When two entities talk to each other, if you see closely, their communication sequence can be logically separated in two parts.
Better explained with an analogy.
Let us say a manager would like to meet a subordinate, probably they would first decide a meeting room or place to meet. And after this initial negotiation or communication, they would eventually meet and proceed with actual discussion.
In telecommunication, when two entities would like to talk to each other, they first do initial communication to decide about transmission channel. And then actually this transmission channel used for user data transfer. (Channel is nothing but a communication link)
When you use your cell to make a call, your cell first does the signaling to setup a communication channel, then uses this communication channel to transfer your voice conversation !
The initial communication is known as signaling and transfer of user data is transmission. In technical terms, we call signaling plane and transmission plane. Note that even though user data is being transmitted over transmission plane, signaling plane can remain (which may be in idle state).
Refer a diagram below.
So when you are on a call and say another person makes a call to your cell, the waiting call (assuming call waiting is activated by your network operator) is informed to your cell over signaling plane or channel.
Note that signaling plane and transmission plane is a logical division and standard/specification specify exact implementation of it (transmission and signaling may be part of same communication channel or may be actually different channel).
Another point which - though apparent - needs mention is during setup of communication channels or links, lower layers act first. For example, before Data link layer could establish reliable data link, Physical layer need to do setup specific to transmission media.
© Copyright Samir Amberkar 2010
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