Wired and Wireless IPTV Access Networks: A

IV International Congress on Ultra Modern Telecommunications and Control Systems 2012

Wired and Wireless IPTV Access Networks: A Comparison Study Alireza Abdollahpouri1,2, Bernd E. Wolfinger1 1

Department of Computer Science - TKRN University of Hamburg - Germany 2 University of Kurdistan - Sanandaj - Iran

{Abdollahpouri, Wolfinger}@informatik.uni-hamburg.de

Abstract: Internet Protocol Television (IPTV) describes a mechanism for transporting streams of video content encapsulated in IP packets over a managed network infrastructure using networking protocols. It provides a so called “walled garden” architecture to guarantee necessary QoE for subscribers. We analyze the different components of an IPTV system with a focus on access networks. Among many variants of access networks, DSL and WiMAX are compared from different aspects (e.g., cost, uplink and downlink bandwidth allocation, QoS, mobility, data rate) and advantages and disadvantages of each are given. Key words- IPTV, xDSL, WiMAX, access network, comparison study

1-Introduction The paradigm shift from push-based media broadcasting to pull-based media streaming has been started for some years and will be accelerated in the next few years. Internet Protocol Television (IPTV) is a good example to illustrate this claim. IPTV describes a mechanism for transporting streams of video content encapsulated in IP packets over the network using Internet protocols. It provides a so called “walled garden” architecture to guarantee necessary Quality of Experience (QoE) for subscribers. The number of households that receive IPTV service is planned to grow 52.2% annually until year 2012 [1]. Currently (i.e., June 2012) in France already more than 17% of the total population is using IPTV. This is partially due to the enormous improvement of networking technologies and partially because of the advances in media encoding and compression techniques. Commercial deployments of IPTV services by Telcos around the world continue to increase. In general, IPTV service can be divided into two classes, namely Video on Demand (VoD) for stored

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contents and Broadband-TV (BTV) for live TV channels. Unlike the native broadcast in traditional TV systems, in IPTV, video streams are distributed toward subscribers using IP unicast and multicast. Typically, unicast is employed in case of video on demand and multicast is employed by BTV service for the delivery of live TV channel streams. Each individual TV channel is mapped into a dynamic multicast stream with a unique multicast address. When a user switches into a specific channel, the set top box (STB) sends a request to join the corresponding multicast stream using IGMP protocol, and then if successful, the user can receive and start to watch the required content after a short buffering and decoding delay. In [2], a new measure (called multicast gain) was introduced and applied which allows one to compare the efficiency of multicast versus unicast. Compared with other Internet services, e.g., online surfing, gaming and VoIP, IPTV service not only consumes higher bandwidth, but also requires a higher degree of end-to-end Quality of Service (QoS) guarantees throughout the delivery path. Thus, user QoE is sensitive to packet delay, jitter and loss. Challenges of IPTV system have been widely studied in the literature [3][4]. In [5], authors discussed admission control problems in heterogeneous access networks, namely, xDSL, WiMAX and UMTS. Garcia et al. proposed a system consisting of different wireless access networks in order to obtain higher values of subscriber’s QoE parameters for IPTV [6]. They compared delay, jitter and loss of packets between WiMAX and IEEE 802.11a/g networks in a real environment and argued about which type of access network should be used in different network conditions. Design and implementation of MHAQ, (Merged Hybrid Adaptive FEC and ARQ) for reliable wireless IPTV multicast is done in [7]. This system uses strong application layer FEC and hybrid ARQ with a simple adaptation algorithm to deliver IPTV streams over WLAN. In [8], Uilecan et al. identified the challenges

related to delivery of IPTV services over WiMAX networks and proposed a framework that could be used in implementing a system capable of delivering IPTV services over an IP based WiMAX network. They also offered some solutions especially related to the Physical and MAC layers.

3. Distribution Network (Metro backbone): The distribution network typically serves a region or a metropolitan area. It inserts local content such as local TV channels or commercial advertisements into the IPTV streams and provides on-demand video services to the clients located in its region. Typical equipment in this part consists of the encoders for local TV channels, local advertisement inserters and video servers to stream on-demand video services.

Another interesting subject is modeling user activities and channel popularity in IPTV systems [9][10][11]. Channel switching or zapping delay is a key QoE metric in IPTV systems and has attracted attentions in recent years [12][13][14].

