Getting to know cables, part 2: The 100-Mbps networking technology

The 100-Mbps networking technology is, far and away, today's networking standard. Understanding the foundation of this technology can be difficult. Our TechProGuild cable guide is here to help.

In “Getting to know cables, part 1: The 10-Mbps arena,” we explained the various cable technologies available in the 10-megabits-per-second arena. Of course, the 10-Mbps technology is far too slow for today’s needs. So let’s enter the speedier world of 100 Mbps! In part two of this cable guide, we’ll take a look at the various technologies surrounding 100-Mbps networking.

100 Mbps history and overview
Today's standard is the 100-Mbps networking technology, which can carry Ethernet frames up to 100 Mbps. When the IEEE standardization committee started working on a faster Ethernet system, it developed two approaches. The first approach is to simply speed up the Ethernet system to 100 Mbps, keeping the original Carrier Sense Multiple Access with Collision Detection (CSMA/CD) medium-access control mechanism. This method is called 100Base-T, or Fast Ethernet. This new standard is based on the original IEEE 802.3 standard.

The second approach is to create an entirely new medium-access control mechanism. This new mechanism is based on hubs that control access to the medium using a "demand-priority" mechanism. The new access control system transports standard Ethernet frames, but it does so with a new medium-access control mechanism and allows for the transport of token-ring frames as well. This approach, called 100VG-AnyLAN, is based on a newer IEEE 802.12 standard.

The 100-Mbps technology follows a naming scheme similar to that for 10 Mbps. For example, 100Base-T can be broken down as follows:
  • 100—Speed
  • Base—Type of signaling
  • T—Segment type

Using the ever-popular star topology, 100Base-TX easily enjoys the most widespread use today. What is 100Base-TX? It is used for your standard LAN, such as your office. Take a look at the Category 5 cable coming out of your machine and running into either the wall jack or the hub—more than likely, that is 100Base-TX.

This technology is based on the ANSI Twisted Pair-Physical Medium Dependent (TP-PMD) physical media standard and operates over two pairs of wires: one pair for receiving data signals and the other pair for transmitting data signals. By using two pairs of wires, you know that four of the eight pins in the RJ-45-style medium dependent interface (MDI) connector will be used. (We’ve listed standard pin usage later in this section.) In accordance with the ANSI TP-PMD standard, 100Base-TX provides for the use of either unshielded twisted-pair or shielded twisted-pair cable. (The most popular is unshielded twisted-pair cable.)

The standard 100Base-T Ethernet connection varies somewhat from the 10Base technology. This type of media starts with the computer containing an Ethernet interface. This interface is attached to the system by means of either an outboard Media Independent Interface (MII) cable with a Physical (PHY) layer transceiver via twisted-pair RJ-45-style jack or a fiber-optic cable/connector. With the 100Base-T technology, there is no longer a need for drop cables or taps. In this case, the twisted-pair cable (common Category 5 cable) plugs directly into the network interface.

With this twisted-pair cable, the two wires can be twisted together (along the entire length of the wire run) within half an inch of any connector or wire termination point. (This is a standard method for improving the signal-carrying characteristics of an unshielded wire pair.)

The 100Base-TX technology offers a segment length of 100 meters when you’re using data-grade unshielded twisted-pair wire that has a characteristic impedance of 100 ohms and meets the Electronic Industry Association/Telecommunications Industry Association (EIA/TIA) Category 5 wire specifications. (The 100BASE-TX standard also accommodates shielded twisted-pair cabling with a characteristic impedance of 150 ohms.) Two 100-meter segments can be joined together with either a Class I or Class II repeater.

This standard uses eight-pin (RJ-45 style) connectors with the following layout:
  • Pin 1: Transmit Data+
  • Pin 2: Transmit Data-
  • Pin 3: Receive Data+
  • Pin 4: Unused
  • Pin 5: Unused
  • Pin 6: Receive Data-
  • Pin 7: Unused
  • Pin 8: Unused

It is also possible to use nine-pin D-type connectors with the following layout:
  • Pin 1: Receive Data+
  • Pin 2: Unused
  • Pin 3: Unused
  • Pin 4: Unused
  • Pin 5: Transmit Data+
  • Pin 6: Receive Data-
  • Pin 7: Unused
  • Pin 8: Unused
  • Pin 9: Transmit Data-

Useful applications: The 100Base-TX standard is best employed for short runs where speed is key. This technology is probably one of the most widely deployed within a standard LAN topology. Going from workstations to network closets or server farms is your best bet.

