The Fast Ethernet standard specifies support for a wide variety of media
Paul Cunningham
Fast Ethernet, also known as IEEE 802.3u or 100Base-T, has the potential to quickly become the successor to Ethernet as the networking topology of choice. This year about 1 million Fast Ethernet network interface cards (NICs) will ship, according to market-research firm International Data (Framingham, MA). And next year, the number is expected to increase to about 4 million, about four times as many NICs as is predicted for the other 100-Mbps Ethernet alternative, 100VG-AnyLAN.
The reason for Fast Ethernet's popularity is that it offers 10 times the speed of Ethernet for a small premium in price over today's 10Base-T adap
ter cards and hubs. In addition, Fast Ethernet is compatible with traditional Ethernet, using the same CSMA/CD method to arbitrate access to the cable. However, questions still remain in the minds of many about how to implement Fast Ethernet -- and what type of cables to use with it.
With such concerns in mind, the IEEE standard for Fast Ethernet details cabling specifications for a wide range of copper cabling, as well as optical fiber. The Fast Ethernet standard provides for three distinct cabling systems (see the table
"100Base-T Fast Ethernet Cabling Guide"
). Each of these systems has its merits and is appropriate for different situations. In addition, each standard specifies the use of cabling types, which are themselves based on industry-approved definitions.
The standards for twisted-pair copper cabling are called 100Base-TX and 100Base-T4; the standard for fiber-optic cabling is called 100Base-FX. These standards can be intermixed within an organization, and in combin
ation they address virtually all cabling needs.
There are two general sources of specifications for twisted-pair cabling: the Telecommunications Industry Association (TIA) branch of the Electronic Industries Association (EIA), which is referred to as the EIA/TIA (or simply the EIA), and IBM. The EIA is responsible for the "Category" cable standards (see the table
"EIA/TIA Cable Specifications"
). It's important to note that the terms
Category
and
Level
are sometimes used interchangeably. IBM is responsible for the "Type" cable standards, such as Type 1, Type 2, and so forth.
Strictly speaking, a cable specification does not include the connector used to terminate the cable; however, both IBM and the EIA define specific connectors for use with twisted-pair cable. For example, with Category 3, Category 4, and Category 5, the connector standard is EIA-568, which specifies an RJ-45 connector. For Type 1 cable, the connector type is DB-9.
Most Ethernet install
ations use cable based on EIA standards, and most installations of IBM or Token Ring equipment tend to use cable that meets IBM cable standards. The notable exception to this is that many Token Ring installations use Category 4 unshielded twisted-pair (UTP) cable. The 100Base-FX standard specifies the use of the same type of fiber-optic cabling system that's used by the fiber-optic Ethernet standard (10Base-FL) and Fiber Distributed Data Interface (FDDI).
Flexibility Is the Key
The 100Base-TX method requires two pairs of high-quality, data-grade twisted-pair wiring, one for transmission and one for reception. This cable type can be either UTP or shielded twisted-pair (STP), provided it meets the required performance specifications. The most common examples of cable that meet these requirements are Category 5 UTP and IBM Type 1 STP.
The 100Base-T4 method requires four pairs of ordinary-quality twisted-pair wiring: one dedicated to transmission, one dedicated to reception, a
nd two bidirectional pairs. Essentially, the 100-Mbps data signal is divided over three pairs of cable. Thus, the effective data rate remains the same as with other methods, but the cable frequency is much lower.
This means that you can use a less sophisticated cable with this method. This type of cable is sometimes referred to as
voice-grade cable
. Most often, this means Category 3 UTP. It's important to note, however, that 100Base-T4 is not limited to voice-grade cable. It can also be used with better-quality cable, including Category 4 UTP, Category 5 UTP, and Type 1 STP.
Pluses and Minuses
Each of the cabling specifications has advantages and disadvantages. The advantages of 100Base-T4 are that it will operate on virtually any preexisting twisted-pair cabling, its adapters are about 10 percent less expensive than those for other specifications, and it can use less-expensive Category 3 cabling. Meanwhile, 100Base-TX uses only two pairs of twisted-pair cabling a
nd supports full-duplex mode for up to 200-Mbps rates into the servers.
The disadvantages of each of these two specifications are virtually the strengths of the other. For instance, 100Base-T4 cannot support full-duplex mode (which might be desirable for servers but is unneeded in workstations) and requires four pairs of wiring. Meanwhile, 100Base-TX suffers from a temporary shortage of Category 5 cabling, which was caused by a resin-supply problem. Additionally, for 100Base-TX to be used, all patch panels and jumper blocks must be rated Category 5.
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 runn
ing 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 fi
ber-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 bein
g 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 cab
ling, 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.
Cabling specification 100Base-TX
Cable type Category 5 UTP (uses two pairs) or IBM Type 1 STP
Advantages Full-duplex mode offers 200-Mbps transfer
rate; requires only two pairs of cable.
Disadvantages Category 5 cabling might not be installed; patch
panels and jumper blocks must be rated for
Category 5.
Cabling specification 100Base-T4
Cable type Category 3, 4, or 5
UTP (uses four pairs)
or IBM Type 1 STP
Advantages Operates on virtually any existing twisted-pair
cabling; adapters are less expensive.
Disadvantages Requires four pairs of cabling; cannot support
full-duplex operation.
Cabling specification 100Base-FX
Cable type Multimode fiber (uses one pair of 62.5-/125-
micron fiber)
Advantages Allows for extended distances between devices;
immunity to electromagnetic interference;
added security; uses same cabling as FDDI.
Disadvantages Connectors are more expensive than copper
alternatives.
Cable type Common uses
Category 1 Telephone service and low-speed data
Category 2
ISDN and T1/E1
Category 3 Data up to 16 MHz (including 10Base-T at
10 Mbps and 100Base-T4 at 100 Mbps)
Category 4 Data up to 20 MHz (including Token Ring at
16 Mbps and 100Base-T4)
Category 5 Data up to 100 MHz (including 100Base-TX and
100Base-T4 at 100 Mbps)
Paul Cunningham is director of product marketing for Cogent Data Technologies, Inc. (Friday Harbor, WA). He can be reached on the Internet at
paul@cogentdata.com
or on BIX c/o "editors."
Untangling Fast Ethernet Cables
