User's Manual

Fig. 2.2. LibraPlus P-MP System Components
2.1.4.1. Point to Multipoint based on CSMA
A CSMA/CA (Carrier Sense Multiple Access With Collision Avoidance) system will require that all
remote stations be able to hear one another. If any one station in the system cannot hear all the rest of
the stations, collisions will most likely occur. A CSMA station will first listen to the air, if it hears no
other station transmitting, then it will start transmitting. If a CSMA station hears another station's signal
it waits until the transmission halts. If any one station is "out of range" of another station it is quite
possible for them both to attempt to transmit at the same time, resulting in collisions.
The CSMA/CA protocol has severe limitations in many multipoint applications. The biggest problems
arise when the data traffic load is high; that is, when every station has a lot of data to send at the same
time. This can lead to every station being in Collision Avoidance mode and the link is unlikely to be
optimally utilized.
CSMA/CA does not share the available bandwidth fairly; a site with a strong signal has a great advantage
over a site with a weak signal. This is termed "capture effect" and results in one site gaining more
share of the available bandwidth during transmission. Finally, CSMA/CA bridging devices also exper-
ience problems when some sites in a network are not within listening distance of some other sites on
the network. This can lead to two or more sites attempting to transmit at the same time, resulting in
transmission collisions.
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Description