SIMULATION OF A MAC PROTOCOL FOR WIRELESS MULTIMEDIA TRAFFIC

Abstract

The demand on information exchange has pushed the development of wireless communication in an unprecedented pace. The traffic in the next-generation wireless networks will be a mixture of voice, data, and video messages. These traffic classes have their own characteristics of requirements such as maximum tolerable bit error rate, transmission rate, and timeout specification. Bandwidth is an important issue in wireless networks. So a highly flexible MAC protocol is needed, that can easily adapt to the changing conditions and requirements of multimedia traffic, so that, bandwidth is used efficiently. The thesis reviews and evaluates Wireless multimedia access control protocol with BER scheduling(WISPER), which has potential to meet the above requirements. WISPER is basically a novel MAC protocol proposed for CDMA based systems, which tailors the transmission of multimedia packets according to their BER requirements. The objectives of the WISPER protocol are to maximize the throughput, and to decrease the packet losses. Throughput is maximized by incorporating a novel packet scheduler which accommodates the packets with equal or similar BER requirement in the same slots. Packet losses are minimized by using packet prioritization scheme which schedules the packets with higher priority for transmission first. Priority values are determined according to the packet timeout values and number of packets ready for transmission. iii Figure 3.1-a An example of capture effect In (Figure [3.1-a]), both terminal A and terminal B want to transmit signals to terminal C. Although A and B have the same transmitting power, terminal B is closer to terminal C, so the received signal power from node B is greater and is the only transmission picked up by C. The capture effect can be beneficial even though it is unfair. In this example, a collision would have occurred if the capture effect did not take place. The capture effect can be minimized however, by using a sophisticated power controller. Hidden termilals A hidden terminal is one that is within the range of the intended destination but out of range of the sender . Hidden terminals can cause the problem of collisions on data transmission[15]. Exposed terminals Exposed terminal are complementary to hidden terminals. An exposed terminal is one that is within the range of the sender but out of range of the destination. If the exposed terminals are not minimized, the bandwidth is underutilized[15]. 3.2 MAC PROTOCOL PERFORMANCE PARAMETERS There are several issues important to a MA(: protocol that are also metrics for measuring its performance. Most of them apply to both wired and wireless MAC protocols, although some are wireless specific. The following is a brief discussion of these parameters: 19 Delay Delay is usually measured as the average time a packet spends in the queue. It is the time from the generation of a packet until it is transmitted successfully. The delay requirement is usually dependent on the type of traffic being transmitted. A long delay is not always bad. If the payload of each transmission is large, then the overall transmission rate is not affected. If the traffic being carried is delay sensitive (e.g. real time voice), then a long delay is not suitable. Throughput Throughput is the fraction of the total channel capacity that is used for data transmission. If the throughput is high, then the bandwidth wastage is small. The goal of a good MAC protocol is to maximize throughput, while minimizing access delay. Fairness A fair MAC protocol ensures that each user has the same opportunity to access the channel. Unfairness can produce dominators in the system and cause the system to be unpredictable and unbalanced. If a user close to the target user has a large amount of data to transmit, then the user further away from the target must wait for a long time before transmitting its packets to the target user. Stability A network is required to be stable at all times. This includes the occasional heavy load on the channel that is greater than the maximum transmission capacity. An unstable MAC protocol can fall apart during a heavy load. The average delay can rise dramatically and cause the channel to be jammed for a very long period. A stable system should handle heavy loads without a long delay........