About the Radio | 27
Aprisa SRx User Manual 1.3.0
Data Link Layer / MAC layer
The Aprisa SR PHY enables multiple users to be able to share a single wireless channel; however a DLL is
required to manage data transport. The two key components to the DLL are channel access and hop by
hop transmission.
Channel Access
The Aprisa SR radio has two modes of channel access, Access Request and Listen Before Send.
Option
Function
Access Request
Channel access scheme where the base stations controls the
communication on the channel. Remotes ask for access to the
channel, and the base station grants access if the channel is not
occupied.
Listen Before Send
Channel access scheme where network elements listen to ensure
the channel is clear, before trying to access the channel.
Access Request
This scheme is particularly suited to digital SCADA systems where all data flows through the base station.
In this case it is important that the base station has contention-free access as it is involved in every
transaction. The channel access scheme assigns the base station as the channel access arbitrator and
therefore inherently it has contention-free access to the channel. This means that there is no possibility
of contention on data originating from the base station. As all data flows to or from the base station, this
significantly improves the robustness of the system.
All data messages are controlled via the AG (access grant) control message and therefore there is no
possibility of contention on the actual end user data. If a remote station accesses the channel, the only
contention risk is on the AR (access request) control message. These control messages are designed to be
as short as possible and therefore the risk of collision of these control messages is significantly reduced.
Should collisions occur these are resolved using a random back off and retry mechanism.
As the base station controls all data transactions multiple applications can be effectively handled,
including a mixture of polling and report by exception.
Listen Before Send
The Listen Before Send channel access scheme is realized using Carrier Sense Multiple Access (CSMA). In
this mode, a pending transmission requires the channel to be clear. This is determined by monitoring the
channel for other signals for a set time prior to transmission. This results in reduced collisions and
improved channel capacity.
There are still possibilities for collisions with this technique e.g. if two radios simultaneously determine
the channel is clear and transmit at the same time. In this case an acknowledged transaction may be used.
The transmitter requests an ACK to ensure that the transmission has been successful. If the transmitter
does not receive an ACK, then random backoffs are used to reschedule the next transmission.
Hop by Hop Transmission
Hop by Hop Transmission is realized in the Aprisa SR by adding a MAC address header to the packet. For
802.15.4, there are 2 addresses, the source and destination addresses.
