Packet Reservation Multiple Access (PRMA)
- Packet reservation multiple access (PRMA) is a transmission protocol proposed for packet voice terminals in a cellular system
- PRMA is a time division multiplex (TDM) based multiple access protocol that allows a group of spatially dispersed terminals to transmit packet voice and low, bit rate data over a common channel
- The key feature of this protocol is the utilization of user transmission to gain access to the radio resources
- PRMA is a derivative of reservation ALOHA, which is a combination of TDMA and slotted ALOHA.
- A reservation protocol like PRMA has an advantage in that it can utilize the discontinuous nature of speech with the help of a voice activity detector (VAD) to increase capacity of the radio channel
- The raw channel bit stream is divided into time slots with each slot designed for a single packet of information
- The time slots are grouped as frames, which are repeated cyclically over the channel. In a frame, the individual slots are accessed by the mobile for communication with the base.
- Selection of the frame duration is based on the fact that a speech terminal can generate exactly one packet per frame
- The allotted time slot is fixed within the frame until the conversation is over.
- The problem of contention is taken care of by designing the system using a probabilistic model based on trunking theory to predict the availability of time slots.
- The availability of time slots depends on the usage of the network, and if there are too many users, call set-up will be prolonged.
- If congestion at a base station is encountered from many mobile users, data packets are dropped, and speech packets are given priority, since speech requiresthat the packets be delivered in order
- A feedback signal from the base station to the mobiles concerning the previous transmitted packet is multiplexed along the stream of data from the base station
- Based on ARQ error correction, the packets are retransmitted if a mobile receives a negative acknowledgment