You know how to operate equipment designed for message transmissions. You understand the basics of radio and can connect and route wiring to get a signal. At higher levels, you're good at working with satellite and digital signals. You also know how to bug a room. Combined with Mathematics, you're a wizard at cryptography. Combined with TV/AV, you can setup complex theatre systems and handle all the different standards.
This skill does not exist in the past. In the future, the technology and range of communications is likely quite great. Even in the future, this skill is meant to represent the character's ability to handle communications devices and any systems based on optics (or a futuristic equivalent).