Sierra Wireless DART 200 CDPD Modem - Manual (Page 35)

DART 200 CDPD Modem User’s Guide 2 Installation and Setup
PN1197-00 Revision 1.0 2-11
3. If the channel is not available for CDPD use, the DART
continues searching, dropping the allowable signal level by 10
dB after each pass through all the cellular channels on the side it
is searching, until it finds a usable channel or reaches the
weakest allowable signal level (-110 dBm).
4. If an available CDPD channel was not found, the search starts
over again at -60 dBm and the modem repeats the spiral search
process indefinitely until an available CDPD channel is found.
5. When a CDPD channel is found the modem checks that the
block error rate (BLER) is acceptable.
6. If the BLER is adequate, the DART extracts the network
operating parameters (continuously being broadcast by the
MDBS) from the data stream, then stays on the channel as long
as possible awaiting commands from the Mobile Application
Subsystem (MAS); the attached device. If the BLER is not
acceptable the search continues.
7. While on the CDPD channel, the modem accepts operating
parameters (called XID parameters) broadcast by the base
station, including thresholds, threshold time limits, and adjacent
channel lists. The modem continually monitors its radio
environment and compares the current signal quality to the
thresholds and time limits; if any of the thresholds are exceeded
for longer than their permissible time limit, the modem must go
find 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.
This process allows a modem to stay on an acquired channel, until
the signal strength or BLER fails the threshold test. In a situation
where the M-ES is mobile, the modem can continue to use a channel
far from its original acquisition point (a phenomenon known as cell
dragging). In many cases this causes interference with voice
channels on intervening base stations. One of CDPD Version 1.1’s
main objectives is to avoid cell dragging.
• CDPD Version 1.1 mode operation
In the system types already described the channel acquisition process
is:
1. The DART 200 powers up and immediately searches for a
CDPD channel to use. It searches sequentially, from the lowest
number channel to the highest, on the side (A or B) it is
configured to use recording the signal strength (RSSI) of every
active cellular channel encountered.
2. The DART then sorts the channels in descending order of
strength and evaluates them to find the best available (strongest
signal) CDPD channel (in a dedicated system many in the list are
voice only channels).