WIT2410P
© 2000- 2008 Cirronet™ Inc
12
M-2410P-0000 Rev G2
2.3.3. Low Power Mode and Duty Cycling
To conserve power, WIT2410 remotes power down the receiver and transmitter between
hops when not in use. Base stations must remain active all the time to handle any
transmission from any remote. Remotes can save even more power by enabling the duty
cycle feature. This feature causes a remote to power down for 2
N
frequency hops where
1/2
N
is the duty cycle. Rather than attempting to transmit on every frequency hop when
data is in the transmit buffer, a remote will attempt to transmit only every 2
N
hops.
Roughly speaking, this will proportionately reduce the average power consumption while
increasing average latency. When there are more than 16 remotes being operated, duty
cycling must be enabled since a maximum of 16 time slots is available per hop.
When a remote radio is powered up but is out of range of a base station, it will
continuous scan the frequency bands for the presence of a base radio. During this
scanning the radio can consume up to 80mA of current. A low power seek mode is
available in which the remote radios seek base stations only 50% of the time. This will
reduce current consumption by about 50% but will double the time it can take a remote to
link with a base up to 4 seconds.
2.3.4. RF Flow Control Mode
Because of slight differences in baud rates between transmitting and receiving hosts,
when sending large amounts of data (100’s of KB) in one direction in a point-to-point
application, it is possible to overrun the receive buffer of the receiving radio. For example
a nominal 115.2Kbaud at the transmitting radio’s host might really be 115,201 and at the
receiving radio’s host it might be 115,199. This is similar to a situation where the
transmitting radio is sent data at a higher baud rate than the baud rate at which data is
received by the receiving host. To compensate for the variations in nominal baud rates,
the WIT2410 supports an RF flow control mode for point-to-point operation. In this
mode, when the receive buffer of the receiving WIT2410 is close to full, the receiving
WIT2410 stops acknowledging transmissions. The transmitting radio is set to infinite
retries which invokes the RF flow control mode (See
Set Packet Attempts Limit
in Section
5.3). The receiving radio will not begin acknowledging transmissions from the
transmitting radio until more room in the receive buffer has become available. This will
cause data in the transmit buffer of the transmitting radio to back up. If it backs up to the
point where the transmit buffer fills up, the transmitting radio will deassert CTS stopping
data from the transmitting radio’s host device. Once room is available in the receiving
radio’s buffer, the receiving radio will begin acknowledging transmissions from the
transmitting radio allowing the transmitting radio’s buffer to begin to empty which will
cause the transmitting radio to reassert CTS. Either one or both of the radios in a point-to-
point installation can be configured for the RF flow control. If this mode is invoked in a
point-to-multipoint installation, communications with all radios will be stopped when any
one radio’s receive buffer becomes full.
