Bit Map Protocol
This is how the Basic Bit-Map Protocol works.
- Assume N stations are numbered from 1 to N.
- There is a contention period of N slots (bits).
- Each station has one slot time during the contention period, numbered 1 to N.
- Station J sends a 1-bit reservation during Jth slot time if it wants to transmit a frame.
- Every station sees all the 1-bit reservation transmitted during the contention period, so each station knows which stations want to transmit.
- After the contention period, each station that asserted its desire to transmit sends its frame in the order of station number.
Issues with this scheme:
- Stations’ access to the network is unfaire: the low numbered stations have implied priority over high numbered stations. That is, if station i and station j both want to transmit, and i < j, then station i gets the automatic bid.
- On the other hand, low numbered stations have to wait longer than high numbered stations for the reservation to complete. For example, the shortest time station 1 has to wait is N bits, the longest time station 1 has to wait is 2N – 1 bits; the shortest time station N has to wait is 0 when no one else in the network wants to transmit and the reservation slot just arrived at station N, the longest time station N has to wait is N-1 bits.
- Efficiency: at low load, the protocol efficiency is low. For example, at the extreme case, one frame transfer per contention period, thus efficiency is d/(d+N) where d is the number of bits in frame, and N is the number of stations on the network. At high load, the protocol efficiency is high. At the extreme case, all stations have frames to transmit at all time, then the efficiency is Nd/(Nd + N) which is d/(d + 1). Typically, d >> N.
- The average waiting time is : plus the waiting time inside the station queue.