Cellular/ Mobile phones

What are the new digital phone networks capabilities for data communications? [ie. Can I use my One2One/Orange phone?]

GSM systems support a single 9600 baud data connection or two simultaneous 4800 baud data connections, theoretically.

You'll be VERY lucky to get a normal modem to work over GSM/PCN voice channels, the compression etc squishes the signal nicely (and adversely affects the modem's encoding). You also get problems with the TDMA signals squirting out the aerial knocking out nearby electrical equipment! (The GSM set is not the problem, but the nearby equipment isn't RF immune. This works both ways- if something is vulnerable to RF pickup it is likely to also be generating RF noise).

For two simultaneous half-rate channels (Lm + Lm) the phone needs half rate codecs - they don't seem to be available yet. When they do become available, you might be able to use speech on one channel while sending data on the other ... the specs allow for this.

For a single full rate channel (Bm) the specs support data rates at 9600, 4800, 2400 ... but no other simultaneous use. I.e. using a data-rate of 4800 baud does not leave any spare capacity for use at the same time.

There are modems available for GSM, but are proprietary, use weird encoding, and are slow ...

The issue here is they all use clever speech-encoding schemes to turn voice into the lowest possible bit rate. ISDN encodes speech using conventional PCM into a 64kbps data stream, and CT2 networks squeeze it tighter into 32kbps using ADPCM. The encoding used on GSM and PCN networks is smarter still, and codes speech into a 13kbps channel.

The problem with all this super-efficient coding is that it preserves an analogue speech signal, but wreaks havoc with the analogue signal modems turn your data into. For example, the phase-encoding used with any modem faster than 1200bps is highly likely to be lost. Quite what the limits are I don't know - I have heard that you can get 2400 V.22bis and possibly 9600 V.32 through a 32kbps ADPCM link, but no chance for 14.4kbps V.32bis. Information theory clearly constrains you from sending 14.4kbps through a 13kbps channel as well. On the other hand, 64kbps ISDN preserves V.32bis data no problem. I would be surprised if you can get more than 2400bps from a GSM/PCN phone.

What is really needed is the mobile equivalent of ISDN access - instead of turning data into an analogue signal using a modem, and then feeding it through a codec to turn it into a higher rate digital signal again, you want to be able to feed data straight into the network, bypassing the modem and codec. That way you get the full data rate, like ISDN.

A large proportion of the GSM specifications deal with this. GSM IS designed as a sort of mobile equivalent of ISDN. It is not the specifications that cause problems, it is the priorities of the marketing people (which are based on their perception of the priorities of the customers). This then influences the implementors, although, in practice, they have to provide speech, so this gets first priority.

Something to bear in mind is that such access is still circuit- switched, simulating a telephone call across the air. Radio waves are a broadcast medium, just like ethernet, however, and there is a strong case that such media are better suited to datagram traffic rather than circuit-switched. This would appear to suit Internet access, as IP very much datagram-oriented. This is where pure mobile data networks come in, with providers such as RAM, Cognito, Paknet and Hutchison.

What do One2One PCN aerials look like?

They look like normal radio masts, but half way up the mast (not at the top) there are horizontal arms coming out, with vertical aerials on top of the poles.

How do mobile phones work in tunnels?

Using 'leaky feeders'. Basically run a length of thick coax and cut sections of the outer shielding off (about 6" to a foot) every now and again. This is being done in the Channel Tunnel too.

How does the network find the mobile phone?

With TACS (Total Access Communication System) and ETACS (Extended TACS), as used on Vodafone and Cellnet, when a mobile is switched on it sends a registration message to a local cell (usually the one with the strongest signal). Data about where that mobile is can then be sent to (& stored in) a special database called a "home location register", usually kept at the Network Control Centre. This database can be interrogated by any switch on the network that is trying to call that mobile.

When mobiles move from one cell to another, they can detect that the control channel has changed and then they re-register on the new cell - and the Home Location Register is updated.

At regular intervals (between 15 minutes and an hour - depending on how busy the local area is) the network sends out a data message asking all phones to re-register: this ensures that the Home Location Register is fairly uptodate. When a call is made to a mobile, calling signals will be sent out to the area in which the mobile last registered. If that signal isn't acknowledged by the mobile, the network then tries to find the mobile by calling in all areas. Obviously if it had to do this on every call, there would be a tremendous overhead of unnecessary signalling !

How does the network distinguish between 'switched off' and 'not responding' when giving messages to the caller?

In the GSM system, when the power is switched off, the mobile sends a detach message to the Mobile Switching Centre (MSC) that you were connected to, before the power actually disappears. If your mobile is switched on, but you don't pick it up, a 'not responding' message is given?

When an ETACS phone is turned on it transmits a power-up message to the network. However when its turned off it doesn't transmit a shutdown message so the network has to try and 'guess' when you've turned your phone off.

It does this via the periodic 'pages' which it sends to each phone. If the network doesn't receive a response to a number of these pages then it assumes your phone is off.

How do I program my XXX brand of mobile phone?

For the Nokia 101

 To enter programming mode, key in *60312#12345
 Key in M
 Key in 00
 If it's worked it'll say STORE NOT DONE
 If it hasn't, it'll say NOT ALLOWED

The information is edited and stored in the same way as for normal code memories - the power-up name is in memory location 02. (If you intend to change this, write down the information in memory 01-05, in case you mess things up!)

The telephone number in memory location 02 is your full mobile number... By using ABC as per normal memory locations, you can enter a power-up name, too. Store the information back in memory location 02.

Then exit programming mode by turning the 'phone off.


Version: 2.10 Last-modified: Time-stamp: <96/05/12 23:04:34 jrg>