Wednesday, April 20, 2011

Multimedia over Mobile WiMAX

Mobile worldwide interoperability for microwave access (WiMAX) technology based on recent IEEE 802.16e specifications and Orthogonal Frequency Division Multiple Access (OFDMA) physical layer air interface has become one of the most promising broadband wireless protocols to support high throughput and mobility over large coverage areas. This technology can provide fast and inexpensive broadband access to markets that lack infrastructure such as rural areas and unwired countries. 

WiMAX can also serve as backhaul networks for client accesses to hot spots using different technologies including 802.11a/b/g as well as 802.16d/e. The most prominent characteristics of mobile WiMAX is that it can provide large distance data services for up to 31 mi with high data rate transmissions. With rapid growth of online multimedia services, supporting sensitive multimedia streaming with low latency over wireless networks especially mobile WiMAX becomes a focus of many research and development activities. 
Figure 1 illustrates an example of multimedia delivery applications over mobile WiMAX networks for railroads. The high-speed 802.16d protocol is applied to backhaul network, and mobile WiMAX (802.16e with SOFDMA) is applied to client access networks.

Figure 1: Multimedia applications over mobile WiMAX networks.

Multimedia streaming is bandwidth intensive, delay sensitive but loss tolerant, and bit errors or packet losses are inevitable in WiMAX networks due to the error-prone characteristic of wireless channel. Another important characteristic of multimedia streaming is unequal importance, where different packets in the stream have different perceptional values in terms of reducing distortion (i.e., some packets in the stream may be much more important than other packets). Fortunately, the connection oriented medium access control (MAC) layer of mobile WiMAX is designed to provide flexible quality of service (QoS) to different applications, which lays foundations to support multimedia streaming over WiMAX networks in two aspects: traffic differentiation among connections and resource allocation adaptation inside each connection. Each application traffic flow (e.g., video, voice, data) can be mapped to one or multiple service flows, and each service flow is further mapped into a logical connection with a unique 16-bit connection identifier (CID). 

The Service Data Unit (SDU, e.g., an H.264 video frame or a JPEG2000 image packet) from upper layer is dispatched with proper CID by SDU classifier, and MAC common part sublayer performs fragmentation and retransmission as well as QoS control. There are five QoS classes defined in mobile WiMAX: Unsolicited Grant Service (UGS), extended real-time polling service (ertPS), real-time polling service (rtPS), non real-time polling service (nrtPS), and best effort (BE). Such WiMAX flow scheduling architecture is illustrated in Figure 2, and the attributes of each service class in terms of traffic differentiation are described as follows. Example applications are also shown for each type of service class.

Figure 2: Packet delivery process in mobile WiMAX.

UGS: The UGS service class is especially designed to support real-time service flows which generate fixed-size data packets on a periodical basis, and the UGS service class offers real-time periodic bandwidth grants, which eliminates the bandwidth request overhead and latency . Typical applications for UGS are T1/E1 data flows, G.711 based voice-over-IP (VoIP) traffic without silence repression, etc. 

ertPS: IEEE 802.16e standard introduces extended real-time polling service, which allows 802.16e to manage traffic rates and transmission policies, as well as improves latency and jitter performance, where the ertPS service class is built on the efficiency of both UGS and rtPS. The base station (BS) provides unicast bandwidth grants in an unsolicited manner similar to UGS. The difference between UGS and ertPS is that UGS bandwidth allocations are fixed while ertPS allocations are dynamic. The advantages afforded by ertPS are especially important in support of VoIP applications without silence repression, which generate variable size data packets on a periodic basis.

rtPS: The rtPS service class is designed to support real-time service flows with variable data size packets on a periodical basis, and it offers unicast and periodical request opportunities real-timely, which allows the subscriber station (SS) to specify the size of desirable bandwidth grant. rtPS incurs more bandwidth request-grant overhead than UGS, but it improves data transmission efficiency and bandwidth resource utilization. Typical applications for rtPS include Moving Pictures Expert Group (MPEG) video streaming, video conferences, and IPTV.

nrtPS: The nrtPS service class is designed to assure service flows receiving bandwidth request opportunities even during network congestions, where BS offers unicast polling service to SS on a regular basis. It is especially suitable for delay tolerant data streams such as HTTP-based Internet Web browsing, FTP-based file transferring, etc.

BE: The intent of BE service class is to support data streams without minimum bandwidth allocation requirement. The BE service (for instance, e-mail service) is on a resource available basis, where no throughput or delay guarantees are provided.

Overall, the versatile QoS framework in WiMAX has significant flexibility to differentiate application streams (e.g., voice, video/image, data) and to provide different services to these streams. More important, the flexible scheduling architecture provides considerable advantages to the resource allocation adaptation and optimization in each media stream.
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