无线局域网的VoIP QoS研究
摘要
WLAN技术和VoIP技术的发展，使得利用无线局域网来进行廉价的VoIP(VoWLAN，Voice over WLAN)通话成为可能。但传统的IP网络采用尽力而为的服务方式，同时无线网络的带宽资源有限，很难有效满足语音等实时业务的服务质量要求。影响VoIP服务质量的主要因素有延时、抖动、丢包率以及带宽。
目前应用得最广的无线局域网技术是IEEE 802.11b，绝大部分的802.11b设备采用DCF方式竞争信道。DCF平等地对待语音和数据，不对实时业务提供接入优先级，这使得对延时敏感的语音业务的服务质量得不到保障。本文详细分析了DCF下的基本接入机制和RTS／CTS接入机制的工作原理，通过实验仿真得出VoIP不宜采用四次握手方式的RTS／CTS机制的结论。得出可从排队延时、帧间隔和随机退避时间三方面来控制VoWLAN的延时，并由此引出EDCA信道接入方式。分析了EDCA的QoS保障技术，并通过仿真模拟了EDCA对VoIP性能的改善。
鉴于MAC层的CRC机制要求分组中不能存在任何错误，否则会引起MAC重传，但实际的语音业务采用小包发送，能够容忍一定程度的语音比特位错误，提出了一种改进的MAC层CRC机制，让CRC不再保护语音数据，减少MAC重传，从而降低VoIP延时和抖动。仿真结果表明，该机制有效改善了延时和抖动。
VoWLAN的容量是一个和QoS紧密联系且非常重要的问题。论文从理论上分析了理想信道条件下802.11b的VoIP容量，仿真结果和理论分析值相符。
The development of Wireless LAN technology and VoIP technology makes it possible to transmit voice over Wireless LAN. But because of the IP network's best-effort service and the limited bandwidth of wireless network, QoS requirements of real-time applications such as voice can't be met. The main factors that affect the performance of VoIP are delay, jitter, packet loss rate as well as bandwidth.
Currently, the most widely used WLAN is IEEE 802.11b. Most 802.11b devices adopt DCF to contend for the channel. DCF treats voice and data equally, and doesn't provide any access priority to the real-time applications. This can't guarantee the QoS of delay-sensitive voice application. This paper analyzes the principles of basic access scheme and RTS/CTS scheme under DCF in detail, draws a conclusion that the four handshakes RTS/CTS scheme is not suitable for VoIP through simulation, and concludes that the delay of VoWLAN can be reduced by the control to the queueing delay, IFS and the random backoff time. On this basis, another method to contend for the channel-EDCA is presented. After the analysis of EDCA's QoS support, experiment is performed. The result shows that EDCA greatly improves VoIP's performance.
Considering that CRC of MAC requires error free packets, otherwise MAC retransmission will occur, but voice transmitted in the form of small packets can tolerate some bits error, so a modified CRC scheme is brought forward. The modified CRC doesn't protect voice data any more, so MAC retransmission is less. This results in reduced delay and jitter of VoIP. Simulation result shows that the modified CRC scheme can improve delay and jitter.
Capacity of VoWLAN is related with QoS of VoWLAN, and it is also a very important problem. The thesis analyzes VoIP capacity of 802.11b network theoretically in ideal channel condition, and simulation result matches the theory.
