1、CCITT FASCICLE 1V.L * 4862591 0505381 2 -, Recommendation M.496 FUNCTIONAL ORGANIZATION FOR AUTOMATIC TRANSMISSION RESTORATION This Recommendation is a description of the functional organization for three general types of automatic - - - The terminology and general principles of transmission restora
2、tion are described in Recommenda- tion M.495. Specifications for equipment of the 1+1 restoration system type can be found in Recommenda- tion G.181 i. Specifications for equipment of the N + M direct transmission restoration or automatic rerouting system type can be found in Recommendation G.180 12
3、1. transmission restoration systems: 1 + 1 transmission restoration: N + M direct transmission restoration (protection link switching); N + M automatic rerouting (protection network switching). 1 1 + 1 transmission restoration 1 . Purpose of I + 1 transmission restoration 1.1.1 1 + 1 restoration is
4、used for the restoration of one group or digital block or link on one restoration link that is dedicated to its transmission restoration. 1.1.2 This type of restoration is generally reserved for specific services with a need for a very high availability. 1.1.3 As this method of restoration is rather
5、 expensive (duplication of transmission links), it is often effected, at the present time, at low hierarchichal levels (for example, group or primary digital block). This is a preventive protection, adapted to specific services, whereas restoration at the highest order group or digital block is a co
6、rrective protection for part of the network. Figure 3/M,495 illustrates such a configuration. 1.2 Method of transmission restoration 1.2.1 The transmission signal is sent on the normal link and generally also on the restoration link at the same time. 1.2.2 In order to ensure the best availability of
7、 transmission, it is recommended to have the restoration link routed on a transmission route different from the route of the normal link. 1.2.3 Control equipment or a control function implemented in equipment with other functions, at both ends of the link, ensures link supervision and control and de
8、tects the occurrence of such conditions that may need a restoration action. Generally, there is no control circuit in such a transmission restoration system: control and switching can be done at both receive ends of the signal. When a fault has been detected and confirmed, the switching equipment re
9、ceives a command for a switching action. 1.2.4 If a switchback function is provided, when the normal link becomes available again for transmission, it is advisable to perform the switchback at a time when there is the least impact on the traffic concerned. At that time, a switching command is sent b
10、y the control equipment. The switching equipment switches back the transmission to the normal link. This switching is normally effected in such a way that it has minimal impact on transmission quality and availability. .- 1.3 -Restoration time Restoration time should be kept as short as possible, in
11、 order to have minimal impact on service availability. - 4 Fascicle IV.1 - Rec. M.496 + CCITT FASCICLE 1V.L ft m 4862571 0505382 4 m 2 N + M direct transmission restoration (protection link switching) 2.1 Purpose of N+ M direct transmission restoration N + M direct transmission restoration systems p
12、rovide M restoration links for N normal links. All the links have their terminal equipments located at the same locations. Figure 1/M.495 illustrates such a configuration. 2.2 Method of transmission restoration 2.2.1 The M restoration links may be routed on the same transmission route as all, or par
13、t, of the N normal link; but preferably, some restoration links may be routed over a different route, so that a fault of a whole transmission route allows the restoration of some links. 2.2.2 This type of transmission restoration can be effected at all hierarchical levels. It is often used at the tr
14、ansmission system level. 2.2.3 At both ends of the links, control equipment (or a control function implemented in equipment) ensures link supervision and control, and detects the occurrence of a failure. The control circuits for this function might be on a restoration link, or on another link which
15、is not one of the N normal links, or duplicated on at least 2 of the N normal links. 2.2.4 restored in priority and can use a restoration link on a preemptive basis. This means that: Some of the N normal links might have a higher priority. In this case, when one of them is in fault, it is in case of
16、 a simultaneous fault of several links, and if a complete restoration is not possible, only the highest priority links are restored; if all restoration links are in use and if a normal link having a priority higher than the priority of one of the restored links has a failure, the lower priority rest
17、ored link is interrupted so that the link in fault can be restored. - - 2.2.5 When a restoration action has been detected, confirmed and accepted (restoration link available or priority link in fault), the switching equipment receives a command for a switching action. Switching might be effected at
18、both ends systematically, but it is also possible to switch only the faulty direction, if necessary. 2.2.6 In the case of automatic switchback, after the normal link is available again for a normal transmission, the control equipment sends a switchback command. In this case, the switching equipment
19、switches back the signal transmitted from the restoration link to the normal link. This switching is normally effected in such a way that it has a minimal impact on transmission quality and availability. 2.3 Restoration time N+M direct transmission restoration is characterized by a requirement to de
20、tect a degraded or faulty normal link and switch to a restoration link in a time Interval that is short enough not to cause established telephone calls to be released. 2.4 Other considerations 2.4.1 A restoration link might be used, when a restoration is not needed, for other purposes such as planne
21、d outages or non-priority traffic. In this case, it can be preferable that the N normal links have a preemption of the restoration link when they are in fault. 2.4.2 The maximum number N of normal links for one restoration has to be dimensioned correctly to avoid a too large number of non-restored f
22、aults. When a large number of links have to be restored, N+M direct transmission restoration (with M 1) is necessary; in this case M restoration links can be. used for the restoration of N normal ones. 2.4.3 semi-automatic (remote manual) actions, in order to force switching or inhibit restoration.
