Why Are MAC Layer Design Issues Important

In all wireless networks, nodes must share a single medium for communication. Network performance largely depends upon how efficiently and fairly the nodes can share this common medium. Note that the packet transmission is directly handled by the MAC layer. Compared to a wired medium, a significant portion of the node's energy is spent on radio transmissions and on listening to the medium for anticipated packet reception. On the other hand, wireless networks always have restricted power sources; thus, careful design of the MAC scheme is necessary for the optimal performance and extended lifetime of the network.

In the context of WSNs, this requirement is extremely critical. According to the characteristics highlighted previously, nodes of a WSN carry extremely low energy resources and remain unattended after deployment; therefore, the node lifetime depends entirely on how energy is conserved during communication. Although some exhausted nodes could be compensated using redundant neighboring nodes, certain situations may arise rendering a part of the network completely inactive due to low connectivity and insufficient coverage, or making that part of the network inaccessible and isolated from the other parts. Such scenarios could be averted by avoiding unnecessary transmissions and longer listening periods — activities that consume the highest amount of power in nodes.

Another related issue is the high node density in WSNs. Although the transmission ranges are lower, a fairly high number of nodes can contend for the medium, at least in certain portions of the network. By the same token, transmissions from each node would increase the background noise for a large number of nodes, which may disrupt their own receptions. Thus, the MAC schemes for WSNs should be carefully designed to achieve the optimum performance toward the intended application. Previous surveys [1, 5] discuss some issues related to medium access in WSNs and WAHNs.

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