One-upping 100Base-TX is the fiber equivalent, 100Base-FX. Again, we are using the 100-Mbps speed with a baseband signaling (we are sending only Ethernet signals) in a star topology. A 100Base-FX segment can reach up to 412 meters using two strands of multimode fiber-optic cable (multimode involves using LED instead of laser-generated light) per link—one for transmitting data and one for receiving data—with the transfer-to-receive signal crossover performed within the link. With a 100Base-FX line, you can have up to two multistation access units (MAUs) per segment.

The standard fiber-optic cable is a graded index multimode fiber cable with a 62.5-micron fiber-optic core and 125-micron outer sheath (otherwise dubbed 62.5/125).

A 100-Mbps fiber segment consists of the following components:
  • Fiber cable (as mentioned earlier), which could be either a simple two-strand fiber cable or a larger interbuilding cable carrying multiple pairs of fiber.
  • The Medium Dependent Interface Connector, which could be one of three types: a duplex Subscription Channel (SC) connector, a Fiber Distributed Data Interface (FDDI) Media Interface Connector (MIC), or a Straight Tip (ST) connector. (The most common is the SC connector.)

If the distance traveled exceeds 412 meters, a repeater will be used.

An interesting note: Segment length will change given the type of repeater used. Regardless of type, the fiber segment length will be reduced when you introduce a repeater. If a single Class 2 repeater is used, the length between DTEs (Data Terminating Equipment, which is a terminal or computer) is dropped to 320 meters; if a Class 1 repeater is used, the distance between DTEs will drop to 272 meters; if two Class 2 repeaters are used, the distance then drops to 228 meters.

Fiber, of course, is not perfect. You are dealing with an entirely different beast than with twisted pair. You have to take into consideration the greater number of connections you have. Also, the longer your cable, the higher the optical loss will be. Standard fiber will allow for approximately 1 to 2 decibels (dB). With each fiber connection (or splice), you risk losing between 0.5 and 2.0 dB (depending on how well the connection was made). Contributing factors to higher loss include poor splices and finger oil or dust on connector ends.

Obviously, fiber can be very temperamental. Fortunately, the benefits far outweigh the issues (as long as the technicians making your splice clean their fingers after eating their greasy cheeseburger).

Useful applications: 100Base-FX is a very useful technology for running LAN lines between buildings and possibly campus-wide networks.

The 100Base-T4 technology, otherwise known as Fast Ethernet, operates over four pairs of wires (EIA/TIA Category 3, 4, or 5 unshielded twisted-pair cable, although Category 5 is recommended for best signal). The 100Base-T4 technology can work effectively over copper cabling. Because this signal type uses four pairs of wires, the connector scheme must look like this:
  • Pin 1: Transmit Data 1+
  • Pin 2: Transmit Data 1-
  • Pin 3: Receive Data 2+
  • Pin 4: Bidirectional Data 3+
  • Pin 5: Bidirectional Data 3-
  • Pin 6: Receive Data 2-
  • Pin 7: Bidirectional Data 4+
  • Pin 8: Bidirectional Data 4-

Of course, if two computers are connected directly together, the connection must be crossed over. To do this, you must adhere to the following:
  • Pin 1 —-> Pin 3
  • Pin 2 —-> Pin 6
  • Pin 3 —-> Pin 1
  • Pin 4 —-> Pin 7
  • Pin 5 —-> Pin 8
  • Pin 6 —-> Pin 2
  • Pin 7 —-> Pin 4
  • Pin 8 —-> Pin 5

You may also be using a hub (or repeater) that would have its own crossover capabilities. In that case, you would not need to run the above crossover scheme.

The specifications for 100Base-T4 cabling technology resemble those of 100Base-FX. The segment length of this technology is 100 meters, although the EIA/TIA cabling standard recommends a segment length of only 90 meters. The 100Base-T4 technology uses standard RJ-45-type connectors (see the lists above) and uses special transceiver circuits that constantly monitor the status of a connection. If the physical transceiver circuit detects that the network is idle, it will send link pulses (called fast-link pulses) over the segment to verify link integrity. These pulses are also used in auto-negotiation, which allows multispeed hubs to detect the connection speed and adjust the hub to match.

Useful applications: 100Base-T4 is a great way to upgrade a copper infrastructure to a faster topology. Like 100Base-FX, 100Base-T4 is best suited for internal LANs. Because of its Fast Ethernet capabilities, this particular technology makes upgrading a 10-Mbps network much cheaper than going with the more expensive 100Base-FX.

So we bring to a close our two-part cable guide, in which we examined the 10-Mbps and 100-Mbps technologies. I hope you’ve gained an understanding of what makes each of these technologies unique and where to best employ them.
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Jack Wallen is an award-winning writer for TechRepublic and He’s an avid promoter of open source and the voice of The Android Expert. For more news about Jack Wallen, visit his website

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