4. Access Network: The access network is an essential part of the IPTV structure and acts as a border between service provider and subscriber. It provides last mile access for IPTV subscribers and can be based on wired technologies (e.g., xDSL, FTTx) or wireless technologies. In Fig. 1 two access networks based on xDSL (lower one) and WiMAX (upper one) technologies are shown.

The rest of this paper is organized as follows. In Section 2, we will describe different components of typical IPTV systems. Section 3 takes a closer look at access networks in an IPTV system. In Section 4, a comparison of two access networks is given. Section 5 is an assessment of the comparison and finally, Section 6 concludes the paper.

5. Customer Network: The customer network provides TV, IP phone and Internet services to subscribers. It connects the home computer(s), the IPTV STB and IP phone device to a broadband service which is provided by the access network via a home gateway. This network may also support Voice over IP (VoIP) services. In this paper, our focus is on the access networks which play an important role in an IPTV system structure. One wired and one wireless type of access networks will be discussed and compared.

2- IPTV system structure Fig. 1 depicts a typical IPTV service network architecture which provides triple play (i.e., voice, video and data) using an xDSL access network and quad play (i.e., triple play and mobility) using a WiMAX access network. IPTV system mainly consists of five different parts as depicted in the figure. In the following we discuss these components in detail. 1. IPTV Head-end: This part of IPTV system is responsible for acquiring, processing, encoding, and managing video content. It receives video from a variety of sources such as satellites. Video content is typically compressed using either MPEG-2 or MPEG-4 codec and then it is sent in MPEG transport stream (MPEG-TS) packets delivered via IP Multicast in case of live TV or via IP Unicast in case of video on demand. A typical IP packet for transporting MPEG video contains seven 188-byte MPEG-TS packets. The head-end also manages access to on-demand videos and remote operations as well.

Fig. 1- A typical IPTV system architecture

3- IPTV access network technologies

2. Core Network: The core network is the central portion of an IPTV system. It primarily provides interconnection between several metro networks. In order to reduce latency between the clients and the streaming servers and guarantee the required level of experienced QoS, core networks use fiber optic links. Meanwhile, multicast enabled routers are employed to deliver TV channels to the distribution network. Traffic engineering techniques using MPLS can also help the subscribers to experience a better video quality.

Access network is a critical part of the IPTV system and provides last mile access for IPTV subscribers to get the TV service through STBs or mobile terminals. Nowadays, the most widely used access network technologies are mainly wired, like xDSL and fiber-tothe-node (FTTN), or wireless such as WiMAX, High Speed WiFi and 3G. Due to the bandwidth limitation of the access networks, multicasting has been widely adopted to provide for the customers a simultaneous access to the

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TV channels. Multicast allows the sender to transmit a message (destined for multiple receivers) only once, instead of sending it to each end-user separately and clogging up the bandwidth with multiple transmissions of the same data. Therefore, it can reduce the steady state bandwidth requirement in TV streaming systems from one stream per viewer to one stream per TV channel. A single stream of each TV channel is shared among a group of subscribers who demand the same content. Data is replicated only at appropriate branching locations when this is necessary. In 3G networks, multicasting is supported by using the Multimedia Broadcast Multicast Service (MBMS). In xDSL access networks replication of TV streams is done in DSLAMs. In WiMAX access networks multicasting is supported by Multicast Broadcast Service (MBS) [15] which will be discussed later in detail. In the following, we will discuss two access networks which are based on ADSL and WiMAX technologies.

IGMP Router: The BRAS routes traffic into an ISP’s backbone network. Meanwhile, it receives and processes IGMP messages to forward multicast packets. Session termination: The BRAS provides the logical termination of Point-to-point Protocol (PPP) sessions. Subscriber management functions: The BRAS acts as an access server and provides the services for subscriber authentication, authorization and accounting (AAA). IP address assignment: The BRAS is also responsible for assigning session parameters such as IP addresses to the clients. In most cases, the BRAS is the first IP hop from the perspective of the client. Policy management and QoS: The service provider can insert management and Quality of Service (QoS) policies at the BRAS. In xDSL networks, the connections are typically PPP over Ethernet (PPPoE), PPP over ATM (PPPoA) or IP over Ethernet (IPoE) where the connection endpoints are customer premises equipment (e.g., STB) and the BRAS. This connection is required for user authentication and authorization during service attachment. In addition, IGMP messages are sent through this connection. BRAS can monitor individual members by correlating IGMP messages with the PPPoE connection from which they were received (see Fig. 2).

