1、ITU-T RECNN*U.LZ 93 4862591 0594140 818 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU TELEGRAPH SWITCHING SPECIFIC SIGNALLING SCHEMES AND INTERWORKING BETWEEN SIGNALLING SYSTEMS u.12 (03193) TERMINAL AND TRANSIT CONTROL SIGNALLING SYSTEM FOR TELEX AND SI
2、MILAR SERVICES ON INTERNATIONAL CIRCUITS (TYPE D SIGNALLING) ITU-T Recommendation U.12 (Previously “CCITT Recommendation”) ITU-T RECMN*U-L2 93 486259L 0594L4L 754 FOREWORD The Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The IT
3、U-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 Conference (WTSC), which meets every four years, established the topics for
4、study by the -T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recommendation U.12 was revised by the ITU-T Study Group IX (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 T
5、elecommunication 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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In orde
6、r not to delay publication of this Recommendation, no change has been made in the text to references containing the acronyms “CCIT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation will contain the proper terminology relate
7、d 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 reserved. No part of this publication may be reproduced or utilized in any form o
8、r by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. ITU-T RECMN*U=12 93 4662571 0594342 690 II CONTENTS 1 Signal conversion . 2 General switching and signalling principles 3 Specific signalling characteristics Appendix I - Possi
9、ble sequences of network selection signals Appendix II - Examples of network selection signals . II.1 First example (minimum sequence of network selection signals) 11.2 Second example (a sequence of network selection signals including closed user-group characters) . Appendix III - Through-connection
10、 procedure References . Recommendation U.12 (03/93) Page 1 1 2 19 20 20 20 21 24 1 ITU-T RECMN*U.12 93 Li862591 05941Li3 527 0 Recommendation U.12 TERMINAL AND TRANSIT CONTROL SIGNALLING SYSTEM FOR TELEX AND SIMILAR SERVICES ON INTERNATIONAL CIRCUITS (TYPE D SIGNALLING) (Geneva, 1972; amended at Gen
11、eva, 1976, 1980 and Malaga-Torremolinos, 1984 and Helsinki, 1993) The CCITT, considering (4 that new networks are being introduced based upon stored programme control techniques; (b) similar services or these services in combination with data traffic; that these networks, which may be synchronous or
12、 anisochronous, are being provided to carry either telex and (c) available on existing types of telex network; that the equipment provided for these networks facilitates an enhanced range of facilities compared with those (4 be handled, as far as practicable, by common processes; that these factors
13、justify the establishment of a new type of signalling, enabling both telex and other traffic to (e) that, for interworking between these anisochronous networks for telex and similar switched telegraph services, a signalling standard (designated type D) has been adopted, based upon that described in
14、Recommendation X.70 11 for start-stop data services on anisochronous networks; (f) described in Recommendation X.71 2, that the decentralized signalling to apply on connections between synchronous public data networks is unanimously declares the view 1 Signal conversion 1.1 Subclause 1.1KJ. 1 concer
15、ning the responsibility for signal conversion, should be the ultimate aim for interworking between networks using type D signalling on the one hand and type A, B or C signalling on the other hand. 1.2 However, in order to avoid unnecessary inconvenience during the introductory stages of the new sign
16、alling system, it is recommended that countries employing type D signalling systems should provide for incoming international traffic type A or B signalling and possibly for transit working type C signalling. The question as to when 1.1KJ.1 will become fully effective is yet to be resolved. 1.3 Rule
17、s for interworking Recommendation U. 12 signalling with signalling standards defined in Recommendations U. 1 and U.11 are given in Recommendation U. 15. 2 General switching and signalling principles 2.1 transfer. Decentralized signalling will apply, the same channel being used for control signalling
18、 and information 2.2 signalling control of calls will be adopted. Both terminal and transit operation will be required. Due to the inclusion of transit operation, link-by-link Recommendation U.12 (03/93) 1 ITU-T RECMN*U.32 93 4862593 O594367 TbO 2.3 Onward selection from transit and incoming termina
19、l centres should be arranged to overlap the receipt of selection signals, this in order to minimize call Set-up times. Selection signals will be transmitted by the originating country at automatic speed in a single block that includes an end-of-selection signal. It is nevertheless necessary to coord
20、inate the transmission of signals on the forward path to allow sufficient time for retransmission or conversion of backward path signals. 2.4 numerical codes will be used for network identification purposes. The schedule of telex destination codes laid down in Recommendation F.69 3 will apply. The s
21、ame 2.5 Alternative routing will be permitted. The principle of a few high usage circuits will be adopted, with overflow on to adequately provided routes between centres. In order to prevent repeated alternative routing causing traffic to circulate back to the originating point, alternative routing
22、will be restricted to once per call. 2.6 Both-way operation will be assumed and inverse order testing of circuits on both-way routes, or a close approximation to it by testing the route in small groups in fixed order always starting the search from the same position, will be specified in order to mi
23、nimize head-on collisions. 2.7 information. In all cases (including transit switching) the originating network will be responsible for recording accounting 2.8 The grade of service for the provision of circuits should not be worse than one lost call in 50 for routes carrying overflow traffic or from
24、 which overflow is not permitted. For high-usage direct links, circuits would be provided at a grade of service to be agreed bilaterally, but should not be worse than one lost call in ten. 2.9 0.4% of calls in the busy hour, and only then when congestion has been positively identified. Sufficient sw
25、itching equipment will be provided to ensure that congestion will not be signalled on more than 3 Specific signalling characteristics NOTES 1 X denotes the international centre that originates the call under consideration on the international link concerned. Y denotes the international centre that r
26、eceives the call under consideration on the international link. Centre X and centre Y will provide any necessary signalling conversion to the type of signalling employed on the preceding and succeeding links if these do not use type D signalling. 2 Timings shown are within the centre concerned with
