On the Efficiency of Multicasting for IPTV Delivery over IEEE 802.16j Networks Alireza Abdollahpouri 1,2 , Bernd E. Wolfinger 1 1 Department of Computer Science - TKRN University of Hamburg - Germany 2 University of Kurdistan - Sanandaj - Iran {Abdollahpouri, Wolfinger}@informatik.uni-hamburg.de Abstract- The main objectives of relaying are to extent coverage area and increase throughput without deploying expensive base stations. Mobile multi-hop relay (MMR), which is an amendment of the current mobile WiMAX standard, provides relaying capabilities. WiMAX could be a leading solution to deliver bandwidth-hungry IPTV services to mobile users, due to characteristics such as: multicast support, wide coverage range, high bandwidth and mobility support. The smallest logical bandwidth allocation unit in OFDMA-based WiMAX, is a slot. In this article, we use the term slot to introduce a new performance criterion, called multicast gain, which can be used for computing multicast efficiency. We analyze multicast transmission for delivery of live TV channels in mobile multi-hop relay networks, and determine the gain of access and relay links in order to quantify the benefits of multicasting. This quantification can be used in access control, resource provisioning or decision support mechanisms in delivery of different TV channels in an IPTV system. Keywords- IPTV; multicast gain; WiMAX; performance metrics; relay networks I. INTRODUCTION Internet Protocol TV (IPTV) describes a mechanism for transporting streams of video content encapsulated in IP packets over the network using networking protocols [1]. IPTV is one of the fastest growing services in the Internet. This rapid growth is partially due to the enormous improvement of networking technologies and partially because of the advances in media encoding and compression techniques (e.g., H.264/AVC). Worldwide Interoperability for Microwave Access (WiMAX) technology which is based on IEEE 802.16 air- interface standard, provides a specific QoS class for bandwidth-hungry and delay-sensitive applications and therefore, it can be a leading solution to deliver IPTV streams to fixed and mobile subscribers [2]. A WiMAX cell consists of a number of subscriber stations or mobile stations (SS or MS) served by a base station (BS), which controls the access to the wireless medium in a centralized manner. Due to the signal attenuation, subscribers located near the cell edge, can not obtain the required signal-to-noise ratio (SNR) to receive high data rates. As a solution, the service provider can shrink the size of the cell by increasing the number and density of base stations. This requires a large infrastructural cost for wired backhaul for the additional BSs. Using relays is a cost-effective alternative solution. Relays have significantly simpler hardware and software architecture, and hence lower cost. In addition, they operate at lower powers and without connection to any wired backhaul. Inserting relays into the cell can extent coverage area and increase throughput. Such networks are called multi-hop relay networks. IEEE 802.16 has formed a task group to extend the IEEE 802.16e-2005 standard to include multi-hop communication, and address the problems of relaying. This amendment is called IEEE 802.16j or mobile multi-hop relay (MMR) and is fully compatible with 802.16e mobile and subscriber stations [3]. Relays in MMR are organized in tree structure, rooted at the BS. Multicasting is a key technology that may provide beneficial services for applications like, IPTV and video conferencing which require communication among a group of users. Multicast allows the sender to transmit a message (destined for multiple receivers) only once, instead of sending it to each end-point separately and clogging up the bandwidth with the multiple transmissions of the same data. An interesting feature of WiMAX which makes is quite suitable for IPTV services, is support of multicast using Multicast Broadcast Service (MBS) [4]. MBS allows the transmission of common data (e.g., a TV channel) to a group of users using shared system resources. To manage overall operations of MBS, an MBS controller (server), is needed in the system. Note that, like a regular IP host, an MBS server which provides IPTV service for subscribers, joins all multicast groups (all TV channels) using Internet Group Management Protocol (IGMP). Using a multicast session, usually outperforms multiple unicast sessions in terms of bandwidth efficiency. However, there are several challenges in multicasting as mentioned in [5] and [6]. Therefore, the efficiency of multicasting must be formally specified to find out whether it is worthwhile to use multicasting or not. For example, for a rarely watched and unpopular TV channel, multicasting is not reasonable. In [7], J. Aaltonen et al. evaluated the multicasting gain over unicast in cellular networks, for cells with a specific target call blocking probability. They used Monte-Carlo simulation for multicast sessions and the traditional Engset model for the unicast traffic and modeled a mobile cell as a single transmission link and calculated the number of users SPECTS 2011 151