1、 TSB-160 March 2007R/11-2012Telecommunications IP Telephony Infrastructures Synchronization in IP Networks- Methods and User Perception NOTICE TIA Engineering Standards and Publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchase
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22、TS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TSB-160 TABLE OF CONTENTS LIST OF FIGURES . III LIST OF TABLES . III 1 INTRODUCTION .1 2 SCOPE1 3 REFERENCES 2 4 DEFINITIONS, ABBREVIATIONS AND ACRONYMS .3 4.1 DEFINITIONS .3 4.2 ABBREVIATIONS AND ACRONYMS3 5 IP-BA
23、SED SERVICES AND NETWORK CONFIGURATIONS4 5.1 NETWORK MODELS .4 6 TIMESTAMP METHODS .6 6.1 RTP BASED MEDIA TRANSPORT 6 6.1.1 RTP Control Protocol (RTCP) 7 6.1.2 Functionality of RTP Timestamps.8 6.2 NETWORK TIME PROTOCOL OR NTP.9 6.2.1 Possible Sources of Errors in NTP 12 6.2.2 Achievable Performance
24、 with NTP .13 6.3 PRECISION TIME PROTOCOL OR IEEE 1588 14 6.3.1 Protocol Implementation of PTP .16 6.3.2 Limitations of PTP Protocol17 6.3.3 Possible Sources of Error in PTP 18 6.3.4 Achievable Performance with PTP19 6.4 CLOCK RATE AND STATE BASED SYNCHRONIZATION .19 6.4.1 Positive and negative feat
25、ures of SyncUTC 20 6.4.2 Achievable Performance in SyncUTC.21 7 PERCEPTION-BASED APPLICATION REQUIREMENTS21 7.1 AUDIO22 7.2 VIDEO 22 7.2.1 Animation 22 7.2.2 Audio and Video22 7.3 DATA AND AUDIO23 7.3.1 Pointers and Audio 23 7.3.2 Still pictures and audio 24 7.3.3 Text and Audio 24 7.4 DATA AND VIDE
26、O.24 7.4.1 Text and video .24 7.4.2 Still pictures and audio 24 8 ANALYSIS OF TIMESTAMP METHODS .25 8.1 CLOCK FREQUENCY STABILITY.25 8.2 TIMESTAMP READING ERRORS 25 i TSB-160 ii 9 MEDIA SYNCHRONIZATION IN SERVICES OFFERED OVER A COMBINATION OF TDM AND IP NETWORKS 27 ANNEX A (INFORMATIVE) - BIBLIOGRA
27、PHY.28 TSB-160 LIST OF FIGURES Figure 1 - IP-based Implementation of a Multimedia Service4 Figure 2 - Multimedia Service Offering over a Combination of TDM and IP Networks.5 Figure 3 - Media Play-out Mechanism involved in a Multimedia Service .5 Figure 4 - Typical Scenarios of Media Transport using
28、RTP .8 Figure 5 - NTP Hierarchical Master-Slave Configuration10 Figure 6 - Request and Response in NTP .12 Figure 7 - Master-Slave Hierarchy in IEEE 1588.16 Figure 8 - Offset and Delay Measurements using IEEE 1588 Protocol .17 Figure 9 - Clock in a Typical IP Network.18 Figure 10 - Example Scenario.
