Limited-Contention Protocols

Limited Contention Protocols are the media access control (MAC) protocols that combines the advantages of collision based protocols and collision free protocols. They behave like slotted ALOHA under light loads and bitmap protocols under heavy loads.


In computer networks, when more than one station tries to transmit simultaneously via a shared channel, the transmitted data is garbled, an event called collision. In collision based protocols like ALOHA, all stations are permitted to transmit a frame without trying to detect whether the transmission channel is idle or busy. In slotted ALOHA, the shared channel is divided into a number of discrete time intervals called slots. Any station having a frame can start transmitting at the beginning of a slot. Since, this works very good under light loads, limited contention protocols behave like slotted ALOHA under low loads.

However, with the increase in loads, there occurs exponential growth in number of collisions and so the performance of slotted ALOHA degrades rapidly. So, under high loads, collision free protocols like bitmap protocols work best. In collision free protocols, channel access is resolved in the contention period and so the possibilities of collisions are eliminated. In bit map protocol, the contention period is divided into N slots, where N is the total number of stations sharing the channel. If a station has a frame to send, it sets the corresponding bit in the slot. So, before transmission, each station knows whether the other stations want to transmit. Collisions are avoided by mutual agreement among the contending stations on who gets the channel. Limited contention protocols behave like slotted ALOHA under low loads.

Working Principle

Limited contention protocols divide the contending stations into groups, which may or not be disjoint. At slot 0, only stations in group 0 can compete for channel access. At slot 1, only stations in group 1 can compete for channel access and so on. In this process, if a station successfully acquires the channel, then it transmits its data frame. If there is a collision or there are no stations competing for a given slot in a group, the stations of the next group can compete for the slot.

By dynamically changing the number of groups and the number of stations allotted in a group according to the network load, the protocol changes from slotted ALOHA under low loads to bit map protocol under high loads. Under low loads, only one group is there containing all stations, which is the case of slotted ALOHA. As the load increases, more groups are added and the size of each group is reduced. When the load is very high, each group has just one station, i.e. only one station can compete at a slot, which is the case of bit map protocol.

The performance of limited contention protocol is highly dependent upon the algorithm to dynamically adjust the group configurations to the changes in network environment.

Example − An example of limited contention protocol is Adaptive Tree Walk Protocol.