Installed Base Maintainability Selecting Appropriate LAN Technology

100 Most seriously, if there are problems with one or more devices connected to a workgroup switch, then the only noticeable result will be performance problems for the other devices in that workgroup. The workgroup switch will not pass bad frames to the Access switch, [1] and it cant complain about the bad frames it receives from its end devices. It is possible to have a serious problem that simply will never be seen unless the users are diligent about complaining. [1] It is customary to use the word frame when talking about the Layer 2 view of a chunk of data and the packet at Layer 3.

4.1.2 Installed Base

Installed base is another facet of cost effectiveness. The chances are slim that you are building a new network from scratch. In most cases, there is existing equipment, existing applications, servers, and a cable plant. A significantly cheaper alternative network technology may be available. If migrating to that means that you have to absorb a high cost in changing your installed base, then simply staying with the existing technology may be more cost-effective. For example, a large company may make extensive use of native Token Ring protocols to connect to legacy mainframe equipment. Token Ring equipment is more expensive than Ethernet equipment, but after factoring in the cost of replacing the mainframe, rewriting the applications to use TCPIP, and changing every end device to use this new application, they probably wont want to make the change. This is where it is useful to have a long-term strategic information technology vision for the entire organization. If you have a long-term goal to phase out these legacy applications, then you need to build a network that can accommodate a phased-in migration to the target technology. Perhaps you will migrate the Core of the network from Token Ring to Fast and Gigabit Ethernet with TCPIP routing and use DLSw to tunnel the native Token Ring protocols. Then, when the new servers and applications are available, you can migrate user devices in relatively small groups. An installed base doesnt need to cripple a network, but it can limit your design options temporarily.

4.1.3 Maintainability

One of the biggest potential hidden costs in a network is maintenance. I have mentioned how using unmanageable workgroup devices in the Access Level of the network can make it harder to find problems. I previously mentioned that the design principle of simplicity makes network maintenance easier. Remember that these are not just annoyance factors for the engineer who gets stuck with the ultimate responsibility for running the network. There are costs are associated with these issues. The trouble is that quantifying these costs can be extremely difficult. How can you tell, for example, that cutting a particular corner will result in needing an extra staff member to keep the network going? Only experience can tell you what the hidden costs are. In general, since your design goals are centered on reliability, the more corners you cut, the less reliable the results will be. Lower reliability generally translates into higher maintenance costs.

4.1.4 Performance