1、CCITT FASCICLE Iv.1 * m 42593 0505373 T m Recommendation M.495 TRANSMISSION RESTORATION AND TRANSMISSION ROUTE DIVERSITY : TERMINOLOGY AND GENERAL PRINCIPLES 1 Purpose of transmission restoration and transmission route diversity The purpose of transmission restoration and transmission route diversit
2、y is to protect the continuity and quality of international telecommunication services by minimizing the effects of potential effects of a transmission failure. This Recommendation applies to both analogue and digital transmission. Note - This Recommendation may also apply in the case of hazardous c
3、onditions. 2 Causes of transmission failures The causes of transmission failures can be divided into three major categories: - - - equipment failure: this can be reduced by improving equipment reliability: outages due to the operating organization. For example, maintenance work or human errors; exte
4、rnal causes which are very difficult to prevent and for which specific protection might be needed. For example weather conditions or excavation work. In this Recommendation, failures or faults that are referred to may be either total or partial failures or faults. The relevant terminology concerning
5、 failures and faults can be found in Supplement No. 6 i. 3 Definitions concerning transmission restoration and transmission route diversity The purpose of this terminology is to define a vocabulary which can be used in connection with transmission restoration and transmission route diversity. Note -
6、 In this terminology, the term “link“ is used as a generic term for digital section, digital path, group link or section, supergroup link or section, mastergroup link or section, supermastergroup link or section, line section, section and line link. 3.1 Basic concepts 3.1.1 transmission restoration
7、The different actions taken in order to restore the transmission of a signal affected by a transmission fault. 3.1.2 transmission restoration function The ability to perform under stated conditions and within given time constraints the transmission Note I - This function is aimed at increasing the t
8、ransmission availability; it can provide transmission link supervision and control, the sending and receiving of control and check signals, and the changeover from normal to an alternative link, if necessary by assembling links, Note 2 - This function can allow the restoration of failed transmission
9、 systems, links, groups, digital blocks, equipment, etc., as well as the restoration for maintenance purposes such as planned outages, or to remedy conditions that affect transmission such as fading. Note 3 - The transmission restoration function can be implemented by equipment that is dedicated to
10、it, restoration. or by equipment that has other functions, such as, for example, automatic digital distribution frames. 292 Fascicle IV.l - Rec. M.495 ! CCITT FASCICLE IV.3 * m 4862593 0505372 3 m 3.1.3 transmission restoration function : direct fransmission restoration (protection link switching) D
11、irect transmission restoration is that category of transmission restoration function in which one transmission link between two stations is substituted for another between those two stations. Note - This reflects a configuration in which M links proctect N links, or in which N+M links give redundanc
12、y to a relation requiring N links, with the extremities of all links in the same locations. It is recommended to use the expression N + M direct transmission restoration to designate such a configuration. See Figure 1/M.495. Station B Station A Normal links (N) - - Il I l u T0400820-87 L Restoration
13、 links (M) FIGURE UM.495 N + M direct transmission restoration system (protection link switching) 3.1.4 transmission restoration function : automatic or semi-automatic transmission rerouting (protection network switching) Automatic or semi-automatic transmission rerouting is that category of transmi
14、ssion restoration function in which transmission links are assembled together and substituted for another link. Note - This reflects a configuration in which a certain number of links form a restoration network and protect normal links. Within a given transmission station, or for a given switching e
15、quipment, M links protect N links. It is recommended to use the expression N+M automatic transmission rerouting to designate such a configuration. N normal. A link between A and B can be restored, for example, directly or via C. Figure 2/M.495 shows un example. In Station A, M restoration links can
16、be used for restoration of Normal links 1, Station B I I I ,I U I I Station C TO40083047 FIGURE 2M.495 N 4- M automatic transmission rerouting system (protection network switching) Fascicle IV.l - Rec. M.495 CCITT FASCICLE 1V.L It qBb259L 0505373 3 -1 3.1.5 transmission restoration function : 1 + 1
17、restoration 1 + 1 restoration is that category of transmission restoration function in which one transmission link is substituted for another associated link, generally on another transmission route. See Figure 3lM.495. Normal link - - - - n - A Restoration link T0400840-87 FIGURE 3M.495 1 + 1 resto
