Company Confidential
32
Raveon Technologies Corp.
10.2. TDMA Overview
TDMA, or
Time-Division-Multiple-Access
is a very effective way of allowing a lot of
radios to share one radio channel. Used extensively in GSM cellular and APCO
public-safety systems, TDMA excels at allowing quick and reliable access to radio
channels. It allows 2-10 times more radios to share a radio channel than
conventional carrier-sense methods. This allows 2-10 times more tracking radios
on one channel, as compared to radios that do not have TDMA capability.
The following diagram illustrates how it works.
When a
RF-7800W
wants to report its position and status, it waits until its
assigned time-slot, and then transmits its data. By default, TDMA time slot
positions are assigned by unit-ID, so
RF-7800W
with ID 1 uses the first slot, and
ID 2 uses the second slot, and so on.
A TDMA “
Frame
” time is the time it takes all units to transmit once. This is
configured with the
TDMATIME xx
command. The factory default is 1 second, so
every 1 second, each
RF-7800W
may transmit. The TDMA frame must be set
long enough for all units to transmit. For example, if you have 5
RF-7800W
s, and
use 100mS TDMA slots, then the
TDMATIME
should be set to 1 seconds. The
simplest way to set the
TDMATIME
is to make it equal to the
TXRATE
, the rate
you wish to report position.
The duration of a TDMA time slot is programmed into the
RF-7800W
with the
SLOTTIME
command. If
SLOTTIME
is set to 100 milliseconds (factory default),
then every 1 second, the RF-7800W will have a 100mS window to report its
position in.
All TDMA frames are synchronized automatically in all
RF-7800W
transponders to
the top of the minute. Slot 0, frame 0 is at the top of each minute.
10.3. Configuring TDMA Operation
Step 1.
Determine your over-the-air baud rate. See section 11.3 for
information on baud-rate selection. Raveon recommends 4800 baud
for narrow-band and 9600 baud for wide-band radios.
Step 2.
Determine the position transmission time, in milliseconds. Again,
review the table in section 11.3 to determine how long a position
transmission at your selected baud rate will take.
Step 3.
Repeaters. If a repeater will be used in the system, then the
transmission time must be doubled, because each position will be
