Specifications

10. Channel access

Method of accessing the radio channel may significantly affect the overall reliability of packet transmis-

sion. Even in a simple polling-type application, which never generates more than a single packet at a

time, collisions may occur when repeaters are used. The goal of channel access is either to eliminate

collisions completely, or to reduce their probability while ensuring that systematic repeated collisions

never happen. RipEX provides different channel access methods in different modes and optimum

configuration can be found for every communication scheme and network layout.

10.1. Collisions

What is so special about collisions that they deserve that much attention? Well, they are a special case

of interference (“friendly fire”, a military reporter would say), which may very seriously harm network

performance.

A collision happens, when two (or more) transmissions in the network overlap in time. Radio modem

A transmits a packet for B, C transmits for D. In well designed network the respective signal levels (i.e.

A received at B, C received at D) do ensure error-less reception. For the period of time when these

two transmissions overlap, signal from C at receiver input B and signal A at D act as interference signals,

reducing the SNR (Signal to Noise Ratio). If B and D are in the same area, the difference in signal

strength is small and so is the resulting SNR at both receivers. Consequently the BER (Bit Error Rate)

at both receivers jumps to unacceptable level and none of the packets is successfully received. That

is the basic principle of a collision.

There are two very harming features of collisions:

The first is a systematic repeated collision. No application generates a totally random traffic pattern.

So it may happen (and it does happen), that a certain sequence of packets in a certain network layout

generates a collision and it generates this collision repeatedly, in fact always. The result is that certain

specific packets are never delivered, regardless of number of retries set at the application level. Imagine

a SCADA system never capable of performing one specific task, while all communication tests report

that links are in perfect shape. It would be very tempting to blame the SCADA, while the true problem

is a systematic collision, i.e. wrong network design. Ways to avoid such collisions are described further

in this document.

The second dangerous feature of collisions is just a direct consequence of probability laws. The most

effective communication scheme for many applications is the report-by-exception mode, which can

vastly reduce the amount of mainly useless traffic generated by polling-type systems. Report-by-excep-

tion means though, that collisions can never be ruled out completely, hence a collision-solving system

must be an integral part of the protocol in the radio channel (RipEX in router mode provides such protocol

of course). Solving a collision means retransmission, typically a delayed retransmission. Consequently

the probability of another packet being generated by the application in the meantime increases by the

delay, and it increases at both parties involved in the collision. That results in an increased probability

of next collision to happen...and so on. This principle makes report-by-exception networks very sensitive

to bursty loads. Whenever the load increases over certain limit (we may say “normal” network capacity),

number of collisions grows exponentially, reducing the instant network capacity well below normal

situation. Series of lost packets and very long delivery times are the result from the application point

of view. While the network for report-by-exception application has to be designed to provide maximum

capacity possible, it is recommended to take measures to avoid burst load generation at the application

level. Limiting the possible load generated by a single device can help to avoid the whole network col-

lapse just because one remote unit goes suddenly “crazy” (e.g. generates hundreds of “exceptions”

per second).

Channel access