Network Reliability Considerations

Clearly, one would like to build a network that supports the expected
demand and do so at the lowest overall cost—including both capital and
operating costs. Reducing capital cost often involves a centralized design
where equipment is deployed in fewer locations, thereby taking advantage of efficiencies of scale. This also helps to reduce some aspects of operating
costs as it reduces the number of locations and the number of network elements
that need to be managed. On the other hand, a centralized design
can lead to greater operational costs caused by increased transmission
requirements, particularly if transmission facilities are leased and paid for
on a monthly basis.

Cost is not the only important factor, however. Network reliability also
plays a big role. The fewer the number of locations used to support a given
subscriber base, the greater the impact if one of those locations becomes
inoperative due to some major catastrophe. If for example a single city is
served by a single location and that location suffers some catastrophe, such
as an earthquake or tornado, it is likely that service to the city will be
degraded if not completely halted. If, however, that location also serves a
number of other cities, then service in those cities will suffer equally. Thus
in parts of California, one might want to limit the number of markets
served by a particular location so that damage is somewhat contained in
the event of an earthquake. Similar considerations might apply in parts of
the eastern United States that are subject to hurricanes.

We understand that the foregoing discussion does not provide any real
rules for determining the placement of network elements and for establishing
the overall network topology. In reality, there are no hard and fast
rules that can be applied to any network. Therefore, we have attempted to
provide a description of the issues that need to be considered in the network
design exercise. Each network or operator will vary in terms of geographical
service area, quality and reliability objectives, capital and operating
expenditure limitations, offered services, service packaging, equipment
capacities, technology roadmap, and so on. All of these aspects must be considered
in determining the initial network design and how that design
should evolve over time. It is possible to include some of these factors in
software-based network design models, to which network design experience
should be added in developing the optimum design.