2.2 Fast Ethernet
2.2.2

Full duplex and half duplex

This chapter began with discussion about legacy Ethernet and CSMA/CD. Legacy Ethernet uses CSMA/CD because it operates on a shared media where only one device can talk at a time. When a station talks, all other devices must listen or else the system experiences a collision. In a 10-Mbps system operating at half-duplex, the total bandwidth available is dedicated to transmitting or receiving, depending upon whether the station is the source or the destination.

The original LAN standards operate in half-duplex mode, allowing only one station to transmit at a time as shown in Figure . This was a result of the early physical media Ethernet implementations, such as 10BASE-5 and 10BASE-2, where all stations were attached to the same cable or "bus." With the introduction of 10BASE-T, networks deployed hubs and attached each station to a hub on a dedicated point-to-point link. Stations do not share the wire in this topology. The 100BASE-X Ethernet uses hubs with dedicated point-to-point links. Because each link is not shared, a new operational mode becomes feasible. Rather than running in half-duplex mode, the systems can operate in full-duplex mode, which allows stations to transmit and receive at the same time, as shown in Figure , eliminating the need for collision detection. This provides a tremendous asset of possibly the most precious network commodity-bandwidth. When a station operates in full-duplex mode, the station transmits and receives at full bandwidth in each direction.

The most bandwidth that a legacy Ethernet device can expect to enjoy is 10 Mbps. It either listens at 10 Mbps or transmits at 10 Mbps. In contrast, a 100BASE-X device operating in full-duplex mode sees 200 Mbps of bandwidth-100 Mbps for transmitting and 100 Mbps for receiving. Users upgraded from 10BASE-T to 100BASE-X have the potential to immediately enjoy a twentyfold or more bandwidth improvement. If the user was previously attached to a shared 10-Mbps system, he/she might practically enjoy only a few megabits per second of effective bandwidth. Upgrading to a full-duplex 100-Mbps system might provide a perceived one-hundredfold improvement.

The IEEE 802.3x committees designed standards for full-duplex operations for 10BASE-T, 100BASE-X, and 1000BASE-X. The 802.3x standards also defined a flow-control mechanism, which allows a receiver to send a special frame back to the source whenever the receiver buffers overflow. The receiver sends a special packet called a pause frame. In the pause frame, the receiver can request the source to stop sending for a specified period of time. If the receiver can handle incoming traffic again before the timer value in the pause frame expires, the receiver can send another pause frame with the timer set to zero. This tells the receiver that it can start sending again.