3.1 DSL Digital Subscriber Line (DSL) refers to a group of technologies that employ the unused bandwidth in the existing copper local loops to deliver high-rate data services to the end users. The main reason for success and popularity of DSL technology is that it requires almost no upgrading because it uses the existing telephone infrastructure which has already been widely installed in homes and office buildings. There are many variations of DSL (e.g., ADSL, SDSL, VDSL), each aimed at a particular purpose but designed to accomplish the same basic goals. As depicted in Fig. 1, the xDSL access network is composed of two main network elements. The Broadband Remote Access Server (BRAS) and Digital Subscriber Line Access Multiplexer (DSLAM). DSLAM aggregates traffic from hundreds of subscriber premises and connects to BRAS which again aggregates outputs from its downstream DSLAMs to the high speed metropolitan backbone. In the following, these devices are discussed in more detail.


Digital Subscriber Line Access Multiplexer (DSLAM) The DSLAM aggregates the requests coming from numerous home-gateways onto a single high-capacity uplink (ATM or Gigabit Ethernet) towards the BRAS and replicates TV streams toward subscribers. Each DSLAM typically can provide services for few hundred subscribers and uses multiplexing techniques. Based on the type of the uplink port, DSLAMs can be divided into ATM DSLAMs or Ethernet DSLAMs. In general, DSLAMs function as layer 2 devices but some new DSLAMs have limited layer 3 capabilities. The DSLAM must be able to make the decision to transmit a given multicast stream on a DSL line. Most new DSLAMs have the ability to listen to IGMP messages sent by the STB (IGMP snooping) and build multicast forwarding tables accordingly. IGMP snooping implies that IGMP packets are read and then forwarded to the upstream router transparently without any manipulation. In addition, most new DSLAMs can perform IGMP proxy function. IGMP proxy transmits only one message per multicast group to the BRAS, using aggregation and suppression of upstream IGMP messages. IGMP snooping at the DSLAM enables the BRAS to identify individual members based on the packets’ IP or MAC addresses.


Broadband Remote Access Server (BRAS) The BRAS sits between access and distribution networks and provides access to the Internet and related services to the customers. The specific tasks of a BRAS include: Aggregation point: The BRAS aggregates the output from multiple DSLAMs in the access network.

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WiMAX MAC layer supports five QoS service types. The real-time Polling Service (rtPS), which is designed to support real-time service flows that generate variable size data packets on a periodic basis (e.g., MPEG video), is ideal for IPTV applications. ASN-GW (Access Service Network Gateway) acts the role of IGMP router. It is the first-hop router from the perspective of MS and is also in charge of providing AAA services for WiMAX clients. As depicted in Fig. 1, downstream connections can be used to transmit the same content to a group of MSs, using multicast CIDs (MCIDs). Multicast CIDs are in the range 0XFE9F to 0XFEFE and suited for IP multicast data transmission [17]. Still there are some unresolved issues with the use of MCID, such as the low transmission efficiency of MCIDs for small multicast groups.

IGMP proxy, however, prevents BRAS from identifying individual group members since the DSLAM, which in this case generates the IGMP messages, would act as an IGMP router for users and as an IGMP client for the BRAS). The typical DSL protocol stack is depicted in Fig. 2. Here, DSLAM is connected to BRAS through an ATM backbone.

Fig. 3 depicts the protocol stack of a connection between a WiMAX MS and ASN-GW. Here, IP-CS is a term used in WiMAX to describe a process which allows IP packets to be directly carried in the 802.16 PDU (Protocol Data Unit).

Fig. 2- DSL protocol stack

3.2

WiMAX

Worldwide Interoperability for Microwave Access (WiMAX) technology which is based on IEEE 802.162004 and IEEE 802.16e-2005 air-interface standards, provides a wireless last-mile broadband access for fixed and mobile subscribers in a Metropolitan Area Network (MAN) [16]. It is capable to deliver data rates up to 70Mbps and to cover ranges of about 50km and can provide secure delivery of content and support mobile users at vehicular speeds. WiMAX operates in MAC and Physical layers of OSI reference model. In order to reduce inter-symbol interference (ISI) and provide resiliency to multi-path fading, orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) techniques are used in fixed and mobile WiMAX, respectively. Adaptive Modulation and Coding (AMC) is used based on MS’s SNR to provide a tradeoff between throughput and robustness. Thus, inside the Base Station (BS) coverage area, each Mobile Station (MS) can use the most suitable modulation and coding scheme irrespective of the others. Advanced antenna systems like MIMO, sectorized antennas and beam-forming techniques can also be used in physical layer to provide a better SNR for MSs. WiMAX MAC layer is divided into three distinct sublayers: convergence sublayer, common part sublayer and security sublayer. In WiMAX, MAC layer is connection oriented and each connection is identified by a unique 16-bit Connection Identifier (CID). There exist several connections between SS/MS and BS for uplink and downlink data transmission and management message transmissions.

