1、CCITT RECNN*X-21 92 m 4862593 0574906 O00 m I INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE DATA COMMUNICATION NETWORK: INTERFACES x.21 (09/92) INTERFACE BETWEEN DATA TERMINAL EQUIPMENT AND DATA FOR SYNCHRONOUS OPERATION ON PUBLIC DATA N
2、ETWORKS CIRCUIT-TERMINATING EQUIPMENT Recommendation X.21 CCITT RECINxX.21 92 = 4Bb259L 0574907 TY7 = FOREWORD The CCllT (the Intemational Telegraph and Telephone Consultative Committee) is a pemianent organ of the International Telecommunication Union 0. CC” is responsible for studying technical, o
3、perating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCIIT which meets every four years, establishes the topics for study and approves Recommendations ppared by its Study Groups. The approval o
4、f Recommendations by the members of CC between Plenary Assemblies is covered by the procedure laid down in CC Resolution No. 2 (Melbourne, 1988). Recommendation X.21 was revised by Study Group W and was approved under the Resolution No. 2 procedure on the 10th of September 1992. 1) telecommunication
5、 Administration and a recognized private operating agency. 2) In this Recommendation, theexpression “Administration” is used for conciseness to indicate both a A list of abbreviations used in this Recommendation can be found in Annex J. O ITU 1993 All rights reserved. No part of this publication may
6、 be reprouced or utilized in any form or by any means, electronic or mechanical, including photocopying and mimfilm, without permission in writing from the ITU. CCITT RECNN*X-ZL 92 w 4862593 0574908 983 Recommendation X.21 INTERFACE BETWEEN DATA TERMINAL EQUIPMENT AND DATA CIRCUIT-TERMINATING EQUIPM
7、ENT FOR SYNCHRONOUS OPERATION ON PUBLIC DATA NETWORKS (Geneva, 1972; amended at Geneva, 1976 and 1980, Malaga-Torremolinos, I984 and Melbourne, 1988, revised in 1992) CONTENTS Preface 1 Scope 2 DTE/DCE physical integace elements 3 Alignment of call control characters and error checking Elements of t
8、he call control phase for circuit switched service 4 5 Data transfer phase 6 Clearing phase 1 Test loops Annex A - Interface signalling state diagrams Annex B - Interface signalling sequence diagrams and time-out operations Annex C - DTE Ume-limits and DCE time-outs Annex D - Formas of selection, ca
9、ll progress and DCE-provided information signals Annex E - Interworking between DTEs conforming to Recommendations X.21 and X.21 bis Annex F - Coding of call progress signais and DTE provided information Annex G - Facility request, indicator and parameter coding Annex H - Information content of DCE-
10、provided information Annex i - Reference and transition tables Recommendation X.21 (09/92) 1 Preface The CCITT, considering (a) that Recommendations X.l and X.2 define the services and facilities to be provided by a public data network; (b) that Recommendation X.92 defines the hypothetical reference
11、 connections for synchronous public data networks; (c) that Recommendation X.96 defines call progress signals; (d) that the necessary elements for an interface Recommendation should be defmed in architectural levels; (e) that it is desirable for characteristics of the interface between the data term
12、inal equipment (DTE) and data circuit-terminating equipment (DCE) of a public data network to be standardized, unanimously declares the view that the interface between the DTE and DCE in public data networks for user classes of service employing synchronous transmission should be as defined in this
13、Recommendation. 1 Scope 1.1 This Recommendation defines the physical characteristics and call control procedures for a general purpose interface between DTE and DCE for user classes of service, as defined in Recommendation X.l, employing synchronous transmission. 1.2 Recommendation. The formats and
14、procedures for selection, call progress and DCE-provided information are included in this 1.3 The provision for duplex operation is covered. 1.4 The operation of the interface for half duplex operation when the data circuit interconnects with Recommendation X.21 bis DTEs is described in Annex E. Hal
15、f duplex operation between X.21 DTEs is for further study when such new facilities are identified. 2 DTE1IDCE physical interface elements 2.1 Electrical characteristics 2.1.1 Data signalling rates of 9600 bids and below The electrical characteristics of the interchange circuits at the DCE si those b
16、its are binary 1. During data transfer (state 13), c = ON, i = ON and data are transferred on circuits T and R. Data transfer may be terminated by clearing, as defined in P 6, by either: i) theDCE, or ii) any connected DTE. The action to be taken when circuit C is turned OFF during data transfer (st
17、ate 13), except when the DTE is signalling DTE clear request (state 16) by t = O, c = OFF, is for further study except for the procedures for half-duplex operation between DTEs conforming to Recommendations X.21 and X.21 bis as described in Annex E. 5.2 Leased circuit service - Point-to-point (see F
