Reservation ALOHA

The R-ALOHA scheme can be viewed as combination of slotted ALOHA and TDM protocols. There are many versions of the R-ALOHA scheme. Here we briefly describe one of them due to [Pah95].





Fig. 6.1. The example of reservation ALOHA (M=4, R=5).


The example of R-ALOHA scheme is presented in Fig. 6.1. The system has two modes of operation: unreserved mode and reserved mode. In the unreserved mode, the time axis is divided into short equal-length subslots for making reservations. Users with data to send, transmit short reservation requests in the reservation subslots using the slotted ALOHA protocol. The reservation request can ask for a single slot or multiple slots. After transmitting a reservation request a user waits for positive acknowledgment (ACK). The reservation acknowledgment advices the requesting user where to locate its first data packet. The system then switches to the reserved mode.

In the reserved mode the time axis is divided into fixed-length frames. Each frame consists of M+1 equal-length slots of which the first M slots are used for message transmission (message slots) and the last slot is subdivided into R short reservation subslots used for reservation. A sending user that has been granted a reservation sends its packets in successive message slots, skipping over the reservation subslots when they are encountered. When there are no reservations taking place, the system returns to the unreserved mode.

In the R-ALOHA system, the contention is limited to the short reservation subslots, while the transmission in the message slots is contention-free. The choice of the number of reservation subslots relative to the number of message slots is a design tradeoff issue. The number of reservation subslots should be small enough to keep system overhead low, but large enough to serve the expected number of reservation requests.

In the R-ALOHA scheme the control of the system is distributed among all users in the network. Because all reservation messages are heard by all users in the network, each user maintains information on the queue of outstanding reservations for all other users in the network as well as for its own reservation. When the queue length drops to zero, the system returns to the unreserved mode, in which there are reservation subslots only.

In the example presented in Fig. 6.1 the user reserves three message slots. The reservation acknowledgment advices the user when to send its first data packet. The user knows that the slot next to the first packet slot comprises five reservation subslots, so it does not transmit its packet during this time. The second and third data packets are sent in the following two slots. Because there are no more reserved slots, the system returns to the unreserved mode.