From Academic Kids

100BASE-TX is the predominant form of Fast Ethernet, providing 100 Mbit/s Ethernet.

100BASE-TX runs over two pairs of wires in category 5 cable. Like 10BASE-T, the proper pairs are the orange and green pairs (canonical second and third pairs) in the TIA-568B wiring standard.

In TIA-568B, wires are in the order 1, 2, 3, 6, 5, 4, 7, 8 on the modular jack on each end. The colour-order would be orange/white, orange, green/white, blue, blue/white, green, brown/white, brown:

RJ-45 Wiring (EIA/TIA-568B)
Pin Pair Wire Color
1 2 1 Missing image
Pair 2 Wire 1

2 2 2 Missing image
Pair 2 Wire 2

3 3 1 Missing image
Pair 3 Wire 1

4 1 2 Missing image
Pair 1 Wire 2

5 1 1 Missing image
Pair 1 Wire 1

6 3 2 Missing image
Pair 3 Wire 2

7 4 1 Missing image
Pair 4 Wire 1

8 4 2 Missing image
Pair 4 Wire 2


Each network segment can have a maximum distance of 100 metres. Capable of 100 Mbit/s throughput (200 Mbit/s in full-duplex configurations). See IEEE 802.3 for more details.

The configuration of 100BASE-TX networks is very similar to 10BASE-T. When used to build a local area network, the devices on the network (computers, printers etc.) are typically connected to a hub or switch, creating a star network. Alternatively it is possible to connect two devices directly using a cross-over cable.

Technical details

Missing image
Image:Ethernet 100-Base-TX Signal.jpg

With all 100BASE-T Ethernet (including 100BASE-TX), the raw bits of a packet to be transmitted -- a series of 0 and 1 bits at 100 Mbit/s -- are typically transferred 4 bits at a time clocked at 25 MHz to the Ethernet hardware.

This limits the theoretical maximum data bit rate to 100 Mbit/s. The data signaling rate actually observed on real networks is far less than the theoretical maximum, to account for the header and trailer (addressing and error-detection bits) necessary on every packet, the occasional "lost packet" due to noise, and time waiting after each packet for your turn after other devices on the network finish talking.

With 100BASE-TX hardware, Inside the Ethernet hardware, the raw bits go through 4B5B binary encoding to generate a series of 0 and 1 bits clocked at 125 MHz. Those bits go through MLT-3 encoding. The final voltage going out on a twisted-pair is a trinary signal at a symbol rate of 125 MHz, and a maximum cycle time of 31.25 MHz. Each symbol has a typical voltage of (nominally) -1 V, 0 V, or +1 V. (The ____ pair typically carries data from a computer to a network switch. The ____ pair typically carries data from the network switch to a computer.)

See also

Missing image
25 Pair Color Code Chart


This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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