Classification of MAC Protocols Given figure shows a classification of wireless MAC protocols. Wireless MAC protocols can be broadly classified into two categories, distributed andcentralized, according to the type of network architecture for which they are designed.Protocols can be further classified based on the mode of operation into random accessprotocols, guaranteed access protocols, and hybrid access protocols. In a randomaccess protocol, nodes contend for access to the medium. When only one node makesa transmission attempt, the packet is delivered successfully. When multiple nodesmake a transmission attempt, a collision results. Nodes resolve the collisions in anorderly manner according to rules defined by the contention resolution algorithm(CRA). ALOHA was the first protocol proposed for packet radio networks, and it is aclassic example of a random access protocol. The protocol operates as follows: A nodethat has data to send transmits it. If the transmission collides with anothertransmission, it retries after a random period. The maximum throughput of thisprotocol is 18 percent. If the medium is slotted and transmission attempts are made atthe beginning of the slot, the vulnerable period of a transmission is halved, doublingthe efficiency of the system. This slotted version of ALOHA is called S-ALOHA. In aguaranteed access protocol, nodes access the medium in an orderly manner, usuallyin a round-robin fashion. There are two ways to implement these protocols. One is touse a master-slave configuration, where the master polls each node and the nodesends data in response to the poll. These protocols are called polling protocols. Thesecond is to operate in a distributed manner by exchanging tokens. Only the stationwith the token can transmit data. Each station, after transmitting data, passes thetoken to the next station. These protocols are called token-passing protocols. Hybrid
access protocols blend the best qualities of the above two protocols to derive moreefficient MAC protocols. Most hybrid access protocols are based on request-grantmechanisms. Each node sends a request to the base station indicating how much timeor bandwidth is required to send the data currently resident in its buffer. The requestis sent using a random access protocol. The base station then allocates an upstreamtime slot for the actual data transmission and sends a grant to the node indicatingthat time slot. Depending on the intelligence at the BS, the hybrid access protocolscan be further classified into Random Reservation Access (RRA) protocols and DemandAssignment (DA) protocols. In an RRA protocol, the BS has implicit rules for reservingupstream bandwidth. An example of a rule is: A successful request results in aperiodic reservation of an upstream slot. On the other hand, in a DA protocol the BScontrols upstream data transmissions according to their QoS requirements. It collectsall the requests from the nodes and uses scheduling algorithms to make bandwidthallocations. Even though wireless MAC protocols handle issues very differently fromwire line networks, some of the principles in hybrid access protocols are similar to theprinciples in the protocols developed for hybrid-fiber coax systems. Hybrid accessprotocols and polling protocols by their mode of operation require a central node.Therefore, they fall into the category of centralized MAC protocols. Random accessprotocols can operate in either architecture. Token passing protocols could be used asdistributed protocols but are not because of robustness considerations. Due to thetime varying nature of the wireless channel, token loss would be common and tokenrecovery is a huge overhead. As a result, all proposed distributed MAC protocols arerandom access protocols. Distributed Mac Protocols: With the exception of ALOHA, all distributed MAC protocols are based on principles ofcarrier sensing and collision avoidance. Carrier sensing refers to listening to thephysical medium to detect any ongoing transmissions. Recall that location-dependentcarrier sensing results in hidden and exposed nodes. Such nodes play a dominant rolein CSMA protocols. Collisions that occur at the destination node are not necessarilyheard by the sender, so the destination needs to relay feedback to the sender.
Consider the example in figure when nodes A and C transmit simultaneously to B.This results in a collision at B but not at either A or C. Therefore, B has to transmitthis collision information back to A and C. However, because wireless transceiversoperate in half-duplex mode, nodes cannot listen while transmitting and the feedbackinformation has to be sent using out of band signals or the node has to stop and listenfor feedback. As a result, most distributed MAC protocols use collision avoidancetechniques wherein mechanisms are built into the protocol to minimize the probabilityof a collision. There are two mechanisms that can be used: the already mentioned out-of-band approach and the handshaking approach. These two approaches are described below. 1. Collision Avoidance Mechanisms. Busy tone multiple access (BTMA) is an example of a protocol that uses an out-of-band busy tone signal to preventhidden nodes. Any node that hears an ongoing transmission transmits busytone; any node that hears a busy tone does not initiate transmission. Thus, allnodes in a 2R radius of the transmitting node are inhibited from transmitting,where R is the range of the transmitting node. Although this solution eliminateshidden nodes, it increases the number of exposed nodes. In receiver initiated-busy tone multiple access (RI-BTMA), a node transmits a busy tone only after itdecodes the transmission and identifies itself as the intended\ receiver. As aresult, only the nodes within radius R of the receiving node are inhibited. In RI-BTMA the destination has to decode and match the destination address todetect a collision. As a result, takes a long time to initiate the busy tone.Therefore, the packet transmission is vulnerable to collision for a durationmuch longer than the round trip. This results in a higher probability of collisionand hence lower throughput. RIBTMA does not completely eliminate hiddennodes but minimizes exposed nodes. 2. Collision Avoidance with Control Handshaking. Multiple access with collision avoidance (MACA) uses a three-way handshake as a solution to the hidden nodeproblem. A node that has data to send transmits a short Request to Send (RTS)packet. All stations within one hop of the sending node hear the RTS and defertheir transmissions. The destination responds with a Clear to Send message(CTS). All nodes within one hop of the destination node hear the CTS and alsodefer their transmissions. On receiving the CTS, the transmitting node assumesthat the channel is acquired and initiates the data transmission. Thishandshaking mechanism does not completely solve the hidden terminalproblem, but it does prevent it to a large extent. Enhancements to RTS-CTScontrol handshaking and more complete single-channel solutions can be foundin. In these techniques there is a tradeoff between the overhead of handshakingand the number of hidden nodes eliminated.