A medium access control protocol is needed for a multi-access network. Such
protocols are designed to address the problem of how to share a common
broadcast channel. The techniques that have been used for packet radio
networks can be termed random access or contention
techniques. They are random access in the sense that there is no
predictable or scheduled time for any station to transmit: Stations
generate packets for transmission at random times. They are contention in
the sense that no central control is exercised to determine whose turn it
is. All stations contend for time on a network.
There are three medium access protocol appropriate
for packet radio networks: ALOHA, slotted ALOHA, and
Carrier Sense Multiple Access (CSMA). This subsection introduce
CSMA very briefly, for more comprehensive discussion about these topics
see [Stallings 91].
The maximum channel utilization is only 0.37 with slotted ALOHA
(see [Stallings 91]). The reason for this poor performance is
that both ALOHA and slotted ALOHA fail to take advantage of one of
the key properties of packet radio networks, which is that the
propagation delay between stations is insignificant compared to packet
transmission time.
If the station-to-station propagation time is extremely small compared to
packet transmission time, then, when a station launches a packet, all
the other stations know it almost immediately. So, if they had any sense,
they would not try transmitting until the first station was done. In that
case, collision would be rare since they would occur only if two stations
began to transmit almost simultaneously. In other word the short delay
time provides the stations with better feedback about the state of the
system.
This observation led to the development of a technique known as carrier
sense multiple access (CSMA) or listen before talk (LBT). A station
wishing to transmit first listens to the medium to determine if another
transmission is in progress. If the medium is in use, the station backs
off some period of time and tries again. If the medium is idle, the station
may transmit. To avoid collisions, a station waits a reasonable amount of
time after transmitting for an acknowledgment, taking into account the
maximum round-trip propagation delay. If there is no acknowledgment, the
station assumes that a collision has occurred and retransmits.
With CSMA, an algorithm is needed to specify what a station should do if the medium is found to be busy. The following three algorithms are in use (see [Stallings 91]):
Both non-persistent and p-persistent algorithms are built into most TNCs, but the default algorithm is non-persistent because it is simpler to use.