When internet devices communicate over a shared medium, a protocol is needed so that signals - the messages sent over medium - do not interfere with each other.
Shared medium is for example: an ethernet's coaxial cable used to transmit messages. It's called 'shared', because multiple network cards can access it simultaneously which often results in a message collisions. Just like sending message into ether, where everyone and anyone can hear it.
There's the difference between a signal and a message, as far as the Computer Sciences and the Internet Theory goes. These words have multiple meanings, depending on context. For purpose of this post, however, you can think of a signal as of a mean of transmitting information (message) over a medium (physical link between internet devices that can be shared by more than two of such devices).
Message collision are such... most often it's known who (which device) sends which message to who. But collisions result in malformed messages... just like a wet envelopes with unreadable addresses and damaged letters. The Internet is fast, so malformed messages can be retransmitted and protocols used to help to avoid such damaging collisions and a message loss at the cost of speed.
There are three classes of medium access protocols:
* channel partitioning,
* random access,
* taking turns.
In a wireless LAN (Local Area Network) and cellular technologies a protocol named CDMA (Code Division Multiple Access) is prevalent and important. CDMA belongs to the family of channel partitioning protocols.
(to be elaborated in other posts).