Technology

Why can't I tether my phone to my laptop with two out of three of my cables?

I have an Android phone with a USB-C port and a laptop with (several) USB-C ports.

I have three cables that I carry with me: a USB-C-to-USB-C cable, a second USB-C-to-USB-C cable, and a USB-A-to-USB-C cable. None of these are charging-only power cables, and I've used them for data connections.

One of the USB-C-to-USB-C -- an unmarked cable -- permits for USB tethering to be used.

The other two do not.

The other USB-C-to-USB-C cable even has USB 3.1 symbol.

I don't know why.

Looking more-closely, it looks like the other two don't have a data connection established between the Android phone and the laptop from the laptop's perspective. They've let me do so with other devices.

Checking what data transfer rates a given cable supports electrically

As far as I can tell, there isn't a way to query the "e-marker" on a USB cable from Linux today; I found a comment from someone saying that kernel support is still being worked on. You can use lsusb -t to show the negotiated speed between two devices, so can use them to infer the speed, as long as you have fast-enough devices at both ends of the cable.

https://lemmy.world/post/18014298

What USB PD rates does a USB cable or power consumer or charger support?

I don't know of a good way to determine this from a user standpoint. Note that this is a matrix of voltages and currents, so it isn't just "I support up to rate X". Also, not all devices display the rate of power that they are providing or consuming -- in fact, most don't. My Android phone, a reasonably-sophisticated device and one with a display and capable of both providing or consuming power, doesn't show the rate of power consumption or provision, just "slow" or "fast", without additional software. I understand that that software doesn't work on all Android hardware.

I have -- had -- a laptop that just won't charge if a charger doesn't support a certain USB PD profile, which its provided charger did but not all charging devices did.

When I plug in two devices that both support USB PD, which is the consumer and which the provider?

When I'm in my car, I typically I have three devices that have USB PD ports and can either provide or consume power -- a large powerstation, a laptop, and a phone. I eventually learned a few facts:

First, the direction in which power is being provided via USB PD is independent of which device is operating as a USB host or device using USB OTG ports; it's possible for the direction to be different from the direction of power provision.

Second, apparently the direction of host/device order is random, and devices just remember the host/device direction for a certain amount of time, so that if you plug two USB OTG devices into each other and the direction is not what you want, the idea is that you can figure it out from one or more of the devices indicating this and then unplug them and plug them in again to get transfer in the other direction.

Third, as best I can tell empirically, USB PD does the same random thing.

This creates all kinds of fun if one device powers off and then on again or something; my laptop can start draining its power to my powerstation (generally not what I want), or my phone to my laptop, since all the USB PD ports in question support USB PD in both directions.

Which end is which on an active USB cable?

I have an active optical USB cable, which I obtained so that I could put my computer in a closet, a long way from the rest of my devices; USB on copper has very limited range at present-day speeds without a repeater. It functions in only one direction in terms of data transfer (and obviously can't move power). That particular manufacturer labeled it, though there's no standard for labeling that.

In sum

USB does have reasonably good fallback, so most cables and most devices tend to sort of do something to some degree -- they move some amount of power and some rate of data, though some devices have hard demands on what they need and there isn't a great way to assess what a cable or device supports in most cases from an end user standpoint. But it definitely could be a lot better from my standpoint.