1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU 1.480 (03/2000) SERIES I: INTEGRATED SERVICES DIGITAL NETWORK ISDN user-network interfaces - Aspects of ISDN affecting term i na I req u i remen ts 1 +I protection switching for cell-based physical layer ITU-
2、T Recommendation 1.480 (Formerly CCITT Recommendation) ITU-T I-SERIES RECOMMENDATIONS INTEGRATED SERVICES DIGITAL NETWORK GENERAL STRUCTURE Terminology Description of ISDNs General modelling methods Telecommunication network and service attributes General description of asynchronous transfer mode Sc
3、ope General aspects of services in ISDN Common aspects of services in the ISDN Bearer services supported by an ISDN Teleservices supported by an ISDN Supplementary services in ISDN Network functional principles Reference models Numbering, addressing and routing Connection types Performance objective
4、s Protocol layer requirements General network requirements and functions Application of I-series Recommendations to ISDN user-network interfaces Layer 1 Recommendations Layer 2 Recommendations Layer 3 Recommendations Multiplexing, rate adaption and support of existing interfaces SERVICE CAPABILITIES
5、 OVERALL NETWORK ASPECTS AND FUNCTIONS ISDN USER-NETWORK INTERFACES MAINTENANCE PRINCIPLES B-ISDN EQUIPMENT ASPECTS ATM equipment Transport functions Management of ATM equipment I. 1 10-1.1 19 I. 120-1.129 1.130-1.139 I. 140-1.149 I. 150-1.199 1.200-1.209 1.210-1.219 1.220-1.229 1.230-1.239 1.240-1.
6、249 1.250-1.299 1.3 10-1.319 1.320-1.329 1.330-1.339 I.34CI.349 1.350-1.359 1.36CI.369 1.370-1.399 1.420-1.429 1.430-1.439 1.440-1.449 1.4504459 1.460-1.469 1.500-1.599 1.600-1.699 1.730-1.739 I. 740-1.749 1.750-1.799 1.470-1.499 For further details, please refer io the list of ITU-TRecommendations.
7、 ITU-T Recommendation 1.480 1+1 protection switching for cell-based physical layer Summary This ITU-T Recommendation, “1 + 1 protection switching for cell-based Physical Layer“, provides architecture and mechanisms for 1+1 protection at the Physical Layer for ATM cell-based transmission systems. Thi
8、s ITU-T Recommendation defines the transmitter and receiver operation, the synchronization and switching process. It is based on the mechanisms described in ITU-T Recommendation 1.432.2 for ATM cell-based transmission systems. It provides hitless protection switching at the Physical Layer between tw
9、o sources and sinks of F3 OAM flows. Source ITU-T Recommendation 1.480 was prepared by ITU-T Study Group 13 (1997-2000) and approved under the WTSC Resolution 1 procedure on 1 O March 2000. FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field
10、 of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T 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 Worl
11、d Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSC Resolution 1. I
12、n some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a reco
13、gnized operating agency. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed In
14、tellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendati
15、on. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. O ITU 2001 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechani
16、cal, including photocopying and microfilm, without permission in writing fi-om the IT. CONTENTS Page 1 2 3 4 5 6 7 7.1 7.2 7.3 8 8.1 8.2 8.3 8.4 Scope References Definitions Symbols and abbreviations . Field of application . 1+1 protection switching principles . Transmitter operation . Transmitter b
17、lock diagram . Synchronization process in the transmitter . TC and PMD operation in the transmitter Receiver operation Receiver block diagram TC and PMD operation in the receiver . Link synchronization process . Block selection process 1 1 1 2 2 ITU-T Recommendation 1.480 1+1 protection switching ,a
18、r cell-based physical layer 1 Scope This ITU-T Recommendation provides architecture and mechanisms for 1+1 protection at the Physical Layer for ATM cell-based transmission systems. This ITU-T Recommendation defines the transmitter and receiver operation, the synchronization and switching process. It
19、 is based on the mechanisms described in ITU-T Recommendation 1.432.2 13 for ATM cell-based transmission systems. It provides hitless protection switching at the Physical Layer between two sources and sinks of F3 OAM flows. It applies at the TB reference point for 155 520 and 622 080 kbit/s cell-bas
20、ed Physical Layer. 2 References The following ITU-T Recommendations contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations are subject to revision; all users of this Re
