The most widely used method of networking computers together within a single building or campus is called Ethernet. There's no need for most network users to understand in detail how Ethernet works, but it's useful to understand a few specifics. Ethernet transmits data about 1000 times as fast as a typical telephone line. However, unlike a telephone circuit (which is normally used for a single session, transmitting data between two computers), Ethernets typically connect dozens of computers, any one of which can send messages to any or all of the others.
One defining feature of Ethernet is that it doesn't limit when any of the connected computers can "talk". Because of this characteristic, it's always possible that some computer will start transmitting at the same time as another one. This usually doesn't cause a problem, though, because the Ethernet senses the conflict and tells both computers to wait a short, but random, period and try again. But, as the number of the computers on the Ethernet grows, there are more "data collisions", triggering more retransmissions and even more collisions. In a business environment, this usually occurs when there are 30 to 60 computers on the LAN (the exact number may be larger or smaller, depending on the computers' activity levels). Fortunately, there's a way to relieve this network overcrowding, as described below.
There's a second common LAN structure called Token Ring. It's a bit faster than Ethernet, but because the electronic components needed to use it are more expensive, it's less popular. Although Token Ring is designed to prevent "data collisions", it too can get overcrowded. However, the overcrowding can be resolved by breaking it up into pieces, much the same way as with Ethernet.
The difference between "bridges" and "routers" is that bridges (which are less expensive) broadcast all of the data message traffic around the network indiscriminately, while routers actually "route" it just to the segments it needs to pass through to reach its destination. This indiscriminate transmission isn't a problem in LANs with only two or three segments, but in larger LANs, bridges create unnecessary data traffic (i.e., by sending messages to segments where they're not needed). Thus, the more segments or rings in a network, the more benefit there is to connecting them with routers instead of bridges.
With the first generation of "switching hubs", which are now commonly available, network managers can move computers from segment to segment manually whenever a segment becomes too congested. Newer products that have just come to market within the last year (including both hubs and routers), allow LAN managers to do the switching via software from a PC at their desks rather than with manual switches. Eventually, the network electronics will do optimized segment switching "on the fly", based on parameters set by the network manager. However, that's still in the future.
Linking several LANs in different locations adds complexity and creates a potential problem with network speed. As mentioned previously, LANs transmit data about 1000 times as fast as normal telephone lines. However, since it's illegal to run network cables across a public right-of-way except under unusual circumstances, most LAN-to-LAN connections have to be done with telephone circuits. Wireless links (such as microwave) or other common carriers are sometimes used as well, but they're usually relatively slow and can be unreliable. In actuality, most wide area network linking is done with special digital telephone circuits that are five to ten times as fast as normal dial-up phone lines. But even this is still less than 2% of the speed of a typical LAN. The problem is that trying to link LANs with a telephone circuit is kind of like trying to connect fire hoses with a drinking straw.
Once again, however, segmentation is the answer. Each location has its own LAN, made up of one or more network segments. Remote bridges, like the local bridges described above, can be used to connect the segments (or rings), although "routers" are becoming much more common. The reason for this preference for routers is that space on the slow inter-network links is scarce and routers are "smart" enough to "know" which messages need to go across which links, and which ones don't. Thus, by "screening out" the unnecessary messages, they can keep the traffic on the slower communications links down to a minimum.
Internet security issues aren't as much of a concern for E-mailing as they are for wide area networking, though, because there's no remote log-on when it's transmitted or received. However, even E-mail users need to be cautious, as it is possible, although very unlikely, that an E-mail message would be intercepted. Thus, it's always wise to encrypt (i.e., scramble) any confidential message. The Guest Viewpoint column in Bulletin 95.3 discussed encryption briefly, and it will be covered in more detail in an upcoming Bulletin.
No firewall offers perfect security (there's no such thing) but, unless your LAN controls access to large amounts of cash or marketable securities, or you think you could be a target for espionage or terrorist activity, there's probably a firewall somewhere along the spectrum that can provide most companies with adequate protection at an affordable price.
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