Elsevier

Computer Networks

Volume 54, Issue 2, 15 February 2010, Pages 257-277
Computer Networks

Application-aware scheduling for VoIP in Wireless Mesh Networks

https://doi.org/10.1016/j.comnet.2009.05.014Get rights and content

Abstract

Wireless Mesh Networks (WMNs) are seen as a means to provide last mile connections in Next Generation Networks (NGNs). Because of their auto-configuration capabilities and the low deployment cost WMNs are considered to be an efficient solution for the support of multiple voice, video and data services in NGNs. This paper looks at the optimal provision of resources in WMNs for Voice over IP (VoIP) traffic, which has strict performance requirements in terms of delay, jitter and packet loss. In WMNs, because of the challenges introduced by wireless multi-hop transmissions and limited resources, providing performance quality for VoIP comparable to the voice quality in the traditional circuit-switched networks is a major challenge.

This paper analyses different scheduling mechanisms for TDMA-based access control in mesh networks as specified in the IEEE 802.16-2004 WiMAX standard. The performance of the VoIP applications when different scheduling mechanisms are deployed is analysed on a variety of topologies using ns-2 simulation and mathematical analysis. The paper concludes that on-demand scheduling of VoIP traffic – typically deployed in 802.11-based WMNs – is not able to provide the required VoIP quality in realistic mesh WiMAX network scenarios and is therefore not optimal from a network operator’s point of view. Instead, it is shown, that continuous scheduling is much better suited to serve VoIP traffic. The paper then proposes a new VoIP-aware resource coordination scheme and shows, through simulation, that the new scheme is scalable and provides good quality for VoIP service in a wide range of network scenarios. The results shown in the paper prove that the new scheme is resilient to increasing hop count, increasing number of simultaneous VoIP sessions and the background traffic load in the network. Compared to other resource coordination schemes the VoIP-aware scheduler significantly increases the number of supported calls.

Introduction

Internet users increasingly require flexibility and mobility when using network services. This can be achieved using wireless access networks, which have become very popular in recent years. One viable solution for providing wireless access are Wireless Mesh Networks (WMNs). The main advantage of WMNs is the ability of wireless nodes to communicate over multiple wireless hops which increases radio coverage area, provides coverage in shadowed areas and enhances system performance. This multi-hop communication enables network connectivity between stations that are outside of their typical single-hop transmission range. It is interesting to note that multi-hop communication is also a feature of wireless ad hoc networks where the focus is on mobility support, low power operation and end user device design. Compared to wireless ad hoc networks, WMNs are based on static infrastructure components with unlimited power supply and the focus is typically on network reliability and network capacity.

Today, the Internet is not just responsible for delivering traditional data services but also telephony services using Voice over IP (VoIP) technology and television services using television via IP (IPTV). These real-time services have strict Quality of Service (QoS) requirements in terms of delay, loss, jitter and bandwidth. It is generally regarded that WMNs are not able to meet the strong QoS requirements as these networks increase the packet delay and packet loss due to their wireless multi-hop transmission. This paper shows that this is not necessarily true and analyses the VoIP quality in WMNs in detail, by measuring delay, jitter and packet loss of VoIP application in a range of scenarios.

The potential of WMNs generated a lot of activity at various levels in the networking community. Firstly, the vendors are pushing their proprietary mesh products. For example BelAir [1], Saxnet [2] and Cisco [3] are offering mesh solutions for small- and large-scale Internet connectivity. Secondly, a number of community mesh networks have been implemented, providing connectivity and increased capacity for wireless users. The MIT Roofnet [4] and the Freifunk in Berlin [5] are examples of community networks currently containing up to 200 access points and are continuously growing in size. Thirdly, research testbeds such as MagNets [6], RescueMesh [7] or MeshBed [8] have been developed to experimentally evaluate mesh networks as well as to understand their limitations and their full potential. Finally, the standardisation activities, especially within the Institute of Electrical and Electronics Engineers (IEEE) 802 Working Group, focus on multi-hop mesh networks for broadband wireless access. Examples of these include the IEEE 802.11s and the IEEE 802.16 Worldwide Interoperability for Microwave Access (WiMAX) [9] initiatives.

