1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 - TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU-!).4%.!.#%G0 ).42/$5#4)/.G0G0!.$G0G0%.%2!,G0G002).#)0,%3$)4!,G0G0,/0“!#+G0G0-%#(!.)3-3)45G134G0G0RecommendationG0G0- (Extract from the “LUEG0“OOK)NOTES1 ITU-T Recommendation M.125 was published in Fascicle I
2、V.1 of the Blue Book. This file is an extract fromthe Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below).2 In this Recomm
3、endation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic ormechanical, i
4、ncluding photocopying and microfilm, without permission in writing from the ITU.Fascicle IV.1 - Rec. M.125 1Recommendation M.125Fascicle IV.1 - Rec. M.125DIGITAL LOOPBACK MECHANISMS1 GeneralLoopback can be one of the mechanisms which may be applied to fault localization and failure detection. ThisRe
5、commendation provides digital loopback definitions and describes loopback applications related to the maintenancephases of Recommendation M.20.2 Digital loopback definitionsA digital loopback is a mechanism incorporated into a piece of equipment whereby a bidirectionalcommunication path may be conne
6、cted back upon itself so that some or all of the information contained in the bit streamsent on the transmit path is returned on the receive path.The loopback point is the location of the loopback.The loopback control mechanism is the means by which the loopback is operated and released from theloop
7、back control point.The loopback control point is the point which has the ability to directly control loopbacks.The loopback control point may receive requests for loopback operation from several loopbackrequesting points.The loopback requesting point is the point which requests the loopback control
8、point to operate loopbacks.Note 1 Loopback requests should be subject to identification and authorization.Note 2 Possible locations of loopback requesting points are: the network, or a telecommunicationsmanagement network (TMN), or a maintenance service provider (MSP).The loopback test pattern is th
9、e test information transmitted during the operation of the loopback in thechannel or channels which are to be redirected by the loopback.Note 1 The generation of the test pattern used over the loopback may or may not take place at the controlpoint.The loopback application is the maintenance phase fo
10、r which the loopback operation is used, as defined inRecommendation M.20.2.1 Loopback typesThe following three types of loopback mechanisms are defined:a) complete loopback A complete loopback is a physical layer 1 mechanism which operates on the fullbit stream. At the loopback point, the received b
11、it stream shall be transmitted back towards thetransmitting station without modification.Note The use of the term “complete loopback” is not related to implementation since such a loopbackmay be provided by means of active logic elements or controlled unbalance of hybrid transformer, etc. Atthe cont
12、rol point only the information channels may be available.b) partial loopback A partial loopback is a physical layer 1 mechanism which operates on one or morespecified channels multiplexed within the full bit stream. At the loopback point, the received bit streamassociated with the specified channel(
13、s) shall be transmitted back towards the transmitting station withoutmodification.c) logical loopback A logical loopback acts selectively on certain information within a specified channelor channels and may result in some specified modification of the looped information. Logical loopbacksmay be defi
14、ned to apply at any layer 1, depending on the detailed maintenance procedures specified.2 Fascicle IV.1 - Rec. M.125For each of the above three types of loopback mechanisms, the loopback may be further categorized as eithertransparent or non-transparent:i) A transparent loopback is one in which the
15、signal transmitted beyond the loopback point (the forwardsignal) when the loopback is activated, is the same as the received signal at the loopback point. SeeFigure 1 a)/M.125.ii) A non-transparent loopback is one in which the signal transmitted beyond the loopback point (theforward signal) when the
16、 loopback is activated is not the same as the received signal at the loopbackpoint. The forward signal may be defined signal or unspecified. See Figure 1 b)/M.125.Note Whether or not a transparent loopback is used, the loopback should not be affected by facilitiesconnected beyond the point at which
17、the loop is provided, e.g., by the presence of short circuits, open circuits or foreignvoltages.Annex A to this Recommendation shows some examples of loopbacks.d01-scFIGURE 1/M.125.D01Fascicle IV.1 - Rec. M.125 33 Loopback applications3.1 Failure detectionIn order to detect failures related to netwo
18、rks maintained by different maintenance organizations, loopbacksshould be applied at the borderline separating the maintenance responsibilities. Loopbacks should be located in themaintenance entities (ME) adjacent to the borderline and as close as possible to the borderline. Part of the bit stream c
19、anbe involved in failure detection. Figure 2/M.125 shows an example with failure detection originated in locations A andB.d02-scFIGURE 2/M.125.D023.2 Fault localizationThe localization of faults in networks consisting of n maintenance entities requires at least n + 1 loopbackmechanisms. The loopback
20、 point should be as close as possible to the in- and output ports of the ME in order to includeas much as possible of the ME in the loopback mechanism. (See example in Figure 3/M.125.) Part of the bit stream orthe complete bit stream can be involved in fault localization, originated in locations A o
21、r B.d03-scFIGURE 3/M.125.D033.3 VerificationVerification can require performance tests and measurements of the complete bit stream.The same loopback location can be used as for fault localization.4 Loopback operation and releaseLoopbacks can be operated/released locally or remotely. Remote operation
22、/release can be based on in-serviceaddressing (e.g., layer 1 protocols) or it can require separate loopbacks addressing systems.5 Loopback examples (under study see Annex A)4 Fascicle IV.1 - Rec. M.125ANNEX A(to Recommendation M.125)d04-scFIGURE 4/M.125.D04Fascicle IV.1 - Rec. M.125 5Reference1 CCITT Recommendation Reference model of open system interconnection for CCITT applications, Vol. VIII,Recommendation X.200.