Fig. 3- WiMAX protocol stack

The interface between wireless device and BS is represented by R1 reference point and R6 reference point. It defines the interface between BS and ASNGW as depicted in the Fig. 3 [18]. IP packets between the BS and the ASN-GW are transmitted over a Generic Routing Encapsulation (GRE) tunnel. A great advantage of WiMAX is to support multicasting using MBS. Similar to unicast services in IEEE 802.16, the MBS service flows are managed through a DSx (Dynamic Service Addition/ Deletion/ Change) messaging procedure used to create, remove, and change a service flow for each MS. Some important service flow information such as QoS, service flow identifier (SFID), and multicast connection identifier (MCID) are exchanged between MS and BS via the DSx handshaking procedure. In the three-way DSA procedure, the uplink transmission takes most time, because the uplink bandwidth is shared among multiple MSs via bandwidth request and grant mechanisms. When a user wants to watch a specific TV channel the following steps should be carried out (see Fig. 4): 1. The MS tries to join the multicast group of the desired TV channel by sending the IGMP Join message

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to the ASN-GW. In addition, a three-way dynamic service addition (DSA) mechanism at MAC layer between MS and BS is used to establish a multicast connection with a specific multicast CID at MAC layer. Therefore, there is a one-to-one mapping between the MAC layer multicast connection and the IP layer multicast stream [19]. DSx messages are transmitted on primary management connection and IGMP messages are transmitted on secondary management connection of the MS.

To summarize, some features that make WiMAX to be a good candidate as an IPTV access network are including but not limited to: QoS support, Multicast Broadcast Service (MBS), wide coverage range, high bandwidth, power saving mode (necessary for handheld devices) and mobility support. 4- Comparison of DSL and WiMAX access networks In this section DSL and WiMAX access networks are compared from different perspectives and advantages and disadvantages of each are discussed.

2. Depending on the situation of the TV channel, the ASN-GW may forward the IGMP join request to the upstream router or itself reply to the request. ASN-GW also stores the multicast IP address of the channel and MAC address of the BS which covers the MS.

4.1 Mobility Mobility support is an essential feature of the next generation networks. WiMAX can provide ubiquitous services for both fixed users and mobile users at vehicular speed (up to 120 km/h). The IEEE 802.16e standard which is also known as mobile WiMAX, defines a framework for supporting mobility management. Power management and handover are two important issues for mobile applications. Energy saving features help the MS to reduce the power consumption and allow the MS to enter a pre-negotiated sleep mode or periodic idle mode when there is no data destined for it. Meanwhile, handover procedures like “hard handover”, “Macro Diversity Handover” and “Fast BS Switching handover”, provide uninterrupted and seamless services for clients during movement from one location to another. In addition, beam-forming and smart antennas can be used to provide higher SNR for mobile subscribers. Obviously, those who use DSL as an access network and use STB, have TV sets with SD or HD resolution and mobility is not a consideration. For the subscribers who use DSL modems with wireless capability, some limited mobility can be supported.

3. A multicast stream corresponding to desired TV channel will be transmitted to the MS via the connection with the assigned MCID. After a short delay for buffering, receiving a suitable play point (e.g., Iframe) and decoding, the user will be able to watch the channel. Since there is a one-to-one mapping between multicast connections in MAC and IP layers, and in order to not disrupt the sleep mode of Mobile subscribers, ASN-GW can reply to IGMP query messages on behalf of the mobile nodes. If no active client watching a specific TV channel exists in the downstream, ASN-GW does not reply to a query message belonging to that specific channel and therefore, the edge router removes that channel from the list of multicast groups [19]. Leaving a channel is done in a similar manner. Each channel change consists of a channel leave and a channel join.

