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A USB connection is often much faster and cheaper than Ethernet.

Why is the idea of connecting two computers to each other via USB so unpopular?

Much cheaper? You can get a gigabit Ethernet card for $12, which is about the same as USB3 card.

Faster? Theoretical maximum USB3 speed is 5Gbit/s, but it is very difficult to achieve this speed in practice. Ethernet, on the other hand, has no problem sustaining communication at full rate. If you only have USB2 ports (on either side), your theoretical maximum speed is only 480Mbps, which is more than 10-times slower. With USB3.1 and newer, the compatibility, cabling and other issues are notoriously problematic and unreliable.

USB cable length limitation is 3m in practice. Ethernet is in the range of 100m. If your computers are not in the same room (and probably next to each other), 3m will not be enough. With fiber cable you can run Ethernet even further, to about 30km.

With USB, you can only connect 2 computers together. With Ethernet, you can connect as many as you want. A simple task such as connecting 2 computers to internet router would be very difficult with USB. Even less to connect a room full of servers.

An USB cable always has a “master” and a “slave” side. A computer is always acting as master, so you can only plug one end of the cable in a computer. You would need to modify the plugs, sockets, USB electrical circuitry, OS drivers and OS USB stack if you wanted to connect computer to computer. Not sure how well this would work (i.e. it could work on one computer, but not on another).

Most Ethernet cards have a circuitry to offload certain functions from CPU to the card (i.e. checksums), so the CPU is free to do other things. This translates to higher performance. USB has no such function.

With Ethernet cards, you can easily drop and replace the adapter with 10Gbps which costs about $100. If your USB chips are attached to motherboard, you need to replace entire computer for upgrade.

With USB, you have about 20 different types of plugs and sockets, with Ethernet you have just one. Note that 20 different types means 380 possible combinations.

USB, by specification, MUST be able to provide power to devices, which mean you need a more powerful power source even if you aren’t using it. Ethernet does not have this. On the other hand, Ethernet CAN provide power, but because it is optional, it does not require anything if you don’t plan use it.

All in all, USB and Ethernet were designed for different purposes. USB was designed to connect peripherial devices to a computer, whereas Ethernet was designed for building networks. The differences are thus fundamental, not just superficial. As a matter of fact, I would be much in favour of abolishing USB and replacing it with Ethernet everywhere.

The speed/price ratio is probably more in favour of USB-C compared to 10Gbps Ethernet, but then again not that much, probably not for long and you are still stuck with other limitations.

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