UNIX System Administration Handbook - Evi Nemeth [233]
Existing networks
Computer networks are the focus of this discussion, but many organizations already have CATV networks and telephone networks capable of transmitting data. Often, these include fiber links. If your organization is ready to install a new telephone system, buy lots of extra fiber and have it installed at the same time.
We had that opportunity several years ago and asked the contractors if they would string some fiber for us. They said, “Sure, no charge,” and were a bit miffed when we showed up with a truckload of fiber for them to install.
Expansion
It is very difficult to predict needs ten years into the future, especially in the computer and networking fields. Therefore, it is important to design the network with expansion and increased bandwidth in mind. As cable is being installed, especially in out-of-the-way, hard-to-reach places, pull three to four times the number of pairs you actually need. Remember: the majority of installation cost is labor, not materials.
Even if you have no immediate plans to use fiber, it is wise to install some when wiring your building, especially if it is hard to install cables later. Run both multimode and single-mode fiber; the kind you will need in the future is always the kind you didn’t install.
Congestion
A network is like a chain: only as good as its weakest or slowest link. The performance of Ethernet, like that of many other network architectures, degrades as the network gets loaded.
Diskless nodes, terminal concentrators, mismatched interfaces, and low-speed links can all lead to congestion. It is helpful to isolate local traffic by creating subnets and by using interconnection devices such as routers. Subnets can also be used to cordon off machines that are used for experimentation; it’s difficult to run an experiment that involves several machines if there is no easy way to isolate those machines both physically and logically from the rest of the network.
Maintenance and documentation
We have found that the maintainability of a network correlates highly with the quality of its documentation. Accurate, complete, up-to-date documentation is absolutely indispensable.
Cables should be labeled at all termination points and also every few feet so that they can easily be identified when discovered in a ceiling or wall.9
It’s a good idea to keep copies of local cable maps posted inside communications closets so that they can be updated on the spot when changes are made. Once every few weeks, someone should copy the changes down for entry into an electronic database.
Joints between major population centers in the form of repeaters, bridges, routers, or even connectors can facilitate debugging by allowing parts of the network to be isolated and debugged separately. It’s also helpful to put joints between political and administrative domains.
15.12 MANAGEMENT ISSUES
If the network is to work correctly, some things need to be centralized, some distributed, and some local. Reasonable ground rules and “good citizen” guidelines need to be formulated and agreed on.
A typical environment includes:
• A backbone network among buildings
• Departmental subnets connected to the backbone
• Group subnets within a department
• Connections to the outside world (e.g., Internet or field offices)
Several facets of network design and implementation must have site-wide control, responsibility, maintenance, and financing. Networks with charge-back algorithms for each connection grow in very bizarre but predictable ways as departments try to minimize their own local costs. Prime targets for central control are:
• The network design, including the use of subnets, routers, switches, etc.
• The backbone cable itself, including the connections to it
• Host IP addresses, hostnames, and subdomain names
• Protocols, mostly to ensure that they interoperate
• Routing policy to the Internet