近年來，以IEEE 802.11為基礎的無線區域網路在全球已越來越熱門，這是因為其相當便利的特性以及相對於細胞式網路而言較低的收費。因此，人們設計了越來越多的應用於IEEE 802.11無線區域網路中。舉例來說，網路語音通話，VoIP。VoIP不僅用於朋友間的社交生活中，也提供了更便宜的通話方式。然而，VoIP的語音容量和網路情況的影響有非常重大的關係。當在IEEE 802.11無線區域網路中評估VoIP 通話容量時，大部分的研究作了某些以下假設: 1.網路中僅有一種站台，即語音站台。 2.每個站台流量皆為飽和。換句話說，每個站台的佇列中，永遠有封包在等待傳送。 3.只有MAC進接之效能指標被納入考量。換句話說，佇列中的效能指標總是被忽略。 4.僅考慮一次動差的效能指標，例如平均封包進接延遲。 5.完美的通道情況。 6.無捕獲效應。 為了更接近真實的情況，首先，我們考慮了多種服務的無線區域網路，其中包含了語音封包、資料封包以及短回應封包。第二，我們假設佇列長度是有限的，且封包的抵達是依照Poisson 隨機程序；亦即我們考量非飽和流量的情境。第三，我們將MAC進接以及佇列中之效能指標皆納入考量。第四，當評估VoIP 通話容量時，我們不僅計算了封包延遲，也計算了延遲抖動。我們建立了考量多種服務的數學模型，並且研究RTS/CTS、資料站台數、基礎競爭式窗大小、佇列長度、通道雜訊、以及捕獲效應對於VoIP 通話容量之影響。我們所計算的效能指標包含在MAC進接以及佇列中之封包遺失率、平均延遲以及延遲波動。

In recent years, wireless local area networks (WLANs) based on IEEE 802.11 standards [1]-[2] have been more and more popular all over the world due to their convenience of mobility and relatively low cost compared with cellular networks. As a result, more and more network applications are designed based on the advantages of WLANs, such as Voice over IP (VoIP), which is commonly used not only for social life between friends but also for cheaper communications. However, the call quality of VoIP depends strongly on the conditions of the network. When evaluating the VoIP capacity over IEEE 802.11 WLAN, most studies make some of the following assumptions: (1) there is only one type of stations, e.g., VoIP station, (2) the traffic of each station is saturated, i.e., the station always has a packet ready to be transmitted in the queue, (3) only the MAC access performance measures are taken into account, i.e., the performance measures of queue are ignored, (4) only the first moment performance measurements are considered, e.g., mean MAC access delay, (5) perfect channel condition, and (6) no capture effect. To be closer to real-life situations, first, we consider the multi-service WLAN, where there are three types of packets, i.e., VoIP, data, and ack packets. Second, we assume the queue size is finite and the packet arrivals at each station follow a Poisson process, i.e., we consider the non-saturated traffic scenarios. Third, we take account of the performance measures both at MAC access layer and at the queue. Fourth, not only the mean delay but also the delay jitter is taken into account when evaluating the VoIP capacity. We derive the analytical model of the considered multi-service WLAN, and study the effect of RTS/CTS, number of data stations, minimum contention window size, buffer size, channel error, and capture effect on VoIP capacity. The performance measures of interest are loss rate, mean delay, and delay jitter both in queue and in MAC access layer.

[1] "IEEE Standard for Information technology - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications," IEEE Std 802.11-2007, pp. C1-1184, 2007.
[2] B. P. Crow, I. Widjaja, L. G. Kim, and P. T. Sakai, "IEEE 802.11 Wireless Local Area Networks," IEEE Communications Magazine, vol. 35, pp. 116-126, 1997.
[3] G. Bianchi, "Performance Analysis of the IEEE 802.11 Distributed Coordination Function," IEEE Journal on Selected Areas in Communications, vol. 18, pp. 535-547, 2000.
[4] H. Wu, Y. Peng, K. Long, S. Cheng, and J. Ma, "Performance of Reliable Transport Protocol over IEEE 802.11 Wireless LAN: Analysis and Enhancement," IEEE INFOCOM, vol. 10, pp. 599-607, 2002.
[5] F. Daneshgaran, M. Laddomada, F. Mesiti, and M. Mondin, "Unsaturated Throughput Analysis of IEEE 802.11 in Presence of Non Ideal Transmission Channel and Capture Effects," IEEE Transactions on Wireless Communications, vol. 7, pp. 1276-1286, April 2008.
[6] M. A. R .Siddique and J. Kamruzzaman, "VoIP Service over Multihop 802.11 Networks with Power Capture and Channel Noise," IEEE ICC 2010, pp. 1-6, May 2010.
[7] P. Raptis, V. Vitsas, P. Chatzimisios, and K. Paparrizos, "Voice and Data Traffic Analysis in IEEE 802.11 DCF Infrastructure WLANs," Second Interational Conference on Advanced in Mesh Networks , pp. 37-42, June 2009.
[8] C. Brouzioutis, V. Vitsas, and P. Chatzimisios, "Studying the Impact of Data Traffic on Voice Capacity in IEEE 802.11 WLANs," IEEE ICC 2010, pp. 1-6, May 2010.
[9] M. A. R .Siddique and J. Kamruzzaman, "VoIP Call Capacity over Wireless Mesh Networks," IEEE GLOBECOM., pp. 1-6, Dec. 2008.
[10] Hadzi-Velkov, Z. and Spasenovski, B., "Capture effect in IEEE 802.11 basic service area under influence of Rayleigh fading and near/far effect," The 13th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, vol. 1, pp. 172-176, Sep, 2002.
[11] D. Gross and C. M. Harris, Fundamentals of Queueing Theory, 4th.: John Wiley, 1998.
[12] R. Caputo, CISCO Packetized Voice and Data Integration. McGraw-Hill, 2000.