1、 TS46-50 Page E 1 (old reference: T/CS 46-50 E) Recommendation TIS 46-50 (Copenhagen 1987) SUPPORT OF PACKET MODE TERMINAL EQUIPMENT BY AN ISDN (X.31/1.462) Recommendation proposed by Working Group T/WG 1 1 Signalling, Protocols and Switching (SPS) Text of the Recommendation adopted by the “Telecomm
2、imications” Commission: 1. 2.1 GENERAL SERVICE ASPECTS Two main services are available to packet-mode terminals connected to the ISDN, namely: (a) access to the data transmission services provided by PSPDNs (PSPDN services), (b) use of an ISDN virtual circuit bearer service. In case (a) an ISDN tran
3、sparent circuit connection, either permanent (i.e. non-switched) or demand (Le. switched), is used. The corresponding ISDN bearer service is a 64 kbit/s service as described in Recommen- dation 1.211. The service available to the user is that of the PSPDN described in X.25 (permanent access) and X.3
4、2 (demand access), as well as in other X-Series Recommendations (e.g. X.2, X.121). In case (b) an ISDN virtual circuit bearer service is used, as described in Recommendation 1.21 1. The service available is described in I-Series Recommendations. In case (a) only B-channels can be used to access the
5、packet switched service at the user-network interface, while in case (b) both B- and D-channels can be used. In the latter, a network may provide access through B-channels, or both. Either service (a) or (b) may be exploited by the user according to specific national conditions. The detailed service
6、 characteristics for both cases are described in section 3. Note 1. For CEPT networks the detailed specification of procedures for D-channel access will be provided at later stage. REFERENCE CONFIGURATIONS The configurations given below are the basis on which the support of X.25 DTEs and TEls by the
7、 ISDN should be standardized. These configurations are also the basis on which the support of packet mode TEls by ISDN has been standardized, since an X.25 and its Terminal Adaptor (TA) is always equivalent to a packet-mode TE1 at the S/T interface. Therefore, every reference in this text to the com
8、bination of an X.25 DTE and its TA should always be considered as being applicable to a packet-mode TE1. However, some TEls may have more capabilitiy than that available from a X.25 DTE and its TA. Similarly, this Recommendation covers the support of NT2s operating in the packet-mode. Note. Packet-m
9、ode operation through the ISDN (i.e., through the B- and D-channels) will use the X.25 packet-layer protocol. This Recommendation only applies to packet-mode operations carried out independently on a single ISDN network connection. Configuration when accessing PSPDN service This configuration (Figur
10、e 1 (T/S 46-50) refers to service (a), thus implying a transparent handling of packet calls through an ISDN. Only access via the B-channel is possible. In this context, the only support that an ISDN gives to packet calls is a physical 64 kbit/s circuit-mode permanent or demand transparent network co
11、nnection type between the appropriate PSPDN port and the X.25 user TA or TE1 at the customer premises. In the case of permanent access, the X.25 DTE-I-TA or TE1 is connected to the corresponding ISDN port at the PSPDN. The TA, when present, performs only the necessary physical channel rate adaption
12、between the user at the R reference point and the 64 kbit/s B-channel rate. Edition of February 29, 1988 CEPT T/S*46-50*E 7 2326434 0008953 8 f/S 46-50 E Page 2 II S i I - B-channel is used D-channel is used IP, IP ISDN interworking unit ports I i TA Terminal adaptor NT Network termination Reference
13、 points ! ! I (see Recammendation 1.41 1 _- Packet proEedures apply at this point although this does not imply a physical realisation of Recommendation X.25 Note 1. This figure is only an example of many possible configurations and is included as an aid to the text describing the various interface f
14、unctions. Note 2. See Recommendation X.325 for interworking guidelines. Figure 1 (T/S 46-50), Configuration when accessing PSPDN services. In the case of demand access to PSPDNs, which is illustrated in the upper portion of Figure 1 (T/S 46-50), the X.25 DTE+TA or TE1 is connected to an ISDN port at
