CDPD

CDPD is a packetized data service utilizing its own air interface standard
that is utilized by the cellular operators. CDPD is functionally a separate
data communication service that physically shares the cell site and cellular
spectrum.
CDPD has many applications but are most applicable for short, burstytype
data applications and not large file transfers. CDPD application of the
short messages would consist of e-mail, telemetry applications, credit card
validation, and global positioning, to mention a few potentials. CDPD is a
pure data service designed for mobility; however, it cannot, nor was it ever
designed to, supply data speeds needed for 3G services.
CDPD does not establish a direct connection between the host and
server locations. Instead it relies on the OSI model for packet-switching
data communications, and the model routes the packet data throughout
the network. The CDPD network has various layers that comprise the system.
Layer 1 is the physical layer, layer 2 is the data link itself, and layer
3 is the network portion of the architecture. CDPD utilizes an open architecture
and has incorporated authentication and encryption technology
into its airlink standard.

The CDPD system consists of several major components, and a block diagram
of a CDPD system is shown in Figure 3-42.
The Mobile End System (MES) is a portable wireless computing device
that moves around the CDPD network, communicating to the MDBS. The
MES is typically a laptop computer or other personal data device that has
a cellular modem.

The Mobile Data Base Station (MDBS) resides in the cell site itself and
can utilize some of the same infrastructure that the cellular system does for
transmitting and receiving packet data. The MDBS acts as the interface
between the MES and the MDIS. One MDBS can control several physical
radio channels, depending on the site’s configuration and loading requirements.
The MDBS communicates to the MDIS via a 56-Kbps data link.
Often the data link between the MDBS and MDIS utilizes the same facilities
as that for the cellular system, it but occupies a dedicated time slot.
The Mobile Data Intermediate System (MDIS) performs all the routing
functions for CDPD. The MDIS performs the routing tasks utilizing the
knowledge of where the MES is physically located within the network itself.
Several MDISs can be networked together to expand a CDPD network.

The MDIS also is connected to a router or gateway, which connects the
MDIS to a Fixed End System (FES). The FES is a communication system
that handles layer-4 transport functions and other higher layers.
The CDPD system utilizes a Gaussian minimum-shift keying (GMSK)
method of modulation and is able to transfer packetized data at a rate of
19.2 Kbps over the 30-kHz-wide cellular channel. The frequency assignments
for CDPD can take on two distinct forms. The first form of frequency
assignment is a method of dedicating specific cellular radio channels to be utilized by the CDPD network for delivering the data service. The other
method of frequency assignment for CDPD is to utilize channel hopping
where the CDPD’s Mobile Data Base Station (MDBS) utilizes unused channels
for delivering its packets of data. Both methods of frequency assignments
have advantages and disadvantages.
Utilizing a dedicated channel assignment for CDPD has the advantage
of the CDPD system not interfering with the cellular system it is sharing
the spectrum with. By enabling the CDPD system to operate on its own set
of dedicated channels, no real interaction takes place between the packet
data network and the cellular voice network. However, the dedicated channel
method reduces the overall capacity of the network and, depending on
the system loading conditions, this might not be a viable alternative.
If the method of channel hopping is utilized for CDPD, and this is part of
the CDPD specification, the MDBS for that cell or sector will utilize idle
channels for the transmission and reception of data packets. In the event
the channel that is being used for packet data is assigned by the cellular
system for a voice communication call, the CDPD MDBS detects the channel’s
assignment and instructs the Mobile End System (MES) to retune to
another channel before it interferes with the cellular channel. The MDBS
utilizes a scanning receiver or sniffer, which scans all the channels it is programmed
to scan to determine which channels are idle or in use.

The disadvantage of the channel hopping method involves the potential
interference problem to the cellular system. Coexisting on the same channels
with the cellular system can create mobile-to-base-station interference.
This kind of interference occurs because of the different handoff boundaries
for CDPD and cellular for the same physical channel. The difference in
handoff boundaries is due largely to the fact that CDPD utilizes a BER for
handoff determination and the cellular system utilizes RSSI at either the
cell site, analog, or MAHO for digital.

This chapter covered numerous radio access platforms that were built to
improve the efficiency of mobility systems offering voice services. The
advent of the Internet during the time that these services were beginning
to be deployed has resulted in a desire to have a wireless mobility system capable of handling high-speed data traffic. However, as was the case with
migrating from 1G to 2G, the path to 3G is not straightforward. It is hoped
that the inclusion of the 2G systems will facilitate the introduction of 3G
systems and the interim platforms that are currently being deployed, which
are referred to as 2.5G.