目前应用得最广的无线局域网技术是IEEE 802.11b，绝大部分的802.11b设备采用DCF方式竞争信道。DCF平等地对待语音和数据，不对实时业务提供接入优先级，这使得对延时敏感的语音业务的服务质量得不到保障。本文详细分析了DCF下的基本接入机制和RTS／CTS接入机制的工作原理，通过实验仿真得出VoIP不宜采用四次握手方式的RTS／CTS机制的结论。得出可从排队延时、帧间隔和随机退避时间三方面来控制VoWLAN的延时，并由此引出EDCA信道接入方式。分析了EDCA的QoS保障技术，并通过仿真模拟了EDCA对VoIP性能的改善。
鉴于MAC层的CRC机制要求分组中不能存在任何错误，否则会引起MAC重传，但实际的语音业务采用小包发送，能够容忍一定程度的语音比特位错误，提出了一种改进的MAC层CRC机制，让CRC不再保护语音数据，减少MAC重传，从而降低VoIP延时和抖动。仿真结果表明，该机制有效改善了延时和抖动。
VoWLAN的容量是一个和QoS紧密联系且非常重要的问题。论文从理论上分析了理想信道条件下802.11b的VoIP容量，仿真结果和理论分析值相符。
The development of Wireless LAN technology and VoIP technology makes it possible to transmit voice over Wireless LAN. But because of the IP network's best-effort service and the limited bandwidth of wireless network, QoS requirements of real-time applications such as voice can't be met. The main factors that affect the performance of VoIP are delay, jitter, packet loss rate as well as bandwidth.
Currently, the most widely used WLAN is IEEE 802.11b. Most 802.11b devices adopt DCF to contend for the channel. DCF treats voice and data equally, and doesn't provide any access priority to the real-time applications. This can't guarantee the QoS of delay-sensitive voice application. This paper analyzes the principles of basic access scheme and RTS/CTS scheme under DCF in detail, draws a conclusion that the four handshakes RTS/CTS scheme is not suitable for VoIP through simulation, and concludes that the delay of VoWLAN can be reduced by the control to the queueing delay, IFS and the random backoff time. On this basis, another method to contend for the channel-EDCA is presented. After the analysis of EDCA's QoS support, experiment is performed. The result shows that EDCA greatly improves VoIP's performance.
Considering that CRC of MAC requires error free packets, otherwise MAC retransmission will occur, but voice transmitted in the form of small packets can tolerate some bits error, so a modified CRC scheme is brought forward. The modified CRC doesn't protect voice data any more, so MAC retransmission is less. This results in reduced delay and jitter of VoIP. Simulation result shows that the modified CRC scheme can improve delay and jitter.
Capacity of VoWLAN is related with QoS of VoWLAN, and it is also a very important problem. The thesis analyzes VoIP capacity of 802.11b network theoretically in ideal channel condition, and simulation result matches the theory.
引文
[1] 吴春雷，周忠丽，邓文红．基于软交换体系下的VoIP分析与实现．中国民航飞行学院学报，2005，16(3)：24～28
[2] PTSN.IP电话的概念和基本原理．http://www.ptsn.net.cn/xueyuan/show_knowledge.php?auto_id=49
[3] 潘倩倩．基于WLAN的VoIE集成电路应用，2004年5月：68～73
[4] 张敏丽，王丽芳，谢昀．引发企业通讯革命的VoIP技术．中国科技信息2005，(9)：68～68
[5] ChinaITLab.com搜集整理．VolP的定义．http://www.chinaitlab.com/www/news/article_show.asp?id=14534
[6] 杨仁忠，侯紫峰，孙国滨．一种改善802．11无线局域网语音传输性能的方法．微电子学与计算机，2004，21(5)：85～89
[7] 谭钦红，邱洪云．无线局域网内的语音数据传输．信息技术与信息化，2005， (1)：54～57
[8] 陈福，刘冬梅．VoIP的QoS研究．中国数据通信，2001，(11)：21～25
[9] 杨仁忠，孙国滨，侯紫峰．基于IEEE 802．11无线局域网的VoIP技术综述．计算机工程，2005，31(4)：1～3
