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

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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?

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18 Answers

Anonymous 0 Comments

balanced vs unbalanced cable as well. electrical wires are carrying a signal, and “noise” from environment/system enters the signal. a balanced cable has another wire inside in reverse phase, the signal from which , at cables end, is compared against the “intended” signal. the “noise” part of the signal gets cancelled out , “phase cancelling” its really neat.
if the cable is “unbalanced” the phase cancelling wire is not present, and over increasing length of cable the signal to noise ratio quickly becomes unacceptable for use.

Anonymous 0 Comments

electrically usb is voltage coupled. think of voltage as pressure, and the cable (at this length) is like a very long sponge. if you tap the pressure on one end of the tube will you detect to at the other end? yes or no? usb uses this method.

another problem is electrical noise over distance. think of that long tube getting bumped or vibrated causing pressure changes these add up over a long distance sort of like a giant antenna

in contrast ethernet is magnetically coupled. you force more electricity into and out of the cable via current flow that alternates direction. the result is current flow on the other end of the cable causes a magnetic change in the coil at the other end of the cable. this is easier to detect. this type of transmission is immune to those pressure bumps but current flow is very different and requires very strong magnets to cause noise signals.

electo magnets are like the coil of wire around a nail you did in middle school science class

Anonymous 0 Comments

Its actually super complicated so im going to do my best to ELI5 without getting too technical from what i know form basic system engineering.

At its simplest form Ethernet was built for long range and has decay protection built into it and all its parts along any give path. Its also made to handle multiple parts along a path. It takes a lot of processing power and extra parts and software and hardware reading that data to do it

USB was designed for end to end with short runs so it was simply never build with any long range decay protection in place or have its data constantly repeated though different devices to achieve the long range. ITs low processing power because there is no need to for backups and loss detection and systems that double check the data before passing it onto the next guy because by design there is no next guy so its lower power , short distance minimum complexity

You use the best tool for the job. You can use a wrench to hammer in a nail…but its not the most effective way and no one is out there trying to figure out how to make a wrench a better hammer because hammers exist. No one really focuses on long range USB data transfer because there are better tools for the job for that. ( i know some joker will link me to those hammer wrench things but you get the point)!

Best way i can explain it without getting more complicated is ethernet has a lot of working parts and routers and such things that help control data and power in smart way. So you send lower power signals further because you have a lot of tech that can figure it out on each end if things get lost and repair lost data on the fly.

USB is simpler and also higher power…with that higher power comes a problem of trying to regulate and control it . It gets more unstable the further it goes and more subject to decay

ethernet is designed for data and power was added on as something extra they found it can do.

USB was designed for data and power and over the years its focus shifted more to power with an aside of having high data transfer as well. It controlling hardware at the ends was never dsigned with keeping long range data intact so there is no decay protection

Anonymous 0 Comments

Because what they are used for is different.
Ethernet is meant for communication that may or may not be occurring.
The computer may not be on. The internet may not be sending data.

Also most internet/ Ethernet communication isn’t done in a serial manner. Is it doesn’t wait for computer A to talk then move to computer B etc

Universal
Serial
Bus

This is an internal communication channel in the computer that gives each device a slot of time and then moves to the next. Longer cables mean longer time for comm to device and back from.
Cables get too long and the communication comes back and the computer is already listening to the next device on the bus.

Anonymous 0 Comments

“Because they were designed that way” isn’t very helpful, but is basically correct.

Ethernet cables are designed for connecting things throughout a building. In something like an office building, you need long run lengths (e.g connecting upper floors to the servers in the basement), and you need many km of cables so they can’t be too expensive. Long cable runs are subject to a lot of interference so pushing the data rate too high can lead to errors unless you physically bulk up the cable and voltages, or switch to fibre optics. Speed is theoretically nice, but realistically you don’t normally need to be transferring large files around anyway, and the bottleneck is usually the server or router having to deal with dozens or hundreds of users simultaneous requests and not the actual network speed. Reliability and practicality are more important, and ethernet is a good middle ground that’s cheap, easy to work with, fast enough for most purposes.

