Sierra Wireless DART 200 CDPD Modem - Manual (Page 36)

DART 200 CDPD Modem User’s Guide 2 Installation and Setup
Sierra Wireless, Inc. 2-12
3. If the CDPD channel’s BLER is acceptable, the DART acquires
the power product, current CDPD operating parameters, and
adjacent channel lists from the MDBS. If the channel BLER is
not satisfactory, the modem goes to the next best CDPD channel
in the list and repeats this step using the next best channel.
4. Once a channel is acquired, there is no guarantee that the modem
stays there very long. In a sniff-and-hop system it is forced to
continuously hop from one channel to another because voice
traffic has priority. Even in a dedicated system, if the modem is
mobile, it is forced to frequently change channels as it travels
from one cell coverage area to another.
5. The XID parameters broadcast by the base station, in addition to
the thresholds, threshold time limits, and adjacent channel lists
used with CDPD 1.0, include an evaluation frequency (scan
time), and a signal strength change (scan delta) value. As in
CDPD 1.0 mode, the modem continually monitors its radio
environment and compares the current BLER value to the XID
threshold and time limit; if the threshold is exceeded for longer
than the permissible time limit, the modem finds a better
channel. To speed up this search, the modem makes use of the
adjacent channel lists picked up from the base station along with
the operating parameters.
CDPD Version 1.1 requires that the DART periodically evaluates
alternative channels to ensure that it is still operating on the best
available channel. Also, CDPD Version 1.1 requires that if the DART
detects an RSSI changes of more than a predetermined amount, from the
initial acquisition value, it must check that it is still using the strongest
channel in the area. These features, scan time (usually 90 seconds), and
scan delta (usually +/- 8 dB), help to keep the M-ES on the best available
channel, and avoids the cell dragging phenomenon common to mobile
M-ESs operating in CDPD 1.0 mode.
Whenever the RSSI threshold or scan delta is exceeded, or the scan timer
expires the modem must locate a better channel if possible. To speed up
this search, the modem makes use of the adjacent channel lists picked up
from the base station along with the operating parameters.
Problem determination
There are three ways to determine if a channel was acquired:
• By observing the OPR light (simplest way). At power on this light
flashes at the rate of about once per second. When the DART 200
acquires a channel the rate of flashing speeds up to at least twice per
second and flashes faster as the signal strength increases. A very
strong signal causes the light to glow steadily
• By viewing S-Register 101 with the ATS101? command. A value of
1 indicates that a channel has been acquired