Bit-map protocol is a collision free protocol that operates in the Medium Access Control (MAC) layer of the OSI model. It resolves any possibility of collisions while multiple stations are contending for acquiring a shared channel for transmission.
In this protocol, if a station wishes to transmit, it broadcasts itself before the actual transmission. This is an example of Reservation Protocol.
In this 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.
Suppose that there are 10 stations. So the number of contention slots will be 10. If the stations 2, 3, 8 and 9 wish to transmit, they will set the corresponding slots to 1 as shown in the following diagram:
Once each station announces itself, one of them gets the channel based upon any agreed criteria. Generally, transmission is done in the order of the slot numbers. Each station has complete knowledge whether every other station wants to transmit or not, before transmission starts. So, all possibilities of collisions are eliminated.
In general, high numbered stations have to wait for half a scan, i.e. slots, before starting to transmit. Low numbered stations have to wait on an average () slots.
This protocol works best under high loads. During high loads, if all the stations want to transmit, the N bit contention time is distributed over all the stations. So, the overhead is just 1 bit per frame. If d is the data transmission time, the efficiency is .
However, during low loads, if only 1 station has to transmit, the contention time has to be borne by only that station. The overhead becomes N bits per frame. Here the efficiency drops to.