There are currently two main Medium Access Control (MAC) protocol concepts under consideration for WMNs. The random access protocols, such as the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) used in the IEEE 802.11 standard [10] are not capable of meeting the “carrier-grade” requirements of network operators because of increased network contention and thus increased delay and packet loss in highly loaded networks [11]. This is the main reason why coordinated access protocols, such as the Time Division Multiple Access (TDMA), used in IEEE 802.16 networks, are currently under consideration for Wireless Mesh Networks. The IEEE 802.16-2004 standard specifies the resource control and the reservation mechanisms to realise QoS support for real-time applications. However, the standard provides only a framework for the mesh mode and in its current state leaves a number of open issues. For example, the standard defines two different scheduling approaches, namely “centralised” and “distributed”, but the exact mechanism for assigning and distributing slots for the data transmissions within the mesh network (the actual scheduling process) is not defined.

This paper adopts the mesh framework of the IEEE 802.16-2004 standard [9] and analyses the operation of TDMA-based WMNs in chain and grid topologies. The paper provides a detailed ns-2 simulation and analytical investigation of the network performance for VoIP traffic in terms of quality and scalability. The paper then introduces scheduling mechanisms for mesh networks and gives their detailed evaluation. Two different scheduling approaches are presented: on-demand and the continuous scheduling. For the continuous scheduler, two well known resource coordination schemes are considered and a new, application-aware scheduling is proposed. Analytical models are developed for the analysis of the VoIP quality and network scalability for various TDMA configurations. To the best of our knowledge, this is the first paper proposing application-aware scheduling for VoIP in TDMA-based WMNs and analysing in detail the influence of scheduling on the VoIP performance in such systems.

The results and investigations in this paper, while focusing on the IEEE 802.16-2004 network standard, are general in nature and can be applied to any other TDMA-based multi-hop system. The rest of the paper is organised as follows. Section 2 discusses related work. Section 3 analyses the resource scheduling in the TDMA-based WMNs and describes the different scheduling approaches that are used within this paper, including the new VoIP-aware scheduler. Section 4 provides an overview of the observed network scenarios and the simulator, including the simulation parameters, the analytical models, and the use of the International Telecommunication Union (ITU)-T E-Model as a quality metric for the VoIP applications. Section 5 presents the results of the quality and scalability investigations. Finally, Section 6 concludes the paper.

Section snippets

Related work

The support for real-time applications like VoIP in wireless access networks has been a major research challenge in the recent past. A substantial amount of work investigating the support for VoIP in single-hop wireless networks has been published. For example, in [12], [13] the performance of VoIP in IEEE 802.16 based backhaul networks operating in the Point-to-Multipoint (PMP) mode is investigated. [12] evaluates the impact of the scheduling mechanism on the quality of the VoIP service as

The mesh TDMA MAC layer

The TDMA frame structure used within this paper is based on the IEEE 802.16-2004 standard (see Fig. 1). The length of the TDMA frame is defined by the Frame Length (FL) parameter (e.g. 4 and 10 ms) and the frame is divided into the control subframe and the data subframe.

A slot in the control subframe is called a Transmission Opportunity (TO). The TOs can only be used for the transmission of signalling messages. There are two types of control subframes: the network control subframe and the

VoIP quality

For VoIP applications the Quality of Experience (QoE) is the most important performance metric from a user’s point of view. To measure the QoE, the ITU-T E-Model [30], [31] provides an algorithm to estimate the voice quality based on different parameters and aspects of voice quality impairment. The R-score considers these aspects and expresses the voice quality on a scale between 0 and 100, see Table 1. The R-score is given by:R=100-Is-Id-Ief+A.In this equation Is represents the signal-to-noise

Performance evaluation

In this section, the performance and quality of VoIP in TDMA-based WMNs is investigated. The on-demand scheduling mechanism is compared with the continuous scheduling mechanism and the different resource coordination schemes. Particular attention is paid to the influence on quality and scalability. First, the results for the chain topology are discussed followed by the results for the grid topology.

