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Half-duplex connections were popular at the time of 10Mbps and 100Mbps Ethernet, and, according to standards, it is allowed also in case of 1Gbps Ethernet.

Am I correct that half-duplex mode support in Ethernet chipsets was crucial in case either a network hub (hub is internally a single wire) or some other shared Ethernet medium (for example, 10BASE-5) was used?

Are there any reasons for half-duplex connections in Ethernet environments where twisted-pair cabling is used and hubs are not used?

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    Ethernet was derived from en.wikipedia.org/wiki/ALOHAnet which used UHF radio; an inherently half-duplex medium when operating on the same frequency. – Dale M Jul 14 '15 at 2:45
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    It's worth noting that while the various 802.11 standards are not Ethernet and thus are not directly applicable to the question, 802.11 based wireless networks still remains a half-duplex standard. There are significant differences in collision handling (avoidance vs detection) between Ethernet and 802.11, but it's a good modern lens to view historical challenges through. – user24313 Jul 18 '15 at 12:30
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The reasons for half-duplex ethernet are as you understand them. In fact, there was a movement to not include half-duplex for 1000Base-T, but it still made it into the standard. For 10 Gb ethernet, half-duplex was dropped so there is no such thing as 10 Gbps half-duplex ethernet as a standard.

Unless you still have a hub (they are still around) or a device that doesn't support full-duplex (they exist, especially for 10Base-T), 10Base-T or 100Base-TX on UTP don't really need half-duplex.

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Ethernet was derived from https://en.wikipedia.org/wiki/ALOHAnet which used UHF radio; an inherently half-duplex medium when operating on the same frequency.

Early wired networks used a coaxial bus until well into the 1980s.

4

In the case of partial wiring damage, half duplex can be the difference between a slower connection and NO connection (until the damage is repaired.)

That can be a BIG difference.

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    Please explain this answer. For UTP, a minimum of two pairs are required for either full- duplex or half-duplex at 10 or 100 Mb, and 1 Gb requires all four pairs for either full-duplex or half-duplex. I don't see how partial wiring damage has anything to do with the allowed duplex on UTP. – Ron Maupin Jul 14 '15 at 1:39
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    I have a lightning-damaged overhead wire that is proving organizationally difficult to get replaced with fiber. It works on 10 half duplex. It does not work on 10 full duplex. 4 wires are still electrically connected end to end, but the connection will not come up on anything but 10 half duplex. – Ecnerwal Jul 14 '15 at 1:50
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    Obviously, both pairs are working since you can both send and receive. A damaged pair (or one wire in a pair) would allow a link on one side but not the other. You may have damage in one of the devices. At the very least, test the cable with a high-end Fluke. – Ron Maupin Jul 14 '15 at 1:59
  • A high end Fluke costs more than my mile and a half of fiber network backbone cost; Or about twice my entire annual budget. Not going to happen. Presumably there is crosstalk between pairs that shouldn't be there, but is there anyway, post lighting-strike. The end equipment was all replaced. I have aerial fiber in hand to simply replace it, as copper between building links are off my list of acceptable techniques for precisely this reason. – Ecnerwal Jul 14 '15 at 2:11
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    You can rent a Fluke by the hour or get a cable vendor with one for an hour. It will tell you right where any cable problem is. It may be as simple as re-terminating or cutting some off the end of one of your service loops. You need to know if it is the outside-plant cable, or if it is in the connection to the inside plant cable, or the inside-plant cable on either end (easy fix). It would be a shame if this problem is only 5 feet into your 10 meter service loop on one end, and you just put up with the problem. – Ron Maupin Jul 14 '15 at 2:20
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The reason for half duplex, was due to cost, initial 10Meg ethernet shared the same coax cable only two conductors 10Base2. At the time it was not imagined that any thing as fast as 10Mbit/s could be transmitted on a unshielded cable.

One port on a switch/hub could have 20 pc's connected in daisy chain, thus CDMA was used to listen for traffic, start sending, back-off for random time if someone started transmitting at the same instance and send again.

This is also the reason for the length restriction and minimum packet size, as you had to be sure before the last bit was sent that someone on the other end of the same cable did not also start to transmit at the same instant.

As electronics became faster and cheaper, we moved to Cat5, individual cables/ports for each pc, and then to switches, where the packet can be stored and forwarded in both directions at the same time (Full Duplex)

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Rich Seifert gives the answer (for Gigabit Ethernet) in his book "Gigabit Ethernet" and in an usenet post:

Quote:

The answer is more political than technical. Gigabit Ethernet was developed under the auspices of the IEEE 802.3 Working Group. By definition, 802.3 networks must include the capability of CSMA/CD operation. [Note: This was true at the time of the writing of the Gigabit Ethernet stamdard; it is no longer the case.] If Gigabit Ethernet offered a full-duplex-only solution, it would have been difficult to justify its development within the IEEE 802.3 Working Group. [...]

  1. The resulting standard would have had difficulty calling itself “Ethernet,” since it would not use CSMA/CD (even as an option) and it would not have been developed as part of IEEE 802.3, the recognized “owner” of the Ethernet name.
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Am I correct that half-duplex mode support in Ethernet chipsets was crucial in case either a network hub

Yes

hub is internally a single wire

It would be more accurate to say a hub simulates a single wire, it's actually a bit more complex than that.

or some other shared Ethernet medium (for example, 10BASE-5) was used?

Yes

Are there any reasons for half-duplex connections in Ethernet environments where twisted-pair cabling is used and hubs are not used?

The other big one is old end devices.

10 Megabit gear is unlikely to support autonegotiation, so full duplex mode cannot be selected automatically. You could potentialy force both the device and the switchport to full duplex mode but doing that risks duplex mismatches happening down the road, so it's usually more trouble than it's worth.

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