4.2 Cost WiMAX can be widely used to provide broadband Internet connectivity to sparsely populated rural areas where using technologies like DSL or cable modems is not economically beneficial. Meanwhile, for the developing countries in which there is not an extensive wired networking infrastructure, establishing network coverage through WiMAX base stations can be more cost-effective. On the other hand, for the urban environments in which there exists a telephone infrastructure, DSL is often seen as a fairly cost effective way to offer more services to customers. In addition, DSL allows competitive operators to offer a variety of broadband services, without having to deploy their own infrastructure.

Fig. 4- IPTV signaling mechanism in WiMAX

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From another perspective, DSL is difficult for upgrade for sake of the complex infrastructure and high expense. To summarize, WiMAX is an appropriate technology to install at urban area or remote villages where DSL has difficulty to reach by high expense and long term deployment. It only needs just a few wireless base stations and some small Customer-premises equipment (CPE) mounted on the building or home to provide coverage in the surrounding area.

4.5 Channel switching delay TV viewers usually like to surf the channels especially during some periods of time. For example during a commercial advertisement or between half-times of a football match, most users change the channel to find a more favorite program. Channel changing in a cable television system is very fast because all channels are present on the wire. In an IPTV system, due to the limited capacity of the last-mile, only one TV channel can be transmitted to the STB. Therefore, one could experience a switching delay of about a few seconds, because of the following factors: -Multicast latency for “leaving” the old channel and “joining” the new channel (IGMP leave and join latency) -Program Clock Reference (PCR) and sequence header information -Random access point (such as I-frame) acquisition delay -Network buffer delays, including delays caused by error-mitigation techniques -MPEG decoder buffer delay Switching delay of about two seconds has been reported in DSL access networks [5]. Since still there is no widely operational IPTV system over WiMAX, no actual data regarding channel switching delay exists in the literature. But regarding to what we mentioned about contention-based uplink transmission, multicast join and leave requests (in MAC and IP level) which are transmitted via uplink, may experience more latency than in DSL networks. On the other hand, taking advantage of MBS capability may provide a good opportunity in WiMAX networks to shorten the switching delay [14]. Therefore, all of the factors must be considered in determining the switching delay.

4.3 Display resolution Since we assume WiMAX subscribers use PDA or mobile phones which have low display resolution and small sizes, encoding standards like QCIF (176×144) or CIF (352×288) can be used. Display resolution directly affects bandwidth requirement for TV streams. For example, for a TV program in CIF format, bandwidth requirement is about 400 Kbps. For users who use DSL to subscribe IPTV services, SD (720×480) with MPEG-2 encoding which requires about 3 Mbps of bandwidth can be used. Since bandwidth is shared between different TV channels in a WiMAX system, the number of different TV channels is limited by both the required bandwidth for each TV channel and the range of multicast CIDs (0XFE9F to 0XFEFE). An example of number of concurrent users (or different TV channels in MBS) is given in [20]. Up to 45 users per transmitter can be served in a 10 MHz bandwidth channel, with a video resolution of 320×240 pixels (QVGA format), and frame rate of 15-30 fps at a 300 Kbps bit rate. 4.4 Uplink and downlink techniques As depicted in Fig. 6, in DSL technologies there is a dedicated bandwidth for uplink and downlink transmission. Although the bandwidth for uplink and downlink may not be equal (e.g., in ADSL), it is dedicated and no contention is necessary between subscribers to send data. In addition, both uplink and downlink connections are point-to-point (between DSLAM and STB) in DSL access networks. On the other hand, in WiMAX networks, the downlink allocation of bandwidth is a process accomplished by the BS according to different parameters. WiMAX takes advantage of MBS for multicasting. Therefore, for TV channels, downlink is a point-to-multipoint connection. The MS obtains the uplink unicast bandwidth by the random backoff-based contention mechanism. In addition, random access may work in combination with polling, which is referred to as a grouping mode [21]. Therefore, each WiMAX subscriber should content for the bandwidth during a contention period in uplink subframe (see Fig. 5).

4.6 QoS Quality of service (QoS) is the ability to provide different priority to different flows or applications to guarantee a certain level of performance. QoS guarantees can be characterized by: delay, jitter, bandwidth, and error rate. The WiMAX architecture provides five different classes of service to assure that the required QoS is obtained. Four classes of service (UGS, rtPS, nrtPS and BE) are defined in IEEE 802.16d, and IEEE 802.16e introduces another QoS class called Extended RealTime Polling Service (ertPS). As mentioned previously QoS class rtPS is suitable for encoded videos. In DSL networks, configuring some QoS service policies such as class-based weighted fair queuing or low latency queuing, in a PPPoE session is possible. Meanwhile, most DSLAMs support IEEE 802.1p Layer two QoS protocol for traffic prioritization and dynamic multicast filtering at MAC layer. In ATM-based xDSL, QoS mechanisms provided by ATM can be used as well.

