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New Member

Half duplex question - Hubs.

OKay apologies in advance for this question but I am unable to find a definitive answer anywhere..

I understand that Hubs are half duplex, but I cant figure out why? Some people say its because clients use the same wires to send and recieve, but this doesnt seem right according to the other articles I read.  Or is it simply because if a client is sending it cannot recieve as well because the same communication channel (within the hub) is used within to send and recieve (i.e. there are not seperate channels for each).

3 REPLIES
Cisco Employee

Half duplex question - Hubs.

Hi,

The half duplex nature of hubs is given by their technical construction. Think of a following scenario: a hub with three PCs A, B, and C. PCs A and B want to communicate over the hub in full duplex. What will the result be?

  • A sends data to B, hub replicates the signal to C as well
  • B sends data to A, hub replicates the signal to C as well

Oops - C is now getting a mix of signals from both A and B - and that's a collision, right? We got the collision because A and B thought they can communicate in full duplex over the hub - and in reality, they can not.

Best regards,

Peter

New Member

Half duplex question - Hubs.

Okay with that said then, if you only have two devices connected to a hub can they operate in full duplex mode? because seperate wires are used for tx/rx?

Cisco Employee

Re: Half duplex question - Hubs.

Hi,

Okay with that said then, if you only have two devices connected to a hub can they operate in full duplex mode?

No, not even two devices connected to a hub can operate in full duplex. The explanation I gave earlier is simple and illustrative but somewhat imprecise.

You see, the way a hub works is to actually reinterpret incoming signals as expressing binary 0 or 1, i.e. read in the individual bits and retransmit them as electrical impulses. This is done to avoid amplifying the noise along with the signal.

Now, my understanding is that a hub is not constructed to process signals coming in from multiple ports at the same time. It either reads in the bits from one port or the other. If signals arrive at two or more ports, the hub is not constructed to read from ports simultaneously. All such occurences are considered to be collisions by the hub. You may even think of it as the hub having a single internal bus that carries the outbound signals. Signals received from one port are - after decoding the 0s and 1s and regenerating the corresponding signals - sent over this bus to all other ports in the hub. If two stations transmit at the same time, the collision occurs inside the hub circuitry.

Best regards,

Peter

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