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Top1. Introduction
IEEE 802.16 air interface standard (Jeffrey et al., 2007) provides the details of physical layer and Media Access Control (MAC) layer of wireless communication system, which aims to build a multi-service network with Worldwide Interoperability for Microwave Access (WiMAX) technology. The different standards of WiMAX networks are IEEE 802.16a, IEEE 802.16d, IEEE 802.16e and IEEE 802.16j. There is a constant evolution of different standards, along with the demand for real-time multimedia applications such as Constant Bit Rate (CBR) based traffic, video stream and Voice over Internet Protocol (VoIP) and non real-time multimedia applications such as file transfer, web browsing and email. To support these multimedia applications, the standards must be sensitive to the need for bandwidth with high speed access, large network coverage and the provision of good Quality of Services (QoS) to a large number of users.
The basic topology of a WiMAX network consists of two participating entities, called Base Station (BS) and Subscriber Station (SS). The BS is the central node, responsible for coordinating all the communication and providing connectivity to the SSs. The BS is the station that provides access to the public network. This network can be categorized into two operating modes: a mandatory Point-to-Multipoint (PMP) mode and an optional mesh mode. In PMP mode (Akashdeep et al., 2014; Ali et al., 2009), all SSs are only one-hop away from the BS and communicate to the BS directly in a centralized manner and not indirectly with other neighboring SSs. In optional mesh mode (Akyildiz et al., 2005; Kas et al., 2010), all SSs are one hop or more than one hop away from the BS and so the SSs communicates with the BS directly or indirectly (i.e. with other SSs) in a distributed manner. In this study, we focus on a centralized PMP mode, which provides better QoS performance compared to distributed mesh mode (Ni et al., 2007).
There are two ways of providing QoS (Sekercioglu et al., 2009), they are user-centric QoS and network-centric QoS. The user-centric QoS comprises the degree of satisfaction of a user for the service. The network-centric QoS comprises the ability to control the mix of bandwidth, average end-to-end delay and average jitter in the network in order to deliver a network service such as CBR based traffic, file transfer and VoIP. This study is primarily concerned with the network-centric QoS. The QoS parameters such as bandwidth, packet delivery ratio, average end-to-end delay, average jitter and throughput are generally used to measure the effect of multimedia streams on the level of QoS. These QoS parameters are considered only for the network-centric QoS, but not on the user-centric QoS. The WiMAX network (Carvalho et al., 2013) mainly depends on parameters such as bandwidth, delay, jitter and throughput for efficient communication. This influences the QoS with a certain level of end-to-end quality for multimedia applications through the management of MAC and network layer for the provision of better QoS services.
QoS (Cicconetti et al., 2006; Grewal et al., 2010; Liu et al., 2006) plays a major role in IEEE 802.16 MAC layer. The MAC layer uses connection oriented approach. For performing the data transmissions over the particular link, the downlink (from BS to SS) and uplink (from SS to BS) connections are controlled by the particular BS. Each connection is identified by a connection identifier (CID) and the connection in the network is associated with a Service Flow Identifier (SFID) that is composed of a set of QoS parameters namely bandwidth, average end-to-end delay, average jitter and throughput. The BS performs the functionality of issuing the SFID and mapping it to unique CIDs and sending the packet to the appropriate SS.
To support real-time and non-real-time multimedia applications, WiMAX network defines five scheduling service classes (Kuran et al., 2007; Li et al., 2007; Sekercioglu et al., 2009). The five scheduling service classes are Unsolicited Grant Service (UGS), Real Time Polling Service (rtPS), Non-Real Time Polling Service (nrtPS), Best Effort (BE) and Extended Real Time Polling Service (ertPS). These scheduling service classes define the nature of the data service supported and the following are its detail description: