Page 126 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes The utility decided to base their applications on TCP/IP communications, so this quickly
disqualified the BellSouth Wireless Data and ARDIS networks which do not directly
support TCP/IP. Moreover, the utility believed that a packet-based approach would better
support the frequent communications that workers in the field require. This requirement
eliminated circuit-switched cellular connections. Since packet-based services are not yet
available for digital PCS networks, the remaining choices were CDPD and the Metricom
Ricochet* network. CDPD and Metricom Ricochet* are both IP-based packet networks.
However, data services for GSM and CDMA digital PCS networks are expected to be
deployed in the 1999 time frame and so may be candidates in the future.
Figure 1: The Utility Chose a Wireless Network that is IP-packet Based for
Greatest Flexibility

Since wireless data services are evolving rapidly, the utility decided to implement an
architecture that insulates its applications from the actual network used to the maximum
extent possible. Using an IP-based approach, where applications make no assumptions
about the nature of the physical connection, achieves this goal. This is not unlike Internet-
based communications, where packets may flow across copper cable, one moment; fiber
optic cable, the next; and a satellite. It should be possible to deploy applications using one
wireless network; and with minimal effort, migrate the application to another wireless
network in the future, should that network become more desirable.
Migrating between network types is indeed possible, though some adjustments may be
necessary for each network. For example, CDPD uses fixed IP addresses and Metricom
Ricochet uses dynamically assigned addresses. This difference could affect how firewalls
are configured. The effective throughput rates of Ricochet and CDPD also differ, with
Ricochet operating at 20 to 30 Kbps and CDPD at about 10 Kbps.
Software Approaches
In an ideal world, a computer connected over a wireless network would work just like a
computer on a LAN. But wireless networks operate at lower speeds with higher latency, and
connections can be lost at any moment, especially when mobile. The utility has considered
a number of software approaches, seeking to strike a balance among these factors: ease of
use, performance, reliability, and cost of deployment. To complicate matters, it discovered
that the best approach for supporting one application is not always the best approach for
another.
The first approach is to use all applications in their native form, with client software
installed on the mobile computer and communicating using TCP/IP protocols. Because
some workers will be working with the same applications both in an office environment
and in the field, the advantage of this approach is that the user interface stays the same
in both environments. Also, IT managers can set up mobile computers in the same way
as desktop computers. A disadvantage is that this approach does not address some of
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