer requires from the physical layer. 0 5 provides an overview of the data link layer structure. O Edition of December 15, 1987 CEPT T/S*46-20*E 2326434 O008350 O T/S
18、46-20 E Page 3 2 Concepts and termlnology 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
19、in Figure 1M.920. System management I Hiahest lauer I H - H M H (N+ 1 ) layer (N) layer (N- 1 ) layer LM I Lowest 1 a uer I LM Layer management (see Figure 1 OlQ.920) FIGURE 1/Q.920 Layer lng Entities exist in each layer. Entities in the same layer, but in different systems which must exchange infor
20、mation 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. A data link
21、 layer Service Access Point (SAP) is the point at which the data link layer provides services to layer 3. Associatied with each data link layer SAP is one or more data link connection endpoint(s). See Figure 2Q.920. A data link connection endpoint is identified by a data link connection endpoint ide
22、ntifier as seen from layer 3 and by a Data Link Connection Identifier (DLCI) as seen from data link layer. Edition of December 15, 1987 CEPT T/S*46-20*E 2326434 0008351 2 = 3 TIS 46-20 E Page 4 Layer 3 entity Layer 3 Data link layer service access point endpoi nt Data link layer entity Data link lay
23、er FIGURE 2M.920 Entitles, servlce access points and endpoints 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 laye
24、r 3 entities in the data link layer using a data link layer protocol. This association is called a data link connection. Data link connections are provided bv the data link laver between two or more SAPS (see Figure 3/Q,920). I I I servi ce access / / I /- point * Datalink connection endpoi nt Data
25、link connection /- FIGURE 3K1.920 Peer-to-peer relatlonshlp Data link layer message units are conveyed between data link layer entities by means of a p.pical connection. Layer 3 requests services from the data link layer via service primitives. The same applies for the interaction between the data l
26、ink layer and the physical layer. The primitives 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. Edition of December 15, 1987 Data link layer peer - to- peer protocol CEPT
27、 T/S*4b-20*E W 2326434 0008352 4 TIS 46-20 E Page 5 The primitives that are exchanged between the data link layer and adjacent layers are of the following four types (see also Figure 4/Q.920): a) REQUEST; b) INDICATION; c) RESPONSE; and d) CONFIRM. Note 7 - The same principle applies for data link l
28、ayer - physical layer interactions. FIGURE 4Q.920 Piimitlve action sequence 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 a
29、ctivity which is service related. The INDICATION primitive may be the resuk of an activity of the lower layer related to the primitive type REQUEST at the peer entity. The RESPONSE primitive type is used by a layer to acknowledge receipt, from a lower layer, of The CONFIRM primitive type is used by
30、the layer providing the requested service to confirm that the primitive type INDICATION. the activity has been completed. Layer-to-layer interactions are specified in Recommendation (2.921. Edition of December 15, 1987 t CEPT T/S*4b-20*E = 232b4L4 0008353 b TIS 46-20 E Page 6 Information is transfer
31、red, in various types of message units, between peer entities and between entities in adjacent layers that are attached to a specific SAP. The message units are of two types: - - message units of a peer-to-peer protocol; and message units that contain layer-to-layer information concerning status and
32、 specialized service requests. The message units of the layer 3 peer-to-peer protocol are carried by the data link connection. The message units containing layer-to-layer information concerning status and specialized service requests are never conveyed over a data link connection or a physical conne
33、ction. This Recommendation specifies (see also Figure 5K2.920): a) The peer-to-peer protocol for the transfer of information and 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
34、 physical layer. Data link layer service CCQSS point Data link layer/ : layer 3 I primitives i I Data link lager 7 IL Physical layer I ! Physical U layer entity P h ysical Physical connection FIGURE 5m.920 Data link layer reference model Edition of December 15, 1987 CEPT T/S*46-20*E 232b414 0008354
35、8 TIS 46-20 E Page 7 3 Overview descrlption of LAPD functions and procedures 3.1 General The purpose of LAPD is to convey information between layei user-network interface using the D-channel. Specifically LAPD will support: multiple terminal installations at the user-network interface; - - multiple
