1、 International Telecommunication Union ITU-T J.211TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (11/2006) SERIES J: CABLE NETWORKS AND TRANSMISSION OF TELEVISION, SOUND PROGRAMME AND OTHER MULTIMEDIA SIGNALS Interactive systems for digital television distribution Timing interface for cable modem t
2、ermination systems ITU-T Recommendation J.211 ITU-T Rec. J.211 (11/2006) i ITU-T Recommendation J.211 Timing interface for cable modem termination systems Summary The timing interface for cable modem termination systems (DTI) defined in ITU-T Recommendation J.211, supports the accurate and robust tr
3、ansport of the DTI server 10.24-MHz master clock, 32-bit cable modem termination timestamp, and time of day, to the DTI client within the modular cable modem termination system (M-CMTS) cable network. The DTI protocol is structured to minimize the complexity and cost of the DTI client clocks, and th
4、e per-port cost of the shared server function while supporting all S-CDMA and TDMA timing requirements. Source ITU-T Recommendation J.211 was approved on 29 November 2006 by ITU-T Study Group 9 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. J.211 (11/2006) FOREWORD The Inter
5、national Telecommunication Union (ITU) is the United Nations specialized agency in the field 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 Recommenda
6、tions on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), 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 appro
7、val of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administrati
8、on“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance
9、with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is requ
10、ired of any party. 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 Intellec
11、tual 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 Recommendation. Ho
12、wever, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2007 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the
13、prior written permission of ITU. ITU-T Rec. J.211 (11/2006) iii CONTENTS Page 1 Scope 1 1.1 System requirements 1 1.2 TDM services consideration. 2 1.3 Modular implementation requirements 2 1.4 Architecture 2 1.5 Synchronization needed for TDM services deployment 4 2 References. 4 2.1 Reference acqu
14、isition . 4 3 Terms and definitions . 5 4 Abbreviations and acronyms 6 5 Conventions 7 6 Physical layer requirements 7 6.1 Introduction 7 6.2 Physical connector description. 8 6.3 Cable requirements. 8 6.4 Electrical description 8 7 DOCSIS timing protocol 9 7.1 DTI timing entities . 9 7.2 DTI timing
15、 structure. 10 7.3 Traceability of DOCSIS timestamp 11 7.4 DTI frame structure requirements 13 7.5 DTI server-client protocol interaction 22 8 DTI client and server operation 24 8.1 DTI server modes . 24 8.2 DTI client operation . 30 8.3 DTI distribution fallback strategies 37 Annex A Ranging wander
16、 qualification filter. 40 A.1 Chip timing jitter for synchronous operation . 40 Appendix I DTI server functional description 43 I.1 Server DTI signal processing . 44 Appendix II DTI client functional description 45 II.1 DTI client block diagram 45 II.2 DTI client PHY. 45 II.3 DTI client frame proces
17、sor. 46 II.4 DTI client clock processor 47 iv ITU-T Rec. J.211 (11/2006) Page Appendix III DTI jitter budget 51 III.1 Model description. 51 III.2 Analysis 52 Appendix IV Symbol clock synchronization 54 Appendix V DTI high-speed clock considerations. 56 ITU-T Rec. J.211 (11/2006) 1 ITU-T Recommendati
18、on J.211 Timing interface for cable modem termination systems 1 Scope The requirements for timing and synchronization of the DOCSIS system come from the following areas. existing DOCSIS specification and testability requirements; remote PHY system requirements; implementation requirements; services
19、like T1 or E1 and wireless. These requirements place definitions and constraints on the use of the DOCSIS master clock and the DOCSIS timestamp, which is delivered in the SYNC message. The DOCSIS specification originally envisioned the M-CMTS-CORE, EQAMs, and upstream receive functions on one assemb
20、ly, fed with a common clock. The timestamp counter resides in the M-CMTS-CORE function. The M-CMTS Remote PHY architecture may result in three components: the M-CMTS-CORE, the upstream receiver, and the EQAM being located in a different chassis, and potentially at different physical locations. As a
21、system, the three components comply with the DOCSIS specification and any existing CMTS equipment. The DOCSIS timing protocol (DTI) defined in this Recommendation, supports the accurate and robust transport of the DTI server 10.24-MHz master clock, 32-bit DOCSIS timestamp, and time of day, to the DT
22、I client within the DOCSIS M-CMTS cable network. The DTI protocol is structured to minimize the complexity and cost of the DTI client clocks, and the per-port cost of the shared server function while supporting all S-CDMA and TDMA timing requirements. To support regional differences in master clock
23、frequency, two DTI client options are defined, one producing a 10.24-MHz master clock output, and another producing a 9.216-MHz master clock output. A single DTI server master clock frequency of 10.24 MHz supports DTI clients of both types. 1.1 System requirements The DTI system requirements refer t
24、o the DOCSIS timing requirements as outlined in the DOCSIS specification. These requirements are presented independent of the CMTS architecture. The clauses of the DOCSIS specification ITU-T J.122 that are of interest are: 6.2.11.2 Mini-slot numbering 6.2.21.8.2 Chip timing jitter for synchronous op
25、eration 6.3.7 CMTS timestamp jitter 6.3.8 CMTS clock generation 6.3.9 CMTS downstream symbol clock jitter for synchronous operation 6.3.10 CMTS downstream symbol clock drift for synchronous operation 9.3 Timing and synchronization 2 ITU-T Rec. J.211 (11/2006) 1.2 TDM services consideration To mainta
26、in compatibility with the TDM service synchronization hierarchy, the DTI server clock operates with the specifications detailed in clause 8.1 which integrate both the DOCSIS timing system requirements and the existing legacy synchronization network clock consistent with ITU-T G.812 and T1.101. This
27、is done to ensure that the CM supporting TDM services can derive its clocking and meet ITU-T G.823 or ITU-T G.824 jitter and wander requirements for both traffic-bearing and synchronization-bearing transport clock sources. Support of TDM services will require that the master clock and the downstream
