4RF Aprisa SR+ User Manual - page 88
Managing the Radio | 85
Aprisa SR User Manual
Note that this includes the wireless protocol header (18 to 20 Bytes) and security payload (0 to 16 Bytes).
The length of the security header depends on the level of security selected.
An example wireless protocol frame structure is illustrated below. This uses maximum security of
16 Bytes. The length of the header varies depending on whether the user data must be segmented. User
data will be segmented if it is larger than the wireless packet payload length. In this case, this limit is
213 Bytes. If the user data packet is larger than this, then the header increases to 20 Bytes to account for
the transport of the required segmentation and reassembly data.
Note that when the security setting is 0, the maximum user data transfer over-the-air is 1516 Bytes.
When encryption is enabled, the entire packet of user data (payload) is encrypted. If authentication is
being used, the security frame will be added (up to 16 bytes). The wireless protocol header is then added
which is proprietary to the Aprisa SR. This is not encrypted.
Packet Time to Live (ms)
This parameter sets the time a packet is allowed to live in the system before being dropped if it cannot be
transmitted over the air.
The default setting is 5000 ms.
ACCESS REQUEST
Maximum Access Attempts
This parameter sets the maximum number of attempts the MAC will try to acquire the channel for the
packet to be transmitted before it is dropped.
The default setting is 3.
Maximum Back-Off Count
This parameter sets the random back-off period before the MAC tries to acquire the channel to transmit a
packet.
The default setting is 4.