1、 TfCS 20-26 Page 1 E Recommendation T/CS 20-26 (Innsbruck 1981) ADDITIONAL SIGNALLING Recommendation proposed by Working Group T/WG 11 “Switching and Signalling” (CS) Text of the Recommendation adopted by the “Telecommunications” Commission: “The European Conference of Postal and Telecommunications
2、Administrations, considering - that in accordance with the principles outlined in Recommendation T/CS 20-01 i, the exchange and network features which are required for the implementation of services and facilities should be identified and specied; - that in the past the signalling capability was opt
3、imised on the basis of the signals required for call set-up; - that, however, lack of signalling capacity in certain cases constrained rapid implementation of new services or new forms of services; - that modern signalling systems were deliberately developed with a view to increased signalling capac
4、ity and that common channel signalling systems, as referred to in Recommendation T/CS 14-05 2, provide ample signalling capability irrespective of the condition of the users call, O recommends to the members of the CEPT, the use of the following definitions, arrangements and specifications for the a
5、dditional signalling feature.” 1. DEFINITION Additional signalling is defined as a network feature by which means it is possible to transmit between exchanges, in both directions, information relating to the call-handling and control of supplementary services, irrespective of the condition of any ca
6、lls concerned. 2. DESCRIPTION For basic telephone call-handling, the signalling process between exchanges is, except for line supervision and metering information, limited to the call set-up phase and only a limited number of signals is required. For supplementary services call-handling, the possibi
7、lity of transmitting a wide range of information is required irrespective of the condition of the related calls, in some cases even without a call path being or having been set-up. This possibility is to be offered by the feature additional signalling. Where a call path is involved, information bein
8、g transmitted in the direction of call path set-up is in the form of forward service instructions. Information transmitted in the opposite direction is in the form of backward service instructions. Where no call path is involved, the information being transmitted is in the form of circuit independen
9、t service instructions. Certain service instructions may have a fixed meaning, while others may have a variable meaning with respect to one or more parameters which are included in the sesvice instruction. Examples : - calling-line-identity-request is a backward service instruction with fixed meanin
10、g; - calling-line-identity is a forward (service instruction with variable meaning) message in which the variable information is the included telephone number of the calling line. 3. TECHNICAL CHARACTERISTICS 3.1. Those parts of the network on which additional signalling will be provided, will have
11、to be able to interwork with parts on which the feature has not been provided. Therefore, the signalling protocols should ensure that no confusion can arise from such interworking between both parts of the network. This can be achieved by acknowledgement signals or messages. 3.2. The best way of ach
12、ieving additional signalling is by means of a common channel signalling system 2. Note: The studies on this series (T/CS 20) of Recommendations for the features in an analogue environment has now been terminated. A continuation of the studies has started with the aim to amend these existing features
13、, where necessary, and to specify new features for the ISDN. These Recommendations will be gathered in a new series of Recommendations. e - Edition of May 15, 1986 CEPT T/CS*20-2b*E 3 W 2326434 O004564 T = TICS 20-26 Page 2 3.3. 3.4. 3.5. 4. 4.1. 4.2. 4.3. 4.4. For some applications, additional sign
14、alling is only required during the call set-up phase. Some channel associated signalling systems, (e.g. System R2) allow transmission of information about the progress of the call in the backward direction, and about the nature and/or address of the origin in the Forward direction during the call se
15、t-up phase. If the speech path is used For the transmission of additional signalling information, it is necessary to disconnect the subscribers lines involved from this speech path during the signalling process. This is in order to protect the network from undesirable signals sent from the subscribe
16、rs line, and to protect the user(s) from disagreeable or confusing audible effects. Certain supplementary services may be controlled From an exchange other than the one to which the subscribers line is connected. For such services, two possibilities exist. (a) The user frst dials a special numerical
17、 access code For the control of services in the remote exchange. After receipt of an acknowledgement, e.g. an audible indication, the user may control the service by means of the push-button tele-phone according to the normal control procedures, in which an identity check is included. For this type
18、of control no additional signalling is required. (b) The subscribers line protocol is translated into inter-exchange messages. In this case, additional signalling is required. It will, however, be sufficient to have additional forward signals for * and + only, the other characters of the control pro
19、cedures being numerical. Furthermore, the subscribers line identity check is an inherent capability of additional signalling. Thus remote control by means of additional signalling requires neither a separate access code nor identification of the subscribers line to be included in the subscriber cont
20、rol procedure and is, therefore, the preferred Form of remote control in future systems. UTILISATION If the A- and B-subscribers lines involved are connected to the same exchange, then the Feature will not be required for any supplementary service provided all other features required are also offere
21、d by that exchange. If the A- and B-subscribers lines involved are not connected to the same exchange, then additional signalling is indispensable for certain supplementary services, whereas for others it is only desirable if a better service performance is required than can be obtained without it.
22、Moreover, certain supplementary services require additional signalling only during the call set-up phase. Such services can be realised with signalling systems as described in Section 3.3. These different requirements are under study and will be marked in Table 1 (T/CS 20-26) for the Co-operative se
23、rvices and other supplementary services. For remote control of supplementary services, additional signalling provides the best way of realising the control procedures in the network (see Section 3.5.). In future systems, the best service performance should be aimed for, and additional signalling sho
24、uld be provided throughout the network. Call progress information. Malicious call identification. Absent subscriber do not disturb service. Diversion of calls to another number. Completion of calls to a busy subscriber. Three-party service. Alarm call service. Call interception. Conference call. Low cost access. Closed user groups. Table 1 (T/CS 20-26). Supplementary services implying use of additional signalling. References i Recommendation T/CS 20-01. Exchange and network features. 2 Recommendation T/CS 14-05. Use of corninon channel signalling. Edition of May 15, 1986
