Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 6
2.2.7. Cell Handoff
Using a typical seven-cell, three-sector channel reuse pattern, call handoff is straightforward,
allowing a cellular handset to move through and between cells while smoothly continuing a call.
In this example, the cellular subscriber makes
(or receives) a call in sector A3. They move to
sector A1 and experience a handoff at location 1
where they are handed off to a new radio
channel in sector A1. From sector A1 they
travel to sector A2 and are handed off at
location 2. In the cases of the handoffs at
locations 1 and 2, the cellular subscriber has
been handed off to radio sectors within the same
cell site.
As the cellular subscriber travels from sector A2
to C1, they are handed off to a new radio
channel being served by a different cell site.
This handoff procedure continues until the
cellular subscriber eventually arrives in sector
B2 after having been handed off at location 5.
In all cases, the cellular subscriber is assigned a radio channel available within a certain sector and
that radio channel is de-assigned from that subscriber once they have been handed off by the
cellular switch to the new sector. The de-assigned radio channel is then made available for
another user within that sector.
Although the cellular handset uses six different radio frequencies and communicates with three
different base stations over the course of this portion of the call, the continuity of the call is not
affected. Other than possible short delays during the five handoffs, the call continues
transparently.
2.3. Cellular Data Transmission
The introduction of the AMPS cellular system in North America in 1983 coincided with the first
public popularity of long-distance data transmissions between personal computers. Using
modems (modulator-demodulators) and POTS wireline telephones, personal computer users could
convert data streams into audible tones that could be transmitted through the phone system,
initially at 110 or 300 bits per second (bps). Like fax before it, such data transmission took
advantage of a system designed purely for voice to transmit something else.
It would not be long before users of notebook computers, once they were widely available, wanted
to be able to connect modems wirelessly, perhaps to hook up to an online service, a corporate
bulletin board system (BBS), or even a home computer. Once again, a system designed for
voice—in this case, the AMPS cellular network—would be used for data.
2.3.1. Circuit-Switched vs. Packet-Switched Data
While public safety bodies and large corporations such as IBM and FedEx created their own
proprietary wireless data networks in the 1970s and ’80s, individual users who really wanted to
move data in a mobile environment developed methods of connecting their conventional wireline
modems to their cellular phones, with mixed results. This groundswell of demand led engineers to
develop modems specifically for connections through cellular telephones, yielding a fairly reliable
service capable of offering data at 9600 bps (9.6 kbps).
The ad-hoc solution of connecting a modem to a cellular phone works, but the end user pays by
the minute with long distance charges where applicable, because each call remains circuit-
Figure 2-4: Cell handoff in three-sector cells