Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 3
called handsets—can both receive signals from, and send signals to, the base station. All the cell
sites for one cellular service provider connect into an automated central management system, and
from there into the wider PSTN; so AMPS users can seamlessly call both traditional wireline
phones and other mobile phones (whether on their network or not), and vice versa.
AMPS has been remarkably successful. In the 1960s, its developers predicted perhaps a million
users in North America by the year 2000. In reality, that number was in excess of 50 million.
AMPS, an analog system, is the oldest of the North American cellular phone technologies. Newer
digital systems may use different radio frequencies and encode voice information differently, but
as a whole they operate fundamentally the same way.
2.2.1. Why Cellular?
A cellular system lets the same limited range of radio frequencies get used over and over again.
Even though there are only 832 conversation channels available in the 50 MHz of radio bandwidth
assigned to AMPS networks, tens of thousands of simultaneous conversations can take place.
Many cellular telephones across a city may be using the same channels at one time, but because
each cell base station has a limited range (and because of other limitations imposed on the system,
see section 2.2.6 below), they do not interfere with one another.
Since cellular telephones must be relatively close to their base stations to operate, the phones can
be quite small and use low-power transmitters. Early AMPS phones were suitcase-sized—similar
to their non-cellular counterparts—but technological improvements mean that today’s analog
cellular phones, and especially their digital descendants, can be small enough to fit in a shirt
pocket.
2.2.2. Analog FM Using Different Channels to Send and Receive
AMPS is an analog standard, which means that voice conversations are directly represented in the
radio transmission as changes in the radio waveforms. Digital systems, by contrast, encode voices
as binary digits which are then modulated into the radio waveform. AMPS uses frequency
modulation (FM), the same technique implemented in FM radio broadcasts, but an AMPS
cellular phone channel has a far smaller slice of bandwidth (30 kHz) than an FM radio station (200
kHz)—so a cellular phone call is obviously of lower quality than an FM radio broadcast. (To
avoid interference and crosstalk, the frequency range of the voice transmissions themselves is only
3 kHz, slightly less than the 4 kHz of a wireline POTS phone call.)
Cellular networks use a portion of the radio
frequency spectrum assigned by government
regulators. For AMPS cellular phones, that range
is between 824 and 894 MHz. A connection
consists of two 30 kHz channels, widely separated
in frequency: a receiving channel (also known as
the forward channel) from the base station to the
phone, and an independent sending channel
(known as the reverse channel) from the phone to
the base station. Since the channels are separated
by frequency, the AMPS technology is also known as Frequency Division Multiple Access
(FDMA). The frequency separation allows AMPS calls to be full duplex: both parties can speak,
and be heard, at the same time.
2.2.3. A-side and B-side Carriers
The 50 MHz of spectrum assigned to AMPS cellular phones has been further divided by
regulatory bodies. In each region served by AMPS services, there can be two competing cellular
phone providers, arbitrarily known as the A-side and B-side carriers. One carrier is usually the
same company that provides local wireline telephone service, and the other is a separate firm,
most often one that does not provide wireline phone service.
Figure 2-1: Forward and reverse channels