Towards the Quality of Service for VoIP traffic in IEEE 802.11 Wireless Networks

The usage of voice over IP (VoIP) traffic in IEEE 802.11 wireless networks is expected to increase in the near future due to widely deployed 802.11 wireless networks and VoIP services on fixed lines. However, the quality of service (QoS) of VoIP traffic in wireless networks is still unsatisfactory. In this thesis, I identify several sources for the QoS problems of VoIP traffic in IEEE 802.11 wireless networks and propose solutions for these problems. The QoS problems discussed can be divided into three categories, namely, user mobility, VoIP capacity, and call admission control. User mobility causes network disruptions during handoffs. In order to reduce the handoff time between Access Points (APs), I propose a new handoff algorithm, Selective Scanning and Caching, which finds available APs by scanning a minimum number of channels and furthermore allows clients to perform handoffs without scanning, by caching AP information. I also describe a new architecture for the client and server side for seamless IP layer handoffs, which are caused when mobile clients change the subnet due to layer 2 handoffs. I also present two methods to improve VoIP capacity for 802.11 networks, Adaptive Priority Control (APC) and Dynamic Point Coordination Function (DPCF). APC is a new packet scheduling algorithm at the AP and improves the capacity by balancing the uplink and downlink delay of VoIP traffic, and DPCF uses a polling based protocol and minimizes the bandwidth wasted from unnecessary polling, using a dynamic polling list. Additionally, I estimated the capacity for VoIP traffic in IEEE 802.11 wireless networks via theoretical analysis, simulations, and experiments in a wireless test-bed and show how to avoid mistakes in the measurements and comparisons. Finally, to protect the QoS for existing VoIP calls while maximizing the channel utilization, I propose a novel admission control algorithm called QP-CAT (Queue size Prediction using Computation of Additional Transmission), which accurately predicts the impact of new voice calls by virtually transmitting virtual new VoIP traffic

VOIP WITH ADAPTIVE RATE IN MULTI- TRANSMISSION RATE WIRELESS LANS

“Voice over Internet Protocol (VoIP)” is a popular communication technology that plays a vital role in term of cost reduction and flexibility. However, like any emerging technology, there are still some issues with VoIP, namely providing good Quality of Service (QoS), capacity consideration and providing security. This study focuses on the QoS issue of VoIP, specifically in “Wireless Local Area Networks (WLAN)”. IEEE 802.11 is the most popular standard of wireless LANs and it offers different transmission rates for wireless channels. Different transmission rates are associated with varying available bandwidth that shall influence the transmission of VoIP traffic

Voice-over-IP (VoIP) over wireless local area networks (WLAN).

