Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 4
A-side carriers use Band A of the AMPS spectrum. Their cellular phones transmit in the
frequency ranges 824-835 MHz and 845-846.5 MHz, and they receive in the 869-880 MHz and
890-891.5 MHz ranges. These correspond to cellular channels numbered 1-333, 667-716, and
991-1023. B-side carriers use Band B, which transmits at 835-845 MHz and 846.5-849 MHz, and
receives at 880-890 MHz and 891.5-894 MHz, corresponding to cellular channels 334-666 and
717-799. Each carrier has 416 pairs of 30 kHz channels available, although the number for calls is
smaller, since each cell requires one or two control channels to manage the operation of the
network.
Each carrier has its own infrastructure, and sets up transceiver base stations at cell sites on towers,
in buildings, or on hilltops. In any particular coverage area, each of the two carriers organizes its
cells so that telephones can move from one to another without losing contact with the network.
2.2.4. Cell Sites Coordinated by the MTSO
The coverage areas of each base station—the cells that give the cellular network its name—are
roughly hexagonal (see Figure 2-3), although in rural areas and those of rough terrain, cell shapes
may differ. The interaction between sites is managed by the carrier’s central Mobile Telephone
Switching Office (MTSO). At the heart of the MTSO is the cellular switch, which also links
into the voice circuits of the PSTN. For cellular phone users, the cellular switch makes the
cellular network a nearly seamless part of the PSTN.
Figure 2-2: A cellular telephone system
The cellular switch is the central coordinating element for all of the cell sites for one carrier in one
area, such as a city. It performs all call processing functions and supports certain aspects of
network accounting and management.
Purchasers of cellular handsets generally activate them with their carrier, and with a particular
local cellular switch for that carrier, providing that handset with a home cellular network.
Arrangements within that carrier and between it and other carriers in other cities may allow the
handset to operate outside the home region when the subscriber travels. When the handset
communicates with a different carrier than normal, it is said to be roaming.
2.2.5. Assigning and De-Assigning Frequencies
When a cellular subscriber originates or receives a call, the MTSO assigns the subscriber an
available radio channel from the group of channels assigned to that carrier. Once assigned this
channel, the call progresses until:
• the subscriber terminates the call, when the MTSO de-assigns the radio channel from the cell
site and makes it available for new calls.
–or–