LANs and WANs

Local Area Networks (LANs) and Wide Area Networks(WANs) are generic terms referring to two important basic types of networks. Let me try to summarize the characteristics of each, and then discuss their importance to the network engineer.

LAN/WAN Comparison

	Local Area Networks (LANs)	Wide Area Networks (WANs)
Most commonly:	Ethernet, Token Ring, FDDI	Leased lines, serial links, ISDN, X.25
Advantage:	speed	distance
Cost center:	dense installation (about one interface per room)	length of long-haul lines (about one interface per 100 miles)
Current Speed:	10-100 Mbps (mostly 10 Mbps)	0.01 to 45 Mbps (mostly clustered around 1 Mbps)
Common uses:	File sharing	Email and file transfer (including Web)
Common problems:	Cable disruption by users	Cable disruption by backhoes
Conceptually:	A bunch of lines hooking users together	A bunch of lines hooking cities together

The Internet can be thought of as a bunch of LANs interconnected by WANs. An average packet will run across a company's local Ethernet (LAN), up an ISDN or leased line or PPP link (WAN) to an Internet Service Provider. The ISP has Ethernet too (LAN), that transports the packet to the right router for delivery to a cross-country provider (WAN). The packet begins bouncing from one LAN site to another over WAN links.

A good networking design must answer both the LAN and WAN needs of its users. WAN links tend to operate with tight bandwidth margins, but many LAN applications depend on lots of surplus bandwidth. This is especially true of Ethernet, which begins to show performance degradation once you exceed about 20% "theoretical capacity", don't expect standard Ethernet to carry more than about 2 Mbps. A network's biggest startup cost is the labor needed to install it. So don't just install two-pair cable; install eight-pair and leave six unused. Don't just install one Ethernet cable; install two or three, and maybe run some fiber alongside it. Be ready to expand your LAN capacity as this becomes needed.

On the other hand, consider your WAN needs. Do you want global email and Web access? If so, you'll need some form of WAN connection, but what kind? Probably the best advice here is the same - plan for expansion, but in a different way. Plan so that you can upgrade your WAN service without changing your LAN configuration. Dialup SLIP or PPP is fine for one or two computers. Once you have a half dozen computers in regular use, I suggest shifting to a router configuration, even if the router is still using PPP. It much easier to track six LAN links and one WAN link than track six LAN links and six WAN links. As much as possible, I suggest static IP address assignment, and intelligent inverse nameserver entries.