29、27 LIST OF TABLES Table 1 Real-Time Multimedia Services and Applications . 22 Table 2 Acceptable-based Timing Differences between Different Media. 24 iii TSB-160 iv FOREWORD (This foreword is not part of this TSB.) This document is a TIA Telecommunications Technical Services Bulletin (TSB) produced
30、by Subcommittee TR-41.4 of Committee TR-41. This TSB was developed in accordance with TIA procedural guidelines, and represents the consensus position of the Subcommittee, which also served as the formulating group. The TR-41.4 IP Telephony Infrastructures Subcommittee (Chair: Avaya, Joanne McMillen
31、, Vice Chair: Nortel, Roger Britt and Secretary: Avaya, Joachim Pomy) acknowledges the written contributions provided by the following individual in the development of this TSB. Organization Representative TSB-160 Nortel Radha Telikepalli Editor The European version of this joint work is the “Synchr
32、onization in IP Networks Methods and User Perception“ technical report. Within ETSI it has the number ETSI TR 103 010 v1.1.1. There is one annex in this TSB. Suggestions for improvement of this TSB are welcome. They should be sent to: Telecommunications Industry Association Engineering Department Su
33、ite 300 2500 Wilson Boulevard Arlington, VA 22201 ( http:/www.tiaonline.org ) TSB-160 1 1 Introduction Traditionally, TDM voice networks have had service performance requirements based on the end users quality of service expectations. In the TDM network, synchronization is a physical layer parameter
34、 that has to be designed to meet specific performance standards. Without proper clock synchronization, a service offered over the TDM network experiences errors, i.e., missing data that contributes to reduced service quality and availability. More and more real-time services are now being offered ov
35、er internet-based networks, where timestamp based synchronization is utilized for billing, maintenance, call control, one way delay measurements and intra/inter-media stream synchronization. Real-time applications offered over the internet include voice, video and some data that have been traditiona
36、lly carried over circuit switched networks. These services are offered by utilizing new equipment and new protocols exclusively designed for this purpose. The protocols permit integration of previously dissimilar voice and data services, creating new applications such as integrated voice mail and em
37、ail, white boarding that combines voice call with data transfer, desktop video calling etc., from the use of a single integrated network. An entity to perform data/signaling conversion is required when these services are supported across disparate networks. Internet services can also be offered by c
38、onnecting existing TDM islands using Internet Protocol (IP) network (TDMoIP or Circuit Emulation over IP) that enables backward compatibility. In TDMoIP, data and signaling from TDM islands will be encapsulated or de-encapsulated in the inter-working functions situated at the interfaces of TDM and I
39、P networks. Service quality requirements are expected to be the same as those for TDM service as the end user is not aware of the IP transport. The same argument can be extended to physical layer synchronization requirements. The IP network is an asynchronous network with no knowledge of the physica
40、l layer and it was solely used for data transport until recently. With the introduction of real-time IP services, the need arose to set specifications for QoS related parameters delay, delay variation and packet loss. Main sources of packet loss are bandwidth limitations at the edges, network conges
41、tion, clock related impairments and large delay variations that cause the jitter buffer to drop or add. ITU-T has published ITU-T Recommendation Y.1541 on performance objectives for Layer 3 based network parameters end-to-end packet delay, packet delay variation and packet loss based on different cl
42、asses of service. Efforts are ongoing to set additional service classes with reduced packet loss objectives. However, synchronization at the physical layer is a topic of discussion only for TDMoIP services in ITU-T Recommendation G.8261 and ANSI standards group. This TSB reviews the time stamping me
43、thods available in IP networks and assesses the performance of these in achieving satisfactory services as perceived by end user. 2 Scope The purpose of the document is to examine whether accuracies achievable by currently available synchronization methods in IP networks are adequate to ensure end u
44、sers quality of service expectation for a particular multimedia service. This TSB will discuss the issues related to inter-stream synchronization when all the concerned media are offered using: IP networks from end-to-end; A combination of TDM and IP networks. Services that will be covered, but not
45、limited to: Video conferencing: uses video and voice; White boarding: uses data and voice. TSB-160 2 When a multimedia service is offered in an IP-based network, the play-out mechanism involves de-multiplexing of different media in end node and subsequent play-out of the media based on the timing in
46、formation carried over by the media. Depending on this timing information, one media may precede the other resulting in user dissatisfaction. For each media, the timing relationship between different packets (intra-stream synchronization) is preserved by proper presentation at the end user in which
47、play-out buffer management plays a crucial part. Size of the play-out buffer can be a fixed value or can be adaptively set based on one way delay measurement using timestamps. This document gives an overview of synchronization methods by time stamping and the other means that are available to IP-bas
48、ed real-time applications and the effect of these methods on service performance as experienced by end user. However, actual implementation of time stamping is outside the scope of this TSB. The reference point at which inter-stream synchronization will be examined is considered to be at the interfa
49、ce of the end device involved, where the play-out buffers are normally located. 3 References The following standards contain provisions that are referenced in this TSB. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this document are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below, or their successors. ANSI and TIA maintain registers of currently valid national standards published by them. (1) ANSI T1 technical