15、 the PSPDN (AU). The AU is also able to set up the physical B-channel through the ISDN. In this type of connection, originating calls will be set up over the B-channel towards the PSPDN port using the ISDN signalling procedure prior to starting X.25 layer 2 and layer 3 functions. This can be done by
16、 exploiting either hot-line (e.g. direct call) or complete selection methods. Moreover, the TA, when present, performs user rate adaption to 64 kbit/s. In the complete selection case, two separate numbers are used for dial-in access to the PSPDN: - the ISDN number of the access port of the PSPDN, in
17、dicated in the SETUP message; - the address of the called DTE indicated in the X.25 call request packet. The corresponding service requested in the SETUP message is ISDN circuit-mode bearer service (see section 6.1 .). For calls originated by the PSPDN, the same considerations as above apply. In fac
18、t, with reference to Figure 1 (T/S 46-50), the ISDN port of the PSPDN includes both rate adaption (if required) and path setting-up functions. When needed, DTE identification may be provided to the PSPDN by using the call establishment signalling protocols in Recommendation T/S 46-30. Furthermore, D
19、CE identification may be provided to the DTE, when needed, by using the same protocols. 2.2. Configuration for the ISDN virtual circuit bearer service This configuration refers to the case where a packet handling (PH) function is provided within the ISDN. The configuration in Figure 2 (T/S 46-50) re
20、lates to the case of X.25 link and packet layer procedures conveyed through the B-channel. In this case, the packet call is routed, within an ISDN, to some PH function where the complete processing of the X.25 call can be carried out. Edition of February 29, 1988 CEPT T/S*4b-50*E 7 W - TIS 46-50 E P
21、age 3 S ! /-. A- !- I - ., or internai network i s protocol Ip - - ! .s I! D P i E 1 Note 3 i X.25 S ! ISDN -a- . /- /. NT - 6-channel is used - D-channel is used Reference points ii (see Recommendation 1.41 1) i Note 1. Figure 2 (T/S 46-50) is only an example of many possible configurations and is
22、included as an aid to the text describing the various interface functions. Note 2. In some implementations the PH functions logically belonging to the ISDN may reside physically in a node of the PSPDN. The service provided is still the ISDN virtual circuit service. Note 3. Recommendation X.75 applie
23、s for intenvorking with PSPDN, see also Recommendation X.325. Figure 2 (T/S 46-50). Configuration for the ISDN virtual circuit bearer service (access via B-channel). Multiple X.25 DTE + TAS or TEls, or a combination thereof, may be supported at the customer premises. Multiple X.25 DTEs may be multip
24、lexed at layer 3 by an NT2 onto a single B-channel. Multiplexing at layer 2 is for further study. Multiple TAS or TEls are able to use the B-channel, one at a time, on a per-call basis. The PH function may be accessed in various ways depending on the related ISDN implementation alternatives. In any
25、case, the B-channel connection is set up towards a PH port supporting the necessary processing for- B-channel packet calls, standard X.25 functions for layer 2 and layer 3 as well as possible path setting-up functions for layer 1 and possible rate adaption. . X.25 . s _- D-channel is used .y.- D net
26、work (see Note 3) -.- -1 protocol e i-/ i*) Switched or non-switched ! Reference points II (see Recommendation 1.411) i Note 1. Figure 3 (T/S 46-50) is only an example of many possible configurations and is included as an aid to the text describing the various interface functions. Note 2. In some im
27、plementations the PH functions logically belonging to the ISDN may reside physically in a node of the PSPDN. The service provided is still the ISDN virtual circuit service. Note 3. Recommendation X.75 applies for interworking with PSPDNs, see also Recommendation X.325. Figure 3 (T/S 46-50). Congurat
28、ion for the ISDN virtual circuit bearer service (access via D-channel). Edition of February 29, 1988 “r CEPT T/S*46-50*E 7 W 2326414 0008755 1 W TIS 46-50 E Page 4 The configuration in Figure 3 (T/S 46-50) (see Note 4 refers to the case of X.25 packet layer procedures conveyed through the D-channel.