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4.7 Bandwidth allocation strategy Bandwidth allocation in WiMAX frames consists of time slots in one dimension and frequency subchannels in the other. WiMAX supports both Time Division Duplex (TDD) and Frequency Division Duplex (FDD) operation. In the case of FDD, the uplink and downlink sub-frames are transmitted simultaneously on different carrier frequencies. TDD uses only one channel for UL and DL transmission, and can dynamically adjust the downlink/uplink ratio to efficiently manage the asymmetric demand for bandwidth. As depicted in Fig. 5, the TDD frame structure is divided into DL and UL sub-frames separated by transition gaps to prevent DL and UL transmission collisions. Typical frame duration is about 5ms. In the DL sub-frame, the preamble is used for synchronization; the Frame Control Header (FCH) provides the frame configuration information such as MAP message length and coding scheme and usable sub-channels. The DL-MAP and UL-MAP provide sub-channel allocation, the type of modulation and coding used by the bursts containing the data information, in DL and UL respectively, as well as other control information. In the UL sub-frame the UL Ranging sub-channel is allocated for MS to perform closed-loop time, frequency, and power adjustment as well as bandwidth requests. The bandwidth requests can be granted by the BS per connection or per subscriber. In the latter case, the Subscriber Station (SS) decides to allocate the bandwidth among multiple streams. In Fig. 5, a combined multicast–unicast in downlink sub-frame is shown which can be used for live TV and VoD services. As mentioned previously, DSL makes use of the existing telephone lines employing previously unused frequency bandwidth above the voice band. It separates the available bandwidth for voice communications (PSTN) and data (Upstream/Downstream). Although PSTN and ADSL occupy distinct channels, they might influence one another. Telephone devices can possibly experience audible disruption whilst ADSL signals can experience interference resulting in throughput deterioration. To avoid this mutual interference, a splitter is required. The most likely approach is called the DMT (Discrete MultiTone) which is an OFDM-based technique. It divides the available 1.1 MHz spectrum on the local loop into 256 independent channels of 4.3125 kHz each. Therefore, DMT based modems can be thought of as 256 “mini modems” running simultaneously. Each channel is modulated using QAM. The first channel is used for traditional PSTN voice transmission and the next five channels are not used to avoid interference of voice and the data signals. The remaining 250 channels are available for the user data.

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It is up to the service provider to determine how many channels should be allocated for upstream and downstream. Although equal share is possible, because ordinary users download more data than they upload, most providers allocate more channels for downstream. For example in ADSL, 26 channels are used for uplink and 224 channels for downlink as shown in Fig. 6.

Fig. 5- WiMAX frame structure in TDD scheme

Fig. 6- Division of ADSL bandwidth

4.8 Data rate In general, WiMAX data rate depends on many factors such as distance, type of modulation, numbers of active subscribers and whether they are in line-of-sight (LOS) or non-line-of-sight (NLOS) situation. A commonly held misconception is that WiMAX can provide data rate up to 70Mbps, over 50km to a mobile subscriber. Each of these is true individually, given ideal circumstances, but they are not simultaneously true. For example, if one could receive data at rate of 4 Mbps in line-of-sight environments at 10km, in an urban environment and non-line-of-sight condition, the same rate can be delivered only at the distance of 1 km. For WiMAX systems operating on the sub-10 GHz frequency bands, the average data rate in a coverage area of a BS would be around 10 Mbps. But the subscribers in the cell edge utilizing BPSK modulation are capable of transmitting/receiving at data rates of less than 3 Mbps. This means that higher rates can only be offered for subscribers located sufficiently close to the base station, utilizing e.g., 16-QAM or 64-QAM modulation.

In this respect, WiMAX has some similarities to xDSL technology, where one can either have high bandwidth or long reach, but not both simultaneously (distance sensitivity). In ADSL for example, subscribers having too long local loops from their premises to the telephone exchanges are not able to utilize data rates in excess of 1 or 2 Mbps. All the users within one sector share the same data transfer rate in WiMAX, whereas, every user could get their bandwidth exclusively in xDSL. As a result, when there are huge number of users, xDSL could supply a better performance than WiMAX.

