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I understand that hub always supports for half-duplex communication. In that case we should use one out of four UTP pairs. However, I read that for 10BaseT ethernet two pairs of UTP cables are used with hubs. Does that mean in 10BaseT with hubs we can get full-duplex mode?

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Hubs do use two pairs for 10BASE-T or 100BASE-TX. One pair is for transmit, and one for receive; that is how the device interfaces are made, and the hub is really just a cable extension. What you need to understand is that in the hub, the transmit pair of one interface is connected to the receive pairs of all the other interfaces.

Hubs cannot do full duplex because two devices transmitting at the same time will have the signals collide on all the other interfaces because the transmit pairs of the two sending devices are wired to the receive pairs of all the other interfaces, so the other interfaces will receive both signals at the same time, resulting in garbage.

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  • Theres a bit more to hubs than "just a cable extension". – Peter Green Jan 18 '18 at 17:09
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    @PeterGreen I disagree. There's absolutely no intelligence, all packets are just broadcasted to all ports, hence only enhancing the electrical signal. – user36472 Jan 18 '18 at 17:23
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No. There's a misunderstanding.

A UTP cable consist of 8 wires, arranged in pair, so you have 4 pairs of wire in a single cable. Each pair is twisted, to reduce crosstalk and some kind of interferences, hence the name UTP: Unshielded Twisted Pair

10BasteT only use 2 of those 4 pairs.

But nowadays you rarely see hubs or 10BaseT network.

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10BASE-T or 100BASE-TX always use one pair for each direction, two pairs in total.

A hub is a simple repeater, working on the physical bitstream. Since it receives and retransmits at the same time there can be no other communication (within the collision domain). Full-duplex communication is not possible - this is a limitation of the repeater, not the cable.

Full duplex is only possible when there are exactly one transmitter and one receiver. You can combine two repeaters in a dual-simplex fashion to extend the range or convert a signal, but only limited to two ports. That's the usual configuration of media converters.

10BASE-T and 100BASE-TX are full-duplex capable (in contrast to inherently half-duplex PHYs like 10BASE5 or 10BASE2) but full duplex requires a switch or a direct NIC-to-NIC link.

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Hubs are half-Duplex by definition because All the ports are in one collision domain, When Hub receives a data, it will broadcast to all the other ports. It does not depend on cable type or no of pairs, Always half-duplex communication. If devices want to communicate it has to wait until medial get idle.

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