1、ITU-T RECNNUT.70 93 4862591 0593b04 4T4 I NTE R NATIONAL TELECOM M U N I CATION U N I ON ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU TELEMATIC SERVICES TERMINAL EQUIPMENTS AND PROTOCOLS FOR TELEMATIC SERVICES T.70 (03193) NETWORK-INDEPENDENT BASIC TRANSPORT SERVICE FOR THE TELEMATIC SERVIC
2、ES ITU-T Recommendation T.70 (Previously “CCITT Recommendation”) ITU-T RECNN*T.?O 93 4862593 0593605 330 FOREWORD The IT Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying technical, operating an
3、d tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their turn, p
4、roduce Recommendations on these topics. ITU-T Recommendation T.70 was revised by the ITU-T Study Group VIII (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITU), the CCITT ceased to
5、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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recommendation,
6、 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. Future editions of this Recommendation will contain the proper terminology related to the new ITU structure. 2 telecommunication a
7、dministration and a recognized operating agency. In this Recommendation, the expression “Administration” is used for conciseness to indicate both a O ITU 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, includi
8、ng photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*T*70 93 4862593 0593606 277 3 4 5 CONTENTS 1 Scope 2 Transport service 2.1 Transport service objectives . 2.2 General structure of the transport service . Transport service implementation for different types of ne
9、tworks 3.1 Terminals connected to a PSPDN . 3.2 Terminals connected to the PSTN 3.3 Terminals connected to a CSPDN 3.4 Terminais connected to an ISDN Interworking between networks . Transport layer procedure 5.1 5.2 5.3 5.4 5.5 Annex A A . 1 A.2 Annex B B.l B.2 AnnexC - C . 1 c.2 c.3 c . 4 AnnexD -
10、D . 1 D.2 D.3 D.4 Transport functions . Description of connection establishment and termination procedures . Description of data transfer procedures Treatment of procedure errors Formats . Transport and network service State transition diagrams for the basic transport layer procedures . State tables
11、 Lists of events. actions and conditions . Recommendations for implementation of Recommendation X.21 . General Receipt of call progress signals O1 or O4 Receipt of call progress signal 03 . Receipt of call progress signals of the group 2 to 8 Service definitions and state transition diagrams for the
12、 HDLC procedure and the network layer defined for CSPDN . Service definitions State transition diagrams HDLC . Summary of frame definitions X.21 service. controlled by the network layer Recommendation T.70 (03/93) Page 1 1 1 2 3 3 3 4 8 8 9 9 10 12 13 13 22 22 27 30 30 30 40 40 40 40 41 41 41 46 53
13、53 i ITU-T RECMN*T*70 93 W 4862591 0573607 LO3 Recommendation T.70 NETWORK-INDEPENDENT BASIC TRANSPORT SERVICE FOR THE TELEMATIC SERVICES (Geneva, 1980, amended at Malaga-Torremolinos, 1984; Melbourne, 1988 and Helsinki, 1993) The CCITT, considering (a) that the Teletex service will be introduced in
14、 different types of network, i.e. circuit-switched public data networks (CSPDN), packet-switched public data networks (PSPDN) and the public switched telephone network (PSTN); (b) that there is a need for international interworking between terminals belonging to the same or different types of Telema
15、tic services: unanimously declares the following view 1 Scope 1.1 Group 4 facsimile terminals connected to the types of network mentioned in (a) above in terms of This Recommendation defines the network-independent basic transport service applicable to Teletex and a) the transport services provided
16、to the higher layer the transport services are provided by the transport layer (layer 4) in association with the underlying services provided by the supporting layers 1 to 31; the transport layer procedure (see clause 5 below). b) 1.2 Clause 2 describes the transport service. Clause 3 describes the
17、transport service implementation for different types of networks. Clause 4 outlines the guidelines for interworking between networks. Clause 5 specifies the transport layer procedure, and Annexes A and B provide associated state transition diagrams and tables, respectively. 2 Transport service 2.1 T
18、ransport service objectives 2.1.1 The purpose of the transport service is to provide two communicating session entities within two terminais with transport services, .e. the means for transparent and reliable end-to-end transfer of data between them irrespective of the particular type of network use
19、d. 2.1.2 i.e. the session entity, are: The main requirements of the transport service to be provided by a transport entity to the local transport user, Network independence - The transport service shall be homogeneous, while allowing a suitable wide variety of underlying communications media, protoc
