1、CCITT RECMN*G.706 91 INTERNATIONAL TELECOMMI 4862593 05bLb2lI 4 NICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONS U LTATIVE COMM ITTEE GENERAL ASPECTS OF DIGITAL TRANSMISSION SYSTEMS; TERMINAL EQUIPMENTS G.706 FRAME ALIGNMENT AND CYCLIC REDUNDANCY CHECK (CRC) PROCEDURES RELATING TO
2、 BASIC FRAME STRUCTURES DEFINED IN RECOMMENDATION G.704 Recommendation G.706 - CCITT RECVN*G-706 71 4Bb2.591 05bLb29 b INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE GENERAL ASPECTS OF DIGITAL TRANSMISSION SYSTEMS; TERMINAL EQUIPMENTS G.7
3、06 FRAME ALIGNMENT AND CYCLIC REDUNDANCY CHECK (CRC) PROCEDURES RELATING TO BASIC FRAME STRUCTURES DEFINED IN RECOMMENDATION G.704 Recommendation G.706 Geneva, 1991 CCITT RECMN*G.706 91 W 48b2591 05bLb3G e! W FOREWORD The CCIIT (the International Telegraph and Telephone Consultative Committee) is a
4、permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCIT which meets every fou
5、r years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCIT between Plenary Assemblies is covered by the procedure laid down in CCiT Resolution No. 2 (Melbourne, 1988). Recommendation G.706 was prepared b
6、y Study Group XVIII and was approved under the Resolution No. 2 procedure on the 5th of April 1991. CCIT NOTES 1) telecommunication Administration and a recognized private operating agency. In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a 2) A list o
7、f abbreviations used in this Recommendation can be found in Annex D. O ITU 1991 All rights reserved, No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. - CC
8、ITT RECMN*G*70b 93 4862593 05b3b3L LI Recommendation G.706 FRAME ALIGNMENT AND CYCLIC REDUNDANCY CHECK (CRC) PROCEDURES RELATING TO BASIC FRAME STRUCTURES DEFINED IN RECOMMENDATION G.704 (Melbourne, 1988, revised 1990) 1 General This Recommendation relates to equipment which receives signals with ba
9、sic frame structures as defined in Recommendation G.704. It defines the frame alignment, the cyclic redundancy check (CRC) multiframe alignment and CRC bit error monitoring procedures to be used by such equipment. Annex A contains background information about the use of the CRC procedures and their
10、limitations. Annex B gives details of a modified CRC-4 multiframe alignment algorithm which allows automatic interworking between equipment with and without a CRC-4 capability. Annex C gives details regarding the updating of CRC-4 information when an intermediate equipment (Le. between true path ter
11、minating equipments) has a write-access to a message-based data-link facility (see Recommendation G.704, $2.3.3.5.4). 2 Frame alignment and CRC procedures at 1544 kbit/s interface 2.1 Loss and recovery offiame alignment There are two alternative multiframe structures at the 1544 kbit/s interface: a)
12、 24-frame multiframe, and b) 12-frame multiframe. 2.1.1 Loss offrame alignment The frame alignment signal should be monitored to determine if frame alignment has been lost. Loss of frame alignment should be detected within 12 ms. Loss of frame alignment must be confirmed over several frames to avoid
13、 the unnecessary initiation of the frame alignment recovery procedure due to transmission bit errors. The frame alignment recovery procedure should commence immediately once lass of frame alignment has been confirmed. Note - For the 12-frame multiframe described in Recommendation G.704, loss of mult
14、iframe alignment is deemed to occur when loss of frame alignment occurs. 2.1.2 Recovery offrame alignment 2.1.2.1 Frame alignment recovery time The frame alignment recovery time is specified in terms of the maximum average reframe time in the absence of errors. The maximum average reframe time is th
15、e average time to reframe when the maximum number of bit positions must be examined for locating the frame alignment signal. a) 24-frame multiffame The maximum average reframe Ume should not exceed 15 ms. Note - Some existing designs of equipment were designed to a limit of 50 ms. Recommendation .G
16、.706 1 CCITT RECMN*G*70b 91 4862.591 05bLb32 b = b) 12-frame mutiframe Start frame 4 search The maximum average reframe time should not exceed 50 ms. Noce - These times do not include the time required for the CRC procedure for false frame alignment verification defined in 9 2.2.2. 2.1.2.2 Strategy
17、for pame alignment recovery a) 24-frame multiframe Frame alignment should be recovered by detecting the valid frame alignment signal. When the CRC-6 code is utilized for error performance monitoring (see 8 2.2.3), the CRC-6 information may be coupled with the framing algorithm to ensure that a valid
18、 frame alignment signal contained within the 24 F-bits is the only pattern onto which the reframe circuit can permanently lock. This procedure is illustrated in Figure UG.706. b) 12-frame multiframe Overall frame alignment should be recovered by way of simultaneous detection of the frame alignment s
