Sierra Wireless DART 200 CDPD Modem - Manual (Page 34)

DART 200 CDPD Modem User’s Guide 2 Installation and Setup
Sierra Wireless, Inc. 2-10
The values associated with the bit positions are listed below.
Bit Position Value
0 1
1 2
2 4
3 8
4 16
5 32
6 64
7 128
For example, if register bits 0,3, and 5 are on, the register value would be
1+8+32=41.
Channel acquisition
For the DART 200 to acquire a channel for operation it must have a good
antenna and an MDBS near enough to deliver a signal that is strong
enough to be received by the modem. CDPD systems are currently
defined as being either forced hopping or dedicated.
In a forced hopping (often called sniff-and-hop) system, an MDBS has
multiple channels (from 1 to 21, but usually 7) shared by voice calls or
CDPD. CDPD uses any available channel when it is not in use for voice.
If a voice call wants a channel already in use for CDPD, the MDBS notes
that occurrence (sniffs) and moves CDPD to another available channel
(hops).
The modem detects the fact that the current channel no longer supports
CDPD (loss of sync), and searches for an alternate CDPD channel using
the MDBS’s adjacent channel list. In such a system, CDPD operates in
the time between voice calls. During busy periods there is little time
available. Applications requiring quick response times are adversely
affected by the hopping.
In a dedicated system, each MDBS has one or possibly two channels
assigned for CDPD use only. In such a system, contention with voice
calls is not an issue and application response times are much more
predictable.
• CDPD Version 1.0 Mode Operation
In the above system types, the channel acquisition process is:
1. The DART 200 powers up and immediately begins searching for
a CDPD channel to use. It searches sequentially, from the lowest
numbered channel to the highest, on the side (A or B) it is
configured to use, starting at a relatively strong signal level (-60
dBm).
2. Each time the DART finds an active channel, the modem checks
to determine if it is in use for voice or available for CDPD.