21、commendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations listed below. A list of currently valid ITU-T Recommendations is regularly published. i ITU-T Recommendation 1.432.2 (1 999), B-ISDN user-network interface - Physical Layer
22、 specification: 155 520 kbit/s and 622 080 kbit/s operation. ITU-T Recommendation G.826 (1 999), Error performance parameters and objectives for international, constant bit-rate digital paths at or above the primary rate. Definitions s ITU-T Recommendation defines the following terms: working link:
23、The transmission link which is used for ATM cell transmission in normal protection link: The transmission link which is used for ATM cell transmission in global link: This defines the aggregation of the working link and the protection link. synchronization process: The process in the receiver which
24、consists in aligning the two cell selection process: The process in the receiver which consists in selecting between the two tection operation mode, that is when the working link is in error. om the working and the protection link. cell flows the block of cells which has the better performance from
25、a transmission point of view. 4 Symbols and abbreviations This ITU-T Recommendation uses the following abbreviations: Working path 4 b Protection path b AIS B-ET BIP B-NT CB-TC EDC HEC OAM PMD PSN RDI REB REB-G TC VP 4 B-NTl fi B-ET Alarm Indication Signal Broadband-Exchange Terminal Bit Interleaved
26、 Parity Broadband-Network Termination Cell Based-Transmission Convergence Error Detection Code Header Error Check Operation And Maintenance Physical Medium Dependent (sublayer) Payload Sequence Number Remote Defect Indication Remote Error Block Remote Error Block-Global link Transmission Convergence
27、 (sublayer) Virtual Path 5 Field of application This 1 + 1 protection mechanism is dedicated to cell-based user-to-network physical interfaces at the TB reference point. This includes both the 155 Mbit/s and the 622 Mbits cell-based interfaces, as defined in ITU-T Recommendation 1.432.2 i. This mech
28、anism offers a physical protection at the cell level without disruption. It is based on 1+1 redundancy which means the transmitted data is duplicated on two separate physical links. This mechanism is intended to be used only in a point-to-point physical configuration. The reference architecture is s
29、hown in Figure 1. B-NT2 r - I I T131625-99 TB Figure 1I.480 - Reference architecture As specified in ITU-T Recommendation 1.432.2 i, the maximum interface reach at the TB reference point is 2 km on single-mode medium for both the 155 and the 622 Mbit/s interfaces. The reference model for this mechan
30、ism is illustrated in Figure 2. The transmitter sends the same cells on the working and protection entities. These cells are inserted between two F3 OAM cells having the same PSN values. ? T1313960-98 Protection domain 0 Source/sink of F3 OAM flows Protection switching function Figure 2fi.480 - Refe
31、rence model 6 1+1 protection switching principles This 1+1 protection mechanism covers the Physical Layer; the protection is performed on all the transmitted cells. Consequently all the ATM-VPs supported by the physical interface are protected. This method is specific to the cell-based Physical Laye
32、r because it uses the cell structure of the interface and the F3 OAM cells for cell synchronization between the links. It is functionally bit-rate independent so it applies to both the 155 Mbit/s and the 622 Mbit/s cell-based interface. This mechanism offers a physical protection at the cell level w
33、ithout disruption because the two links are continuously synchronized and each time one errored block is detected on one link, the system selects the corresponding block on the other link. The positioning of the protection switching mechanism fi-om a layer view is shown in Figure 3; this e is given
34、for illustration only and it does not imply any particular implementation. Although e protection functions appear as a separate sublayer they are actually included in the Physical Layer of the ATM reference model as well as the TC and the PMD sublayers. In the transmit direction, each TC (CB-TC-A1 a
35、nd CB-TC-A2) generates the same cell flow which is sent synchronously on the two links. In the receiver, the protection sublayer insures synchronization at the Celt level, based on F3 OAM cells, to compensate the length difference between the links. Once the synchronization at the cell level is achi
