Pluses and Minuses
There's no clear right or wrong in choosing between 100Base-TX and 100Base-T4. The right system for a given network depends on several factors, the first of which is the installed cable type. If properly installed Category 5 or Type 1 cable is not available, you must use 100Base-T4 or install new twisted-pair wiring. If the quality of the installed cable is unknown or questionable, 100Base-T4 is a better choice because it offers the flexibility of running on more cable types.

A second consideration is budget. You can save a substantial amount of money if you can use 100Base-T4 adapters and Category 3 cabling. (A quick survey of cable costs shows four-pair Category 3 plenum cable priced at 9 to 25 cents per foot, and four-pair Category 5 plenum cable at 23 to 37 cents per foot.)

Finally, you must take your future needs into account. If you must have full-duplex mode now, or if there's a chance that you might want it in the future, you'll need 100Base-TX.

Meanwhile, 100Base-T4 is best suited for use in workstations, where cost is an important issue. 100Base-T4 also runs over the more commonly installed types of cabling, and workstations are not likely to need full-duplex connections. 100Base-TX, on the other hand, is more desirable for network servers, which can take advantage of full-duplex mode, and in situations where it's easier to control the quality of the cabling.

The Fiber Alternative


The 100Base-FX fiber-optic option for Fast Ethernet offers the same types of advantages in traditional Ethernet and FDDI networks as fiber does. Namely, 100Base-FX offers extended distances, electromagnetic immunity, and increased security.

Like 10Base-T, 100Base-T allows a maximum distance of 100 meters between a repeater (i.e., a hub) and a node. Using fiber-optic cable, 100Base-FX increases this distance to a maximum of 185 meters. Between a server and a workstation, the maximum distance (with no repeater) is increased to 400 meters, and it can be as much as 2 kilometers when full-duplex mode is used.

The standard cable type for 100Base-FX is multimode fiber with a 62.5-micron core and 125-micron cladding. Only one pair of fibers is required -- one for transmission and one for reception. This is the same type of cable that's commonly used in 10Base-FL Ethernet networks with ST bayonet-style connectors.

However, the new EIA-preferred connector is the SC-plug style. An SC connector has the advantage of being a push-on/pull-off connector (with no twisting required). Since it's keyed, there's no possibility of incorrectly connecting the transmit and receive cables. If your installed fiber-optic cable is already terminated with FDDI-compatible MIC connectors, then you can use an inexpensive MIC-to-ST converter.

The 100Base-FX standard will find its primary niche in the interconnection of repeaters to form a fiber-optic backbone. A typical company using this standard will have Fast Ethernet repeaters on each floor or in each department. Each of these repeaters will support 100Base-TX or 100Base-T4 workstations. The repeaters will then be interconnected using 100Base-FX links. When repeaters on different floors are connected, the fiber-optic cabling will provide protection from the electromagnetic noise often associated with elevators, and it will also enable longer cable runs between buildings.

Migration Strategies

Today there are many networks based not only on twisted-pair cabling, but also on thin-coaxial cabling with BNC connectors. Clearly, there must be a strategy that will allow today's networks to smoothly migrate to Fast Ethernet.

Many companies want to protect their investment in the cabling and connectors they've already installed. These companies should consider auto-sensing network adapters for their workstations. Such adapters can be used today for 10-Mbps Ethernet (either coaxial or UTP), and they can be used in the future for 100-Mbps Fast Ethernet. Such auto-sensing cards allow workstations to automatically switch to Fast Ethernet (when, for example, the servers and repeaters are upgraded) without the need for a LAN administrator to pop open each PC on the network and reconfigure DIP switches on the adapter card.

For those who are ready to install Fast Ethernet today -- and need to do so gradually and keep some 10-Mbps devices -- a number of options exist. Perhaps the simplest is to install a Fast Ethernet adapter in the existing server, alongside the 10-Mbps Ethernet adapter, and use this connection to support a Fast Ethernet repeater and workstations.

The Fast Ethernet standard is designed to provide flexible solutions for a wide variety of cabling situations. With the availability of 10/100 adapters, 10/100 adapters with BNC/coaxial support, and 10/100 switches, organizations should be able to migrate in a manner of their choosing.