JPL's Wireless Communication Reference Website

Chapter: Data Networks
Section: Random Access, ISMA

Propagation Delays in ISMA

In Inhibit Sense Multiple Access, the base station continuously broadcasts channel status to terminals. This signalling leads to two types of delays:

Packet arrivals at the receiver


Figure: Non-stationary Poisson arrival rate seen by base station

The arrival rate (as depicted above) can be derived from studying the wake signal propagation model discussed below.

'Wake' channel traffic model


Figure: Time - Space Diagram for signals travelling in ISMA network.
Vertical: distance, Horizontal: time.
Arrows pointing up: signal from terminal travelling to the base station
Arrows pointing down: Busy/Idle feedback signaling from base station travelling to all terminals
Green: Incoming packets as present at the base station receiver.

Total delay in feedback

Delay consist of

Probability of successful transmission

Depends on The probability of successful access is proportional to the duration of the idle period, so it almost linearly decreases with increasing distance.

How to approximate effect of propagation delays?

Literature sometimes says that one can This approximation is acceptable for CSMA. However, for ISMA it is optimistic, i.e., it overestimates throughput. A better approximation for ISMA is to add the maximum round-trip (two way) delay to d1 and not to account for propagation delays in d2.


JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz, 1993, 1995.