36、eved, the protection sublayer monitors the received blocks of cells and switches between the two links when the blocks received on one link are errored. This process insures that the protection is performed without disruption. B-NT2 8-NT 1 I I I 1 I I I I ATM Layer ATM Layer T1316260-99 Link 2 Figur
37、e 3h.480 - Positioning of the protection switching mechanism from a layer view 7 Transmitter operation 7.1 Transmitter block diagram The transmitter block diagram is shown in Figure 4. All the functional modules indicated in Figure 4 are part of the Physical Layer. Splitting of the protection functi
38、ons within the Physical Layer is shown for illustration only and does not imply any particular implementation. The T-SYNC and SELECTION modules are part of the protection sublayer whereas CB-TC-Ax and PMD-Ax modules perfonn the classic cell-based transmission functions at the TC and PMD sublayers, a
39、s defined in ITU-T Recommendation 1.432.2 i. ATM-Cell signal represents the ATM cells being sent by the ATM Layer to the Physical Layer. The Sync signal corresponds to the synchronization signal of the Physical Layer for the egress direction. The REB-G signal represents the number of errored blocks
40、between two consecutive F3 OAM cells for the global link (this signal is provided by the SELECTION module in the ingress direction). Signals Flow1 and Flow2 indicate the binary flows sent by each TC to the underlying PMD sublayer. Transmit Transmit Transmit Transmit Link1 Link2 Egress r T1316270-99
41、Figure 4D.480 - Transmitter block diagram 7.2 The T-SYNC module is responsible for synchronization of the two TC sublayers in the egress direction. The T-SYNC module receives ATM cells fi-om the ATM layer and sends them synchronously to the two TC sublayers (ATM-Cell signal). To insure synchronizati
42、on between the links, the T-SYNC module provides a common clock (Sync) to the two TC sublayers (CB-TC-A1 and CB-TC-A2). This synchronization signal can be provided by the ATM layer or by a local oscillator within the Physical Layer. The tolerance on this synchronization signal shall be as defined in
43、 ITU-T Recommendation 1.432.2 i, that is *20 ppm (for both the 155 and the 622 Mbit/s interface). The synchronization provided by T-SYNC shall also apply at the F3 level which means that CB-TC- Al and CB-TC-A2 shall generate F3 OAM cells with the same PSN value at the same time. Synchronization at b
44、oth the cell level, F3 OAM level and physical level (common transmission clock) insures that the cells transmitted on each link between two consecutive F3 OAM cells having the same PSN number are identical. This condition is necessary for the block selection process in the The T-SYNC module also pro
45、vides the REB-G information to the two TC sublayers for performance reporting at the far end. This information field is generated by the SELECTION module in the ingress direction. Synchronization process ia the transmitter . ingress direction. 7.3 The TC operation shall be as defined in ITU-T Recomm
46、endation 1.432.2 i. CB-TC-A1 and CB-TC-A2 are independent fiom each other although they are synchronized by the same clock (Sync signal) and they receive the same ATM cells f-om the T-SYNC module. They are also synchronized to insure that F3 OAM cells having the same PSN value are generated at the s
47、ame time. CB-TC-A1 and CB-TC-A2 perform the classic cell-based functions in the egress direction (insertion of idle cells, cell scrambling, HEC computation). They also generate F1 and F3 OAM flows for supervision and performance monitoring of the section and transmission path. If there is no regener
48、ator section, the F1 OAM flow shall not be implemented. The content of F1 OAM cells shall be as defined in ITU-T Recommendation 1.432.2 i. The content 3 OAM cells shall be as defined in ITU-T Recommendation 1.432.2 i except that byte 45 of F3 OAM payload shall be no longer reserved and shall be allo
49、cated to the REB-G information field. The REB-G field contains the number of errored blocks detected in one direction of ission for the global link, that is the aggregation of the two protected links. This field shall be the same way as the individual REB fields (byte 46 of F3 OAM payload) which are dedicated to each transmission link. For each F3 OAM cell being sent synchronously on the two links, the fields PSN, EDC-B1 to B-G are identical. The other fields (AIS, RDI and REB) are different because they The PMD sublayer (PMD-A1 and PMD-A2) shall be as described i
