Review
A comprehensive survey on scheduler for VoIP over WLAN

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Abstract

Voice over Internet Protocol (VoIP) technology has observed rapid growth in the world of telecommunications. VoIP offers high-rate voice services at low cost with good flexibility, typically in a Wireless Local Area Network (WLAN). In a voice conversation, each client works either as a sender or a receiver depending on the direction of traffic flow over the network. A VoIP technologically requires high throughput, less packet loss and a high fairness index over the network. The packets of VoIP streaming may experience drops because of competition among the different kinds of traffic flow over the network. A VoIP application is also sensitive to delays and requires voice packets to arrive on time from the sender to the receiver without any delay over a WLAN. To date, scheduling of VoIP traffic is still an unresolved problem. The objectives of this survey paper are to discuss fundamental principles of VoIP-related schedulers and identify current scheduler issues. This survey paper also identifies the importance of the scheduling techniques over WLANs. Related research work for real-time applications specifically for VoIP will also be highlighted.

Introduction

Voice over Internet Protocol (VoIP) is a revolutionary technology that utilizes high-speed packet-switched networks. The IEEE 802.11 standard has increased an importance due to significant research in the last decade. VoIP is a delay sensitive application that runs in packet-switched networks. A VoIP application would expect the network to ensure that each traffic flow is able to provide efficient performance guarantee, real-time voice flow, better throughput and a fair share of the bandwidth. Packet scheduling is important for enhancing the performance of the VoIP over WLANs. Queue management scheduling is a dynamic area of research over a WLAN based on the IEEE 802.11 standard.

This survey paper will discuss the fundamental background principles of the related schedulers and algorithms. This paper also identifies the importance of the scheduling techniques over WLANs. Related research work for real-time and specifically VoIP applications will also be highlighted. A number of traffic schedulers and algorithms are recommend for meeting the requirements of VoIP applications (Wong et al., 2004, Hegde et al.,, Zhang et al., 2008, Lim et al., 2004, Shakkottai and Stolyar, 2002, Demarch and BeckerPlease check the page range in the following Refs.: Demarch and Becker (2007) and Gao et al. (2008)., 2007, Jacobson,, Trad et al., 2004). The traffic scheduling algorithms will be discussed, highlighting their advantages, disadvantages, and challenges.

Scheduling algorithms to support packet scheduling over networks include Class Based Queue (CBQ), Faire Queue (FQ), Weight Faire Queue (WFQ), Generalized Processor Sharing (GPS), Worst-case Fair Weighted Fair Queueing (WF2Q), Deficit Round Robin (DRR), Deficit Transmission Time (DTT), Low Latency and Efficient Packet Scheduling (LLEPS), Credit Based-SCFQ (CB-SCFQ), Controlled Access Phase Scheduling (CAPS), Queue size Prediction-Computation of Additional Transmission (QP-CAT), Temporally-Weight Fair Queue (T-WFQ), Contention-Aware Temporally fair Scheduling (CATS), and Decentralized-CATS (D-CATS) (Nisar et al., 2010). These scheduling algorithms will be classified and discussed in relation to VoIP.

Most of the available traffic scheduling architecture and algorithms are for wired networks. The traditional scheduling techniques are applied without any decision on managing traffic flow for real-time applications such as a VoIP application. This paper will study the most related scheduling algorithms to ensure appropriate QoS particularly for a VoIP application. High-speed packet-switched networks from various flows enter at a switch or router for controlling the voice traffic. The purpose of the scheduling algorithm is to choose each voice traffic flow from the switch or router and transmit it in the output node. An efficient scheduling algorithm manages the various flows in an organized manner and ensures the Quality of Service (QoS) is met (Forouzan, 2007, Stiliadis and Varma, 1996, Kadhum and Hassan, 2009a, Kadhum and Hassan, 2009b).

Section snippets

Classifications of scheduling algorithms

VoIP is end-to-end delay sensitive and requires proper traffic scheduling algorithms over the network. These scheduling algorithms may be classified based on their behaviour over IP-based networks (Wu et al., 2006, Lu et al., 1999, Zhang and Ferrari, 1993). They can be classified into packet-based schedulers, frame based-packet schedulers, bit-by-bit schedulers, priority packet-based schedulers and regulative packet schedulers. Details are as shown in Table 1. The Generalized Processor Sharing

Real-time traffic scheduler

Real-time schedulers provide a guaranteed fairness facility and offer multiple sessions over the same transmission node link. These traffic schedulers divide the bandwidth in the small traffic flows and forward the traffic flow with priority over IP-based networks. Real-time flows require adequate bandwidth allocations and strict delay control within the network.

Real-time traffic scheduling system model and algorithms are designed to share the bandwidth over the network. The motivation behind

VoIP traffic scheduler issues

In WLANs, the voice traffic flow is categorized into different classes (Tao et al., 2005, Wang et al., 2005, Lee, 2005, Cole and Rosenbluth, 2001, Sze et al., 2002). The traffic flow guarantee in WLAN addresses how to assign bandwidth resources among these traffic flows so that each flow can have its own Quality of Service (QoS) requirements satisfied. Most of the schedulers perform well when the network load is not high (Venkatakrishnan and Selvakennedy, 2004, Gao et al., 2008). Some VoIP

Traffic scheduler

The main concern of the VoIP over WLAN is for a well-organized traffic flow on the network. Traffic scheduler techniques manage the multiple flows in a flexible approach with a bandwidth guarantee and it provides QoS over the networks. Some of these traffic schedulers are discussed in the following subsections. It will also show the difference between these schedulers and proposed scheduling system model.

Conclusion

This survey paper presents related traffic schedulers and algorithms for VoIP traffic. Fundamental characteristics of each scheduling technique ((Vaidya et al., 2005; Golestani, 1994; Yu et al., 2006; Antila and Luoma, 2005; Song et al., 2010; Mathy et al., 2000; Lee et al., 2007)) are discussed. The scheduling algorithms are classified into real-time traffic schedulers, VoIP traffic scheduler issues and representative schedulers. Related scheduling algorithms for enhancing the QoS of VoIP

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