Physical connection

For the physical connection, IEEE 100BASE-TX typically needs two channels, each of which has two twisted wires. While it is possible to operate this Ethernet variant with one channel, that happens only rarely in practice because these networks support only simplex or half duplex.

Encoding and decoding

The wire pairs are used to transmit symmetrical differential voltages that represent previously encoded symbols. A sender encodes the symbols based on the desired bit stream. A receiver decodes the symbol stream, thereby recovering the sent bits.

In contrast to IEEE 100BASE-T1, IEEE 100BASE-TX uses a combination of NRZI, 4B5B, and MLT-3 methods for coding and decoding as well as for generating the differential voltages. These methods are implemented in the IEEE 100BASE-TX PHY, which is integrated as a dedicated block in the ECU. The PHY establishes the connection between the physical medium and Ethernet controller.

Cables and connectors

Standardized Cat5 or Cat5e cables are typically used for IEEE 100BASE-TX. Of the eight available wires, four are needed for connecting the two channels. The assignment of the wire pairs is specified in the two standards EIA/TIA-568A and EIA/TIA-568B. These standards also contain the pin assignment for the normally used RJ45 plug connectors and sockets.


Only two nodes are ever connected to one cable. Thus, only point-to-point connection topology is available. More than two nodes can be connected using a coupling element. A switch is normally used here, which as a Layer 2 coupling element allows connection to multiple physical connections and can independently forward messages from branch to branch.

Dual Simplex

If IEEE 100BASE-TX is operated with two channels, information can be transmitted bidirectionally at 100 Mbit/s. For this, a node uses one channel for sending and the second channel for receiving. If a PHY supports only fixed channel assignments, a crossover cable must be used for a connection between two nodes. However, most modern PHYs today have an auto negotiation mechanism that a node uses to automatically detect the channel usage. Auto negotiation eliminates the need for crossover cables.

Last modified: Tuesday, 24 April 2018, 3:16 PM