摘要
ITU-T制定的H.323协议已被大多数公司采用，它成为了IP电话实现和互通的技术依据。但是，IP网络只支持尽力而为的承载业务，难以保证IP电话的服务质量。要保证IP电话的QoS，就应将IntServ／RSVP、DiffServ等机制引入到H.323协议中。
IntServ／RSVP利用RSVP信令协议在端到端之间预留带宽和缓冲区等资源，从而确保所需的服务质量；而作为目前最有发展前途的IP网络QoS支持技术的DiffServ，将具有相同特性的数据流聚合，以向用户提供若干种类的服务。
从理论上来说，IntServ／RSVP确实能保证IP网络提供时延低、抖动小、质量高的实时性能服务，但是，IntServ／RSVP扩展性差等固有的缺点，使其无法应用于大规模网络；而DiffServ对网络上的数据流进行分类，划分优先级，其状态信息量与服务类型数量成正比，而与流的数量无关，伸缩性较好，便于实现，弥补了IntServ／RSVP的不足。
但是DiffServ对数据流优先级的简化会降低其灵活性和能力。改进的方法之一是和IntServ／RSVP方法相结合。如果DiffServ使用RSVP参数或根据专门应用类型对恒定比特率(CBR)数据流进行鉴别和分类，就可能建立完全定义的综合流，它能够被引导成为固定带宽的管道。这样做可以更有效地享用资源并且仍然能够提供有保证的服务。
在IP电话系统的研制过程中(2001.4——现在)，我主要做了以下工作：
1．仔细阅读了H.323协议栈；阅读了大量有关服务质量的文献和RFC文档，
基于H.323协议的IP电话系统的QoS的研究
摘要
并对IP电话的Qos进行了深入的研究;
2.作为项目组主要成员从头至尾参与了一种新型的IP电话网关开发和实
现，该成果顺利通过了信产部专家组的鉴定和验收;
3.深入分析了Intsery/RSVP和Diffsery技术，并将二者有机地结合起来，
用于基于H.323协议的IP电话系统的QoS服务质量控制。
4.深入研究了抖动抑制缓冲技术，使得在IP电话系统中非匀速到达的语音
数据包得以匀速播放。
H.323 protocol, proposed by the ITU-T group, was accepted by most of the tele-communication companies worldwide. However, because IP network provides data transmitting service only in a "best-effort" manner., it does not guarantee the quality of services. We can introduce IntServ/RSVP and DiffServ mechanism into H.323 protocol to guarantee the QoS of IP telephony.
IntServ/RSVP reserves the end-to-end bandwidth and buffers with RSVP signaling protocol to guarantee QoS. As the most promising technology at present. DiffServ aggregates the data streams of the same type to supply all kinds of services for clients.
Theoritically, IntServ/RSVP can guarantee real-time services of low latency, small jitter and high quality supplied by IP network. Because of its intrinsic defect of awful expansibility, IntServ/RSVP cannot apply to the network of large scale. However, DiffServ classifies and divides the data streams in the networks, the quantities of status information are proportional to the types of service and independent of the quantities of stream. It is scalable and easy to implement, thus overcomes the defects of IntServ/RSVP.
But DiffServ simplifies data streams with priority, this can limits its agility and capability. We can improve this method by combining IntServ/RSVP with DiffServ. It is possible to setup integrated streams defined perfectly and can be led to pipe with constant bandwidth, if DiffServ authenticates and classifies data streams of constant bit rate using the RSVP parameters or according to the types of special application.
Hence we can share resources more effectively and provide guaranteed services.
From April 2001 to now, as a member of a development team of IP telephony system, I have accomplished the following works:
1. Read carefully the H.323 protocol stack and RFC documents about QoS of IP telephony system.
2. Implemented IP gateway. It was passed the test and validation of expert group under the Ministry of Information Industry of China.
3. Analysis and compare IntServ/RSVP and DiffServ, then combine them into IP telephony system based on H.323 protocol.
4. Research the technologies of jitter buffer to make the irregular data packets of voice played evenly.
引文
[1] ITU-T. Recommendation H.323 Version 4, Packet-based multimedia communications systems. [EB/OL]. November 2000.
[2] ITU-T. Recommendation H.225.0 Version 4[EB/OL], Call Signalling Protocols and Media Stream Packetization for Packet-Based Multimedia Communication Systems. November 2000.
[3] ITU-T. Recommendation H.245 Version 8[EB/OL], Control Protocol for Multimedia Communication. July 2001.
[4] ITU-T Recommendation H.235 Version 2[EB/OL], Security and Encryption for H Series (H.323 and other H.245 based) multimedia terminals. November 2000.
[5] R.Branden and others. Resource Reservation Protocol (RSVP): Version 1 Functional Specification [EB/OL]. RFC2205, September 1997.
[6] A.Mankin and others. Resource Reservation Protocol (RSVP) Version 1 Applicability Statement, Some Guidelines on Deployment [EB/OL]. RFC2208, September 1997.
[7] RSVP home page [EB/OL]. http://www. isi. edu/rsvp. July 2002.
[9] S. Casner V. Jacobson. Compressing IP/UDP/RTP Headers for Low-Speed Serial Links [EB/OL]. RFC 2508, February 1999
[10] P. Almquist, Consultant. Type of Service in the Internet Protocol Suite[EB/OL]. RFC1349, July 1992.
[11] J. Wroclawski. The Use of RSVP with IETF Integrated Services [EB/OL].RFC 2210, September 1997.
[12] J. Wroclawski. Specification of the Controlled-Load Network Element Service [EB/OL].RFC 2211, September 1997.
[13] J. S. Shenker, C. Partridge, R. Guerin. Specification of Guaranteed Quality of Service [EB/OL]. RFC 2212, September 1997.
[14] K. Nichols and others. Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers [EB/OL]. RFC 2474, December 1998.
[15] J. Heinanen and others. Assured Forwarding PHB Group [EB/OL]. RFC 2597, June 1999.
[16] V. Jacobson and others. An Expedited Forwarding PHB [EB/OL]. RFC 2598, June 1999.
[17] W. Stevens. TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms [EB/OL]. RFC 2001, January 1997.
[18] E. Crawley. A Framework for QoS-based Routing in the Internet [EB/OL]. RFC 2386, August 1998.
[19] H. Schulzrirme and others. RTP: A Transport Protocol for Real-Time Applications [EB/OL]. RFC 1889, August 1998.
[20] H. Schulzrinne. RTP Profile for Audio and Video Conferences with Minimal Control [EB/OL]. RFC 1890, January 1996.
[21] Y. Bernet, P. Ford and others A Framework for Integrated Services Operation over Diffserv Networks [EB/OL]. RFC2998. November 2000.
[22] M. Carlson and others. An Architecture for Differentiated Services [EB/OL]. RFC 2475 December 1998
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