6.4 Scaling Dial-on-Demand Routing
6.4.2 Network design considerations
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).