What is half-duplex ethernet and how does it work?

Question

I read everywhere that half-duplex means that a host in a network can only send OR receive data at the same time.

However, I also read that in 10/100BASE-T ethernet, regardless of being set to half or full duplex, the UTP pins 1,2 in NICs are used to transmit data and 3,6 are used to receive data. So if two hosts are connected directly via a crossover cable, each one has a dedicated electronic circuit to send and receive. In such a situation there is no possibility to have collisions and there is no need for a complicated algorithm like CSMA/CD.

Can they still only send OR receive at the same time when set to half duplex mode? Despite having two dedicated circuts for it?

Or does half-duplex rather mean something like "two different hosts can not send data at the same time to another host"?

Answer 1

In the original Ethernet specification, the network itself consisted of a long coaxial cable that all users were connected to via a variety of different tap off arrangements. The ends of the cable were terminated with special resistive terminators that had the same impedance characteristics as the cable so that the cable appeared infinitely long. This prevented reflections of the signals that could cause distortion. The cable operated at 10 Mbps.



When receiving, each transceiver presented a high impedance to the cable so that it could monitor the signal voltage without causing distortion. A technique called Carrier Sensing Multi Access / Collision Detection (CSMA/CD) was used.



Theoretically, only one client should be transmitting on the cable at any one time. If a client wanted to transmit and there was already a carrier present, then the client had to wait for the current user to finish its transmission. This was Carrier Sensing. However, if two clients started to transmit at the same time, neither would have detected that a carrier was present and the information on the cable would be corrupt. Each transceiver listens to what it is sending, and if it detects corruption it stops and waits a random time before trying again. This was Collision Detection.



Only one user could transmit on the cable at any one time so even with just two clients (or stations) on the cable, the only mode of operation could be half-duplex.



With the introduction of the modern cables such as Cat5 and Cat6 types which support simultaneous transmission in both directions, and modern switches (or routers with built in switch functions), full duplex operation is supported. If two clients transmit at the same time to the same destination, then the switch will buffer one message so the recipient receives them sequentially. If two clients transmit at the same time to different recipients, then the switch will allow both to pass in parallel as the switch will normally only pass a message to the specific client that is to receive it.



If a network adapter can work only in half duplex mode, and it receives a packet while it is transmitting, then the received packet is dropped and higher level recovery mechanisms will have to retransmit the dropped packet.



Wireless networks are half duplex, but although they can carrier sense they do not have the collision detection mechanisms, so they do not know that packets have collided. They must rely on higher level error recovery.



I hope this helps.

Answer 2

First statement is correct.



Second statement is correct as well.



Yes, if the duplex is set to half then that is what is used regardless of whether full duplex is available.



Your last question is technically a yes. In a half duplex connection only one device can be transmitting at a time, in which case by definition two hosts could not be transmitting.

Answer 3

Half-duplex is used to describe communication where only... one side can talk at a time. Once one side has finished transmitting its data, the other side can respond. Only one node can talk at a time. If both try to talk at the same time, a collision will occur on the network.

As you can understand, this method of communication is not very efficient and requires more time to send/receive larger amounts of data.Older networks used to work in half-duplex mode, due to the constraints of the network medium (coax cable) and hardware equipment (hubs).



On the other hand, full-duplex is used to describe communication where both sides are able to send and receive data at the same time. In these cases, there is no danger of a collision and therefore the transfer of data is completed much faster

Answer 4

Half-duplex is mean that only one side of talk at a time, its mean if one host computer send, the other can only receive. UTP cable use different pins (pairs of cable) to send and receive communication signal, so its doesn't fully depend on half or full duplex setting. If you use full-duplex setting your computer and other computer can send and receive signal at the same time. May this helpful.

Answer 5

It does depend on the NICs you have in your PCs. Some older ones cannot send and receive at the same time. Have a look at the NIC properties in Windows, and there may be a "Full/Half Duplex/Auto" setting.

Answer 6

Duplex means it can send and receive data, half duplex means it can send OR recieve data at the time,and full duplex,it can send and receive data at the same time