User's Manual

24 Document Number: 0100SM1401 Issue: 10-14
Dynamic Speed Cache
The dynamic speed cache is used by a radio to record specific external values that are learnt. These values are then used by
other processes within the radio (such as dynamic speed and ARQ) to help ensure optimum performance is achieved over
the radio channel.
External values that are learnt are shown in the table below.
Remote Host MAC Associated Radio Serial Number RSSI level
Remote Host MAC: The MAC address of a device which is connected to a remote radio.
Associated Radio Serial Number: The Unique ID of a remote radio with which the Remote Host Device/MAC is associated with.
RSSI level: The signal strength received from the radio specified within the associated radio serial number field.
A typical data packet structure is shown below. (For the purposes of this example, the preamble, Ethernet type and CRC
fields have been excluded.)
Data packets sent over the air between radios have an additional field appended. An example is shown below.
Destination MAC Source MAC Datagram
Radio Headers
Source Radio
Serial Number
ARQ Flag
Destination MAC Source MAC Datagram
Part D – Feature Detail
Consider now, in the below example where dynamic RF speed selection is enabled. Now that the receiving radio can adjust
its RF speed dynamically, a faster RF speed can be chosen for normal operation. Even though the faster RF speed will not
provide 20dB of fade margin, the system is still reliable because dynamic speed selection will drop down in RF speed when a
signal fade occurs.
Dynamic speed selection derives QoS from both RSSI (Received Signal Strength) and ARQ performance in order to maintain a
radio link operating at the fastest speed possible for the given quality of the link. The two influencing factors are:
Each radio stores a table of destination MAC addresses vs fastest RF speed in a dynamic speed cache, using information
learned from previous transactions over the air. In this way, the radio dynamically learns what RF speed should be chosen for
a transmission. Should the transmission generate an ARQ, the radio will drop down in RF speed to improve reliability of data
delivery.
In the event that radio has not yet learnt the fastest RF speed for a destination MAC, the message will be transmitted out at
the slowest RF speed in the given bandwidth, to provide the highest sensitivity, and therefore the highest reliability.
As broadcast and multicast addresses are potentially destined for multiple radios these types messages will always be
transmitted at the slowest speed.