Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 7
switched—using up the entire bandwidth of a cellular channel for the audible tones used to
modulate the data transmission.
With the growing popularity of the Internet in the 1990s, circuit-switched became even less
appropriate for most data transmissions. Like many computer data transmissions, Internet
connections are “bursty”, with short flows of information interspersed with long idle periods.
The Internet’s standard Transmission Control Protocol and Internet Protocol (TCP/IP) take
advantage of this burstiness by being packet-switched. Data is broken into small packets that are
wrapped with information describing their length and destination (specified in the packet header).
Thousands or millions of these packets can share a transmission medium—whether a wire, a laser
in a fiber-optic cable, a microwave beam, or a radio channel—because each one is targeted to a
destination.
The Internet’s technology infrastructure reads the packet headers and routes them to their
destinations, where the receiving computer can reassemble the packets into a reconstituted version
of the original information. An additional advantage is that packets can include error correction,
such as the Forward Error Correction (FEC) used by CDPD (see section 5.5) to prevent data
loss.
Creating a packet-switched data standard that could be widely used over the AMPS cellular
network, using the emerging Internet standards of TCP/IP for packet encapsulation—and which
could be billed by the packet instead of by the minute—was a reasonable goal for the
telecommunications industry. It led to the birth of CDPD.