Early cellular manufacturers and operators recognized the
need for data communications, and the first modems were very
similar to standard modems used in homes and offices.
However, cellular uses a valuable, shared commodity—spectrum.
(Fixed telephone lines or wires may be shared, but they
can always be increased in number if necessary.) CDPD was
created as a digital packet data service over an analog cellular
telephone: It uses the same analog channels as voice, but with
different modulation applied to the air interface. Traffic channels
not being used for voice calls may be used for CDPD calls.
CDPD was the first digital data application to use packet data
for cellular, and it is still very much in use today by carriers
such as AT&T Wireless Services.
CDPD is fully compatible with analog cellular and is colocated
with AMPS systems. Therefore, the analog infrastructure,
such as frequency spectrum, cell sites, towers, and antennas,
can be shared. The CDPD network elements overlay parallel to
the analog infrastructure (see Figure 2-3). Analog voice or analog
data using an AMPS modem or digital data using a CDPD
modem shares the same frequency spectrum. External modems
are most common for CDPD communications, typically existing
as PCMCIA cards for laptop computers, as accessories for PDA
devices, or as external modems for connection to an analog
phone. Some manufacturers actually include CDPD modems
into their cellular telephone. This makes a 2G digital phone
“Internet ready” because all TDMA and CDMA phones also
include AMPS analog compatibility, and CDPD is carried on
AMPS channels.
Two key design criteria were used to develop the CDPD
protocol. From its inception, it was designed to use TCP/IP, the
Internet protocol, making it transparent to Internet data. It was
also designed to overlay an AMPS network, taking advantage of
existing infrastructure. These design goals make CDPD very
attractive to any carrier that manages an AMPS network. This
becomes even more important when a carrier lacks true 2.5G
or 3G capabilities and finds itself at a competitive disadvantage.
CDPD can make any TDMA or CDMA phone “Internet
ready” with 19.2 Kbps data rates by using TCP/IP in a packetdata
network instead of on a circuit-switched connection.
As of the end of the February 2000, CDPD was available in
184 markets in the United States covering 56 percent of the
United States population and 60 international markets.* As
2.5G and 3G cellular networks launch, CDPD will begin to
fade, but as long as there are 2G or analog phones, CDPD will
retain its usefulness. And although it may compete with SMS
for short text messages, it still has one advantage—CDPD
works across networks with different physical layers. A CDPD
modem in a laptop using CDMA can still send an email to a
phone across the country, which is using TDMA.