Reservation protocols

Reservation protocols are controlled-access MAC protocols where a station must reserve the channel before sending data. This prevents collisions, so they work well when the channel is heavily loaded.

Basic bit-map protocol

Time is divided into a contention period (reservation interval) and a data period. For N stations (numbered 0 to N − 1), the contention period has N mini-slots, one per station. A station with data sets its bit to 1 in its own slot. All stations see every reservation, so they know the order. Then stations transmit frames in numeric order.

 Reservation: | 0 | 1 | 0 | 1 | 1 | 0 | 0 | 0 |   (stations 0-7; 1, 3, 4 ready)
 Data:        |  frame 1 | frame 3 | frame 4 |

• Low load: efficiency is low, d/(d + N), because the N reservation bits are spent on only one frame (d = frame length in mini-slot units).
• High load: efficiency approaches 100%, d/(d + 1), because the N bits are shared by N frames.
• The delay is unequal: a low-numbered station that becomes ready just after its slot waits on average about 1.5N slots, while a high-numbered station waits about 0.5N slots.

Other variants

• Binary countdown: each station sends its binary address bit by bit; the bitwise OR on the channel decides; a station that sees a 1 where it sent a 0 gives up. Higher address wins; efficiency is d/(d + log₂N).
• Reservation in satellite networks (R-ALOHA): slots are reserved by short requests and then used without contention.

Reservation is a controlled-access method, like polling and token passing. It is collision-free, but it adds delay under low load and needs N reservation slots per round.