About the Radio | 27
Aprisa SRi User Manual 1.1.0
Data Link Layer / MAC layer
The Aprisa SRi 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 SRi radio uses frequency hopping in conjunction with a channel access of Access Request (AR)
MAC to maximize the data throughput and performance. With a channel access scheme, the base station
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.
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 radio 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.
Hop by Hop Transmission
Hop by Hop Transmission is realized in the Aprisa SRi by adding a MAC address header to the packet. For
802.15.4, there are 2 addresses, the source and destination addresses.