Ad Hoc Networks
MAC protocols

The important issue regarding ad hoc networks is to design suitable MAC protocols, which main purpose is to control the access to radio channel. The main challenge is to avoid collision in transmission and to effectively utilize radio channel. The MAC protocols can be classified into contention-free and contention based:

- Contention-free protocols - The UE does not have to contend for access to the medium. Instead, some kind of controlled access, such as TDMA, FDMA, CDMA, polling or token based, is employed here. The main advantage of contention-free approach is that there are no collisions in the transmission. Consequently, these MAC protocols are meant to guarantee end-to-end delay and specific QoS requirements. The contention-free scheme is employed, for example, in Bluetooth technology where master-slave MAC mechanism is utilized.
- Contention-based protocols - The contention based protocols introduce specific contention mechanism where the UE has to contend for access to the medium. Basically, all contention methods are based on ALOHA and Slotted ALOHA schemes. In case of ALOHA, the UE accesses the medium as soon as it has data to transmit. Consequently, if two UEs send data at the same time, collision occurs and data retransmission has to take place. Although, Slotted ALOHA partially solves the collision problem, its applicability is still too limited.

The most common access methods used in contemporary wireless networks such as WiFi are based on CSMA (Carrier Sensing Multiple Access). As the name suggests, the UE firstly “sense” the medium before data are sent. If the medium is for the time being used, the UE has to wait until the medium is released. If the medium is free, the UE sends its data immediately. To further minimize the probability of collision, other methods were introduced, such as CSMA-CD (CSMA with Collision Detection) or CSMA-CA (CSMA with collision avoidance).