Full-Duplex Ethernet

Simultaneous bidirectional communication eliminating collision detection needs.

A Tale of Two Conversations: Half-Duplex vs. Full-Duplex

To grasp the significance of Full-Duplex Ethernet, it's essential to first understand the different ways communication can flow. In telecommunications, there are three fundamental modes of transmission, each defined by the direction of data flow.

Simplex

Data flows in only one direction. Think of a standard radio broadcast or a one-way street. One device is always the transmitter, the other is always the receiver.

Half-Duplex

Data can flow in both directions, but not at the same time. This is like a walkie-talkie conversation; one person must finish speaking before the other can begin.

Full-Duplex

Data can flow in both directions simultaneously. This is like a natural telephone conversation, where both parties can talk and listen at the same time.

Early Ethernet, constrained by its physical design using shared media like coaxial cables and hubs, was forced to operate in half-duplex mode. This was a major bottleneck, as devices had to politely wait their turn to "speak" on the network. The shift to Full-Duplex was a revolutionary step that unlocked the true performance potential of Ethernet.

The Half-Duplex Legacy: A World of Collisions

In a half-duplex Ethernet environment, all devices are part of a single . Because there is only one shared channel for both sending and receiving, a device must use the CSMA/CD protocol to coordinate access. The key takeaways from this operational mode are:

  • Listen Before Talk: A device must wait for the line to be silent before it can begin sending data.
  • Listen While Talk: The device must monitor the line as it sends, ready to detect a collision if another device starts transmitting at the same time.
  • Shared Bandwidth: The total bandwidth of the link (e.g., 100 Mbps) is shared for both transmitting and receiving. At any given moment, the link is either transmitting, receiving, or idle. The effective throughput is always less than the nominal link speed due to the time spent waiting and recovering from collisions.

The Enabling Technologies for Full-Duplex

The move from half-duplex to full-duplex was not just a protocol change; it was made possible by two key advancements in network hardware: Ethernet switches and modern twisted-pair cabling.

  1. Ethernet Switches: Unlike a hub that broadcasts signals to all ports, a switch creates dedicated, point-to-point connections. When a PC on port 1 sends a frame to a server on port 8, the switch creates a virtual circuit between just those two ports. This eliminates the shared medium. The connection from the PC to the switch is now a private line, not a party line.
  2. Multi-Pair Cabling: Modern Ethernet standards like 100Base-TX and 1000Base-T use twisted-pair cables containing multiple pairs of wires (typically four pairs). Crucially, different pairs are used for sending and receiving. For example, in 100Base-TX, one pair of wires is dedicated for transmitting data (the TX pair), and a completely separate pair is dedicated for receiving data (the RX pair).

By combining these two technologies, we create a situation where a device has a private, two-lane highway to the switch. Data it sends travels down one lane (the TX pair), and data it receives travels down another (the RX pair). Since the paths are physically separate, head-on collisions between transmitted and received signals are physically impossible.

The Profound Impact of Full-Duplex Operation

Enabling full-duplex mode on a switched, point-to-point Ethernet link fundamentally changes how the network operates and delivers a massive performance boost.

1. The Death of Collisions

Since collisions are now physically impossible on the link, the "Collision Detection" mechanism of CSMA/CD is no longer necessary. A device operating in full-duplex mode disables its CSMA/CD logic. It does not listen before talking and does not worry about detecting collisions. It simply transmits a frame whenever its higher-level protocols have one ready to send.

Eliminating Inefficiency

This change eliminates the primary source of inefficiency in classic Ethernet. All the time that was previously wasted on waiting for the medium to be free, sending jam signals, and executing random backoff algorithms is now reclaimed for productive data transmission. This dramatically reduces network latency.

2. Doubling the Bandwidth

This is the most celebrated benefit of full-duplex communication. Because a device can send and receive data simultaneously on its separate wire pairs, the theoretical bandwidth of the link is effectively doubled.

Half-Duplex 100 Mbps Link

Total shared capacity: 100 Mbps.
(Either sending OR receiving at up to 100 Mbps)

Full-Duplex 100 Mbps Link

Total aggregate throughput: 200 Mbps.
(100 Mbps sending AND 100 Mbps receiving simultaneously)

For applications that have symmetric traffic patterns, like a file server that is both receiving requests and sending data, this doubling of capacity leads to a massive real-world performance improvement.

Full-Duplex in Gigabit and 10-Gigabit Ethernet

The principle of full-duplex continues in faster Ethernet standards, but the implementation becomes even more sophisticated. In 1000Base-T (Gigabit Ethernet), the system cleverly uses all four wire pairs to transmit and receive simultaneously on each pair. This is achieved through complex digital signal processors (DSPs) that use hybrid circuits and echo cancellation techniques, allowing a device to distinguish its own transmitted signal from the one it is receiving on the same pair. Even with this advanced technique, the logical outcome is the same: 1000 Mbps of dedicated bandwidth in each direction, for a total aggregate throughput of 2000 Mbps.

    Full-Duplex Ethernet | Teleinf Edu