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100Base-TX cable, or Fast Ethernet, explained

This tip reviews 100Base-TX cable fundamentals, including a breakdown of the term's meaning, how it works and related cable types that network pros need to know.

The 100Base-TX cable -- sometimes referred to as 100Base-T -- is the most popular cable around, since it replaced the 10Base-T and 10Base-2 coaxial cable types. The 100Base-TX cable provides fast speeds -- up to 100 Mbps -- and is more reliable than the older cables because it uses CAT5e cable. There are also 100Base-T4 and 100Base-FX cables available, which are discussed below as well.

Understanding the terminology

To understand 100Base-TX cables, it helps to break down the term into its constituent parts:

100: The numeral 100 represents the frequency in megahertz (MHz) for which this cable is designed -- that is, for a frequency of 100 MHz. The greater the megahertz, the higher the speeds the cable can handle. If you try to use this type of cable for greater frequencies -- i.e., speeds -- it will either not work or become extremely unreliable. The 100 MHz speed translates to 100 Mbps, which, in theory, means 12 Mbps.

Base: The word base refers to baseband. Baseband is an Ethernet communications standard that enables a network device to use all of the available bandwidth when it is transmitting. Broadband, by contrast, shares the bandwidth that is available. This is why cable modem users notice a slowdown in speed when they are connected on a busy node, or when their neighbor is downloading all the time at maximum speed. Ethernet also experiences slowdown in speed, but it will be smaller in comparison to broadband.

TX/T4/FX: The T refers to twisted pair, the physical medium that carries the signal. This illustrates the structure of the cable and tells us it contains pairs that are twisted. For example, unshielded twisted pair (UTP) has twisted pairs, and this is the cable used in such cases. The 100Base-T is used sometimes to refer to the 100Base-TX cable specification. All 100 Mbps-rated cables, except the 100Base-FX, use CAT5 cable.

100Base-TX cable

The TX -- sometimes referred to simply as T -- means it's a CAT5 UTP straight-through cable using two of the four available pairs. A 100Base-TX cable can support speeds up to 100 Mbps. Its maximum length is 100 meters, and its minimum length between nodes is 2.5 meters.


The T4 in this term means it's a CAT5 UTP straight-through cable that is using all four available pairs and supports speeds up to 100 Mbps. Its maximum length is 100 meters, and its minimum length between nodes is 2.5 meters.


FX, in this term, means it's a two-strand fiber cable and supports speeds up to 100 Mbps. The maximum length for a 100Base-FX cable is usually up to 2 kilometers.


It’s important to keep the following basics in mind about 100Base-TX and other cables:

  • The 100Base-TX/T4 only works for 100 Mb networks and uses unshielded twisted pair cable with RJ-45 connectors at each end.
  • All CAT5 UTP cables have four pairs of cables -- that is, eight wires.
  • The 100Base-TX -- sometimes called 100Base-T -- uses two of the four available pairs within the UTP cable, whereas the 100Base-T4 uses all four pairs.
  • The 100Base-FX also works for speeds up to 100 Mbps, but it uses fiber optic cable instead of UTP.

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This was last published in August 2016

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