18、igure A-3K.21) and packet-switched service (Recommendation X.25, level 1) In this section, for the case of packet-switched service, one of the DTEs must be understood as being the packet network data switching exchange (DSE). Data transmitted by the DTE on circuit T with c = ON are delivered to the
19、remote DTE on circuit R with i = ON. Both DTEs may employ duplex operation for the exchange of data. Any bit sequence may be sent by either DTE during the ON-condition of its circuit C. Note - The entering of the DTEDCE-interface of DTE uncontrolled not ready (state 22) will be signalled on the remo
20、te end as DCE not ready (state 18). In Figure A-3BC.21 a state diagram indicating a possible data connection is shown. Apart from state 13, two additional states, 135 and 13R, can be identified. I 5.2.1 Send data (state 13s) I Data transmitted by the DTE on circuit T with c = ON are delivered to the
21、 remote DTE on circuit R with , i =ON. I 16 Recommendation X.21 (09/92) CCITT RECMN*X.ZL 92 = 4Ab259L 0574924 02b I 5.2.2 . Receive data (state 13R) Data transmitted by a distant DTE with c = ON are received on circuit R with i = ON. 5.2.3 Data transfer (state 13) When c = ON, i = ON, data are trans
22、ferred on circuits T and k. 5.2.4 Termination of data transfer The DTE signals the termination of data transfer by signalling t = 1, c = OFF. The DCE indicates termination of data transfer by signalling r = 1, i = OFF, Note - The action taken by the DCE when the DTE signals c = OFF and t does not eq
23、ual 1, is for further study except for the DTE uncontrolled not ready procedures described in 0 2.5.1.2. 5.3 Leased circuit service - Centralized multipoint See Figure A-3K.21. 5.3.1 Central DTE data transfer 5.3.1.1 Send data (state 13s) Data transmitted by the central DTE on circuit T with c = ON
24、are delivered to all remote DTEs on circuit R with i =ON. 5.3.1.2 Receive data (state 13R) Data transmitted by any remote DTE with c = ON (one at a time as determined by the data link protocol) during state 13s are delivered to the central DTE on circuit R with i = ON. 5.3.2 Remote DTE data transfer
25、 Data transmitted by a remote DTE are not delivered to other remote DTEs. Note - Transmission of data by two or more remote DTEs at the same time may result in unsatisfactory conditions. 5.3.2.1 Send data (state 13s) Data transmitted by remote DTEs with c = ON (one at a time as determined by the dat
26、a link protocol) are delivered to the central DTE on circuit R with i = ON. 5.3.2.2 Receive data (state 13R) Data transmitted by the centrai DTE with c = ON are delivered to the remote DTE on circuit R with i = ON. 5.3.3 Data transfer (state 13) When c = ON, i = ON data transmitted by the central DT
27、E are delivered to all remote DTEs, and data transmitted by a remote DTE (one at a time as determined by data link protocol) are delivered to the central DTE. A remote DTE may send (one at a time as determined by the data link protocol) while the central DTE is sending to all remote DTEs. 5.4 Circui
28、t-switched service - Point-to-multipoint service The different configurations of point-to-multipoint are defined in Recommendation X.2. Recommendation X.21 (09/92) 17 5.4.1 Centralized multipoint All bits sent by the central DTE after indication of ready for data for 16-bit intervals (see 2.4) and b
29、efore clearing procedure bas been started by the central DTE or by the network, will be delivered to all remote DTEs after they have received readyfor data and before they bave received DCE clear indication (provided that the remote DTEs do not take the initiative of clearing). All bits received by
30、a remote DTE, after indication of ready for data for 16-bit intervals (see Q 2.4) and before clearing procedure has been started by the central DTE, by the network or the remote DTE, were sent by the central DTE. Some of those may bave originated as DTE waiting before the central DTE has received re
31、adyfor data; those bits are binary 1. All bits sent by a remote DTE, after indication of ready fur data for 16-bit intervals (see 6 2.4) and before clearing procedure has been started by the central DTE, by the network or by the remote DTE, will be delivered to the centrai DTE after that central DTE
32、 has received ready for data and before it bas received DCE clear confirmation or DCE clear indication, provided that the the other remote DTEs are transmitting binary 1. All bits received by the central DTE, after receiving readyfor data for 16-bit intervals (see 8 2.4) and before receiving DCE cle
33、ar indication or DCE clear confirmation, were sent by one of the remote DTEs. Some of these may have originated as DTE waiting before the remote DTEs have received ready for data; those bits are binary 1. During data transfer (state 13), c = ON, i =ON and data are transferred on circuits T and R. Da
