Mobile Phones: How They Work Part 2 — the Control Channel
Introduction
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Mobile Phones: How They Work Part 2 — the Control Channel
Introduction
In the first part of this series we saw what happens when a mobile phone is first turned on. What hasn't been mentioned so far is that all the communications between the phone and the network discussed in part 1 is actually conducted via a special channel called the control channel. This is a specific frequency employed by each cell so that any phone can communicate with a base station. Indeed, if a phone cannot locate a control channel to listen to then the phone knows that it is out of range of a base station and displays a 'no service' message.
However, if a control channel is actually located then, as well as the security checks already described in first part of this introduction to mobile phones the phone itself also transmits a registration request that the cell network's MTSO (Mobile Telephone Switching Office) uses so that it can track the location of your phone within its database. This way the MTSO knows which individual cell you are in.
How Calls are Made — Control Channels
In terms of being able to make and receive mobile telephone calls then the control channel of the mobile telephone network is all important as it's this that infroms the network where your phone is so that you can connect to the appropriate cell. As mentioned above, it's the control channel that tells the system which cell you are in, so that this information can be stored in the system's database. It is entirely because of this that an incoming call can be routed to your phone. When another person dialls your mobile telephone number then the call is routed via the network's MTSO.
The network then sends a message to your phone saying that there's an incoming call. This message is routed to the cell that represents the last location that the system knows your phone to have been in. Just to be sure the same message is also transmitted to that cell's immediate neighbours, just in case you have moved since the last time your location was recorded. Even if you have wandered out of this cluster of cells the network should have recorded this and updated its database. In fact your mobile phone can be located to within a few tens of metres using the principle of triangulation. A control panel signal can be broadcast to your phone from the three cells nearest to you. The time it takes for the signals to reach your phone is measured and this can be used to pinpoint the location of your phone.
How Calls are Made — Control Channels and Traffic Channels
Once a call is connected via the control channel then the call still has to be maintained. The data are transmitted to and from your phone via traffic channels (for a description of how these work see the next part of this series) and to maximize the number of phones that can be used on a network these traffic channels are different between adjacent cells. Therefore if you move from one cell to the next and your phone wasn't told you had moved then because of the different traffic channels used in each cell your phone actually wouldn't work!
To overcome this problem a process called 'hand-over' is employed. Whilst your mobile phone is in use it actually constantly monitors the control channels of up to sixteen of the neighbouring cells closest to it. The phone then works out which of these cells produce the strongest signals and broadcasts a list of the top six cells by signal strength back to the base station to which it is currently connected. If a phone moves away from the base station then the signal the phone produces is boosted to generate a clearer signal. However, if this boosting of signal strength no longer works then the network consults its database and instructs your mobile phone to switch to another cell. This triggers a handover where the phone adjusts its reception and transmission frequencies so that it can now work with the closest cell that will give the bast signal strength. Generally this handover is seamless and you notice no drop in the quality or reception of your telephone call. However, as anyone who has used a fast-moving tranin has found out the system is not completely infallible and delays in switching can result in calls 'dropping out' or even being dropped entirely.