[10] Sandip Patodia, Xiao-Hong Peng. Implementation and Analysis of VoIP Services in WLANS. Fifth IEE International Conference on 3G Mobile Communication Technologies, 2004: 548～552
[11] 梅旭．VoIP中QoS控制技术的研究与实现：[工程硕士学位论文]．长沙：湖南大学，2005
[12] 黄永峰，李星．VoIP的语音质量分析与控制．控制与决策，2003，18(4)：475～478
[13] 吴一新，肖会会．VoIP中为提高语音质量所采用的关键技术．计算机仿真， 2004，22(4)：153～155
[14] International Telecommunications Union. Pulse Code Modulation (PCM) of Voice Frequencies. Recommendation G.711, ITU-T 1988
[15] International Telecommunications Union. Coding of Speech at 8 Kbit/s Us ing Conjugate-structure Algebraic-code-excited Linearprediction (CS-ACELP ). Recommendation G.729, ITU-T 1996
[16] International Telecommunications Union. Dual Rate Speech Coder for Mul timedia Communications Transmitting at 5.3 and 6.3 Kbit/s. Recommendat ion G.723.1, ITU-T 1996
[17] Brady P. A Model for Generating ON_OFF Speech Patterns in Two-way Conversations. Bell System Technology Journal, 1969, 48(9): 2445～2472
[18] 张瑛．WLAN中语音业务质量保证．电信科学，2004，(8)：44～47
[19] H. Sanneck, N. Le. Speech Property-Based FEC for Internet Telephony A pplications. In Proceedings of SPIE-The International Society for Optical Engineering, 2000, 3969(1): 38～51
[20] Henning Sanneck. Packet Loss Recovery and Control for Voice Transmissi on over the Internet: Ph.D. thesis. Berlin: GMD Fokus/Telecommunication Networks Group, Technical University of Berlin, 2000
[21] Henning Sanneck, Nguyen Tuong Long Le, Martin Haardt, et al. Selecti ve Packet Prioritization for Wireless Voice over IE Proc. of Fourth Intern ational Symposium on Wireless Personal Multimedia Communication, 2001
[22] N. Long Le. Development of a Loss-resilient Internet Speech Transmission Method: Diploma thesis. Berlin: Department of Electrical Engineering, Te chnical University Berlin, 1999
[23] Christian Hoene, Iacopo Carreras, Adam Wolisz. Voice over IP.. Improvin g the Quality over Wireless LAN by Adopting a Booster Mechanism-An Experimental Approach. In: Proc. of SPIE 2001-Voice over IP (VoIP) Tec hnology, 2001:157-168
[24] Antonio Grilo, Mario M. Macedo, Mario S.Nunes. IP QoS support in IEE E 802.11b WLANs. Computer Communications, 2003, 26: 1918～1930
[25] R.Braden, D.Clark, S.Shenker. Integrated Services in the Internet Architecture: an Overview. RFC1633, July 1994
[26] R. Braden, L.Zhang, S.Berson, et al. Resource ReSerVation Protocol (RSVP)-Version 1, Functional Specification. RFC2205, September 1997
[27] S. Blake, D.Black, M.Carlson, et al. An Architecture for Differentiated Services. RFC2475, December 1998
[28] 昂志敏，单永强，施康．基于WLAN的VolP的QoS介绍．合肥工业大等报(自然科学版)，2006，29(1)：10～13
[29] Amit Jain, Daji Qiao, Kang G. Shin. RT-WLAN: A Soft Real-Time Extension to the ORINOCO Linux Device Driver. In: Proc. IEEE PIMRC'03, 2003
[30] Jeonggyun Yu, Sunghyun Choi, Jaehwan Lee. Enhancement of VolP over IEEE 802.11 WLAN via Dual Queue Strategy. IEEE Communications Society, 2004: 3706～3711
[31] Farooq Anjum, Moncef Elaoud, David Fam01ari et al. Voice Performance in WLAN Networks-An Experimental Study. IEEE Globecom , 2003: 3504～3508
