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I know that when hubs were used there was the possibility of collisions whenever two devices would transmit data at the same time. However on that time I believe that communication was done in Half-Duplex mode rather than Full Duplex like today.

Hypothetical Question: Suppose that Full-Duplex communication would have become really popular before the invention of Ethernet Switches. Would Switches have been so necessary in that case. Would it have been possible that switches would have been delayed because they would not be needed AS MUCH as they were because the communication between Network Interface Cards would have been already Full-Duplex and collision would have been already rarer?

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    Feb 19, 2018 at 18:12

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A hub is a rather dumb device, it doesn't have packet buffers it just receives data and immediately retransmits it. If it detects a collision then it takes steps to ensure that the collision is seen thoughout the network.

The possibility of collisions is a fundamental proporty of the hub. If hosts A and B send data at the same time then the hub cannot send both of them to C at the same time.

If the links were in full duplex mode then the hosts would have no way of avoiding or detecting collisions. The end result would likely be very poor utilisation of network resources.

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A repeater hub cannot support full-duplex connections. Full-duplex connections without switches require dedicated connections NIC-to-NIC - it would be possible to run this directly into a router, but you'd need a dedicated router port for each device.

Full-duplex is never possible with a repeater hub. A repeater resends all data (bits) received on one port to all the others. So, given three stations A, B, and C on a hub, A sends a packet to B. Incidentally, C also tries to send to B. The hub resends the packet to B (and A) - simultaneous with the packet from A. The data gets mangled, nothings works. See?

Instead, CSMA/CD causes C (listening in through the hub) to shut up while A talks to B. When this is done, someone else can send.

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  • but what if the hub has itself full duplex connectivity at every port?
    – yoyo_fun
    Sep 1, 2017 at 17:53
  • I was trying to show you that a repeater can't use full duplex. What is it supposed to do when two sources send to the same destination simultaneously? A switch would queue one of the packets (frames), requiring buffer memory - that a hub can't have by definition.
    – Zac67 ♦
    Sep 1, 2017 at 18:18

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