Mobile Phones - The Basics

IV. Channel Names and Functions

Okay, so what do we have? The first point is that mobile phones and base stations transmit or communicate with each other on dedicated paired frequencies called channels. Base stations use one frequency of that channel and mobile phones use the other. Got it? The second point is that a certain amount of bandwidth called an 'offset' separates these frequencies. Now let's look at what these frequencies do, as we discuss how channels work and how they are used to pass information back and forth.

Certain channels carry only cellular system data. We call these control channels. This control channel is usually the first channel in each cell. It's responsible for call setup, in fact, many radio engineers prefer calling it the setup channel since that's what it does. Voice channels, by comparison, are those paired frequencies which handle a call's traffic, be it voice or data, as well as signaling information about the call itself.

A cell or sector's first channel is always the control or setup channel for each cell. You have 21 control channels if you have 21 cells. A call gets going, in other words, on the control channel first and then drops out of the picture once the call gets assigned a voice channel. The voice channel then handles the conversation as well as further signaling between the mobile phone and the base station. Don't place too much importance, by-the-way, to the setup channel. Although first in each cell's lineup, most radio engineers place priority on the voice channels in a system. The control channel lurks in the background.

When discussing mobile phone operation we call a base station's transmitting frequency the forward path. The mobile phones transmitting frequency, by comparison, is called the reverse path. Do not become confused. Both radio frequencies make up a channel as we've discussed before but we now treat them individually to discuss what direction information or traffic flows. Knowing what direction is important for later, when we discuss how calls are originated and how they are handled.

Once the MTSO or mobile switch assigns a voice channel the two frequencies making up the voice channel handle signaling during the actual conversation. You might note then that a call two channels: voice and data. Got it? Knowing this makes many things easier. A mobile phones electronic serial number is only transmitted on the reverse control channel. A person tracking ESNs need only monitor one of 21 frequencies. They don't have to look through the entire band.

So, we have two channels for every call with four frequencies involved. Clear? And a forward and reverse path for each frequency. Let's name them here. Again, a frequency is the medium upon which information travels. A path is the direction the information flows. Here you go:

Forward control path: Base station to mobile phone

Reverse control path: Mobile phone to base station

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Forward voice path: Base station to mobile phone

Reverse voice path: Mobile phone to base station

One last point at the risk of losing everybody. You'll hear about dedicated control channels, paging channels, and access channels. These are not different channels but different uses of the control channel. Let's clear up this terminology confusion by looking at call processing. We'll look at the way AMPS sets up calls. Both analog and digital cellular (IS-54) use this method, CDMA cellular being the exception. We'll also touch on a number of new terms along the way.

The control channel and the voice channel, paired frequencies upon which information flows. Paths indicate direction of flow.

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