[32] Shugong Xu. Advances in WLAN QoS for 802. 11: an Overview. The 14th IEEE 2003 International Symposium on Personal, Indoor and Mobile Radio Communication Proceedings, 2003: 2297～2301
[33] Eustathia Ziouva, Theodore Antonakopoulos. A Dynamically Adaptable Polling Scheme for Voice Support in IEEE 802.11 Networks. Computer Communications, 2003, 26(2): 129～142
[34] Apichan Kanjanavapastit, Hassan Mehrpour. Service Integration Multiple Access (SIMA): A Protocol for Supporting Voice & Data in Wireless LANs. PWC, 2002: 183～190
[35] Andreas Kopsel, Adam Wolisz. Voice Transmission in an IEEE 802.11 WLAN Based Access Network. Proceedings of the 4th ACM International Workshop on Wireless Mobile Multimedia(WoWMoM2001), 2001: 23～32
[36] 赵宁．无线网络接入技术比较.http://51 cmm.csai.cn/ANALYZE/NO000027.htin, 2003
[37] IEEE Standards Board. IEEE Std 802.11b. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher Speed Physical Layer (PHY) Extension in the 2.4 GHz Band. USA: 1999
[38] IEEE Standards Board. IEEE Std 802.11a. Wireless LAN Medium Access Control (MAC) and Physical layer (PHY) Specifications: High Speed Physical Layer in the 5GHz Band. USA: 1999
[39] 张晓敏．无线局域网与IEEE 802.11协议．山东大学学报(工学出版)，2003， 33(2)： 168～171
[40] Woo-Yong Choi. A centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs. International Journal of Electronics and Communications, 2004, 58: 305～309
[41] 刘乃安．无线局域网(WLAN)—原理、技术与应用．西安：西安电子科技大学出版社，2004．82～83
[42] Gaurav Dawra. Voice over Wireless Local Area Network. http://www.cs.montana.edu/～dawra/images/VoWLAN.pdf, 2004
[43] 孟素艳，张杰，孙涛等．VoIP联姻无线接入孕育巨大商机．现代电信科技， 2005，(4)：5～7
[44] 倪奕，苏宏．无线局域网上的VoIP．通信技术，2003，(12)：55～57
[45] 温斌，李亦农．VoIP：IP语音技术．北京：机械工业出版社，2000．45～46
[46] 崔建平．定量评估IP电话语音质量．http://www.ctiforum.com/technology/Voip/2001/04/voip0406.htm, 2001
[47] Tadeus Uhl. Quality of Service in VoIP Communication. International Jour nal of Electronics and Communications, 2004, 58: 178～182
[48] IEEE Standards Board. IEEE Std. 802.11e. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Quality of Service Enhancements. USA: 2005
[49] 王澄，龚玲，杨宇航．IEEE 802.11e基于竞争的信道访问机制性能分析．计算机工程，2006，32(6)：124～126
[50] 王刚，周银东，梅顺良．IEEE 802.11e EDCA网络模型分析．清华大学学报(自然科学版)，2005，45(10)：1389～1392
[51] 张连波，方旭明．802.11e EDCF信道接入机制的仿真研究．西南交通大学学报，2004，39(5)：660～664
[52] 熊桂喜，王小虎．计算机网络．北京：清华大学出版社，1998．402～419
[53] Wei Wang, Soung Chang Liew, Victor O.K.Li. Solution to Performance Problems in VoIP Over a 802.11 Wireless LAN. IEEE Transactions on vehicular technology, 2005, 54(1): 366～384
[54] 徐雷鸣，庞博，赵耀．NS与网络模拟．北京：人民邮电出版社，2003．126～154
[55] The Network Simulator—ns-2. http://www.isi.edu/nsnam/ns/
[56] Tutorial for the Network Simulator "ns". http://www.isi.edu/nsnam/ns/tutorial/index.html
[57] Jae Chung, Mark Claypool. NS by Example. http://nile.wpi.edu/NS/
[2] PTSN.IP电话的概念和基本原理．http://www.ptsn.net.cn/xueyuan/show_knowledge.php?auto_id=49
[3] 潘倩倩．基于WLAN的VoIE集成电路应用，2004年5月：68～73
[4] 张敏丽，王丽芳，谢昀．引发企业通讯革命的VoIP技术．中国科技信息2005，(9)：68～68
[5] ChinaITLab.com搜集整理．VolP的定义．http://www.chinaitlab.com/www/news/article_show.asp?id=14534
