JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random Access

Carrier Sense Multiple Access

For high offered traffic loads, collisions occur frequently in the most basic random-access system; the ALOHA system. This reduces the throughput and may lead to instability if collided packets collide again during their retransmission. A number of other protocols have been proposed to mitigate this problem.

Busy Channel Multiple Access (BCMA) is the class of multiple access schemes in which no new packet transmissions are allowed when the inbound channel is busy. Various strategies have been proposed to acquire information on the channel state. Carrier Sense Multiple Access (CSMA) in its basic form does not employ feedback other than positive acknowledgements of correct reception of a data packet. However all terminals listen to the inbound (terminal to base) channel. No new packet transmission is initiated when the inbound channel is sensed busy by the mobile terminal. This requires that all mobile terminals can receive each others signals on the inbound frequency.

However, in mobile radio nets with fading channels, a mobile terminal might not be able to sense a transmission by another (remote) terminal. This effect, known as the "hidden-terminal" problem, is avoided in Inhibit Sense Multiple Access (ISMA), where the base station transmits a busy signal on an outbound channel to inhibit all other mobile terminals from transmitting as soon as an inbound packet is being received. A disadvantage of ISMA is the necessity of a real-time (continuous) feedback channel.

Similar to ISMA, Busy Tone Multiple Access (BTMA) has been proposed. If the feedback channel only contains a narrowband "busy tone", the mobile terminal may erroneously miss the presence of this tone if the outbound signal happens to be in a deep fade. This may be mitigated by casting an (active) idle tone, rather than an (active) busy tone, or by transmitting busy reports with error control coding.

Even if signalling messages on the feedback channel are always received correctly by all mobile terminals, collisions can nonetheless occur in ISMA for two reasons:


In CSMA-Collision Detect, the receiver continuously informs all terminals about the incoming signal. If a collision is detected, the transmissions are aborted by all terminals immediately. Otherwise the channel would remain occupied without any useful signal being recoverable. This enhances the throughput, compared to a system that only acknowledges reception after transmission of the full message or packet.


CSMA is used in

JPL's Wireless Communication Reference Website 1993, 1995.