USB cables are for connecting peripherals to devices in the same room. Generally you don’t need more than a couple of meters, but you do want high speed for external storage, displays, cameras. You don’t have to worry too much about interference over a short run, nor do you have to have bulky cables and high voltages to counteract with resistance and signal drop off.

Can you do USB3 speeds at Ethernet distances? Absolutely! But either you need expensive, bulky cables and equipment or (much more sensibly) use fiber optics. Fiber is harder to work with, termination and splicing was a really fiddly manual process till relatively recently, and the switching equipment at both ends is expensive, but if you really need it that’s the route to take. Unfortunately it really isn’t suitable for “patch” connections between the wall and a device – a user can kick the crap out of an ethernet patch cable and it’ll still work, but bend a fiber the wrong way and it’s toast – so you often get fiber from the server room to the switch and then ethernet to the desk. Given that the ethernet to the switch is unlikely to be the bottleneck, this works pretty well outside niche applications involving large amounts of raw data, and TBH there you should probably be processing the data in place (using a proper local workstation or client/server model) rather than trying to drag data back and forth across a network.

Anonymous 0 Comments

You can go longer than 3 meter with USB though – I’ve installed devices for extension of USB, where you have one device connected to the computer, which is then connected (ironically, with an ethernet cable) to another device, which recieves the cable and acts as a USB hub. I’ve used this for connecting scientific equipment installed in a radiation area (not friendly to computers, or fleshy computer operators) to a computer installed outside of the radiation area.

EDIT: Example https://www.startech.com/en-eu/cards-adapters/usb2001extv

Anonymous 0 Comments

USB was originally invented as a replacement for various standards (ps/2 , serial, firewire, parallel port) that connected small devices to computers (keyboards, mice, scanners, printers, digital cameras).

They tried to keep it relatively simple by having only two wires to receive and transmit data, and they designed the communication protocol in such a way that it’s important for bits to travel within some time frame across the length of the cable. Longer cables means each bit takes longer to arrive at the other end, so the host (the computer) could mistakenly think the device is behaving erratically or that it’s faulty.

Ethernet is more complex and uses more wires (at least 4 wires for 10/100 mbps, 8 wires for 1 gbps or more), and also uses isolation transformers at each end, and uses much more power to send the bits across the cable, and is less sensitive about timing.

Anonymous 0 Comments

Ethernet transmit signal (for the most part) compare it to yelling. You can deliver a message by sound up to about 100 meters.

Usb transmit signal and power. That power is a load. Like cardboard boxes. If you want to deliver them safely (not safe, as in dropping the box equals loss of power or fire) you would probably be able to stretch something like 3 meters. Maybe with the right tools, 5 meters.

Anonymous 0 Comments

The voltage of the signals is entirely different. See, data is just 1 and 0 transmitted in a certain format. On usb the range for the „1“ Signal is from 5 volt to 1 volt(or even less), everything below is interpreted as 0. if a cable is longer, it naturally has more resistance and a lesser voltage comes out on the other side. on Ethernet the range for the „1“ signal is from 45 volt to 1 volt. So much more voltage can be lost due to length of the cable before the signals can’t be read correctly.

Anonymous 0 Comments

Apart from the protocol technicalities everyone else mentioned, there is also the issue of voltage drop

If you pull 1 ampere via 24 AWG (~0.2 mm^(2)) wires, voltage drop will be about 0.085 volts per meter of round-trip distance, so with a 3 meter long cable you’re looking at about half a volt of drop

A 0.5V drop is pretty significant when you’re carrying 5V

On the other hand, Ethernet, for the most part, doesn’t carry much power, only just enough to transmit the signal

Unless you’re using PoE (Power over Ethernet), but if I recall correctly that is limited to 0.35A (meaning the voltage drop per meter will be about a third of what I said), and carries 48V (meaning you have a much larger margin of extra voltage you probably don’t mind if it’s lost along the way)