[6] 杨仁忠，侯紫峰，孙国滨．一种改善802．11无线局域网语音传输性能的方法．微电子学与计算机，2004，21(5)：85～89
[7] 谭钦红，邱洪云．无线局域网内的语音数据传输．信息技术与信息化，2005， (1)：54～57
[8] 陈福，刘冬梅．VoIP的QoS研究．中国数据通信，2001，(11)：21～25
[9] 杨仁忠，孙国滨，侯紫峰．基于IEEE 802．11无线局域网的VoIP技术综述．计算机工程，2005，31(4)：1～3
[10] Sandip Patodia, Xiao-Hong Peng. Implementation and Analysis of VoIP Services in WLANS. Fifth IEE International Conference on 3G Mobile Communication Technologies, 2004: 548～552
[11] 梅旭．VoIP中QoS控制技术的研究与实现：[工程硕士学位论文]．长沙：湖南大学，2005
[12] 黄永峰，李星．VoIP的语音质量分析与控制．控制与决策，2003，18(4)：475～478
[13] 吴一新，肖会会．VoIP中为提高语音质量所采用的关键技术．计算机仿真， 2004，22(4)：153～155
[14] International Telecommunications Union. Pulse Code Modulation (PCM) of Voice Frequencies. Recommendation G.711, ITU-T 1988
[15] International Telecommunications Union. Coding of Speech at 8 Kbit/s Us ing Conjugate-structure Algebraic-code-excited Linearprediction (CS-ACELP ). Recommendation G.729, ITU-T 1996
[16] International Telecommunications Union. Dual Rate Speech Coder for Mul timedia Communications Transmitting at 5.3 and 6.3 Kbit/s. Recommendat ion G.723.1, ITU-T 1996
[17] Brady P. A Model for Generating ON_OFF Speech Patterns in Two-way Conversations. Bell System Technology Journal, 1969, 48(9): 2445～2472
[18] 张瑛．WLAN中语音业务质量保证．电信科学，2004，(8)：44～47
[19] H. Sanneck, N. Le. Speech Property-Based FEC for Internet Telephony A pplications. In Proceedings of SPIE-The International Society for Optical Engineering, 2000, 3969(1): 38～51
[20] Henning Sanneck. Packet Loss Recovery and Control for Voice Transmissi on over the Internet: Ph.D. thesis. Berlin: GMD Fokus/Telecommunication Networks Group, Technical University of Berlin, 2000
[21] Henning Sanneck, Nguyen Tuong Long Le, Martin Haardt, et al. Selecti ve Packet Prioritization for Wireless Voice over IE Proc. of Fourth Intern ational Symposium on Wireless Personal Multimedia Communication, 2001
[22] N. Long Le. Development of a Loss-resilient Internet Speech Transmission Method: Diploma thesis. Berlin: Department of Electrical Engineering, Te chnical University Berlin, 1999
[23] Christian Hoene, Iacopo Carreras, Adam Wolisz. Voice over IP.. Improvin g the Quality over Wireless LAN by Adopting a Booster Mechanism-An Experimental Approach. In: Proc. of SPIE 2001-Voice over IP (VoIP) Tec hnology, 2001:157-168
[24] Antonio Grilo, Mario M. Macedo, Mario S.Nunes. IP QoS support in IEE E 802.11b WLANs. Computer Communications, 2003, 26: 1918～1930
[25] R.Braden, D.Clark, S.Shenker. Integrated Services in the Internet Architecture: an Overview. RFC1633, July 1994
[26] R. Braden, L.Zhang, S.Berson, et al. Resource ReSerVation Protocol (RSVP)-Version 1, Functional Specification. RFC2205, September 1997
[27] S. Blake, D.Black, M.Carlson, et al. An Architecture for Differentiated Services. RFC2475, December 1998
[28] 昂志敏，单永强，施康．基于WLAN的VolP的QoS介绍．合肥工业大等报(自然科学版)，2006，29(1)：10～13
[29] Amit Jain, Daji Qiao, Kang G. Shin. RT-WLAN: A Soft Real-Time Extension to the ORINOCO Linux Device Driver. In: Proc. IEEE PIMRC'03, 2003
