1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 2 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU4%,%2!0(94%,%2!0(G0G042!.3-)33)/.!54/-!4)#G0G002/4%#4)/.G0G037)4#()./ amended at Melbourne, 1988)The CCITT,considering(a) that Recommendation R.54 lays down a character error rate objective for telegraph comm
2、unication;(b) Recommendation M.201 concerning transmission path restoration for service protection;(c) that the availability and reliability of international telegraph transmission may be improved by providingautomatic protection switching of dual, diversely routed bearers to carry TDM aggregates co
3、nforming toRecommendation R.101;(d) that the principle of automatic switching between dual diversity routed bearers may also apply to othertelegraph channel multiplexers such as TDM systems conforming to Recommendation R.111 or frequency-modulatedvoice frequency telegraph (FMVFT) systems conforming
4、to Recommendation R.35, etc.,unanimously declares the following view1 It may be desirable to take measures to protect the quality and availability of derived international telegraphchannels against bearer breaks or degradation, for example:i) where the bearer is prone to relatively frequent interrup
5、tions (for example, long-haul bearers inintercontinental relations), such that the provisions of Recommendation R.54 may not be met for asignificant proportion of the time;ii) where the number of derived telegraph channels carried on a given telephone-type circuit or other bearerbecomes considerable
6、 (e.g. in excess of 50).2 An effective method of counteracting bearer faults is the use of automatic protection switching between dualdiversely routed bearers. In this technique, a pair of bearers with geographically diverse paths is selected (e.g. onecable, one satellite), ensuring a low probabilit
7、y of simultaneous outages of both bearers. At the sending end for eachdirection, the multiplexer aggregate or aggregates are connected to both bearers continuously. At the receiving end foreach direction, facilities are provided to select automatically either of the two incoming aggregate signals, u
8、sing ascriteria loss of sync or frame alignment from the TDM or loss of line signal (FMVFT or TDM).3 Annex A shows methods of implementing protection switching of telegraph bearers.2 Fascicle VII.1 - Rec. R.150ANNEX A(to Recommendation R.150)Protection switching methods for telegraph aggregatesA.1 S
9、ystem configurationsA.1.1 Figures A-1/R.150 and A-2/R.150 illustrate simple applications of automatic protection switching betweendual, diversely routed telegraph bearers. The aggregate output at each end is transmitted on both bearers continuously.At each end a Bearer Switchover Unit (BSU) automati
10、cally (and independently of action at the distant end) selects oneof the incoming aggregates from the two bearers and feeds it to the multiplexer (TDM or FMVFT).A.1.2 Two of the many further possible configurations using higher order digital multiplexers are shown in blockform in Figures A-3/R.150 a
11、nd A-4/R.150.Fascicle VII.1 - Rec. R.150 3A.2 Bearer routingFor protection switching to be effective, every effort should be made to diversify the routing of the two bearers.On the international portion, one bearer might be carried by cable and the other by satellite for example. Commonequipment nee
12、ds to be avoided in both the international transmission systems and any relevant national extensions tothem.A.3 Bearer switchover unitA.3.1 The BSU splits the multiplexer send path for simultaneous transmission on both bearers. In the case of TDM,the split will be made before or after the modems as
13、required, i.e. the aggregate signal is split in either its digital or itsanalogue form.4 Fascicle VII.1 - Rec. R.150A.3.2 The BSU monitors the appropriate circuit and equipment parameters on the receive path of both bearers. Itswitches the multipexer aggregate input from one bearer to the other as f
14、ollows:a) after a continuous period of between one and two seconds1)when there is: insufficient signal (where the BSU is in the analogue path) or loss of keying (where the BSU is in thedigital path) on the currently selected bearer; and/or loss of local2)sync (Recommendation R.101) or frame alignmen
15、t (Recommendation R.111) within theassociated TDM;Note An optional third condition “the other bearer (the bearer not currently in use) has not been detectedas faulty within the previous two seconds” is left for further study.b) once a switchover has occurred, a further switchover due to bearer failu
16、re on the newly selected path shallbe inhibited for a period of either 8 or 12 seconds1)and an alarm signal given.A.3.3 When TDM systems are used, the BSU switches the received aggregate signal either in its digital or analogueform.When switching the TDM aggregate in digital form, the following circ
17、uits must also be switched: received line signal detector (e.g. Recommendation V.24 circuit 109) if required by the TDM; receiver signal element timing (e.g. Recommendation V.24 circuit 115).A.3.4 The logic controlling the above functions shall be designed to be secure, minimizing the risk of a BSU fault thatcould affect both bearer paths._1)The shortening of this delay for Recommendation R.111 TDM aggregates is for further study.2)Advice from the distant TDM that it has lost sync or frame alignment will not of itself cause the BSU to switch over.