18、ration system 3.1.6 transmission restoration function : manual transmission rerouting Manual transmission rerouting is that category of transmission restoration function in which one transmission link is replaced manually by another when a complete or partial transmission route failure has occurred
19、or when the normal route restoration link is not available due to a previous or simultaneous interruption, or when there is no such restoration link provided. Note - Such rerouting is normally effected using plugs and cords. 3.1.7 transmission restoration control function This is the function which
20、decides whether restoration is necessary on the basis of information from the link supervision system or link alarms. Note - The control function might be included in a specific equipment, or in the transmission restoration equipment itself, or within a restoration control centre. Control decisions
21、can also be taken by people in, for example, a control centre. 3.2 Systems and equipment 3.2.1 transmission restoration system A system that can be used to implement the transmission restoration function. An example is shown in Figure WM.495. 3.2.2 transmission restoration equipment The part of the
22、transmission restoration system that switches the transmission from the normal link to a restoration link. 3.2.3 normal transmission link/equipment ; normal digital block, group, supergroup, etc. A transmission link/equipment or a digital block, group, supergroup, etc., which is used for transmissio
23、n under normal operating conditions. 3.2.4 restoration linklequipment A transmission link/equipment which is used for transmission when the normal link/equipment is not available. Note 1 - A restoration link or equipment is generally idle under normal operating conditions, but might be used under th
24、ese conditions by low-priority traffic for which a lower degree of service availability is accepted. Note 2 - Note 1 may not apply to 1 + 1 type restoration system where both links are carrying the traffic. / -. 294 Fascicle IV.l - Rec. M.495 CCITT FASCICLE IV.1 * = 4862571 0505374 5 3.2.5 restorati
25、on network The network formed by all restoration links. Restoration control centre I ! I Restoration unit TO40085047 a) Line terminai equipments and/or multiplex equipments Note - This illustration is only an example. The structure of a transmission restoration system can be different (for example,
26、the control function might be implemented within a restoration control centre, with no specific equipment). FIGURE 4M.495 Example of transmission restoration system 3.3 Control (see also Figure 5/M.495) 3.3.1 control equipment An equipment that is used to implement the transmission restoration contr
27、ol function. 3.3.2 restoration control centre A centre supervising all or part of normal and restoration transmission systems. Note - A restoration control centre can be included within a control centre which is not dedicated to restoration. 3.3.3 controlled station The station that has its systems,
28、 links and other maintenance elements supervised, where the information and commands for switching are sent to and received from, the control centre, and where the switching is effected. Fascicle IV.l - Rec. M.495 295 F /- CCITT FASCICLE 1V.L *Y 4Bb259L 0505375 7 3.3.4 restoration unit All normal an
29、d restoration links and associated switching equipment capable of being controlled from a particular control centre. Note - Some networks areas may be controlled from more than one control centre. 3.3.5 control circuit A circuit used for the transmission of restoration control information. Restorati
30、on control centre Control circuit Transmission Controlled station U T0400860-87 Controlled station FIGURE 5W.495 Restoration control 3.4 Time intervals associa fed with transmission restoration processes Controlled station The following time intervals are intended to describe the different time comp
31、onents between the failure of a signal and its restoration. These time intervals can be used to characterize those transmission restoration systems, equipment etc. See also figure 6/M.495. - Potential failure Recognition of potential failure Start of restoration procedure Last control signal or comm
32、and received Last restoration operation finished Transmission fully restored b TO40087047 Time Tz L1 -T3 _1 -m T4 T5 TI : Tc r-m 1 Tt b 1 Tr : 1 T, Detection time Tc Confirmation time T, Waiting time Tt Transfer time T, Restoration procedure time Tr Restoration time T, Restoration transfer time T, R
33、ecovery time FIGURE 6M.495 Defined restoration times Fascicle IV.l - Rec. M.495 CCITT FASCICLE 1V.L * 4862591 0505376 9 3.4.1 detection time, TI Time interval between a potential failure of transmission and the recognition of that potential failure. 3.4.2 waiting time, Tz Time interval after the rec
