Wireless Channel Access Protocols

his lecture introduces the problems and challenges in the design of wireless networks and discusses the area of channel access protocols (aka “medium access”, or MAC protocols) in detail.! 11.1 Wireless Networks are Different Communicating over radio is an old idea, and people have been doing it for a long time. Data networks have been around for about four decades. Combining the two is attractive because wireless networks offer important potential benefits over their wired counterparts: they enable mobile communications, they don’t require as much investment in and upkeep of a wired infrastructure, and they enable broadcasts in a natural way. For many years now people have been building data networks that operate over radio channels. One lesson that has been learned is that radios are different from wired links and affect the design of networks in important ways: radios make the design of data networks both interesting and challenging. This lecture and the three following it discuss these issues in detail. Realizing the potential benefits of wireless networks in practical, working systems is difficult for one deceptively simple reason: radios are not wires. To understand the implica-tions of this statement, consider resource sharing in a wired network.1 Placing a “network layer ” over the “link ” and “physical ” layers works well in wired networks because the network designer has to worry about sharing only at the network layer (and higher): links themselves (and therefore the link and physical layers) are shielded from one another and concurrent data transmissions on two or more wired links have no influence on each other unless those transmissions interfere at a switch (i.e., at the network layer). In contrast, wireless transmissions are over a shared medium with very little shielding, especially when omni-directional antennas are used. The problem is that two radio trans-mitters that are near each other will interact adversely with each other, especially if their intended receivers can hear both transmissions. Such “link-layer ” and “physical-layer” 1Sharing, as we have seen time and again in this course, is a fundamental property of all communication networks. 1