(Page 4) Cellular Telephone Basics continued . . .

IV. Channel Names and Functions

Okay, so what do we have? The first point is that cell 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 mobiles 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 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. [See Control channel] Now let's add some terms.

When discussing cell phone operation we call a base station's transmitting frequency the forward path. The cell phone's 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 telephone 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's 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

<-- Reverse control path: Mobile to base station

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--> Forward voice path: Base station to mobile

<-- Reverse voice path: Mobile 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-136) use this method, CDMA cellular (IS-95) and GSM being the exceptions. We'll also touch on a number of new terms along the way.

Still confused about the terms channels, frequency, and path?, and how they relate to each other? I understand. Click here for more: See channels, frequencies, and paths.

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

[Control channel] "Is the control channel important? Actually, I can't think of a case where it would not be. But we don't think of it that way in the business. We have a set-up channel and we have voice channels. They are so different (both in function and in how they are managed) that we never think of the set-up channel as the first of the cell's channels -- it's in a class by itself. If you ask an engineer in an AMPS system what channels he has on a cell, he'll automatically give you the voice channels. Set up channel is a separate question. Just a matter of mindset. You might add channels, re-tune partially or completely, and never give a thought to the set-up channel. If asked how many channels are on a given cell, you'd never think to include the set-up channel in the count." Mark van der Hoek. Personal correspondence.(back to text)

Channels, frequencies, and paths: Cellular radio employs an arcane and difficult terminology; many terms apply to all of wireless, many do not. When discussing cellular radio, which comprises analog cellular, digital cellular, and PCS, frequency is a single unit whereas channel means a pair of frequencies, one to transmit on and one to receive. (See the diagram above.) The terms are not interchangeable although many writers use them that way. Frequencies are measured or numbered by their order in the radio spectrum, in Hertz, but channels are numbered by their place in a particular radio plan. Thus, in cell #1 of 21 in a cellular carrier's system, the frequencies may be 879.990 Hz for transmitting and 834.990 Hz for receiving. These then make up Channel 1 in that cell, number 333 overall. Again, in cellular, a channel is a pair of frequencies. The frequencies are described in Hz, the channels by numbers in a plan. Now, what about path?

 

Path, channel, and frequency, depending on how they are used in wireless working, all constitute a communication link. In cellular, however, path does not, or should not, describe a transmission link, but rather the direction in which information flows.The forward path denotes information flowing from the base station to the mobile. The reverse path describes information flowing from the mobile to the base station. With frequency and channel we talk about the physical medium which carries a signal, with path we discuss the direction a signal is going on that medium. Is this clear?

 

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