19、ignal and the multiframe alignment signal, or of frame alignment followed by multiframe alignment. 4 F-bits I I Frame re-alignment complete iaomi-a7 FIGURE 1/G.706 False frame alignment protectlon using a cyclic redundancy check (CRC) (1544 and 6312 kbit/s) 2 Recommendation G.706 CCITT RECMN*G-706 9
20、1 4862571 0561633 8 2.2 CRC bit monitoring Error monitoring by CRC-6 assumes a signal quality sufficient for frame alignment to be established so that CRC-6 bits can be correctly accessed. 2.2.1 Monitoring procedure i) A received CRC message block (CMB) is acted upon by the multiplication/division p
21、rocess defined in Recommendation G.704 after having its F-bits replaced by binary 1s. ii) The remainder resulting from the division process is then stored and compared on a bit-by-bit basis with the CRC bits received in the next CMB. iii) If the remainder exactly corresponds to the CRC bits containe
22、d in the next CMB of the received signai, it is assumed that the checked CMB is error-free. 2.2.2 Monitoring for false pame alignment (see 5 A.l.l) In the case of the 24-frame multiframe, when the CRC-6 code is utilized for error performance monitoring, it may also be used to provide immunity agains
23、t spurious frame alignment signals. The procedure described in Q 2.1.2.2 a) should be followed. 2.2.3 Error performance monitoring using CRC-6 (see 0 A.1.2) For the purpose of error performance monitoring, it should be possible to obtain indications of each CC message block which is received in erro
24、r. The consequent error information should be used in accordance with the requirements to be defined in respective equipment Recommendations. 3 Frame alignment and CRC procedures at 6312 kbt/s interface 3.1 Loss and recovery offrame alignment For the 63 12 kbit/s hierarchical level, the term “frame
25、alignment” is synonymous of “multiframe alignment”. The last five bits of the 789-bit frame are designated as the F-bits (see Recommendation G.704) and are time-shared as a frame alignment signal and for other purposes. 3.1.1 Loss offrame alignment The frame alignment signal should be monitored to d
26、etermine if frame alignment has been lost. The loss of frame alignment is declared when seven consecutive incorrect frame alignment signals have been received. The recovery of frame alignment procedure should stari immediately once loss of frame alignment has been confirmed. Recommendation G.706 3 C
27、CITT RECMN*G.7Bb 93 4862593 0563634 T 3.1.2 Recovery offiame alignment 3.1.2.1 Frame alignment recovery time The frame alignment recovery time is specified in terms of the maximum average reframe time in the absence of errors. The maximum average reframe time is the average time to reframe when the
28、maximum number of bit positions must be examined for locating the frame alignment signal. The maximum average reframe time should be less than 5 ms. 3.1.2.2 Strategy for fiame alignment recovery Frame alignment should be recovered by detecting three consecutive correct frame alignment signals. In ad
29、dition to this, the CRC-5 code (see 0 3.2) should be coupled with the framing algorithm to ensure that a valid frame alignment signal contained within the F-bits is the only pattern onto which the reframe circuit can permanently lock. This procedure is illustrated in Figure 1/G.706. 3.2 CRC bit moni
30、toring Error monitoring by CRC-5 assumes a signal quality sufficient for frame alignment to be established so that the CRC-5 bits can be correctly accessed. 3.2.1 Monitoring procedure i) A received sequence of 3156 serial bits (i.e. 3151 bits of CMB and 5 CRC bits) is divided by the generator polyno
31、mial defined in Recommendation G.704. ii) If the remainder resulting from the division process is 00000, it is assumed that the checked CMB is error-free. 3.2.2 Monitoring for falseji-ame alignment (see 0 A.l.l) The procedure in 0 3.1.2.2 should be followed when the CRC-5 code is used to provide imm
32、unity against false frame alignment signal. Using the CRC-5 code, it should be possible to detect false frame alignment within 1 second and with greater than 0.99 probability. On detection of such an event, a research for correct frame alignment should be initiated. With a random error ratio of lo-,
33、 the mean time between two events of falsely initiating a search for frame alignment due to an excessive number of errored CRC message blocks should be more than one year. Note 1 -With a random error ratio of approximately lP3, it is almost impossible to distinguish whether CRC errors are caused by
34、the false frame alignment or by transmission bit errors. Note 2 - To achieve the probability bounds stated above, one method is to count the errored CRC-5 message blocks with the understanding that a count of 32 consecutive errored CRC-5 blocks indicates false frame alignment. 3.2.3 Error performanc
