what protocol is used for communicating with drones (quad copters, etc.)?

For outdoor ones, normal model radio control gear.

It's very simple digital system; a string of pulses, one for each channel (servo), with the width of the pulse representing the required servo position.

The pulse width is in the 1 - 2mS range with 1.5mS representing centre position and 1 / 2mS the extreme limits.

A pause after the last channel pulse resets the sequence and the next chain starts back at channel 1.

Older analog style systems send each actual pulse width then a pause.

Newer systems [ppm] just send a "step" pulse, allowing a faster update time:

https://bipeenkulkarni.files.wordpress.com/2013/04...

The radio link frequency varies/ It started out decades ago with a few channels around 27MHz, then 35MHz for flying models (to avoid CB interference) and now typically 2.4GHz using a system something like long-range bluetooth, a frequency-agile system to avoid interference from other stuff in the same band.

With a drone, the radio system works as if its flying a helicopter or fixed-wing plane, with the main joystick and throttle joystick signals being fed to the onboard flight computer and that in turn controlling the lift motor powers to make the machine do what the operator wants.

They use an onboard accelerometer, gyro and altimeter array to continuously monitor the actual movements of the drone and compensate to hold it stationary or moving at the speed commanded by the operator.

Some also have a GPS receiver and can return to the launch point on their own if the radio signal is lost - or go to a sequence of preset waypoints.

This is a typical "high end" flight controller with GPS. It's the size of a small matchbox, about the same as the receiver. Typically all the receiver channel out cables link directly to the channel inputs on the controller.

https://www.dji.com/naza-m-v2

(The image below is the second of the radio pulse system diagrams, that yahoo has decided to include in-line..)