23、The N+M direct transmission restoration is an automatic system, but it should also allow manual or 7 Fascicle IV.l - Rec. M.496 303 CCITT FASCICLE 1V.L * Y8b2591i U505383 b mi 3 N + M automatic rerouting (protection network switching) 3.1 Purpose of N+M automatic rerouting N+M automatic rerouting sy
24、stems provide, on a single switching equipment, M restoration links to N normal ones. The restoration of 1 normal link is effected by a certain number of restoration links that are assembled together. The restoration systems belong to a restoration network. Figure 2/M.495 illustrates such a configur
25、ation. 3.2 Method of transmission restoration 3.2.1 3.2.2 protected by a network of restoration links. restoration control centres. Restoration can be a .specific function of a more. general control centre. At the present time, this type of restoration is generally effected at high hierarchical leve
26、ls. The organization of N+M automatic rerouting systems is generally complex: a network of normal links is There is a supervision and control of every link that can be done by or under the control of one or several 3.2.3 After a failure is detected on a normal link, the restoration is normally effec
27、ted according to certain preestablished restoration plans, if the restoration links are available. It is also possible to have a restoration plan computed after failure detection. A certain number of restoration links are assembled together through switches located at the nodes of the network and co
28、nnected to the faulty link through switches located at its ends. 3.2.4 It should also be possible to have manual or semi-automatic (remote-manual) action or inhibition of N+ M automatic rerouting systems. 3.2.5 If a restoration plan fails or a restoration link used for a restoration has a failure, a
29、ll restoration links involved in the restoration plan should be released. 3.2.6 After the fault of the normal link is removed, there can be a switchback to the normal link which should have a minimal impact on transmission quality and availability, 3.2.7 Certain equipment, such as automatic digital
30、distribution frames, might have a function of N+M automatic rerouting but might not be dedicated to it. 3.3 Restoration time As the operations of N + M automatic rerouting take network conditions into account, they can involve considerable data processing; they may entail all calls being cleared or
31、lost before the operations are compIeted. Restoration times can be in the order of seconds, minutes, or even hours, depending on the complexity of the network and its state at that moment. 3.4 Other considerations 3.4.1 Restoration links might be used under normal conditions by low-priority traffic.
32、 In this case, there is generally a preemption by normal traffic in case of normal link failure. 3.4.2 As the restoration network might not be dimensioned for the total restoration of all transmission route interruptions and multiple failures, it might be necessary to define certain priorities among
33、 normal links. In this case, certain links might be restored in priority with preemption of restoration links used by non-priority links. References CCIT Recommendation, Characteristics of I + 1 type restoration systems for use on digital transmission links, Rec. G.181. CCITT Recommendation, Charact
34、eristics of N+ M tyRe direct transmission restoration systems for use on digital sections, links or equipments, Rec. G.180. i 2 304 Fascicle IV.l - Rec. M.496 CCITT FASCICLE IV-L * m 48b2.591 0505384 8 m 2.5 Routine maintenance of an international carrier system Recommendation M.500 ROUTINE MAINTENA
35、NCE MEASUREMENTS TO BE MADE ON REGULATED LINE SECTIONS 1 Radio-relay regulated line section Measurements should be made as indicated below: 1 . Regulated line section terminal stations: a) b) daily reading of the line pilot level if necessitated by the type of system. It is preferable that such meas
36、urements should always be made at the same time of day; as necessary, readjustment to the nominal value as described in Recommendation M.510. 1.2 Radio-systems terminals 1.2.1 At intervals to be determined by agreement between the Administrations concerned, and based on experience of the reliability
37、 of the system: - measurement of the loss/frequency distortion at frequencies in the baseband (additional measuring frequencies) (permissible limits f 2 dB); - when there is no continuous recording of noise, measurement of the total noise level on the noise-measurement channels outside the baseband
38、in accordance with CCIR Recommenda- tion No. 398 i) i. This measurement can be made without causing any interference in the transmission channel. 1.2.2 When the measurement mentioned in Q 1.2.1 above gives unacceptably high noise values, or more often, when the reliability of the system makes it des
39、irable, check of the following measurements in accordance with the appropriate CCIR Recommendations for the radio-relay system concerned should be made, the radio-frequency channel being switched to the standby equipment, and the measurement results compared with the results of the reference measure
40、ments required by Recommendation M.450, 8 3 : - - the pilot frequency deviation; - - - - the deviation of the frequency at which the level is unchanged by pre-emphasis; the central position of the intermediate frequency in the non-modulated condition of the system; the level of the baseband referenc
41、e frequency (single frequency check); the relative level at the radio reference measurement frequencies (multifrequency check); the level of individual interfering signals in the baseband in the non-modulated condition of the system. 1.2.3 So as to enable the limits for circuit loss variation to be
42、met (see Recommendation M.160), the difference in response between two systems in diversity reception or between a working and standby system should be minimized. 2 Coaxial regulated line section The following measurements should be made at regulated line section terminal stations: a) b) The Adminis
43、trations concerned are left to decide for themselves about measurements at additional Note - Precautions to be taken with additional measuring frequencies: i) is not the same as the end of a line link (Le. when all the groups, supergroups, etc., are through-connected from one regulated line section
44、to another without passing via the through- connection equipment to the basic groups); daily reading of the line pilot level if necessitated by the type of system. It is preferable that such measurements should always be made at the same time of day; as necessary, readjustment to the nominal value a
45、s described in Recommendation M.510. measuring frequencies and about checking the operation of the regulators. When the end of a regulated line section: - 1 Where a protection channel is provided, and if Administrations so desire, noise measurements may be made on that channel with artificial loading, in accordance with CCIR Recommendation 399 2. I Fascicle IV.l - Rec. M.500 3 O5