Wang Wei.Thesis (M.Phil.)--Chinese University of Hong Kong, 2004.Includes bibliographical references (leaves 80-83).Abstracts in English and Chinese.Chapter Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Motivations and Contributions --- p.1Chapter 1.2 --- Organization of the Thesis --- p.4Chapter Chapter 2 --- Background --- p.6Chapter 2.1 --- IEEE 802.11 --- p.6Chapter 2.1.1 --- Distributed Coordination Function (DCF) / Point Coordination Function (PCF) --- p.7Chapter 2.1.2 --- Types of Networks --- p.8Chapter 2.1.3 --- The 802.11 MAC Sublayer Protocol --- p.9Chapter 2.1.4 --- Why CSMA/CA for Wireless LAN? --- p.11Chapter 2.2 --- Voice over IP (VoIP) --- p.13Chapter 2.2.1 --- Speech Codec --- p.13Chapter 2.2.2 --- The H.323 Standard --- p.13Chapter 2.3 --- Related Work --- p.15Chapter 2.3.1 --- Capacity limits of VoIP over WLAN --- p.16Chapter 2.3.2 --- Methods for increasing VoIP capacity over WLAN --- p.16Chapter 2.3.3 --- Interference between traffic of VoIP and other applications --- p.18Chapter Chapter 3 --- VoIP Multiplex-Multicast Scheme --- p.20Chapter 3.1 --- System Architecture --- p.20Chapter 3.2 --- Packet Multiplexing and Multicasting --- p.22Chapter 3.3 --- Header Compression --- p.24Chapter 3.4 --- Connection Establishment --- p.29Chapter Chapter 4 --- Capacity Analysis --- p.31Chapter 4.1 --- VoIP Capacity Analysis for 802. 11b --- p.31Chapter 4.1.1 --- Capacity of Ordinary VoIP over WLAN --- p.32Chapter 4.1.2 --- Capacity of Multiplex-Multicast Scheme over WLAN --- p.33Chapter 4.2 --- "VoIP Capacity Analysis for 802,11a and 802.11g" --- p.34Chapter 4.3 --- VoIP Capacity with VBR Sources --- p.38Chapter 4.4 --- Simulations --- p.38Chapter Chapter 5 --- Delay Performance --- p.41Chapter 5.1 --- Access Delay --- p.42Chapter 5.2 --- Extra Delay Incurred by the Multiplex-Multicast Scheme --- p.47Chapter Chapter 6 --- VoIP Co-existing with TCP Interference Traffic --- p.49Chapter 6.1 --- Ordinary VoIP co-existing with TCP over WLAN --- p.49Chapter 6.1.1 --- Problem Caused by TCP Interference --- p.49Chapter 6.1.2 --- Solutions --- p.52Chapter 6.2 --- M-M VoIP coexisting with TCP over WLAN --- p.53Chapter 6.3 --- 802.11e --- p.56Chapter 6.3.1 --- EDCA --- p.56Chapter 6.3.2 --- ACK Policies --- p.58Chapter 6.3.3 --- VoIP over EDCA --- p.58Chapter Chapter 7 --- Experimental Validation --- p.61Chapter 7.1 --- Transmission Errors --- p.61Chapter 7.2 --- Prototype Implementation --- p.62Chapter Chapter 8 --- VoIP over Ad Hoc Networks --- p.65Chapter 8.1 --- Mobile Ad Hoc Networks (MANET) and Wireless Distributed System (WDS) --- p.65Chapter 8.2 --- The M-M Scheme in WDS --- p.67Chapter 8.2.1 --- Modified System Architecture --- p.67Chapter 8.2.2 --- Delay Performance --- p.68Chapter 8.2.3 --- Analysis of M-M Scheme in WDS --- p.69Chapter 8.2.4 --- Capacity Improvement --- p.70Chapter 8.2.5 --- Delay Improvement --- p.71Chapter 8.2.6 --- Spectrum Reuse --- p.71Chapter Chapter 9 --- Conclusions --- p.76References --- p.8

Adaptive medium access control for VoIP services in IEEE 802.11 WLANs

Abstract- Voice over Internet Protocol (VoIP) is an important service with strict Quality-of-Service (QoS) requirements in Wireless Local Area Networks (WLANs). The popular Distributed Coordination Function (DCF) of IEEE 802.11 Medium Access Control (MAC) protocol adopts a Binary Exponential Back-off (BEB) procedure to reduce the packet collision probability in WLANs. In DCF, the size of contention window is doubled upon a collision regardless of the network loads. This paper presents an adaptive MAC scheme to improve the QoS of VoIP in WLANs. This scheme applies a threshold of the collision rate to switch between two different functions for increasing the size of contention window based on the status of network loads. The performance of this scheme is investigated and compared to the original DCF using the network simulator NS-2. The performance results reveal that the adaptive scheme is able to achieve the higher throughput and medium utilization as well as lower access delay and packet loss probability than the original DCF

An Experimental Analysis of the Call Capacity of IEEE 802.11b Wireless Local Area Networks for VoIP Telephony

