User's Manual

28 Document Number: 0100SM1401 Issue: 12-16
Introduction
In many SCADA and remote Telemetry applications, there exists the potential for over the air data collisions between radios.
This can occur when multiple asynchronous data traffic is present on the radio channel, such as SCADA polling, SCADA
exception reports, SNMP traps, pings and ARP requests.
In two frequency systems, collisions may occur on the receive channel of an Entry Point or Repeater radio, due to two or
more remotes transmitting simultaneously. If this occurs, the radio will receive a corrupted message from both radios and a
re-try will be required. Similarly, in simplex (single frequency) systems, collisions may occur on any receiving radio when two
or more radio transmit simultaneously.
In two frequency systems, collision avoidance minimises the chance of collisions by configuring one radio, as the collision
avoidance master, which informs remote radios when the master’s receive RF channel is busy. Remotes will check whether
the master is allowing access to the channel before a transmission occurs. If the channel is free, the remote will transmit. If
the channel is busy, the remote will buffer the message and execute a small random delay (in case multiple remotes have
data to send), then attempt to access the channel again. By avoiding collisions the SCADA system is able to operate more
efficiently, with fewer retries. Similarly, in simplex (single frequency) systems, remote radios can detect when the Entry Point
or Repeater radio is transmitting, and wait for it finish, before transmitting itself.
For two frequency systems, there are two different modes of collision avoidance:
• Carrier Detect - Remote radios in a carrier detect collision avoidance system, listen for a transmission (carrier)
from the collision avoidance master, to determine if the collision avoidance master is currently busy receiving a
transmission from another remote. When the collision avoidance master receives a transmission from a remote, it
activates its own transmitter, indicating to all other remotes that the channel is busy.
In this mode of operation, remote radios can not distinguish between the collision avoidance master transmitting data
and the collision avoidance master indicating the channel is busy.
Carrier Detect Mode can also be used without a collision avoidance master. This is typically implemented in simplex
systems, or systems with a small number of remotes.
• Digital - Remote radios in a digital collision avoidance system, monitor a channel busy flag in the digital data stream
transmitted from the collision avoidance master to determine if the collision avoidance master is currently busy
receiving a transmission from another remote. When the collision avoidance master receives a transmission from a
remote, it activates its own transmitter and sets the channel busy flag, indicating to all other remote the channel is
busy. However, unlike carrier detect mode, if the collision avoidance master needs to transmit data to remotes, it can
do so and clear the channel busy flag.
In this mode of operation, remote radios can distinguish between the collision avoidance master transmitting data and
the collision avoidance master indicating the channel is busy. Even if the collision avoidance master is transmitting
data, a remote radio can transmit data back to the collision avoidance master. In this way the radio system can fully
utilise the full duplex capabilities of the Entry Point or the Repeater (collision avoidance master). The channel busy
flag consumes a small amount of bandwidth in the collision avoidance master to remote direction. However, as this
direction is one to many, it has negligible impact on radio network capacity.
Digital can be used in two frequency, PTMP and PTMP/R system topologies where the Entry Point or Repeater is full
duplex. It is not available in simplex systems, or where the Entry Point/Repeater is half duplex or in PTMP via multiple
Repeaters system topologies.
Collision Avoidance
Part D – Feature Detail