35、e monitoring using CRC-5 (see P A.1.2) For the purpose of error performance monitoring, it should be possible to obtain indications for each CRC message block which is received in error. The consequent error information should be used in accordance with the requirements to be defined in the respecti
36、ve equipment Recommendations. 4 Recommendation G.706 CCITT RECMN*G-706 93 m 4862593 0563635 II m 4 4.1 4.1. Frame alignment and CRC procedures at 2048 kbi/s interface Loss and recovery offrame alignment Loss offrame alignment Frame alignment will be assumed to have been lost when three consecutive i
37、ncorrect frame alignment signals have been received. Note 1 - In addition to the preceding, in order to limit the effect of spurious frame alignment signals, the following procedure may be used: Frame alignment will be assumed to have been lost when bit 2 in time slot O in frames not containing the
38、frame alignment signal has been received with an error on three consecutive occasions. Note 2 - Loss of frame alignment can also be invoked by an inability to achieve CRC multiframe alignment in accordance with 5 4.2, or by exceeding a specified count of errored CRC message blocks as indicated in a
39、4.3.2. 4.1.2 Strategy for frame alignment recovery Frame alignment will be assumed to have been recovered when the following sequence is detected - for the fist time, the presence of the correct frame alignment signal; - the absence of the frame aiignment signai in the following frame detected by ve
40、rifying that bit 2 of the basic frame is a 1; - for the second time, the presence of the correct frame alignment signal in the next frame. Note - To avoid the possibility of a state in which no frame alignment can be achieved due to the presence of a spurious frame alignment signal, the following pr
41、ocedure may be used: When a valid frame alignment signal is detected in frame n, a check should be made to ensure that a frame alignment signal does not exist in frame n + 1, and also that a frame alignment signai exists in frame n + 2. Failure to meet one or both of these requirements should cause
42、a new search to be initiated in frame n + 2. 4.2 CRC multiffame alignment using information in bit 1 of the basicflame If a condition of assumed frame alignment has been achieved, CRC multiframe alignment should be deemed to have occurred if at least two valid CRC multiframe alignment signals can be
43、 located within 8 ms, the time separating two CRC multiframe alignment signals being 2 ms or a multiple of 2 ms. The search for the CRC multiframe alignment signal should be made only in basic frames not containing the frame alignment signal. If multiframe alignment cannot be achieved within 8 ms, i
44、t should be assumed that fiame alignment is due to a spurious frame alignment signal and a research for frame alignment should be initiated. Note I - The research for frame alignment should be started at a point just after the location of the assumed spurious frame alignment signal. This will usuall
45、y avoid realignment onto the spurious frame alignment signal. Note 2 - Consequent actions taken as a result of loss of fiame alignment should no longer be applied once frame alignment has been recovered. However, if CRC multiframe alignment cannot be achieved within a time limit in the range of 100
46、ms to 500 ms, e.g. owing to the CRC procedure not being implemented at the transmitting side, consequent actions should be taken equivalent to those specified for loss of frame alignment. Recommendation 6.706 5 CCITT RECMN*Gm70b 72 W 4862571 05bLb3b 3 Note 3 - Equipment incorporating the CRC-4 proce
47、dure should be designed to be capable of intenvorking with equipment which does not incorporate the CRC-4 procedure; that is, an ability to continue to provide service (traffic) between equipments with and without a CRC-4 capability. This can be achieved either manually (e.g. by straps) or automatic
48、ally. - For the manual case, the equipment incorporating the CRC-4 procedure should be capable of fixing bit-1 of the frame to the binary “1” (see Table 44G.704 Note 1). - For the automatic case, this can be achieved at the equipment having the CRC-4 capability either: - as a “higher-layer” function
49、 under the control of a network management facility (e.g. a TMN) - the details are for futher study; or - as a “lower-layer function using a modified CRC-4 multiframe alignment algorithm as described in Annex B. 4.3 CRC bit monitoring If frame and CRC multiframe alignment have been achieved, the monitoring of the CRC bits in each sub-multiframe should commence. 4.3.1 Monitoring procedure i) A received CRC sub-multiframe (SMF) is acted upon by the multiplication/division process defined in Recommendation G.704 after having its CRC bits extracted and replaced by Os. ii) The remainde