[30] Jeonggyun Yu, Sunghyun Choi, Jaehwan Lee. Enhancement of VolP over IEEE 802.11 WLAN via Dual Queue Strategy. IEEE Communications Society, 2004: 3706～3711
[31] Farooq Anjum, Moncef Elaoud, David Fam01ari et al. Voice Performance in WLAN Networks-An Experimental Study. IEEE Globecom , 2003: 3504～3508
[32] Shugong Xu. Advances in WLAN QoS for 802. 11: an Overview. The 14th IEEE 2003 International Symposium on Personal, Indoor and Mobile Radio Communication Proceedings, 2003: 2297～2301
[33] Eustathia Ziouva, Theodore Antonakopoulos. A Dynamically Adaptable Polling Scheme for Voice Support in IEEE 802.11 Networks. Computer Communications, 2003, 26(2): 129～142
[34] Apichan Kanjanavapastit, Hassan Mehrpour. Service Integration Multiple Access (SIMA): A Protocol for Supporting Voice & Data in Wireless LANs. PWC, 2002: 183～190
[35] Andreas Kopsel, Adam Wolisz. Voice Transmission in an IEEE 802.11 WLAN Based Access Network. Proceedings of the 4th ACM International Workshop on Wireless Mobile Multimedia(WoWMoM2001), 2001: 23～32
[36] 赵宁．无线网络接入技术比较.http://51 cmm.csai.cn/ANALYZE/NO000027.htin, 2003
[37] IEEE Standards Board. IEEE Std 802.11b. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher Speed Physical Layer (PHY) Extension in the 2.4 GHz Band. USA: 1999
[38] IEEE Standards Board. IEEE Std 802.11a. Wireless LAN Medium Access Control (MAC) and Physical layer (PHY) Specifications: High Speed Physical Layer in the 5GHz Band. USA: 1999
[39] 张晓敏．无线局域网与IEEE 802.11协议．山东大学学报(工学出版)，2003， 33(2)： 168～171
[40] Woo-Yong Choi. A centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs. International Journal of Electronics and Communications, 2004, 58: 305～309
[41] 刘乃安．无线局域网(WLAN)—原理、技术与应用．西安：西安电子科技大学出版社，2004．82～83
[42] Gaurav Dawra. Voice over Wireless Local Area Network. http://www.cs.montana.edu/～dawra/images/VoWLAN.pdf, 2004
[43] 孟素艳，张杰，孙涛等．VoIP联姻无线接入孕育巨大商机．现代电信科技， 2005，(4)：5～7
[44] 倪奕，苏宏．无线局域网上的VoIP．通信技术，2003，(12)：55～57
[45] 温斌，李亦农．VoIP：IP语音技术．北京：机械工业出版社，2000．45～46
[46] 崔建平．定量评估IP电话语音质量．http://www.ctiforum.com/technology/Voip/2001/04/voip0406.htm, 2001
[47] Tadeus Uhl. Quality of Service in VoIP Communication. International Jour nal of Electronics and Communications, 2004, 58: 178～182
[48] IEEE Standards Board. IEEE Std. 802.11e. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Quality of Service Enhancements. USA: 2005
[49] 王澄，龚玲，杨宇航．IEEE 802.11e基于竞争的信道访问机制性能分析．计算机工程，2006，32(6)：124～126
[50] 王刚，周银东，梅顺良．IEEE 802.11e EDCA网络模型分析．清华大学学报(自然科学版)，2005，45(10)：1389～1392
[51] 张连波，方旭明．802.11e EDCF信道接入机制的仿真研究．西南交通大学学报，2004，39(5)：660～664
[52] 熊桂喜，王小虎．计算机网络．北京：清华大学出版社，1998．402～419
[53] Wei Wang, Soung Chang Liew, Victor O.K.Li. Solution to Performance Problems in VoIP Over a 802.11 Wireless LAN. IEEE Transactions on vehicular technology, 2005, 54(1): 366～384
[54] 徐雷鸣，庞博，赵耀．NS与网络模拟．北京：人民邮电出版社，2003．126～154
[55] The Network Simulator—ns-2. http://www.isi.edu/nsnam/ns/
[56] Tutorial for the Network Simulator "ns". http://www.isi.edu/nsnam/ns/tutorial/index.html
[57] Jae Chung, Mark Claypool. NS by Example. http://nile.wpi.edu/NS/