Fast Ethernet

Fast Ethernet offers this as an easy and affordable way to increase the capacity of an Ethernet network to 100 Mbps, by simply increasing the transmission rate on the Ethernet medium from 10 Mbps to 100 Mbps.

Fast Ethernet, has been defined for CAT 5 UTP cable. This is known as “100BT” amd incorporates new physical layer signalling (replacing Manchester encoding) to support 100 Mbps. The standards may be supported by an in-built transceiver or an external transceiver connected via a Media Independent Interface (MII), similar to the AUI in 10 Mbps Ethernet.

Two types of UTP media were originally defined:

• 100BTX for two-pair standard data grade cable using Category 5 UTP (unshielded twisted pair) cable
• 100BT4 for four-pair voice and other types of data grade cable (Category 3, 4, or 5 UTP), now obsolete.

The 100BT standard retains the 100m maximum cable length between the hub and the desktop as specified in 10BT (an important design decision when 100BT was being specified, since many buildings have already been installed with UTP cabling), but some 100BT rules differ from 10BT rules because of the increase in speed.

A 100BT NIC must be connected via point-to-point links to either a 10 Mbps on a hub /switch or a 100 Mbps port on a 100BT switch. To enable backwards compatibility with the older specification, a 10/100 NIC automatically senses the speed of the attached segment by reception of a series of pulses sent along the cable (these pulses do not interfere with normal data transmission).

A Fast Ethernet (100BT) switch is similar to a 10 Mbps Ethernet switch, with the exception that the ports operate at 100 Mbps and the switching engine and memory buffers have been upgraded. Most 100BT switches also can operate any port in half duplex mode using CSMA/CD when a 10BT is connected, but change to operate in full duplex mode when directly connected to a 100BT NIC. In full duplex mode, the media is not shared. This allows simultaneous transmission and reception of Ethernet frames. (i.e. the status of the collision detection (CD) circuitry is ignored).

Although Fast Ethernet supports the same IEEE 802.3 CSMA/CD MAC protocol layer as 10BT, and hence can support use as a shared medium with a Fast Ethernet hub. In practice, 100 Mbps hubs are seldom used. (The 100BT standard defines two classes of repeaters, called Class I and Class II. A collision domain can include one Class I or at most two Class II repeaters. Whether the repeater is Class I or Class II is determined by how much delay is added by the repeater.)

Fast Ethernet over UTP (100BT)

The Unshielded Twisted Pair (UTP) cable provides a low cost Ethernet technology, that supports 10BT. The UTP cabling system uses a RJ-45 connector and 100 Ohm unshielded twisted pair cabling. This connects the computer directly (i.e. using a point to point link) to a wiring hub which acts as a media repeater. The maximum distance of a 10BT link is 100 m.

100BASE-T uses the same cabling to provide 100 Mbit/s Ethernet in either a half-duplex (using CSMA/CD) or full-duplex form (switched). 100BASE-TX runs over two pairs of wires in Category 5 unshielded twisted pair cable. Like 10BASE-T, the normal pairs are coloured orange and green pairs (using pins 1, 2, 3 and 6 of the RJ-45 connector). This cable has a maximum bandwidth of 100-125 MHz. A Manchester encoded waveform would require 200-400 MHz of bandwidth, far in excess of that offered by this type of cable.

A scheme using 4B5B binary encoding therefore generates a series of 0 and 1 bits clocked at 125 MHz; the 4B5B encoding provides DC equalisation and spectrum shaping. 4B5B encoding works by mapping each group of four bits (one 1/2 of a byte) to one group of 5 bits. Since there are (2^5) 32 possible combinations of 5 bits, and there are only (2^4) 16 combinations of 4 bits one half the patterns are unused. The chosen set of 16 5-bit patterns are those with the most transitions, this ensures clocking information is present in the signal (for locking the receiver DPLL). This results in a bandwidth increased of 25%. For consistency with previous versions of Ethernet, the least significant 4 bits of each byte is sent first.

Cross-Talk requirements (i.e. to limit RF Emission) led to the need for a scrambler, this randomises the bit stream to prevent the same set of byte values generating a completely repetitive pattern, which would have stong signal components at some characteristic frequencies.

The data is finally sent as a 3-level physical waveform known as MLT-3. MLT-3 cycles through a set of voltage levels {-1, 0, +1}, to indicate a 1-bit. The signal stays the same when transmitting a 0 bit. It takes four 1 bits to generate a complete cycle, this the maximum fundamental frequency is reduced to one fourth of the baud rate.
This scheme of 45/5b with MLT-3 encoding leads to a waveform of 31.25 MHz, well within the specification for Unshielded Twisted Pair Cabling.

Fast Ethernet over Fibre

Fast Ethernet can be used over fibre cables by using one of the various forms of interface convertors of MII interfaces. 100B-FX uses multimode fibre for connections within a building. A shorter distance version (SX) was also available that used shorter wavelengths reducing component cost. A version (LX) is also available for use with singlemode fibre to reach much longer distances (e.g. 10’s km). The following types of fibre media are defined:


For more information see:

Unshielded Twisted Pair (UTP) cabling

10 Mbps Ethernet over UTP

Gigabit Ethernet


Gorry Fairhurst - Date: 18/03/2012 EG3557