20、ols and mechanisms. End-to-end significance - The transport service shall have end-to-end significance, connecting the end users irrespective of the number of individual communication links used. Transparency - The transport service shall be octet transparent, i.e. not restrict the content, format o
21、r coding of the information (data or control) received from or delivered to the transport user. Error-free delivev - The transport service shall assure error-free delivery. Non-recoverable errors are to be visible to the transport service user. Cost eflciency - The transport service shall optimize t
22、he use of the available communication resources to provide the performance required by each communicating transport user at maximum efficiency. Recommendation T.70 (03/93) 1 ITU-T RECMN*T-70 93 4862593 0593608 04T R 2.2 General structure of the transport service 2.2.1 The general structure of the tr
23、ansport service is shown in Figure 1. To and from transport user (session layer) I t 1 1 Transport iam procedure (Nde 6) Transport ,-a- 4) (Clause 5) Call contrd phase N the called terminal shall take Address B; commands and responses shall be transferred as shown in Figure 2; A and B addresses are
24、coded as follows: Address 12345678 A 11000000 B 10000000 NOTE -The terminal will discard all frames received with an address other than A and B. Calling terminal A terminal T175709#1202 FIGURE 2Jr.70 3.2.2.3 System parameters are: a) timer, T1; b) c) d) maximum number of retransmissions, N2; maximum
25、 number of bits in an I frame, N1; maximum number of outstanding I frames, k. The above system parameters are to be specified by the Administration. However, the possible range of values that may be attributed to each parameter is to be standardized. Such values are for further study. 3.2.3 Network
26、layer procedure 3.2.3.1 See 3.1.3. In addition, for all calls (PSTN only, PSTN-PSPDN, PSTN-PSPDN-PSTN) second stage addressing will apply using X.25 virtual call procedures. The calling terminal should include the called address and the calling address (see Note 2) in call request packets. The forma
27、t of the called address shall conform to: a) b) c) NOTES 1 2 3 the telephone network addressing scheme for PSTN only calls; the telephone network addressing scheme with an X.121 DNIC for PSTN-PSPDN calls (see Note 3); the X.121 addressing scheme for PSTN-PSPDN calls (see Note 1). For other cases of
28、intemetworking the above rule shall apply. In the case of PSTN-PSPDN calls the verification of the calling address by the network requires further study. The The feasibility of such connections is for further study. format of the calling address is for further study. 3.3 Terminais connected to a CSP
29、DN 3.3.1 The DTEDCE physical interface characteristics shall be in accordance with Recommendation X.21, or as an option, Recommendation X.22 for multi-call operation. Physical layer DTE/DCE interface characteristics 4 Recommendation T.70 (03D3) ITU-T RECMN*T-70 93 4862593 059363L 634 B 3.3.2 Link la
30、yer procedure 3.3.2.1 General The link layer procedure shall be used during the data phase of Recommendation X.21 (or X.22) for data interchange over a single physical circuit between two terminals operating in User Classes of Services 3 to 7 and 30 as defined in Recommendation X.l. The link layer p
31、rocedure shall consist of a fully symmetrical HDLC procedure as defined in Recommendation X.75 for single link operation. 3.3.2.2 Link layer address procedure The following describes the application of the link addressing procedures of Recommendation X.75. Link addresses (A and E) shall be assigned
32、dynamically on a per-call basis according to the following rules: a) b) the calling terminal shall take address A; the called terminal shall take address B; c) commands and responses shall be transferred as shown in Figure 3; d) A and B addresses are coded as follows: Address 12345678 A 11000000 B 1
33、0000000 NOTE - The terminal will discard all frames received with an address other than A and B. Commands (B) Responses (B) Calling terminal Commands (A) terminai Responses (A) TO817580-94IdO3 FIGURE 3fl.70 3.3.2.3 Link layer implementation rules In order to achieve full compatibility between differ
34、ent implementations, the rules below for the implementation of Recommendation X.75 shall be followed. 3.3.2.3.1 General rules a) b) c) The 1984 version (Red Book) of clause 2/X.75, shall be used as the reference specification. The term “STE shall be read as “DTE”. At present the non-extended mode of
