From cordless to mobile

Apologies for taking so long to resume my posts. Frankly, it was a bit difficult to feel like posting when it seemed like no one was paying any attention. But, I've decided that that is no reason not to post, and I really appreciate the feedback from my one (perhaps only ) reader -- thank you Leda. Please, everyone, do tell me about what you are interested.

Last post, I talked about going into the details of what makes a mobile/cellular phone different from a cordless phone. There really isn't that much real difference. The primary differences are distance between the handset and the base station and the fact that each base (cell) station will handle more than one phone. In addition, there is a need for handover (handoff) of the phone from one base (cell) station to another as one moves around.

When one uses a cordless phone, it must synchronize with the base station. To do this, it may require cycling between "channels" to find a clear connection. However, it is still a relatively simple connection -- usually no need for identification or security.

A cell phone goes through a number of stages. When it is powered up, it basically "shouts" into the air and these transmissions are omnidirectional (unlike a satellite link, the handset may be in any position relative to the base station) so the signal must go all directions. Although it is possible for carriers to share a pool of transmission frequencies, it is more common for a single carrier to use a set of frequencies. So, the phone shouts out within this bandwidth and waits for a response.

The cell station listens and then, using identification information found in the initial transmission from the mobile handset, sends back a response. It is possible that a further exchange of information (called a handshake) will take place to further identify the specific phone and the services available to that handset. This process is known as registering. Much of the registration information will reside in the memory (or specialized memory -- known as a SIM) of the handset. The first time you use your handset, much more information is exchanged to build up the store of data in the handset.

Once registered, people can call you and you can call others. Signal strength (or number of "bars") will make a difference about the clarity of the call. However, calls are really made between the base station and the other phone -- not the handset. The handset is the final destination/origination of the data (voice or email or whatever) but the call is really between the base station and the other end (peer).

The base station may make an additional jump to a more powerful station or it may directly connect into the wired "landline" phone networks. At some point, almost all mobile calls are actually routed via landlines. It is just more efficient to use the Radio Frequency (RF).

The ability to do the "handover" is what really disctinguishes a mobile phone from a cordless phone. As you move, with your handset, the signal strength from the current base (cell) station will change. If it decreases, it may reach the point where the signal just isn't strong enough to continue the call. When the signal strength decreases, the handset will start shouting again. If it finds a new, stronger, base station, it will initiate the "handover", which is basically the same thing as a "transfer" from a non-mobile phone. The call is transferred from the old base station to the new base station and it will then act as the primary endpoint for the call (with data continuing to be exchanged with the handset).

This transition must be done very quickly to allow an interrupted use of the circuit. Because of this transition delay, the handset and base station may behave a bit differently if a data session is in use -- perhaps setting a lower signal strength trigger for handover and waiting for a pause in data to do the transfer.

Of course, all this doesn't actually work as simply as described. In particular, the RF transmission and reception are very difficult areas to engineer. Although I have taken enough physics and engineering courses to understand, in principle, how the RF works, my general impression is that RF engineers are magicians.

So, that is really the difference between a cordless phone and a mobile phone -- the ability to do that transfer (handover) as smoothly as possible.

I leave you at that point but will try to post on a much more regular basis.