1、 ATIS-0900001 SYNCHRONIZATION OF PACKET NETWORKS TECHNICAL REPORT The Alliance for Telecommunication Industry Solutions (ATIS) is a technical planning and standards development organization that is committed to rapidly developing and promoting technical and operations standards for the communication
2、s and related information technologies industry worldwide using a pragmatic, flexible and open approach. Over 1,100 participants from over 300 communications companies are active in ATIS 22 industry committees and its Incubator Solutions Program. Notice of Disclaimer Timing and synchronization aspec
3、ts of Asynchronous Transfer Mode (ATM) networks.117 ATM Forum, Circuit Emulation Service Interoperability Specification AF-SAA-0032.000.418 ITU-T Recommendation I.363.1, B-ISDN ATM Adaptation Layer Specification: Type I AAL.219 ITU-T Recommendation G.701, Vocabulary of digital transmission and multi
4、plexing, and Pulse Code Modulation (PCM) terms.220 ITU-T Recommendation G.810, Definitions and terminology for synchronization networks.22.3 Additional References 21 ITU-T Recommendation G.711, Pulse Code Modulation (PCM) of voice frequencies.222 ITU-T Recommendation G.726, 40, 32, 24, 16 kbit/s Ada
5、ptive Differential Pulse Code Modulation (ADPCM).223 ITU-T Recommendation G.729, Coding of speech at 8 kbit/s using conjugate-structure algebraic-code-excited linear-prediction (CS-ACELP), in ITU-T Recommendations Series G, Transmission systems and media, digital systems and networks.2 24 ITU-T Reco
6、mmendation G.723.1, Dual rate speech coder for multimedia communications transmitting at 5.3 and 6.3 kbit/s, in ITU-T Recommendations Series G, Transmission systems and media, digital systems and networks.225 ITU-T Recommendation G.822, Controlled slip rate objectives on an international digital con
7、nection, in ITU-T Recommendations Series G, Transmission systems and media, digital systems and networks.226 ITU-T Recommendation G.983.1, Broadband Optical Access Systems Based on PONs (PON), 10/1998 APON, or ATM-based PON.23RFC text is available at . 4This document is available at: . 2 ATIS-090000
8、1 27 ITU-T Recommendation G.983.2, ONT Management and Control Interface Specification for ATM PON.228 ITU-T Recommendation G.983.3, A Broadband Optical Access System with Increased Service Capability by Wavelength Allocation, 03/2001 adds video wavelength - BPON.229 ITU-T Recommendation G.983.4, A B
9、roadband Optical Access System with Increased Service Capability Using Dynamic Bandwidth Assignment, 11/2001 BPON230 ITU-T Recommendation G.983.5, A Broadband Optical Access System with Enhanced Survivability, 01/2002 BPON231 ITU-T Recommendation G.983.6, ONT Management and Control Interface Specifi
10、cations for B-PON System with Protection Features.232 ITU-T Recommendation G.983.8, B-PON OMCI Support for IP, ISDN, Video, VLAN Tagging, VC Cross-Connections and Other Select Functions.233 ITU-T Recommendation G.983.9, B-PON ONT Management and Control Interface (OMCI) Support for Wireless Local Are
11、a Network Interfaces.234 ITU-T Recommendation G.983.10, B-PON ONT Management and Control Interface (OMCI) for Digital Subscriber Line Interfaces.235 ITU-T Recommendation G.984.1, General Characteristics for Gigabit-Capable PONs (GPON).236 ITU-T Recommendation G.984.2, Gigabit-Capable PONs (GPON): Ph
12、ysical Media Dependent (PMD) Layer Specifications.237 ITU-T Recommendation G.984.3, Gigabit-Capable PONs (GPON): Transmission Convergence Layer Specifications.238 ITU-T Recommendation G.984.4, Gigabit-Capable PONs (GPON): ONT Management and Control Interface Specification.2 39 T1.403.01-1999 (R2005)
13、, Network and Customer Installation Interfaces - (ISDN) Primary Rate Layer 1 Electrical Interfaces Specification.140 ITU-T Recommendation G.704, Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44 736 kbit/s hierarchical levels.241 ITU-T Recommendation G.1010, End-user multimedia QoS
14、categories.242 T1.107-2002 (R2006), Digital Hierarchy Formats Specification.143 ITU-T Recommendation G.743, Second order multiplex equipment operating at 6312 kbit/s and using positive justifications.2 44 ITU-T Recommendation G.709, Synchronous multiplexing structure.2 45 ITU-T Recommendation G.1020
15、, Performance parameter definitions for quality of speech and other voice-band applications utilizing IP networks.2 46 IETF RFC 2330, Framework for IP Performance Metrics.347 IETF RFC 2679, A One-way Delay Metric for IPPM.3 3 ATIS-0900001 48 CM-SP-DTI-I04-061222, DOCSIS Timing Interface Specificatio
16、n, Data-Over-Cable Service Interface Specifications Modular-CMTS, CableLabs, December, 2006.549 ITU-T Recommendation Y.1540, IP packet transfer and availability performance parameters.250 IEEE Std 1588 2002, IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and C
17、ontrol Systems.651 IETF RFC 1305, Network Time Protocol (Version 3): Specification, Implementation, and Analysis.352 ITU-T Recommendation J.211, DOCSIS Timing Interface Specification.253 T1.102-1993 (R2005), Digital Hierarchy - Electrical Interfaces.154 T1.404-2002 (R2006), Network and Customer Inst
18、allation Interfaces - DS3 and Metallic Interface Specification.1 55 T1.105.09-1996 (R2008), Synchronous Optical Network (SONET) Network Timing and Synchronization.1 56 GR-2830, Primary Reference Sources: Generic Requirements.757 ITU-T Recommendation G.812, Timing requirements of slave clocks suitabl