27、no allowance being made for propagation and other delays, such as slow sending of selection signals by the originating subscriber. 3 The times for permanent start polarity (A) and stop polarity (Z) are generally indicated in the following signal descriptions as integral multiples of a character (see
28、 Note 4). Compared with Recommendation X.70 I, some other multiples are selected in order to enable simpler interworking with systems operating in accordance with Recommendations U. 1 and U. 1 1. 4 The control signalling code (CSC) used in this signalling system is described in Table 8. 3.1 signalli
29、ng is described in Table 1. The signalling system for telex and similar services between two anisochronous networks using type D 3.2 The incoming equipment may release the connection if the calling signal exceeds the maximum period of two characters, or of four characters in exceptional cases where
30、extension of call signals has been requested by centre Y. Start polarity will be maintained on the backward signalling path from centre Y to centre X. 3.3 The first forward path signal following the calling signal (class-of-traffic signal) is distinctive from the first backward path signal to provid
31、e a guard against head-on collisions in the case of bothway operation. A head-on collision is detected by the fact that centre X receives a first class-of-traffic character instead of the reception-confirmation or reception-congestion signal. When a head-on collision is detected, the switching equip
32、ment at each end of the circuit should make another attempt to select a free circuit, either on the same group of circuits or on a group of overflow circuits, if they exist and there are no free circuits on the primary route. In the event of a further head-on collision on the recall, or on the call
33、attempt via the 2 Recommendation U.12 (03/93) ITU-T RECMN*U*L2 93 m 4b259L 0594344 463 overflow route, no further recall will be made and the call will be cleared down. In the case of a transit centre, the service signai No. 20 (NC) followed immediately by the clearing signal will be returned to the
34、 preceding centre after the reception-confirmation signal and the network identification signai (see Recommendation F.69 3). 3.4 If there is failure to receive the reception-confirmation or reception-congestion signal within 4 seconds from the start of the calling signal or receipt of a spurious sig
35、nal, as indicated by a character other than a first class-of-traffic character, the reception-confirmation signal or reception-congestion signal should then initiate the automatic retest signal on the circuit concerned. In the case of failure to receive the correct reception-confirmation or receptio
36、n-congestion signal, another attempt to select a circuit should be made (once only). If the second attempt is unsuccessful, the service signal No. 20 (NC) followed by the clearing signal will be returned to the preceding centre after the reception-confirmation signal and the network identification s
37、ignal (see Recommendation F.69 3). 3.5 Selection signals can be divided into two parts. The first part, designated as the network selection signals, contains information regarding network and subscriber requirements and may be composed of one or more characters (see Tables 2, 3, 4,4a, 5 and 5a). The
38、 second part comprises the address signals (the called subscriber number, which is preceded by the destination code in the case of a transit call). The network selection signals used in the forward direction (see also Appendix II) are further subdivided and assembled as follows (see 3.5.1 to 3.5.4 b
39、elow) for signalling purposes: 3.5.1 First class-of-traffic character (see Table 2) The calling signal is always followed by at least one class-of-traffic character. The bit functions of this character were so chosen that no further characters are needed for most connections. If there is a need for
40、indication of further requirements, a second class-of-traffic character may be used. Whether the second class-of-traffic and user-class characters follow or not will be indicated by the bits b3 and b4 of the first class-of-trafic character. 3.5.2 User-class character (see Table 3) This character, if
41、 used, will follow the first class-of-traffic character and will be required when, for example, this information cannot be derived from the incoming line. Whether a second user-class character follows or not will be indicated by the bits bl, b2 and b3 of the first user-class character. When seven us
42、er classes in Table 3 are not sufficient, a second user-class character may be added by means of an escape character. Whether a second class-of-traffic character follows or not will be indicated by bit b4 of the first user-class character. 3.5.3 Second and subsequent class-of-traffic characters (see
43、 Tables 4 and 4a) These characters follow any user-class characters required. The number of these class-of-traffic characters depends on the number of user facilities available. The bit b4 of the second or subsequent class-of-traffic characters will indicate whether another class-of-traffic characte
44、r follows or not. 3.5.4 Closed user group character (see Tables 5 and 5a) closed user group is defined as follows: A number of users of a public switched communication service who have the facility that they can communicate with each other but access is barred to and from all other users of the serv
45、ice. NOTES 1 A special facility, permitting a user in a closed group to call any other user connected to a public switched communication service or to any other network with which interworking is permitted, may be offered. This is termed Closed user group with ourgoing access. Access to users of thi
46、s facility is restricted to other members of the closed user group. The start of closed user group character would precede the closed user group number which would be coded into a number of hexadecimal characters up to a maximum of four (see Table 5). 2 Further study is required concerning administr
47、ative aspects of the method to provide the closed user group facility. 3.5.5 class-of-traffic character indicates a terminal call, the Recommendation F.69 3 telex destination code will be omitted. The numerical characters used for the second part of the selection signals are shown in Table 6. When t
48、he first Recommendation U.12 (03/93) 3 ITU-T RECMN*U-12 93 W 4862591 0594145 3TT 3.6 The incoming equipment should maintain start polarity on the backward signalling path by releasing the connection if the first received character is spurious, as indicated by a character other than a valid first cla
49、ss-of-traffic signal. This procedure prevents the possibility of regarding a second selection signal as a first class-of-traffic character and provides a further safeguard against false calls. In the case of receipt of a spurious signal as indicated by a parity error or by a character other than a valid selection signal (with the exception of the first class-of-traffic signal), the incoming equipment should return the service signal No. 20 (NC) to the preceding centre - after the reception-confirmation and the network identification signal (see Recommendation F.69 3)
