duplex for ethernet

Unanswered Question
Feb 12th, 2008

Hi all, can anyone tell me how full and half duplex is put on the wire, for half does it only use 1 pair of wires for tx and rx, and full duplex uses 2 seperate pairs for tx and rx ?

I have this problem too.
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pciaccio Tue, 02/12/2008 - 04:36

Either Half or Full duplex Ethernet uses the same amount of wires (4 or 2 pair). Pairs 1, 2, 3, and 6 on the RJ-45 cable. Half duplex means that it will Xmit and Rcv not at the same time. If you are Xmitting then you cannot Rcv traffic. If you are Rcv traffic then you cannot Xmit traffic on the line. For Full duplex you can Xmit and Rcv at the same time.

Goutam Sanyal Tue, 02/12/2008 - 04:49



A half-duplex system provides for communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying.

An example of a half-duplex system is a two-party system such as a "walkie-talkie" style two-way radio, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit on the same frequency.

A good analogy for a half-duplex system would be a one lane road with traffic controllers at each end. Traffic can flow in both directions, but only one direction at a time with this being regulated by the traffic controllers.


Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is the LAN access method used in Ethernet. When a device wants to gain access to the network, it checks to see if the network is free. If the network is not free, the device waits a random amount of time before retrying. If the network is free and two devices access the line at exactly the same time, their signals collide. When the collision is detected, they both back off and wait a random amount of time before retrying.


A full-duplex system allows communication in both directions, and unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex since they allow both callers to speak and be heard at the same time. A good analogy for a full-duplex system would be a two lane road with one lane for each direction.

Examples: Telephone, Mobile Phone, etc.

Two way radios can be, for instance, designed as full-duplex systems, which transmit on one frequency and receive on a different frequency. This is also called frequency-division duplex. Frequency-division-duplex systems can be extended to farther distances using pairs of simple repeater stations, owing to the fact the communications transmitted on any one frequency always travels in the same direction.

Full-duplex Ethernet connections work by making simultaneous use of all four physical pairs of twisted cable (which are inside the insulation), where two pairs are used for receiving packets and two pairs are used for sending packets, to a directly connected device. This effectively makes the cable itself a collision-free environment and theoretically doubles the maximum bandwidth that can be supported by the connection.

Conclusion: Full duplex means connection that allows communication in two directions simultaneously at once.

Benefits of full duplex i) Time is not wasted; no frames need to be retransmitted as there are no collisions. ii) Full bandwidth is available in both directions because send and receive functions are separated. Iii) Stations/node do not have to wait until other operations complete their transmission as there is only one transmitter for each twisted pair.

Please rate if I clarify u!!

Danilo Dy Tue, 02/12/2008 - 07:26

Hi Carl,

Both 10Base-T and 100Base-T uses 2 pairs (twisted), one pair for transmit (TX) and one pair for receive (RX). See attached table for connection between two hosts (NIC1 for host1 and NIC2 for host2).

From the table

- TX pins 1&2 (pair 2) in NIC1 connects to RX pins 3&6 (pair 3) in NIC2

- RX pins 3&6 (pair 3) in NIC1 connects to TX pings 1&2 (pair 2) in NIC2

Both half-duplex and full-duplex uses different pair for transmit and receive. Full-duplex uses them simultaneuously, NIC1 sending/receiving and NIC2 sending/receiving. For half-duplex, NIC1 is sending while NIC2 is only receiving, when NIC2 is sending NIC1 is only receiving. The full-duplex and half-duplex setting/operation is controlled by host NICs.





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