WiMAX is a new technology and needs time in order to overcome some performance barriers. Eventually, WiMAX deployments are expected to deliver IPTV to rural and underserved regions with high degree of video and audio quality at affordable prices. In a WiMAX network QoS is provided in MAC layer using different service classes (UGS, rtPS, nrtPS and BE), while DSL uses IEEE 802.1q. Fig. 7 depicts a comparison between WiMAX and ADSL access technologies in terms of downlink data rate they offer in best case and worst case. Using a 2X2 MIMO, a high data rate of about 27 Mbps for the users near the BS (using 64QAM 3/4) can be achieved in WiMAX. The lowest data rate is for the users near the border of the cell using a BPSK 1/2 modulation and coding scheme. This data rate is shared among the users and therefore, with the increment of the number of users, the bandwidth portion of each user decreases. In ADSL connections, however, since the access link is dedicated to the user, the number of subscribers does not affect the available bandwidth.

Table 1- Comparison of WiMAX and ADSL access networks

WiMAX Mobility

Cost

Display resolution Uplink Downlink Channel switching delay QoS Duplexing method Data rate

Max range

Technology

ADSL

Fixed - nomadic mobile High in urban environments, Low in rural environments QCIF (176x144) , CIF (352x288) , QVGA (320x240) Point to pointcontention based Point to multi point [22] – shared (for multicast) Not reported

SD (720x480) , HD (1920x1080)

MAC (UGS, rtPS, nrtPS and BE)

ATM, 802.1q

Time and frequency

Frequency

Sensitive to distance from BS and number of subscribers2.8 to 14 Mbps

Distance sensitive 1.5 (5.4 km) to 12 Mbps (300 m)

LOS 30 to 50 km NLOS – 2 to 5 km Mobile WiMAX: OFDMA Fixed WiMAX: OFDM

Fixed Low in urban environments, High in rural environments

Point to point dedicated Point to point dedicated About 2 seconds

5.4 km Fig. 7- Available data rate for WiMAX and ADSL

OFDM

Assume 70% of the available bandwidth is used for IPTV services. In WiMAX networks, overhead for transmitting the control information (FCH, DL-MAP and UL-MAP and MBS overhead) has a significant impact on the overall capacity of the frame (at least 10% reduction) [23]. After taking into account the above-mentioned factors, we want to know how many TV channels (in SD format assumed to require 3Mbps) can be offered by each access technology. Fig. 8 shows the number of TV channels that can be supported in parallel (different channels which are transmitted by means of unicast). The curves are obtained by dividing the available bandwidth by the bandwidth requirement for a TV channel in SD format. The gray area represents the infeasible region in which less than one channel can be supported.

5- Assessment and Discussion From our discussion in Section 4, we can summarize the comparison results in Table 1. Mobile WiMAX can provide services for subscribers moving at vehicular speed. This is a great benefit which can pave the way for quad-play in the next generation networks. The IPTV subscribers who use xDSL technology to access the services, can enjoy wide screens with higher resolution thanks to higher and dedicated bandwidth. Dedicated point to point uplink and downlink in xDSL is an important factor to decrease channel switching delay.

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[6]

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Fig. 8- No of supported TV channels

6- Conclusion

[10]

Access networks play an important role in an IPTV system. Several fixed-line and wireless access networks are available, each of which having their own advantages and disadvantages. xDSL and WiMAX as two candidates among wired and wireless access networks have been compared by us from different perspectives. WiMAX could provide wireless access service to subscribers with high speed and large coverage range just the same as what customer can get from wired access model. On the other hand, xDSL technology is based on the existing copper telephone line. WiMAX provides a flexible bandwidth allocation while in DSL technologies the ratio of DL and UL bandwidths is fixed after tuning by the service provider. The other feature to consider with WiMAX is that available bandwidth is shared between subscribers in a given sector, so if there are many active subscribers in a single sector, each will get reduced bandwidth. Both technologies are distance sensitive. WiMAX has the opportunity to struggle with xDSL technologies especially in rural environments and developing countries. It can be expected that WiMAX (or similar technologies such as LTE [24]) will become a big threat for xDSL in near future as it gets more matured with fast cost descending.

[11]

[12]

[13]

[14]

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