Performance Metrics Given the wide range of protocols that have been proposed, it is necessary tounderstand the metrics that are used to compare the MAC protocols. Delay,throughput, fairness, support for multimedia, and stability are the widely used metricsto compare MAC protocols. Robustness against fading and battery power consumptionare additional metrics used to compare wireless MAC protocols. Following is a briefdiscussion of these metrics. Delay Throughput Fairness Stability Robustness against Channel Fading Power Consumption Support for Multimedia Delay: Delay is defined as the average time spent by a packet in the MAC queue, i.e., from theinstant it is enqueued till its transmission is complete. Delay is a function of protocoland traffic characteristics. Therefore, when comparing protocols, it is necessary tocompare them based on the same traffic parameters. Throughput: Throughput is the fraction of the channel capacity used for data transmission. A MACprotocol's objective is to maximize the throughput while minimizing the access delay.If the average message size is P bits, the average time to transfer a single packet is Tsecs, and C b/s is the capacity of the channel, then the throughput γ given by γ =P/TC. Fairness: A MAC protocol is fair if it does not exhibit preference to any single node whenmultiple nodes are trying to access the channel. This results in fair sharing of thebandwidth. This definition can be biased when traffic with different priorities ishandled. When multimedia traffic is supported, fairness is defined as be enable todistribute bandwidth in proportion to their allocation. Stability: Due to overhead in the protocol, the system may be able to handle sustained sourceloads that are much smaller than the maximum transmission capacity of the channel.A stable system can handle instantaneous loads that are greater than the maximumsustained load when the long-term offered load is less than the maximum. Robustness against Channel Fading:
The wireless channel is time-varying and error-prone. Channel fading can make thelink between two nodes unusable for short periods of time. Such link failures shouldnot result in unstable behavior. Power Consumption: Most wireless devices have limited battery power. Hence, it is important for MACprotocols to conserve power and provide some power saving features. Support for Multimedia: With the convergence of voice, video, and data networks, it is now necessary for MACprotocols to support multimedia traffic Protocols require mechanisms to treat packetsfrom various applications based on their delay constraints. Two common methods areaccess priorities and scheduling- Access priorities provide differentiated service byallowing certain nodes to get access to the network services with a higher probabilitythan others. Scheduling can give delay and jitter guarantees.