19、e for use as node clocks in synchronization networks, in ITU-T Recommendations Series G, Transmission systems and media, digital systems and networks.258 ITU-T Recommendation G.824, The control of jitter and wander within digital networks which are based on the 1544 kbit/s hierarchy.23 DEFINITIONS I
20、n this section definitions of some terms are provided. These definitions are meant to clarify the text in this TR; they are not meant to be normative in connection with other standards. 3.1 UTC (Coordinated Universal Time): The time scale, maintained by the Bureau International des Poids et Mesures
21、(BIPM) and the International Earth Rotation Service (IERS), which has formed the basis of a coordinated dissemination of standard frequencies and time signal. NOTE Time scale is a succession of equal time intervals, with accurate references of the limits of these time intervals, which follow each ot
22、her without any interruption since a well-defined origin. A time scale allows one to date any event. For example, calendars are time scales. A frequency signal is not a time scale (every period is not marked and dated). The reference frequency for network synchronization is the frequency which gener
23、ates the UTC time scale. It is therefore preferable to use the words “UTC frequency“ instead of “UTC“. 5This document is available at . 6This document is available from the Institute of Electrical and Electronics Engineers (IEEE). 7Telcordia documents are available from Industry Direct Sales, Telcor
24、dia, 8 Corporate Place, PYA 3A-184, Piscataway, NJ, 08854-4156, or: . 4 ATIS-0900001 3.2 Primary Reference Source (PRS); Primary Reference Clock (PRC): A device that provides a reference frequency signal to other network elements in a telecommunications network the output of a PRS/PRC is not depende
25、nt on any other network element. The term PRS is used in North America; PRC is the term used in ITU-T Recommendations. The key characteristic of a PRC/PRS is that its output signal is “on-frequency”; the normalized frequency offset (fractional frequency offset), relative to UTC-frequency is less tha
26、n 1x1011. Consequently, the relative frequency offset between two PRSs is less than 2x1011. For all practical purposes, this is considered “small enough” even for the most demanding of telecom applications. A PRS/PRC can be based on a cesium primary atomic reference standard. Other implementations i
27、nclude the use of GPS receivers to provide a frequency and/or time reference. 3.3 BITS and SASE: The term BITS, for Building Integrated Timing Supply, is commonly used in North America; SASE, for Stand-Alone Synchronization Equipment is popular in Europe and other parts of the world. Other terms inc
28、lude: Synchronization Supply Unit (SSU), Timing Signal Generator (TSG), and Node Clock. The underlying concept is that all network elements in a Central Office accept a reference timing signal from the BITS. The BITS accepts one or more reference timing inputs and includes a filtering function and h
29、oldover capability. It is not uncommon for one (or more) of the input reference signals to come from a PRS. 3.4 Timing Signal and Timing Information: A timing signal is a nominally periodic signal, generated by a clock, used to control the timing of operations in digital equipment and networks. Due
30、to unavoidable disturbances, such as oscillator phase fluctuations, actual timing signals are pseudo-periodic ones - i.e., time intervals between successive equal phase instants show slight variations. Mathematically a (sinusoidal) timing signal s(t) is represented by: st A t() sin ()= where A is a
31、constant amplitude coefficient, and (t) is the total instantaneous phase function. The notion of a “square wave” timing signal is simply that of a hard-limited version of s(t). Mathematically, )(sgn)( tsAtsSQSQ= where ASQis the nominal amplitude. Other variations, for example, where the two levels a
32、re (ASQ,0) rather than (+ASQ, ASQ), can be constructed. Timing information refers to the time and frequency domain properties of the timing signal used for its generation or related to the clock in the generating equipment. In principle, this information can be used by the receiving equipment to cha
33、racterize and/or replicate the clock in the generating equipment. The ability to replicate and/or characterize may be impaired by transmission phenomena, such as delay, additive jitter/wander, and errors. For example, a DS1 signal is bipolar in nature with “space” for “0” and alternating polarity “p
34、ulse” for “1”. From this signal, and in particular from the time instants related to waveform edges (rising and/or falling), it is possible to do a clock recovery function to generate a copy of the fundamental clock used to generate the output DS1 signal. In another scenario, the transmitted signal
35、contains information, such as a time-stamp, related to the time of occurrence of an identifiable feature in the transmitted signal such as the launch time of a particular bit. Timing information, in this scenario is the combination of time-stamp and notion of identifiable feature. 5 ATIS-0900001 3.5
36、 Isochronous: The essential characteristic of a time-scale or a signal such that the time intervals between consecutive significant instants either have the same duration or durations that are integral multiples of the shortest duration. Constant bit rate digital signals in telecommunications - e.g.
