How to get data coming from gsm network

Question

I'm trying to see how gsm affects data on a phone call. Here is what I'm trying to do. One person will be talking on a phone and I will record his voice from phone's mic while he speaks and on the other phone I will get the data coming from gsm and compare them. I want to write an android application to get that data. Is that possible on android or can you suggest another way to achieve this?

Answer 1

Some background (you may know this already)...

When you make a GSM call, the analogue signal in the phone microphone corresponding to your speech is converted into a series of digital values and then encoded with a voice codec. This is basically a clever algorithm to capture as much of the speech as possible, in as little data as possible.

The idea is to maintain very good speech quality while saving on the amount of bandwidth needed for a call. Techniques used include not transmitting quite periods (when you are not speaking) and various compressions and predictive encoding algorithms. There have been and still are a number of codecs in use in GSM, but the latest and general preferred codec is called AMR-Narrowband.

Nearly all GSM deployments encrypt speech between the phone and the base station - while there are publicised weaknesses in the various encryption algorithms, I am assuming (hoping...) that decrypting is not what you are looking for.

Your question - 'I want to see that if there will be data loss or corruption when voice reaches over gsm'

Firstly, is it worth noting that speech is 'relatively' tolerant of small amounts of data loss and corruption, at least compared to data. It is quite common to have bursts of packet loss in VoIP networks and it may cause a temporary degradation in voice quality. Secondly, packet loss in a VoIP network will include delayed packets which can be confusing - if the packet arrives too late to be included in the 'sound' being played in the receivers speaker then it is effectively lost from the VoIP point of view, even though other measures may show that it simply arrived late.

To actually measure the loss between the GSM phone and the basestation you would need access to the data received at the basestation, which you will not usually have unless you are the operator.

If you do an end to end test, from one GSM to another, your speech path will traverse other network nodes also, so you will not know if any loss or corruption is happening over the GSM air interface or in one or more of the other nodes.

You would also need to be aware of handover from one cell to another and from 2G to 3G (GSM to UMTS) which may affect your tests (even stationary phones can handover in certain circumstances).

If your interest is purely academic then the easiest thing might be to create your own GSM base station and test on this - there exists several open source GSM 'network in a box' projects which should allow you do this. I have not used it myself, but this one looks the most actively supported at this time - check out the mailing list under the community tab for a good place to maybe follow up your investigations:

http://openbts.org