JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random Access

Inhibit Sense Multiple Access (ISMA)

Compared to ALOHA or CSMA, the Inhibit Sense Multiple Access (ISMA) radio system is supplemented by an outbound signalling of the status of the channel: either "busy" or "idle". An example of such protocol is used in the US CDPD system. When the base station receives an inbound packet, a "busy" signal is broadcast to all mobiles to inhibit them from transmission. In a practical system, this only occurs after a short processing delay d1. The effect of this delay depends on its magnitude relative to the duration of the data packet.. After termination of (all contending) transmissions, the base station starts transmitting an "idle" signal after a delay of duration d2.

In CSMA, the delay is mainly caused by the time a mobile terminal takes to switch from reception to transmission mode (power-up), after sensing the radio channel for carriers from other active terminals.

The busy period is defined as the period during which the base station broadcasts a busy signal plus the preceding signalling delay d1. For memoryless Poisson arrivals, the duration of the idle period, i.e., the time interval starting at the release of the channel until the first packet arrival is exponentially distributed with mean I = 1/G.

The busy period is the time interval between the first arrival of a packet until the moment that the channel becomes idle. During the initial period d1 of the busy period the outbound channel thus still reports an idle inbound channel. The duration of the busy period is at least 1 + d2, but may be longer if a collision is caused by a packet arrival in during the inhibit delay. Moreover, persistent terminals, that sense a busy signal, may start to transmit immediately after the channel becomes idle. In such cases the busy period has a duration longer than two (or more) units of time.

Hence, the average duration of the busy period, B, depends on the signalling delay d1 and d2, and on the persistency p in (re-) scheduling inhibited packets.

Nonpersistent ISMA

For nonpersistent CSMA and ISMA, rescheduling (with random back-off time) always occurs if the channel is busy at the instant of sensing. So, if a packet arrives at a nonpersistent terminal when the base station transmits a "busy" signal, the attempt is considered to have failed. If the feedback channel (or in CSMA the channel sensing mechanism) is imperfect, a transmission may erroneously be started in the period. The packet is rescheduled for later transmission.

1 - Persistent ISMA


Figure: Description of terminal behavior in 1-persistent ISMA/CSMA random access network

p - Persistent ISMA


Figure: Description of terminal behavior in p-persistent ISMA/CSMA random access network

How to compute the throughput of ISMA radio networks?

With probability I/(I + B) a test packet starts at an instant when the channel is idle. A collision can occur if one or more other terminals start transmitting during the time delay d1 of the inhibit signal. This allows us to compute the conditional probability of n transmissions overlapping with the test packet that initiated the busy period.

Alternatively, the test packet itself starts during a period of duration d1 when the channel is busy because of a transmission by another terminal, but seems idle since the inhibit signal is not yet being broadcast. This event occurs with probability d1/(B+I). The test packet thus experiences interference from the packet that initiated the busy period, but possibly also from other arriving packets. The additional contending signals occur with a Poisson arrival rate during the interval d1.

Taking account of the above three possible events, the unconditional probability of successful transmission can be derived.

The above derivation differs from techniques typically used for wireline LANs, because in radio systems we mostly want to be able to consider expressions for capture probabilities, depending on the location of one particular terminal.


JPL's Wireless Communication Reference Website 1993, 1995.