100Base-(T) TX/T4/FX - Ethernet: Cabling tips for network professionals, lesson 5

100Base-(T) TX/T4/FX - Ethernet fundamentals are discussed in this tip.

The 100Base-TX (sometimes referred to 100Base-T) cable is the most popular cable around since it has actually replaced...

the older 10Base-T and 10Base-2 (Coaxial). The 100Base-TX cable provides fast speeds up to 100Mbits and is more reliable since it uses CAT5 cable (see the CAT 1/2/3/4/5 page).There is also 100Base-T4 and 100Base-FX available, which we discuss later.


So what does 100Base-TX/T4/FX mean?

We are going to break the "100Base-T" into three parts so we can make it easier to understand:


The number 100 represents the frequency in MHz (Mega HertZ) for which this cable is made. In this case it is 100 MHz. The greater the MHz, the greater speeds the cable can handle. If you try to use this type of cable for greater frequencies (and, therefore, speeds) it will either not work or become extremely unreliable. The 100 MHz speed translates to 100Mbit per second, which in theory means 12 Mbps. In practice though, you wouldn't get more than 4 Mbps.


The word "Base" refers to Baseband. Baseband is the type of communication used by Ethernet and it means that when a computer is transmitting, it uses all the available bandwith, whereas Broadband (cable modems) shares the bandwidth available. This is the reason 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! Of course with Ethernet you will notice a slowdown in speed but it will be smaller in comparison to broadband.


The "T" refers to "Twisted Pair" physical medium that carries the signal. This shows the structure of the cable and tells us it contains pairs which are twisted. For example, 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. For more information, see the "UTP -Unshielded Twisted Pair" page where you can find information on pinouts for the cables. All 100MB rated cables, except the 100Base-FX, use CAT5 cable.


The TX (sometimes referred as "T" only) means it's a CAT5 UTP straight through cable using two of the four available pairs and supports speeds up to 100 Mb. Maximum length is 100 meters and minimum length between nodes is 2.5 meters.


The T4 means it's a CAT5 UTP straight through cable using all four available pairs and supports speeds up to 100 Mb. Maximum length is 100 meters and minimum length between nodes is 2.5 meters.


The FX means it's a two strand fiber cable and supports speeds up to 100 Mbs. Maximum length is usually up to two kms.

To summarize, keep the following in mind:

  • 100Base-TX/T4 works for 100 Mb networks only and uses unshielded twisted pair cable with RJ-45 connectors at each end
  • All CAT5 UTP cables have four pairs of cables (eight wires).
  • 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.
  • 100Base-FX also works for speeds up to 100 Mb but uses fiber optic cable instead of UTP.

Cabling tips for network professionals series

 Lesson 1: Network history and fundamentals
 Lesson 2: Straight-through UTP cables
 Lesson 3: CAT5 UTP crossover cable
 Lesson 4: 10Base-T/2/5/F/35 - Ethernet
 Lesson 5: 100Base-(T) TX/T4/FX - Ethernet
 Lesson 6: Fiber cable
 Lesson 7: Direct cable connection
 Lesson 8: Serial direct cable connection
 Lesson 9: Parallel direct cable connection
 Lesson 10: USB direct cable connection

This was first published in August 2007

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