Conclusions

This paper analyses the quality and scalability of VoIP in TDMA-based WMNs. Different scheduling approaches are investigated and compared, by measuring the most important metrics for voice: delay, jitter, packet loss as well as the number of supported calls. The main findings of the paper are: (1) On-demand scheduling of VoIP traffic as used in Wireless Local Area Network (WLAN)-based WMNs makes no sense from a network operator point of view as it is not able to provide good VoIP quality in a

Acknowledgement

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 214994.

Nico Bayer received his diploma degree in Electrical Engineering and Information Technology from the University of Applied Sciences Friedberg, Germany, in 2003 and his Ph.D. in Electrical Engineering from the City University London, UK, in 2009. From 2003 till 2008 he worked for T-Systems Enterprise Services GmbH in the department of “Mobile and Wireless Solutions”. Since 2008 he is working for Deutsche Telekom Laboratories in the department of “Seamless Communication” in Darmstadt, Germany. He

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    Nico Bayer received his diploma degree in Electrical Engineering and Information Technology from the University of Applied Sciences Friedberg, Germany, in 2003 and his Ph.D. in Electrical Engineering from the City University London, UK, in 2009. From 2003 till 2008 he worked for T-Systems Enterprise Services GmbH in the department of “Mobile and Wireless Solutions”. Since 2008 he is working for Deutsche Telekom Laboratories in the department of “Seamless Communication” in Darmstadt, Germany. He is involved in different national and international R&D projects in the fields of mobile communications, mesh networks and Fixed/Mobile Convergence (FMC).

    Bangnan Xu is a Research Expert at T-Systems, Deutsche Telekom, leading various research projects in ad hoc/mesh networks, seamless IP mobility, fixed & mobile convergence, and next generation access networks. His current research interests include Next Generation Mobile Network (NGMN, 3GPP LTE SAE), Next Generation Network (NGN, ETSI TISPAN), IMS and beyond, packet optimised fixed & mobile converged (FMC), and Quadrupple play services. He received his Ph.D. in Electrical Engineering and Information Technology from Aachen Univ. of Technology (2002), as well as M.Sc. (1989) and B.Sc. (1986) degrees in Electrical Engineering from Dalian Maritime University. From 1989 to 1996, he was a lecturer on communication systems in Dalian Maritime University. Before joining T-Systems in 2001, he was with the chair of Communication Networks, Prof. Dr.-Ing. B. Walke, Aachen Univ. of Technology, Aachen, Germany. He holds several patents in multihop communications, ad hoc routing, and seamless IP mobility. He has authored and coauthored many technical papers, three of them granted with Best Paper Award.

    Veselin Rakocevic MIEEE currently works as a Senior Lecturer at City University London, UK. His main research interests include wireless ad hoc networks, quality of service in wireless networks, and network reliability. He published over 40 papers in international journals and conferences and has been a member of the TPC for a number of IEEE conferences, including Globecom, MASS and WCNC. He holds a PhD in Telecommunications from the University of London, UK and a Dipl-Ing degree in Electronic Engineering from the University of Belgrade, Serbia.

    Joachim Habermann (Senior Member IEEE) received the Msc. and Ph.D. degrees in Electrical Engineering from Darmstadt Technical University, Germany in 1979 and 1984. In 1984 he joined ABB research centre in Baden, Switzerland, where he worked in the Telecommunications department in the area of mobile communications. Since 1990 he is Professor at the University of Applied Sciences Giessen-Friedberg/Germany.

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