34、ta transfer may be terminated by clearing, as defined in Q 6 below, by either: i) the centrai DTE or, ii) all the remote DES or, iii) the DCEs. 6 Clearing phase See Figure A4X.21. In centralized multipoint calls: - clearing by the cenual DTE imply clearing of the call; - clearing by a remote DTE cle
35、ars the call for this DTE, and has no effect on the caiis which remain establi$ed for the other remote DTEs; - clearing by the last remote DTE stiI1 in the call leads to the clearing of the call. 6.1 Clearing by the DTE (states 16, 17, 21) The DTE should indicate clearing by signalling the steady bi
36、nary condition t = O, c = OFF, DTE clear request (state 16). The DCE will respond by signalling the steady state condition r = O, i = OFF, DCE clear confirmation (state 17), followed by the steady binary condition r = 1, i = OFF, DCE ready (state 21). The DCE ready signal will be sent within 2 secon
37、ds after the receipt of the DTE clear request signal. The DTE shall recognize DCE cZear con.rmution and, except as noted below, shaU then respond to DCE ready, when presented, within 100 miliisecondes by signalling t = 1, c = OFF, ready (state 1). 18 Recommendation X.21 (09/92) CCITT RECMN*X*21 92 =
38、 48b2591 057492b 9T9 = In the case where DCE clear confirmation is either not presented by the DCE or not recognized by the DTE, the DTE shall remain in the DTE clear request state for a minimum of 2 seconds and then go to DTE ready. In this case, the DTE may not respond to DCE ready within the 100
39、milliseconds stipulated above and may be considered by the DCE to be uncontrolled noz ready (state 24) for a finite period of time (until it goes to DTE ready). 6.2 Clearing by the DCE (states 19, 20, 21) The DCE will indicate clearing to the DTE by signalling the steady binary condition r = O, i =
40、OFF, DCE clear indication (state 19). The DTE should signify DTE clear confirmation (state 20) by signalling the steady binary condition t = O, c = OFF, within 500 milliseconds. The DCE will signal r = 1, i = OFF, DCE ready (state 21) within 2 seconds of receiving DTE clear confirmation. The DTE sho
41、uld respond to DCE ready within 100 milliseconds by signalling t = 1, c = OFF, ready (state 1). In the case where DTE clear confirmation is either not presented by the DE or not recognized by the DCE, the DCE shall remain in the DCE clear indication state for a minimum of 500 milliseconds (until the
42、 time T15 has expired) and then signai DCE ready. 7 Test loops The definitions of the test loops and the principles of maintenance testing using the test loops are provided in Recommendation X.150. 7.1 DTE test loop - Type 1 loop This loop is used as a basic test of the operation of the DTE, by loop
43、ing back the transmitted signals inside the DTE for checking. The loop should be set up inside the DTE as close as possible to the DTEDCE interface. While the DTE is in the loop 1 test condition: - circuit T is connected to circuit R inside of the DTE; - circuit C is connected to circuit I inside of
44、 the DTE; - the DCE continues to present signal element timing on circuit S and, if implemented, byte timing on circuit B. The DTE need not make use of the timing information. Loop 1 may be established from either the data transfer or ready state. In some networks, for short routine tests during the
45、 data transfer state, the DTE should either maintain the same status on the interchange circuits as before the test or send the controlled not ready signai. If the loop is established from the data transfer state, the DCE may continue to deliver data to the DTE during the test as though the DTE were
46、 in normal operation. It will be the responsibility of the DTEs to recover from any mors that might occur while the test loop is activated. If the loop is established from the ready state, the DTE should signal one of the not ready states. 7.2 Local test loop - Type 3 loop Local test loops (type 3 l
47、oops) are used to test the operation of the DTE, the interconnecting cable and either all or parts of the local DCE, as discussed below. Loop 3 may be established from any state. Recommendation X.21 (09/92) 19 For testing on leased circuits and for short duration testing on circuit-switched connecti
48、ons the DCE should either continue to present toward the line the conditions that existed before the test (e.g. either data transfer or ready state) or send the controlled not ready state to the remote DTE. Where this is not practical (e.g. in some cases for loop 3a) or desirable (e.g. for long dura
49、tion testing in circuit-switched applications) the DCE should terminate an existing call and, if possible, signal toward the subscriber-line one of the not ready states. Manual and/or automatic control should be provided on the DCE for activation of the test loop. The precise implementation of the test loop within the DCE is a national option. At least one of the following local loops should be implemented: 7.2.1 Loop 3d This loop is used to test the operation of the DTE, including the interconnecting cable, by returning transmitted signals to the DTE for checking. The loop is se
