User's Manual

27
Document Number: 0100SM1401 Issue: 05-15
Data waiting to be
transmitted
Is C/A busy
flag set?
No
Transmit data to
C/A master
Yes
Wait a random
time
Collision avoidance operational examples:
Collision avoidance (C/A) has a number of user configurable parameters. These parameters work together with the specific
mode of C/A chosen to minimise the number of collisions on the radio channel. Interaction of these parameters in C/A is best
explained by reviewing the operational flow charts for common C/A configurations.
ChannelShare+™ Example 1
This flow chart shows the C/A operation in a remote
radio with the following configuration:
• C/A: ChannelShare+
• Backoff Method: Retry After Tx attempt
• Backoff time:
- Max Slots: 16
- Slot Time: 20ms
• Data Priority: Tx Data
As it is possible that there may be more than one radio waiting to transmit data to the C/A master, a random wait time is
applied, to avoid two radios waiting the same time, retrying and then colliding. There are multiple configurable parameters
involved in when the wait time is applied and what amount of time is waited.
• Backoff Method - Defines ‘when’ a radio will implement the backoff time. In this example, the backoff method is
configured to ‘Retry After Tx Attempt’. If the C/A busy flag is clear, the remote will transmit data immediately. If the
C/A busy flag is set, the remote will wait a random Backoff time and try again.
The Backoff time is calculated by choosing a random number between 1 and ‘Max Slots’ (in this example 16) and multiplying
the number by the ‘Slot Time’ (in this example 20ms). In any remote radio, a smaller number of ‘Max Slots’ and ‘Slot Time’
can be configured to reduce the random Backoff time, which will increase the rate at which the radio checks the C/A busy
flag. In effect, a radio with a smaller backoff time has a higher probability of transmitting its data first, in a situation where
multiple remote radios are waiting for access the channel.
As the data priority is configured for Tx Data, the radio will transmit data even when an incoming data packet it being
received.
This collision avoidance configuration provides maximum radio channel efficiency and are recommended when there is
multiple asynchronous data traffic on the radio channel.
When data is ready to be transmitted, the remote radio
checks the C/A busy flag to see if it is set (i.e. is the C/A
master receiver busy). If the C/A busy flag is clear, the
data is transmitted to the C/A master. If the C/A busy
flag is set, the radio waits a random time before trying
again.
Part D – Feature Detail