35、 operation (i.e. modulo 8) and the extended mode of operation (i.e. modulo 128) are defined. The support of modulo 8 is mandatory and modulo 128 is optional. However, the long term objective is to use modulo 128 as the common base modulo. Therefore, it is strongly recommended that the extended mode
36、of operation (Le. modulo 128) be supported, and the terminal supporting the extended mode shall also support the non-extended mode. Only the single link procedure (SLP) shall be used. d) Recommendation T.70 (03/93) 5 ITU-T RECMNxT.70 93 4862593 O593632 570 3.3.2.3.2 Specific rules The following rule
37、s refer to the indicated subclauses and tables of Recommendation X.75. Table I/X. 75 (see Note 1) I-frame must not be sent with an empty I-field. N20 N I N1 - 32 A received empty I-frame shall be treated as a valid I-frame. Subclause 2.3.4.9 Items 5),6) and 7) are not valid (shall not result in the
38、sending of a FRMR). Instead the following actions shall be implemented: - Not expected supervisory frames with the F bit set to 1 shall be ignored. - Not expected UA or DM response shall be ignored. - Frames with an invalid N(S) shall be responded to by sending REJECT. Frames with an FRMR control fi
39、eld shall not be responded by sending a FRMR. Table 7fl.75 Bits W, X, Y and Z set to O indicate that no reason for frame rejection is given. Subclause 2.3.5.3 The DTE and the CSPDN are not octet aligned and the last paragraph is therefore not valid. Subclause 2.3.5.5 Higher layers should be notified
40、 when timer T3 expires (excessive idle state). Subclause 2.4.3 Related to the first paragraph, read instead of “next response” “corresponding response”. Subclause 2.4.4. I In the active channel state, the DTE shall transmit contiguous flags independent of the other DTE. The calling DTE shall initial
41、ize the link by sending a SABM command with the P bit set to 1. Subclause 2.4.4.4. I A condition for entering the disconnected phase is also that no unacknowledged DISC command exists, because of collision cases (see 2.4.4.5Bc.75). In the disconnected phase, it is the calling DTE which may initiate
42、link set up. Subclause 2.4.5.9, fourth paragraph If an RNR is received, the DTE shall remain in the timer recovery condition (because the other DTE is still in the busy condition). Subclause 2.4.5.9, fifrh paragraph If an RNR is received, the DTE shall not resume I-frame transmission or retransmissi
43、on. Subclause 2.4.5.9, last paragraph If the transmission attempt variable is equal to N2, the DTE shall enter the disconnected phase. Subclause 2.4.7.3 In the frame rejection condition, the DTE shall only check the commands and react with an FRMR according to the P bit. The frame rejection conditio
44、n is cleared when the DTE receives an SABM, or, receives or transmits a DISC command. Subclause 2.4.7.3, second paragraph (see Note 2) Only the DTE which caused the FRMR condition may try to reset the link. 6 Recommendation T.70 (03/93) n) Subclause 2.4.7.3, third paragraph (see Note 3) After N2 att
45、empts to get the other DTE to reset the link, the DTE shall enter the disconnected phase. Subclause 2.4.8.1 (see Note 4) The timer T1 shall be started at the end of frame transmission. The value of T1 depends on the data signalling rate, the frame length, the value of N2, and a fixed time representi
46、ng both T2 and the transmission delay see item r). A value is recommended between 2.5 and 7 seconds. Subclause 2.4.8.2 (see Note 4) T1 T2 o) p) Depending on the acknowledgement strategy used, the DTE designer may regard T2 as a decision parameter only, in which case the DTE is not obliged to impleme
47、nt a corresponding timer. q) Subclause 2.4.8.3, second paragraph 30s I T3 560s r) Subclause 2.4.8.4 N2 x T1 2 60s s) Subclause 2.4.8.5 (see Note 5) For the non-extended mode, N1 - 1080 + (n x 1024) bits; n = Oor 1 or 3 or 7 or 15. For the extended mode, N1 - 1096 + (n x 1024) bits;n = Oor 1 or3 or7o
48、r 15. Subclause 2.4.8.6 (see Note 5) For the non-extended mode. k 5 7. t) For the extended mode, k I 127. NOTES 1 Terminals complying with the Red Book version of this Recommendation may react by DL-RESET indication Terminals complying with the Red Book version of this Recommendation may react diffe
49、rently. It is not meaningful to reset the link if the other DTE is not responding for N2 x T1. The acknowledge strategy used by the receiving DTE should be independent of any knowledge about the value of k used by the sending DTE. This can be achieved by acknowledging every correctly received I-frame as soon as possible. The selection of Ni and k values should be performed so that adequate throughput performance can be achieved for a 1;nk with longer transmission delay. The recommended combination of N1 and k values for a link having a higher data rate (e.g. 48 kbi/s or 64 kbivs) ar
