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The following considerations influenced
the design of this network:
Traffic Patterns
An analysis of the anticipated traffic indicated that each remote
site would call the central site an average of four times an hour
throughout the business day. This type of traffic pattern means that
cost savings can be realized at the central site by providing one
telephone line for every 2.5 remote sites (for a total of 48
telephone lines). To spread the calls evenly among the 48 lines, the
remote sites connect through a hunt group. The hunt group provides
an additional benefit in that all of the remote routers dial the
same telephone number to access the central site, which makes the
configurations of the remote site routers easier to maintain.
In order to complete a transaction
initiated by a remote-site, the central site sometimes needs to call
that remote site shortly after it has disconnected from the central
site. To make this possible, the access network must converge
rapidly. The central site also calls the remote sites periodically
to update the transaction processing software on the remote
workstations.
Media Selection
Designers chose asynchronous dial-up technology through the Public Switched Telephone Network (PSTN) for the following reasons:
- Availability---PSTN is
available at all of the remote sites. Potential alternatives,
such as Frame Relay and Integrated Digital Services Network
(ISDN), were not available at some of the remote sites.
- Bandwidth---The transaction
processing software causes a small amount of data to be
transferred between the remote sites and the central site. For
this type of low-bandwidth application, the bandwidth provided
by asynchronous dial-up is acceptable. Occasionally, the central
site dials the remote sites in order to maintain the transaction
processing software on the remote clients. This activity will
occur at night (in the absence of transaction processing
activity), so the bandwidth provided by asynchronous dial-up is
adequate.
- Cost---Given the
low-bandwidth requirement, the cost of installing and operating
Frame Relay or ISDN equipment could not be justified.
Note: Although the network
described in this case study uses asynchronous dial-up technology
over the PSTN, most of the concepts, such as routing strategy and
addressing, also apply when scaling other circuit-switched
technologies (such as ISDN).
Application Protocol
Requirements
The remote workstations run transaction-processing software that
uses the Transmission Control Protocol /Internet Protocol (TCP/IP)
to connect to a database located at the central site. The remote
workstations have no need to run any other network-layer protocol.
Given this requirement, the most cost-effective choice of router for
the remote site is a router that provides an Ethernet interface and
an asynchronous interface, and that supports the Routing Information
Protocol (RIP).
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