36、layer 3 entities. 3 entities across the ISDN All data link layer messages are transmitted in frames which are Uiimitedby flags. (A flag is a unique bit pattern.) The frame structure is defined in Recommendation Q.921. LAPD includes functions for: a) the provision of one or more data link connections
37、 on a D-channel. Discrimination between the data link connnections is by means of a data link connection identifier (DLCI) contained in each frame; frame delimiting, alignment and transparency, allowing recognition of a sequence of bits transmitted over a D-channel as a frame; sequence control, to m
38、aintain the sequential order of frames across a data link connection; detection of transmission, format and operational errors on a data link connection; recovery from detected transmission, format, and operational errors; notification to the management entitiy of unrecoverable errors; and b) c) d)
39、e) f) g) flow control. Data link layer functions provide the means for information transfer between multiple combinations of data link connection endpoints. The information transfer may be via point-to-point data link connections or via broadcast data link connections. in the case of point-to-point
40、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. shows an example of broadcast information transfer. Figure 6/Q.920 shows three examples of point-to-point information transfer. Figure 71
41、0.920 O Edition of December 15, 1987 r CEPT T/S*Yb-ZO*E 232bYL4 0008355 T TE TIS 46-20 E Page 8 ET/NT2 User si de Network side Physical connection -. Data link connection ET Exchange termination TE Terminal equipment NT Network termination SAP Service access point FIGURE 61Q.920 Po in t-to-po int da
42、ta i ink connect ions Edition of December 15, 1987 CEPT T/S*46-20*E 2326434 O008356 1 T/S 46-20 E Page 9 User side Network side FIGURE 7m.920 Broadcast data link connection Two types of operation of the data link layer are defined for layer 3 information transfer: unacknowledged and acknowledged. Th
43、ey may coexict on a single D-channel. 3.2 Unacknowledged operation With this type of operation layer 3 information is transmitted in Unnumbered Information (UI) frames. At the data link layer the Ui frames are not acknowledged. Even if transmission and format errors are detected, no error recovery m
44、echanism is defined. Flow control mechanisms are not defined. Unacknowledged operation is applicable for point-to-point and broadcast information transfer; that is, a UI frame may be sent to a specific endpoint or broadcast to multiple endpoints associated with a specific Service Access Point Identi
45、fier (SAPI). 3.3 Acknowledged operation With this type of operation, layer 3 information is transmitted in frames that are acknowledged at the data link layer. Error recovery procedures based on retransmission of unacknowledged frames are specified. In the case of errors which cannot be corrected by
46、 the data link layer, a report to the management entity is made. Flow control procedures are also defined. Acknowledged operation is applicable for point-to-point information transfer. One form of acknowledged information transfer is defined, multiple frame operation. CEPT T/S*46-20*E m 2326434 0008
47、357 3 m I -Layer 3 information is sent in numbered Information (I) frames. A number of I frames may be outstanding at the same time. Multiple frame operation is initiated by a multiple frame establishment procedure using a Set Asynchronous Balanced Mode Extended (SABME) command. 3.4 Establishment of
48、 information transfer modes 3.4.1 Data link wnnection identification A data link connection is identified by a Data Link Connection Identifier (DLCI) carried in the address field of each frame. The data link connection identifier is associated with a connection endpoint identifier at .the two ends o
49、f the data link connection (see Figure 8M.920). The connection endpoint identifier is used to identify message units passed between the data link layer and layer 3. It consists of the SAPI and the Connection Endpoint Suffix (CES). The DLCI consists of two elements: the SAPI and the Terminal Endpoint Identifier (TEI). The SAPI is used to identify the service access point on the network side or the user side of the user-network interface. The TE1 is used to identify a specific connection endpoint within a service access point. The TE1 is assigned b