37、, DS1, DS3, SONET - are isochronous. Ethernet signals (100M and higher) are also isochronous. In practice, variations in the time intervals are constrained within specified limits. 3.6 Synchronous: The essential characteristic of time-scales or signals such that their corresponding significant insta
38、nts occur at precisely the same average rate. It is common to consider signals as synchronous if their average rate is directly or indirectly traceable to the same reference source. 3.7 Synchronization: The process of adjusting the corresponding significant instants of signals to make them synchrono
39、us. When two signals are adjusted so that they have same underlying frequency, the process is called syntonization; synchronization implies adjusting even the phase (as well as frequency). It has become common in telecommunications to use the term synchronization and, whether it is time or frequency
40、 adjustment, it is usually clear from the context. 3.8 Network Synchronization: A generic concept that relates to the methodology of distributing a common time and/or frequency to all elements in a network. In the telecommunications context, this “common” frequency is UTC traceable frequency. Theref
41、ore the term “network clock” implies a (long-term) frequency offset from UTC frequency of less than 1x1011. 3.9 Synchronous Circuit Timing: A mode of Circuit Emulation Service (CES) timing where the service clock for the transported service may not be preserved; rather, the CES service clock is deri
42、ved from a PRS-traceable clock. This mode requires a common clock frequency (nominally PRS-traceable) at the ingress and egress Interworking Functions (IWF). If the underlying service clock at the ingress IWF is appreciably different from the rate, that data is removed from the buffer in the egress
43、IWF and buffer overflow/underflow events will occur. Note that this use of the term “synchronous” differs from the use of the term in “synchronous clock recovery” method. This method of timing is referred to as “network synchronous operation” in ITU-T G.8261 2. 3.10 Synchronous Clock Recovery Method
44、: A type of clock recovery method that requires synchronized network clocks at the ingress and egress IWFs. More precisely, the timing reference signals at the two IWFs must both be traceable to a Primary Reference Source (PRS). However, in general, it is not required that the timing signals be trac
45、eable to the same PRS. As an example, SRTS (ATM AAL1) is a synchronous clock recovery method that provides asynchronous circuit timing. SRTS, and similar methods - a.k.a., differential methods - preserve the service clock (in terms of frequency offset relative to a common reference). 3.11 Asynchrono
46、us Mode: A mode where clocks are intended to operate in free running mode. Enterprise packet networks - e.g., LANs - typically operate in this mode, with each network element using a free-running oscillator. 3.12 Asynchronous Circuit Timing: A mode of CES timing where the source timing for the trans
47、ported service is preserved. That is, the ingress service clock is reproduced (in terms of 6 ATIS-0900001 frequency offset) at the egress; however, the jitter and wander characteristics may not be preserved. Typical algorithms are based on differential methods - e.g., SRTS - or adaptive methods. Not
48、e that this use of the term “asynchronous” differs from the use of the term in “asynchronous clock recovery” method. 3.13 Asynchronous Clock Recovery Method: A type of clock recovery method that is independent of the network clocks. More precisely, this type of clock recovery does not depend on PRS-
49、traceable network timing signals at the IWFs. ACR algorithms are an example of asynchronous clock recovery method(s). 3.14 Plesiochronous: A special case of asynchronous in which the essential characteristic of time-scales or signals is such that their corresponding significant instants occur at nominally the same rate, any variation in rate being constrained within specified limits. Generally speaking, two signals (such as DS1s) having the same nominal d
