Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 2
2. Telephones and Wireless Data Transmission
2.1. Telecommunications and the Telephone
Today’s wireless data communications standards, including CDPD and more recent varieties,
evolved from technologies in different industries, including radio and data communications.
CDPD’s most direct and well known ancestor is the traditional telephone system, which is where
we begin our history.
2.1.1. Wireline Telephones
The wireline telephones with which we are all familiar evolved from the telegraph system, and are
known within the industry as the Plain Old Telephone System, or POTS. They are connected to
the Public Switched Telephone Network (PSTN). Wireline telephones operate on a circuit-
switched system (see section 2.3.1), which means that in any phone call there is effectively a
single, continuous, dedicated wire connecting one party to the other. In today’s digital-switched
telephone systems, the situation is slightly more complex, but the dedicated circuit remains.
Through the twentieth century, many other technologies piggybacked upon the PSTN, including
telegrams, fax transmissions, credit card authorizations, newswires, various videophone
techniques, corporate PBX telephone exchanges, e-mail, and Internet access. Each adapted itself
to an underlying infrastructure designed purely for the human voice.
2.1.2. Wireless Telephones
Most radio transmissions are broadcasts, where a single powerful transmitter sends signals—such
as music, speech, or television images—to anyone who can receive them in a given (often fairly
large) area. Two-way radio communication has long been used by law enforcement, other public
safety agencies, marine and aircraft navigation, the military, urban dispatchers, and CB and Ham
radio enthusiasts.
Neither broadcast nor two-way radio was initially linked into the vast telephone network. Early
attempts to connect them and create a wireless telephone system were unsuccessful, largely
because they generally used a single large transceiver station for each city. Radio frequencies are
limited, so only a few people could make wireless calls simultaneously, even in a large city.
Conversations had to be patched through an operator who linked the radio transmission into the
PSTN, and the calling phones (to be powerful enough to reach the single central antenna) were
bulky. Also, only one person could speak at a time: the sets could either send or receive, but not
both at once.
For wireless telephones to succeed, engineers and regulators needed to find ways to make the
process simpler and more convenient, subdivide the radio bandwidth, and make smaller phones.
2.2. The Advanced Mobile Phone System (AMPS)
Although originally developed in the 1960s, it wasn’t until 1983 that the Advanced Mobile
Phone System (AMPS) was implemented in North America. AMPS was the first widespread
wireless mobile telephone system, replacing the one or two large and powerful transmitters and
receivers in a city with a constellation of dozens or hundreds of small transceivers, running at
much lower power (originally about 100 W each, but with new technology about 50 W today).
The range of each transceiver—also known as a base station or cell site—is limited, and so it acts
as the hub of a relatively small cell. In that cell, personal transceivers—cellular phones, also