34、ognition of a potential failure and its confirmation as a fault requiring restoration. 3.4.3 restoration procedure time, T3 Time interval between the confirmation of a fault and completion of the processing and transmission of the control signals required to effect restoration. 3.4.4 restoration tra
35、nsfer time, T4 Time interval between completion of the processing and transmission of the control signals required to effect restoration and the completion of transmission restoration operations. 3.4.5 recovery time, T5 Time interval between the completion of transmission restoration operations and
36、the full restoration of Note - This may include the verification of switching operations, re-synchronization of digital trans- failed transmission. mission, etc. 3.4.6 confirmation time, T, The time from the occurrence of the potencial failure to the instant when the fault is confirmed as requiring
37、a restoration: T, = TI + Tz. 3.4.7 transfer time, Tf The time interval after the confirmation that a fault requires a restoration to the completion of the transmission restoration operation; Tt = T3 + T4. 3.4.8 restoration time, T, The time from the occurrence of the failure to the restoration of th
38、e faulty transmission: Note - An apparent fault might be detected by an equipment and not confirmed after the confirmation T, = TI + T2 + T3 + T4 + T5 = confirmation time + transfer time + Ts. operations. In this case, only times TI and T2 are relevant. 3.5 Software related terms 3.5.1 network image
39、 Software description of the transmission network to be protected. 3.5.2 fault definition program Program which collects fault information and defines faulty transmission links. 3.5.3 restoration algorithm Method for forming restoration links for faulty normal transmission links. 3.5.4 restoration c
40、ontrol program A decision making program which controls restoration processes. I Fascicle IV.l - Rec. M.495 297 7 - P CCITT FASCICLE IV-L * 4862593 0505377 O 9 3.6 Route diversity 3.6.1 transmission route A transmission facility on a specific medium used by a certain number of transmission systems b
41、etween two stations. Note I - For example, one cable between two stations could be regarded as one transmission route (whatever the number of systems using this cable might be) and a radio system between these two points could be regarded as an other route. Note 2 - This definition represents a phys
42、ical route: this is different from the term “route” which is defined in the Recommendations E.600 2, Q.9 3 and 2.341 4, which represents a logical route. 3.6.2 transmission route diversity The provision of at least two links between two nodes in a transmission network which are routed over different
43、 transmission routes. Note - In case of a failure of one link, transmission route diversity allows some traffic between the two nodes still to be carried over the remaining link(s). 4 Principles of transmission restoration and transmission route diversity 4.1 General principles 4.1.1 In case of a fa
44、ult of an international transmission system, complete and fast transmission restoration is a maintenance objective. Line and terminal equipment allocated for transmission restoration should be left available to the extent that the objective can be achieved. This equipment may sometimes be used for o
45、ther purposes as required, e.g., planned outages. 4.1.2 restoration. When planning new routes or changes to existing routes, account should be taken of the requirements of 4.1.3 due to a fault or to a planned outage of a transmission link: The responsibility for restoration should be based on the fo
46、llowing principles in the case of an interruption when the fault of an international transmission link takes place on a national section, restoration is solely the affair of the Administration involved; when a fault takes place on an international section of an international route, restoration is th
47、e affair of the Administrations of the two countries directly involved, even if Administrations of other countries are concerned; in the case of a satellite fault, the responsibility to restore the satellite capability rests with the designated satellite system manager; restoration should be effecte
48、d in the transmission network at the highest order of link permitted by the network (group link, supergroup link, etc.) taking into account the service which is carried: it would be desirable to arrive, if possible, at complete restoration based upon bilateral and/or multilateral agreements. Special
49、 consideration is necessary when, in practical cases, complete restora- tion cannot be achieved. When complete restoration is not possible the links to be restored should contain those circuits that satisfy the special needs of the Administrations involved to the extent possible. Sufficient restoration capacity should therefore be provided to reflect the special interests of each Administration involved. Certain services might be considered as priority services by bilateral agreements; in this case, they should be grouped on groups or digital blocks that are rest