28、 symbol clock be locked and upstream and downstream clocks be coherent. 1.3 Modular implementation requirements The M-CMTS-CORE element: uses the DTI server master clock for creating a timestamp; uses the timestamp for MAP generation. The Edge QAM element: uses the DTI server master clock for symbol
29、 rate generation; uses the timestamp for inserting and/or correcting SYNC messages. The Upstream receive element: uses the timestamp and/or S-CDMA frame and the MAP for determining when to look for the start of a receive burst; uses a clock locked to the master clock for reception of symbols in S-CD
30、MA mode. 1.4 Architecture Figure 1-1 shows frequency and timing distribution examples for both headend and hub. The DTI server establishes the reference for the timing distribution network and synchronizes all connected DTI clients via point-to-point connections between the server and each client. A
31、 single protocol initiated by the DTI server permits the client to perform frequency and time synchronization. As shown, upstream receive, Edge QAMs, and the M-CMTS-CORE may have different uses for the synchronized frequency and time, but utilize a common client function. The DTI protocol and server
32、-client interactions are described in detail in clauses 7 and 8. The essential characteristics are: The DTI server initiates the protocol, which the DTI client uses to establish its time and frequency synchronization. Using a ping-pong scheme, the client always immediately replies to the DTI server
33、when it receives a transmission from the DTI server. The server uses this response to auto-compensate any delays with the effect that the client becomes precisely synchronized to the server. The server-to-client-to-server handshake continually repeats, assuring that a tight synchronization can be ma
34、intained. ITU-T Rec. J.211 (11/2006) 3 J.211(06)_F1-1Gate RXburstsGate RXburstsMAPSDemodulatorDemodulatorModulatorModulatorDTI clientDTI clientDTI clientDTI clientDTI clientTx users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of
35、the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T G.812 ITU-T Recommendation G.81
36、2 (2004), Timing requirements of slave clocks suitable for use as node clocks in synchronization networks. ITU-T G.823 ITU-T Recommendation G.823 (2000), The control of jitter and wander within digital networks which are based on the 2048 kbit/s hierarchy. ITU-T G.824 ITU-T Recommendation G.824 (200
37、0), The control of jitter and wander within digital networks which are based on the 1544 kbit/s hierarchy. ITU-T J.122 ITU-T Recommendation J.122 (2002), Second-generation transmission systems for interactive cable television services IP cable modems. ISO/IEC 8802-3 ISO/IEC 8802-3:2000, Information
38、technology Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications. T1.101*)ANSI T1.101 (1999), Synchronization Inte
39、rface Standard. _ *)T1 standards are maintained since November 2003 by ATIS. 2.1 Reference acquisition American Institute of Electrical Engineers, Internet: http:/www.ieee.org/portal/site American National Standards Institute, Internet: http:/webstore.ansi.org ITU-T Rec. J.211 (11/2006) 5 3 Terms an
40、d definitions This Recommendation defines the following terms: 3.1 bridging mode: A short-term operating condition of the DTI clock where the DTI client has recently lost its controlling input and is using stored data, acquired while in normal or fast mode operation, to control its output. While in
41、bridging, the degree of deviation of the output is deemed to be such that DTI client clock is still performing within normal or acceptable limits. If an outage period persists, the DTI client clock will transition to the holdover mode indicating that the DTI client clock output may be degraded. 3.2
42、DTI minimum clock oscillator: An oscillator that supports all the client clock performance requirements with holdover limited to the minimum bridging time. A non-ovenized oscillator can be used to support this oscillator category. 3.3 fast mode: An operating condition of a clock in which it is locke
43、d to an external reference and is using time constants, which are reduced to quickly bring the local oscillators frequency into approximate agreement with the synchronization reference frequency. 3.4 free-run mode: An operating condition of a DTI clock whose output signals are internally controlled
44、by the DTI server. The clock has never had, or has lost, external reference input and has no access to stored data that was acquired from a previously connected external reference during the time after the last power cycle. Free-run ends when the clock output is influenced by an external reference o
45、r the process to achieve lock to an external reference. Free-run may provide needed stability when external reference has been lost or not equipped. 3.5 gpssec: The gpssec is a 32-bit timestamp counter that is incremented every second. GPS system time began on January 6, 1980. The gpssec value was s
46、et to zero at the January 6, 1980 start epoch. 3.6 holdover mode: An operating condition of a DTI clock that has lost its controlling input and is using stored data, acquired while in normal or fast mode operation, to control its output. The stored data is filtered to minimize the effects of short-t
47、erm variations and to establish a predictor of oscillator behaviour during the reference outage. This permits the output deviation from normal operation to be minimized. 3.7 maximum time interval error (MTIE): For a sequence of time delay samples xi, MTIE at observation time (S) is: MTIE measurement
48、: =+=+=+=1111)min()max(max)(jnjiijnjiinNjxxSMTIE where: o = sample period N = number of samples in the sequence n = S/o + 1 S = observation time xi= time delay sample 3.8 normal mode: An operating condition of a clock in which the output signals are controlled by an external input reference. The exp
49、ected mode and state permits each clock within a distribution to have the same long-term average frequency and time. Clocks in this mode are referred to as locked meaning that they are in tight relationship with the DTI root clock. A DTI server clock in a fault-free free-run mode will be considered in normal mode. 3.9 root DTI server: The DTI server that is the source of traceable time and frequency for all subtending DTI servers and clients in a building. 6 IT
