Data Communications Requirements of Distributed Database Systems - High-Performance Web Databases: Design, Development, and Deployment - page 472

a local area network (LAN) configuration can be used. The high throughput
and low cost of LANs make them an attractive alternative for a communi-
cations network for distributed databases. Speeds of as high as 100M b/s
are attainable on LANs using a fiber-optic backbone.
A single LAN often will not reflect the geographic distribution of an orga-
nization. One way of extending the distance over which a LAN may be used
is to employ repeaters, which strengthen and relay messages across a net-
work. The disadvantage of configurations that use repeaters is that as new
nodes are added, network performance decreases.
Another way of extending the physical distance of a network is to use a
bridge to connect two or more LANs. Unlike repeaters, bridges filter traffic
between networks, keeping local traffic local while forwarding only traffic
destined for other networks. In this configuration, data would be distrib-
uted between two or more LANs connected by a bridge.
If several LANs are involved, brouters may be used. Brouters — a type of
smart bridge — not only filter data but perform routing functions as well.
By deciding which route data will take through the network, brouters min-
imize traffic and increase throughput.
Employing any of the previously described devices allows network
designers to extend the distance of a network. In essence, it enables them
to create a Wide Area Network using several LANs. But this strategy could
not be employed over extremely long distances such as between cities. In
these situations, wide area network (WAN) using leased lines would be cho-
sen. For maximum throughput, a network employing a high-speed service
(e.g., T1, T2, T3, or T4) could be used. Because cost increases with perfor-
mance, careful consideration should be paid to the level of service desired.
In a wide area network (WAN) configuration, leased T1 lines would act as
a backbone for LANs at different physical locations. Using a LAN at each
physically distinct site would provide cost and performance advantages,
because most data would be manipulated at a local level. Accessing data
on high-speed LANs allows maximum performance. Using a T1 line also
allows voice to be run along with data, further increasing savings. Although
performance does not match that of a LAN configuration, an acceptable
level of throughput can be achieved.
NETWORK EXPANSION CONSIDERATION
As organizations grow and change, data requirements at new and exist-
ing sites also change. The communications networks and hardware that
support distributed databases should be capable of changing along with
the data they support. To assess possible changes in data communications
requirements, network managers should have a thorough understanding of