Why can USB 3 cables only stretch to 3m when Ethernet cables can be 100m?


USB 3.0/3.1/3.2 Gen 1 aka the kind that can do 5 gbps can only use cables up to 3m in length. Meanwhile Ethernet uses a similar twisted pair copper cable, but can do 10 gbps over 100m.

What gives, why is USB so limited in terms of cable length?

In: 413

It is because Ethernet cables are designed to transmit data between computers which might be far away from the nearest other piece of networked equipment. USB 3 cables are to connect to things on or near your desk.

Think of USB as 2 people communicating without seeing each other. If person A stops talking for a period of time, person B will thing/assume person a has left or no longer wishes to talk. Moving the two people further way from each other increases the time something takes to travel between the two people, and communication starts to break down.

In reality, USB has a maximum tolerance for response times from devices in the standard, if that response time is longer then 1.5 μs, then the standard assumes something went wrong, leading to the 5m length being the maximum.

Ethernet doesn’t wait or care about responses, thus can be as long as you need, though the signal will break down over long distances, which is why repeaters exist.

The kind of simple answer is, long USB cables generally are not needed, so the standard wasn’t developed with long cables in mind. So the goal of USB is to be fast and cheap. USB will acknowledge received messages. So if the cable is too long, you have a host expecting a response, but it hasn’t arrived because the long cable creates a delay. Its an easy way to handle it, with low processing power requirements.

Meanwhile, when developing ethernet, the cables have to be long. A 5-meter max would render ethernet useless, so the standards had to accommodate longer cables. It comes at a greater cost, in terms of equipment, and processing power. (And it tends to be slower, unless you are willing to spend a ton of money.)

Cost and power. USB tries to minimize the cost and keep power usage low. Especially on the peripheral end. It’s not worth driving the cost up and potentially power usage up by magnitudes for distance that is almost never needed. The power and cost of multi-gig copper Ethernet can quite high (relatively), but since you likely only need one of them and distance is a very significant requirement it’s not really as much of an issue.

If you tired to use Ethernet like solution for communications with peripherals then the cost would be very high for very little benefit. You would need expensive controller for every port and in the peripheral plus the power draw simply for transferring would be in the multiple watt range rather than milliwatt range.

USB 3.2 can do 20GBit/s with a USB-C connecote.

USB is designed to be cheap and does not use a lot of power. So the electronics used to integrate the signal are not that complex, the result is you can handle lots of distortion.

Compare taht to a 10Gbit/s ethernet card where the cheapest is at $30 you can compare that to 1Gbit/s cads that start at $14. https://www.newegg.com/p/pl?N=100158104%20600013864%204814&Order=1

Take a look at them too, the heatsinks are not there for looks, you would not add them if they as not needed. If you search for the power usage e you find info like https://edgeoptic.com/products/network-interface-cards/10g-nic-x550-2t/ where a car uses max 13W at 10Gbit/s This is because you need quite a complex signal processing to filter and interpreter the signal.

Compare that to power usage for USB tranciver like https://www.ti.com/lit/ds/symlink/tusb1310.pdf look at page 31 where the sun of all the power pins is 0.43W

A computer typically have multiple USB port so less then 1W each is a lot better than over 10

You do not what to add that all devices that use USB at high speed. The ability for long-range is not really worth the drawback when it is intended for usage on a desktop

They take a look at the cable. Here is a correction of a CAT6 cable https://commons.wikimedia.org/wiki/File:Cross_section_of_a_Cat6_patch_cable.jpg the plastic separator is to keep the cable away from each other. The wires are thicker too.
So you could cant make them as thin and flexible as USB cables, Remember USB-C has 24 wires for multiple purposes, not just 8.

So USB can get away with thinner, more flexible, and cheaper cables

If you really need long-range USB there are USB extenders that use ethernet cables so you can put another device 100 meters away

So USB is designed to provide high speed at a short distance at low cost, low power usage, and thin flexible wires. If you would build it to work at 100m the cost, power usage, and wire size all would increase.

Engineering is about compromises and the important design parameter for USB is not the same as for Ethernet.