Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 11
3.1.4. Base Station Broadcast Parameters
CDPD base stations regularly broadcast information (known as cell configuration messages) to
all CDPD subscriber devices so that they know all channels available both within the cell and in
neighboring cells. This information is then used by the CDPD subscriber device to find new
CDPD channels in the event of a planned handoff or forced channel hop. Other information is
also sent as broadcast messages, allowing CDPD subscriber devices to determine, for instance:
• When they have moved to different areas within the cellular geographic coverage area
• Parameters to control the subscribers’ transmitter power levels
The operating parameters broadcast by the MDBS include:
• Thresholds and threshold time limits
• Available channel lists
• An evaluation (rescan) frequency
• A signal strength change (hysteresis) value
The modem continually monitors its radio environment and compares the current values to the
thresholds and time limits; if any of the thresholds are exceeded for longer than their permissible
time limit, the modem must find a better channel. To speed up this search, the modem makes use
of the available channel lists picked up from the MDBS along with the operating parameters.
In addition, the modem periodically (typically every 90 seconds) evaluates the alternative channels
to ensure that it is always operating on the best available channel. Also, if the Received Signal
Strength Indication (RSSI—see section 3.2.6) changes by more than a predetermined amount
(typically 8 dB) from its initial acquisition value (either up or down), the modem must re-evaluate
the alternative channels to ensure that it is currently using the strongest one in the area. See
section 5.6 for more information.
3.1.5. CDPD Services Provided Over the Airlink
CDPD includes a number of mechanisms to manage the airlink and provide data services over it.
They include Medium Access Control (MAC), the Mobile Data Link Protocol (MDLP), and
the Subnetwork Dependent Convergence Protocol (SNDCP)—discussed in section 5. Services
they provide include:
• Compression of packet header and information fields transmitted over the wireless channel to
reduce the amount of airlink time used.
• Support of many users on the same cellular radio channel at the same time.
• Error correction of data sent over the airlink.
• Encryption of data while transmitted over the airlink.
• Tracking movement of the user from one cell site to another.
• Delivery of properly-sequenced data between user applications over the airlink.
• Multicast service: a company can periodically broadcast company updates to sales and service
people on the road; a news subscription service can transmit its issues as they are published.
Mobility management services within the CDPD network provide continuous communications to
mobile subscribers while their location changes within the coverage area provided by CDPD.
The CDPD network provides a Connectionless Network Service (CLNS), one in which the
network routes each packet individually within the network based on the destination address
carried in the packet and knowledge of the current network topology. It is often referred to as a
datagram service.
3.1.6. A Buffer Between the Internet and the Modem
A key difference between CDPD and more typical Internet data connections is that a CDPD
modem has no fixed location. Pure Internet technologies using TCP/IP cannot handle such
mobility, because they assume that a destination address does not change from minute to minute
or second to second.