Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 12
The CDPD infrastructure provides a buffer so that, to the wider Internet, packets destined for a
CDPD device can be routed as normal, through a fixed series of addresses. The CDPD system
captures these packets and then routes them to the CDPD device according to information the
CDPD network maintains about the modem’s current location. From the user’s point of view,
only the destination address is known, since the CDPD network manages the packet routing (see
section 5.2.1).
3.2. Features of CDPD
CDPD is designed to be flexible, efficient, and open. It is packet-switched, based on standard data
protocols used on the Internet, operates at full duplex, provides good data throughput, supports a
large number of simultaneous users, is straightforward for carriers to implement, and provides
effective data encryption and access control.
3.2.1. Packet-Switched
As mentioned previously, CDPD is a packet-switched system, which allows users to pay only for
data they send and receive, not the time they are connected to the CDPD network. An additional
benefit is that once registered with the network (see section 5.1), a CDPD modem can stay
connected indefinitely at no cost if no data is sent or received—the equivalent of an “always on”
wired Internet connection. Finally, a packet-switched system allows many more users to transfer
data over a single radio channel (see section 3.2.5) than a circuit-switched system, which can
support only one user per channel at a time.
3.2.2. Based on Internet Protocols
Some other wireless data standards (such as ARDIS and Mobitex/RAM) use their own proprietary
protocols, or ones that differ from standard Internet TCP/IP implementations. Connecting them to
the Internet requires protocol translation, adding an extra step to the data connection, and thus
making it slower and potentially less reliable.
CDPD was originally built as an IP-based system, so once a CDPD network is properly set up and
configured, CDPD devices can connect to them the same way as they would to a LAN or dial-up
Internet connection. In many instances, notebook users can simply replace an Ethernet or modem
PC Card with a CDPD modem PC Card and continue working. CDPD is therefore easy to
develop for, and easy to integrate into existing systems and applications. A CDPD subscriber
device can access anything accessible over the Internet. (CDPD can support protocols other than
TCP/IP as well, as long as both ends of the connection do—see section 4.3.8.)
3.2.3. Full Duplex
Like early radio telephones, and like “walkie-talkie”–style two-way voice radio, some wireless
data standards are half-duplex, allowing unidirectional transmission only. If a client device is
sending data, no data can come in; and if data is being received, no data can be transmitted until
the reception is complete.
CDPD takes advantage of the AMPS cellular system’s two-channel setup. Like AMPS voice
calls, As with the separate AMPS voice channels discussed in section 2.2.2, CDPD uses two
widely-separated radio frequencies for any given transfer, one for receiving (forward) information,
and one for transmitting (reverse) information. Sending and receiving can happen
simultaneously—a full-duplex connection.