Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 9
AMPS Voice
Unused
Time
Channel 2
Channel 3
Channel Usage
0
1
2
3
Unused
capacity
Channel 1
Figure 3-1: Cellular radio channel usage within a single cell sector
In this example, the unused channel capacity ranges between 0 and 3 radio channels. The CDPD
concept is based on sending packet-switched data on radio channels within a sector when they are
not used for cellular voice communications. It reuses these unused voice channels by hopping
from one unused voice channel to another whenever that channel is required for cellular voice. In
other words, CDPD reuses the unused channel capacity in a voice cellular network for packet
switched data.
3.1.2. Channel Hopping
Figure 3-2 shows how short data packets can be interleaved between voice calls on the cellular
network, using the idle capacity in the system—known as a “sniff and hop” configuration. is
known as the airlink.
Idle Voice
Packets
Idle
Voice
Voice
Idle Voice
Chan 1
Chan 2
Chan 3
Chan 4
Figure 3-2: CDPD channel hopping
Channel hopping on the airlink—the wireless portion of a CDPD transmission—works well under
typical voice usage, but as the network becomes congested, less room is available for data traffic.
Many CDPD carriers have therefore agreed to guarantee to have channels dedicated to data
transmission only (see section 3.1.3). Otherwise, in an emergency situation like a flood or
hurricane, the cellular system could become completely clogged. Even in a dedicated system,
there is no guarantee that the modem will stay on an acquired channel very long. If the modem is
mobile, it will be forced to frequently change channels as it travels through the carrier’s territory
from cell to cell.
Two types of channel hops can occur in CDPD systems not using dedicated channels:
• Planned channel hops
• Forced channel hops