29、 In this case, a number of DTEs can operate simultaneously through a D-channel by using connection identifier discrimination at layer 2. The accessed port PH is still able to support X.25 packet layer procedures. Note 4. The Figure 3 (T/S 46-50) is kept for completeness although this specification d
30、oes not include yet the case of support of X.25 procedures over the D-channel (see also Note 1, section i.). It is also important to note that the procedures for accessing a PSDTS through an ISDN user-network interface over a B- or D-channel are independent of where the service provider chooses to l
31、ocate packet handling functions, i.e. : (a) in a remote exchange or packet switching module in an ISDN; (b) in the local exchange. However, the procedures for packet access through the B-channel or the D-channel are different (see section 6.). In both cases of B- and D-channel accesses, in service (
32、b), the address of the called DTE is contained in the X.25 call request packet. The establishment of the physical connection from the TA/TE1 and the packet handling functions is done on the basis of the requested bearer capability (ISDN virtual circuit bearer service), therefore the user does not ne
33、ed to provide an access port number (see section 6.2.). 3. ISDN PACKET SWITCHED DATA TRANSMISSION SERVICE CHARACTERISTICS 3.1. 3.1.1. Service characteristics Access to PSPDN services (case A) In this case, the ISDN offers a 64 kbit/s circuit-switched or non-switched transparent network connection ty
34、pe between the TA/TEl and the PSPDN port (AU). In the switched access case the AU must be selected by the information contained in the called party number in the D-channel signalling protocol when the TA/TEl sets up the circuit switched connection to the AU. Since the packet switched service provide
35、r is a PSPDN, some DTEs are PSPDN terminals; they are handled by the PSPDN. Other DTEs may access the PSPDN without being subscribed to the PSPDN permanently. In the first case, the same services as PSPDN services are maintained, including facilities, quality of service (QOS) characteristics and DTE
36、-DCE interfaces. In the case where a DTE is not subscribed to the PSPDN, it will be provided with a limited set of PSPDN facilities (see Recommendation X.32). A number from the X.121 PSPDN numbering plan or possibly a number from the ISDN numbering plan may be attributed to the DTE. The method for X
37、.25 packets to convey numbers from the ISDN numbering plan and the relationship with X.121 requires further study. 3.1.2. User access capabilities In this case DTEs operating at bit rates 2.4, 4.8, 9.6, 19.2, 48 and 64 kbit/s can be supported with no restrictions on the use of Recommendation X.25. T
38、he rate adaption mechanism as well as the TA functiona- lities are described in section 7. 3.1.2.1. Basic rules 3.1 -2.1.1. Packet data communications, when using a switched B-channel, will be established by separating the establishment phase of the B-channel and the control phase of the virtual cir
39、cuits using the X.25 protocol (link layer and packet layer). 3.1.2.1.2. In general an ISDN has no knowledge of the customers terminal equipment or configuration. The incoming B-channel connection establishment will have to employ the D-channel point-to-point and point-to-multipoint signalling proced
40、ure (see sections 6.1.2.3.2., 6.2.2.3.1., and Recommen- dation T/S 46-30). 3.2. 3.2.1. Service characteristics Access to ISDN virtual circuit bearer service (case B) In principle the virtual circuit bearer service provided within the ISDN should not diverge from what is being developed in the X-Seri
41、es Recommendations (e.g. in terms of facilities, quality of service, etc.). Edition of February 29, 1988 CEPT T/S*46-50*E 87 m 2326414 0008956 3 m 3.2.2. 3.2.2. 3.2.2.2. 4. 4.1. 4.2. 4.2.1. 4.2.2. 4.3. 4.3.1. 4.3.2. 5. 5.1. TIS 46-50 E Page 5 The services and facilities provided as well as the quali
42、ty of service characteristics are those of the ISDN. Existing features of the X-Series Recommendations would be enhanced and additional features would also be developed taking into account the new ISDN customer capabilities (e.g. multiterminal arrangements, user rate at 64 kbitls, as well as the pos
43、sible evolution of compatibility checking). A number from the ISDN numbering plan will be attributed to the DTE. In this case both B- and D-channels (see Note I, section 1.) can be used for accessing the ISDN virtual circuit bearer service. Uses access capabilities Access through the B-channel The s
44、ame considerations described in section 3.1.2. apply. Access through the D-channel To be specified at a later stage-(see Note I, section 1.). PACKET COMMUNICATION ADDRESSING AND ROUTING ASPECTS Terminal interface identification This section describes the information necessary to select a TA/TE1 for
45、the completion of an incoming Cali. It is the general requirement that users should be allowed to operate several packet terminals in their multiservice arrangements. It is envisaged that an ISDN would identify, by means of an ISDN address, a specific TA/TE1 within the subscriber premises. Transmiss
46、ion capability information (conveyed in Bearer Capability information element) is essential to select the called TA/TEl. Communication capability information (conveyed in the High Layer Compatibility information element) may be used to select the called TA/TEl. See Recommen- dation 1.331. Note. An I
47、SDN address may contain a subaddress (see Recommendation 1.330). The use of addressing, transmission capability and communication capability information is considered to be sufficient to meet the service requirement for routing a packet call to the required ISDN terminal and/or network service acces
48、s point. Circuit-mode access to PSPDN services Channel type selection Packet calls using this bearer service (i.e. circuit-mode) will always use the B-channel. Addressing scheme for outgoing calls The SETUP message, when used, contains the request for a circuit-mode bearer service. The SETUP message
49、 also contains the ISDN number of the AU of the PSPDN. The X.25 CALL REQUEST packet contains the actual address of the called terminal. Packet-mode access to ISDN virtual circuit bearer service Channel type selection See Note I, section 1. Addsessing scheme for outgoing calls The SETUP message, when used, contains the request for the ISDN virtual circuit bearer service. The SETUP message shall not contain an address. The X.25 CALL REQUEST packet contains the address of the called terminal. INTERWORKIG WITH DEDICATED NETWORKS Circuit-mo