The use of the Internet to make phone calls is growing in popularity as the Voice over Internet protocol (VoIP) allows users to make phone calls virtually free of charge. The increased uptake of broadband services by domestic users will further increase the use of VoIP telephony. Furthermore, the emergence of low cost wireless networks (namely IEEE 802.11a/b/g WLANs) is expected to bring wireless VoIP into the mainstream. As the number of wireless hotspots increases more users will want to use VoIP calls wherever possible by connecting to open access points (AP). A major concern with VoIP is Quality of Service (QoS). In order for VoIP to be truly successful users must enjoy a similar perceived QoS as a call made over a traditional telephone network. There are many factors that influence QoS which include: throughput, packet delay, delay variation (or jitter), and packet loss. This thesis is an experimental study of the call capacity of an IEEE 802.11b network when using VoIP telephony. Experiments included increasing the number of VoIP stations and also increasing the level of background traffic until network saturation occurs. Results show that the network is capable of supporting at least 16 VoIP stations. Due to the operation of the IEEE 802.11 medium access control (MAC) mechanism, the AP acts as a bottleneck for all traffic destined for wireless stations, in that significant delays can be incurred by VoIP packets which can lead to a poor perceived QoS by users. Consequently the performance of the AP downlink is the critical component in determining VoIP call capacity

VoIP capacity over multiple IEEE 802.11 WLANs.

Chan, An.Thesis (M.Phil.)--Chinese University of Hong Kong, 2007.Includes bibliographical references (leaves 80-84).Abstracts in Chinese and English.Chapter Chapter 1 --- Introduction --- p.1Chapter 1.1 --- Motivations and Contributions --- p.1Chapter 1.2 --- Related Works --- p.3Chapter 1.3 --- Organization of the Thesis --- p.4Chapter Chapter 2 --- Background --- p.5Chapter 2.1 --- IEEE 802.11 --- p.5Chapter 2.1.1 --- Basic IEEE 802.11 Standards --- p.5Chapter 2.1.2 --- Types of Networks --- p.7Chapter 2.2 --- Voice over IP (VoIP) Codecs --- p.8Chapter 2.3 --- VoIP over WLAN --- p.9Chapter 2.3.1 --- System Architecture of VoIP over WLAN --- p.9Chapter 2.3.2 --- VoIP Capacity over an Isolated WLAN --- p.10Chapter Chapter 3 --- VoIP Capacity over Multiple WLANs --- p.12Chapter 3.1 --- Topology Settings and Assumptions --- p.12Chapter 3.2 --- Low VoIP Capacity Found in NS2 Simulations --- p.16Chapter 3.3 --- Applying Frequency Channel Assignment --- p.18Chapter Chapter 4 --- Clique Analysis and Call Admission Control --- p.21Chapter 4.1 --- Conflict Graph Model and Cliques --- p.21Chapter 4.2 --- Cliques in Multi-Cell WLANs --- p.22Chapter 4.3 --- Clique-Based Call Admission Control Algorithm --- p.24Chapter 4.3.1 --- Algorithm Description --- p.24Chapter 4.3.2 --- Algorithm Performance Evaluation --- p.27Chapter 4.3.3 --- Clique-Based Admission Control in Three-Frequency- Channel WLAN --- p.29Chapter Chapter 5 --- Time Division Multiple Access (TDMA) on IEEE 802.11MAC --- p.32Chapter 5.1 --- Coarse-Grained Time-Division Multiple Access (CTDMA) --- p.33Chapter 5.1.1 --- Basic Ideas of CTDMA --- p.33Chapter 5.1.2 --- Conflict Graph Modeling of CTDMA --- p.35Chapter 5.1.3 --- Parameter Values in CTDMA --- p.41Chapter 5.2 --- Possible Realization of TDMA on 802.11 Standards --- p.47Chapter Chapter 6 --- Coloring Problem in Wireless Networks: A Theoretical Treatment --- p.52Chapter 6.1 --- Coloring of One-Dimensional Linear Network --- p.53Chapter 6.1.1 --- Network with Same Link Length --- p.53Chapter 6.1.2 --- Network with Variable Link Length --- p.54Chapter 6.2 --- Coloring of Two-Dimensional Network --- p.63Chapter Chapter 7 --- Conclusion --- p.66Appendices --- p.69References --- p.8