100 Mbps Ethernet / IEEE 802.3u including 100 Base-T

- a summary or tutorial about 100 Mbps Fast Ethernet, IEEE 802.3u, including 100 Base-T.

Ethernet IEEE 802.3 Includes:
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100Base-T Ethernet was originally known as "Fast Ethernet" when the IEEE 802.3u standard that defines it was released in 1995. At that time, it was the fastest version of Ethernet that was available offering a speed of 100 Mbps (12.5 MByte/s excluding 4B/5B overhead). Now 100Base-T has been overtaken by other standards such as 1GB and more recently 10 GB Ethernet offering speeds of 10 and 100 times that of the 100Base-T versions. Nevertheless 100Base-T is widely used for most networking applications as it offers a performance that is more than acceptable for many applications. Officially, the 100BASE-T standard is IEEE 802.3u.

100Base-T overview

100BaseT Ethernet, also known as Fast Ethernet is defined under the 802.3 family of standards under 802.3u. Like other flavours of Ethernet, 100Base-T, Fast Ethernet is a shared media LAN. All the nodes within the network share the 100 Mbps bandwidth. Additionally it conforms to the same basic operational techniques as used by other flavours of Ethernet. In particular it uses the CSMA/CD access method, but there are some minor differences in the way the overall system operates.

The designation for 100Base-T is derived from a standard format for Ethernet connections. The first figure is the designation for the speed in Mbps. The base indicates the system operates at baseband and the following letters indicate the cable or transfer medium.

There are a number of cabling versions available:

  • 100Base-TX:   uses two pairs of Category 5 UTP *
  • 100Base-T4:   uses four pairs of Category 3 (now obsolete) *
  • 100Base-T2:   uses two pairs of Category 3 (now obsolete) *
  • 100Base-FX:   It uses two strands of multi-mode optical fibre for receive and transmit. Maximum length is 400 metres for half-duplex connections (to ensure collisions are detected) or 2 kilometres for full-duplex and is primarily intended for backbone use
  • 100Base-SX:   It uses two strands of multi-mode optical fibre for receive and transmit. It is a lower cost alternative to using 100Base-FX, because it uses short wavelength optics which are significantly less expensive than the long wavelength optics used in 100Base-FX. 100Base-SX:   can operate at distances up to 300 metres
  • 100Base-BX:   is a version of Fast Ethernet over a single strand of optical fibre (unlike 100Base-FX, which uses a pair of fibres). Single-mode fibre is used, along with a special multiplexer which splits the signal into transmit and receive wavelengths.

* The segment length for a 100Base-T cable is limited to 100 metres.

Fast Ethernet data frame format

Although the frame format for sending data over an Ethernet link does not vary considerably, there are some changes that are needed to accommodate the different physical requirements of the various flavours. The format adopted for Fast Ethernet, 802.3u is given below:

Ethernet Frame
Ethernet data frame format

Fast Ethernet (802.3u) Data Frame Format

It can be seen from the diagram above that the data can be split into several elements:

PRE     This is the Preamble and it is seven bytes long and it consists of a series of alternating ones and zeros. This warns the receivers that a data frame is coming and it allows them to synchronise to it.

SOF     This is the Start Of Frame delimiter. This is only one byte long and comprises a pattern of alternating ones and zeros ending with two bits set to logical "one". This indicates that the next bit in the frame will be the destination address.

DA     This is the Destination Address and it is six bytes in length. This identifies the receiver that should receive the data. The left-most bit in the left-most byte of the destination address immediately follows the SOF.

SA     This is the Source Address and again it is six bytes in length. As the name implies it identifies the source address.

Length / Type     This two byte field indicates the payload data length. It may also provide the frame ID if the frame is assembled using an alternative format.

Data     This section has a variable length according to the amount of data in the payload. It may be anywhere between 46 and 1500 bytes. If the length of data is below 46 bytes, then dummy data is transmitted to pad it out to reach the minimum length.

FCS     This is the Frame Check Sequence which is four bytes long. This contains a 32 bit cyclic redundancy check (CRC) that is used for error checking.

Data transmission speed

Although the theoretical maximum data bit rate of the system is 100 Mbps. The rate at which the payload is transferred on real networks is far less than the theoretical maximum. This is because additional data in the form of the header and trailer (addressing and error-detection bits) on every packet, along with the occasional corrupted packet which needs to be re-sent slows the data transmission. In addition to this time is lost time waiting after each sent packet for other devices on the network to finish transmitting.

Fast Ethernet using Cat 5 cable

Fast Ethernet can be transmitted over a variety of media, but 100Base-t is the most common form and it is carried over Cat 5 cable. These cables have four sets of twisted pair wires of which only two are used for 10Base-T or 100Base-T. For 10Base-T and 100Base-T one pair of wires is used for the transmitted data (TD) and another for the received data (RD) as shown below. The data is carried differentially over the wires and in this way the "+" and "-" wires carry equal and opposite signals. As a result any radiation is cancelled out.

Pin Wire
1 White + Green +TD
2 Green -TD
3 White + Orange +RD
4 Blue Not used
5 White + Blue Not used
6 Orange -RD
7 White + Brown Not used
8 Brown Not used

Wiring for Cat 5 cable used 100 Base-T Ethernet

Fast Ethernet Applications

Fast Ethernet in the form of 100Base-T, IEEE 802.3u has become one of the most widely used forms of Ethernet. It became almost universally used for LAN applications in view of the ease of its use and the fact that systems could sense whether 10Base-T or 100Base-T speeds should be used. In this way 100Base-T systems could be incorporated steadily and mixed with existing 10Base-T equipment. The higher specification standard would be used once the two communicating elements were both 100Base-T. In addition to this the fibre based version is also used, but in view of the fact that Cat5 cable is so cheap and easy to use, the wired version is more common. However the fibre version has the advantage of being able to communicate over greater distances.

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