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Modems in cellular phones

I'm wondering how modems that are embedded in cellular phones work. I'm confused by the fact that GSM/CDMA is digital and so why would the phone need the modem in the first place since it doesn't need to convert analog signals to digital.

Please correct me if I'm wrong because all along, I'm assuming that GSM/CDMA are both digital systems and so the cellular phones are sending digital data. When one says that the phone is WAP-enabled, does it mean that it is data-enabled AND it contains a modem? If so, what is the function of the modem?

And I'm also confused about the slow 9.6-14.4 kbps internet connection (WAP) using GSM phones. Is it because of the phone power that makes the connection slow? Or that GSM does not have the bandwidth to obtain normal PC dial-up data rates (e.g. 56.6kbps)?

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First, I am not an total expert on mobile phones and 3G technology. I just use them. Personally, I think if you need to dial in via a mobile you are not looking at the problem the right way.

Modems over Digital Mobile Phones
You are right about digital. However, the GSM digital encoding is drastically different from a modem encoding. Thus, conceptually at least, the mobile phone modem translates from your computer to GSM type encoding, whereas your "normal modem" translates from your computer to PSTN analog.

WAP Phones and Integration
A WAP phone actually contains a Web browser program, which uses the Wireless Application Protocol (WAP). WAP has many protocols, including HTTP like commands, transmissions etc., etc. But, at the end of the day, it just turns your mobile phone into a web browser. This doesn't work very well in real life, however, because:
  • you have to write very specialized web pages in WML, which no one wants to do unless someone is going to look at it (chicken and the egg)
  • it's slow, really slow
  • the display screens on mobile phones are very ordinary
  • no one wants to pay for it anyway.

Modem Speed on Mobiles
The speed of WAP is limited by radio frequency bandwidth. Your "normal modem" has an analog circuit all the way to the exchange. This circuit has 64K of available bandwidth. So, factoring in a bit of loss in the circuit, your modem can go at 56K. Today's mobile phone networks (of any flavor) were only ever designed to carry voice which is normally processed into about 8K, including overheads. Worse still, the actual available bandwidth is cellular in nature and must be shared between all simultaneous conversations. SO when you put your modem onto this transmission medium, no matter how hard you try, there just isn't enough bandwidth to go fast than 14K4. And, in real life, transfer rates of about 4-6K are typical.

3G wireless networks are supposed to fix this. I reckon it won't happen any time in the next 5 years. Think again about why you want remote access over a mobile phone, then reengineer the business process to work around it.

This was first published in November 2001

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