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Network Information (cont.)
Cellular Digital Packet Data (CDPD) Network Technology
The Cellular Digital Packet Data (CDPD) network is a two-way wireless data
communication system. This system uses cellular telephone channels to transfer
data seamlessly from a mobile end system to a fixed end system. CDPD operates
within the already existing cellular telephone infrastructure utilizing the idle time
that exists on cellular voice channels to send small packets of data, called datagrams,
to a designated receiver. Researchers estimate that over 30% of a cellular channel’s
airtime is idle, even during heavy traffic.
The CDPD system sends signals down the various cellular channels to locate an
open channel. This process is called “sniffing”. Once the CDPD system has located
an open channel, the modem begins to send datagrams over that channel to the
designated end user. In order that this process does not interfere with the cellular
phone transmissions, the CDPD system has regulated channel “hops” to ensure
that CDPD usage will not block a cellular voice channel for an extended period of
time. Also, if a voice channel is activated, the CDPD system detects this voice
message and automatically “hops” to the next available idle channel. This process
is quite complex. However, the user need not be aware of this process. The CDPD
system manages the routing of the individual data packets to the proper destination.
On a basic level, the CDPD network operates like any other computer network
with the exception that the CDPD mobile end system is free to roam wirelessly and
conduct network operation from other locations using the network of cellular carriers.
The CDPD network is made up of a series of cellular networks which are operated
by various cellular carriers. These cellular carriers provide the user with network
management as well as application services. To increase user security, carriers use
forward error correction, authentication, and encryption. The cellular carrier is
responsible for all accounting services for the CDPD service. The user selects the
CDPD carrier service with which to operate by registering with an individual carrier.
Generally, an area will have two cellular carriers that provide CDPD service. The
user may sign up with either of the two carriers.
CDPD technology provides many benefits to the user including the following:
CDPD is cost-effective. By compressing data before sending it over thecellular
channels, CDPD cuts down the amount of data to be transmitted, and thus, reduces
the cost of cellular usage. Another cost-reducing advantage of CDPD is that the
user is only charged for the data transmitted, not for the airtime used. And by using
the existing cellular infrastructure currently in place for the voice network, CDPD
has few start-up expenditures which further reduces cost to the user. CDPD is fast.
At 19,200 bps, it is one of the fastest wide-area data networks. CDPD is mobile. It
allows free travel between cell cites. The CDPD system automatically tracks the
user’s position and allows the user to maintain communication between cell cites.
CDPD is reliable. It utilizes error correction applications for dependable data
delivery.