9
Medium Access Control Design
The medium access control (MAC) protocols control the access rights of the shared medium. In a half-duplex wireless channel, a node cannot sense the channel while transmitting a frame. Therefore, an efficient MAC protocol becomes necessary. Since collisions cannot be detected by the transmitter, a typical MAC scheme attempts to decrease the probability of a collision using collision avoidance principles. For instance, in IEEE 802.11, a device wishing to access the channel first senses the medium to determine if it is idle. If so, the device waits an additional, randomly selected, period of time and then transmits if the medium is still free. However, if it is determined that the channel is busy, the device may enter into a random backoff period and wait until transmission stops. This prevents multiple devices seizing the medium immediately after completion of preceding transmission. Such carrier sensing based MAC schemes rely on an omnidirectional antenna that receives and transmits radio frequency (RF) energy equally in all directions. While omnidirectional antenna patterns assist an efficient medium access, they create undesired interference at some of the other devices.
The 60 GHz band provides an extremely short wavelength of 5 millimeters that reduces the antenna size significantly. Now it becomes feasible to mount a large array of antenna elements (or directional antennas) on a node. Directional antennas provide some advantages ...
