Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 8
3. Background: Introduction to CDPD
3.1. Cellular Digital Packet Data (CDPD)
In 1991 the U.S. cellular operators began a process to offer packet data technology for services
such as e-mail and telemetry. The result was Cellular Digital Packet Data (CDPD), which the
carriers began to deploy in 1993. Today, regions of CDPD coverage include most of North
America’s population.
CDPD is an open specification. It is fully documented, with the complete specification available
online from www.wirelessdata.org/develop/cdpdspec
.
CDPD shares radio frequency channels with AMPS cellular voice calls, but it has its own
infrastructure that piggybacks upon the AMPS technology. Cellular carriers who choose to
support CDPD must install additional equipment to handle data separately from AMPS voice.
CDPD also requires its own modems for end users, and operates quite separately from cellular
voice handsets—even while sharing channels with them.
Cellular carriers derive the vast majority of their revenues from voice, and are expected to
continue to do so for some time, although data use is growing. The need to optimize voice
revenues therefore drives the development of new technology, so CDPD was developed with the
primacy of voice in mind.
The overall CDPD network operates as a collection—an internetwork—of CDPD service
provider networks, where the CDPD networks of each cellular carrier communicate with one
another, routing data from one CDPD network to another, often through the wider Internet.
CDPD carriers provide services such as:
• Data connection to other networks
• Application services
• Network management
• Network security
• Accounting and billing
Just as cellular carriers ensure that end users see the AMPS network as a nearly seamless part of
the wireline PSTN, they work to ensure that CDPD users are transparently connected to the
Internet, and to each other.
3.1.1. Packet-Switched Data Shared With Voice Calls
Although CDPD operates over the AMPS analog cellular telephone network, CDPD itself is fully
digital, using Gaussian Minimum Shift Keying (GMSK) modulation to encode data on the same
824-894 MHz radio frequency channels as AMPS voice calls. In fact, CDPD is designed as a way
for cellular carriers to capture additional revenue by using the short blank spaces between AMPS
voice calls to transfer data.
There are long periods during which one or more of the radio channels within an AMPS cell sector
are not in use. In other words, there is spare capacity available on the cellular system. Figure 3-1
shows a simplified sample with three channels in a sector allocated for cellular voice use.