GSM and CDMA

GSM
Stands for GLOBAL SYSTEMS for MOBILE Communications.
GSM is a protocol for moblile phone operators (like vodaphone, airtel...) and 80% of the phones in the world use GSM enabled operators.
So what's so special about it?

If you have a GSM connection, roaming around the world within networks becomes easy. In GSM both the signaling and the speech system is digital unlike analogue signals which was used in landline phones a few years ago ago(recent landlines systems like TATA walky use digital signaling). GSM was the first standard to implement the SMS service which is now common among many other systems. This standard includes a number 112, which is an emergency number worldwide. So one can call the emergency services in any country without knowing what exactly the emergency number in that locale is.
An interesting is that, even if your phone is locked, and you just dial 112 in succession (ignoring the continuous messages like ‘press unlock and the *’), the phone immediately gives you the option to call there.

Heard of mobile internet?
Well, thanx to the GPRS technology. GPRS or Global Packet Radio Service which allows users to send data even across countries. ( To activate one has to ask his/her operator for the setting which has to be saved) Thanks to GPRS that now mobile internet is nearing stardom. (the most recent attraction is the one month unlimited internet for 98 bucks brought to you by AIRCEL). Another technology EDGE (which stands for Emhanced Data Rates for GSM Evolution), which is a faster version of GPRS is also gaining momentum.

So what is a cell in the paradigm of network?
Just observe your mobile in standby mode, you'll notice that a place's name always comes up on the screen. This changes when you move from one place to another. This area covered at a time by your network is called a cell e.g., right now I am in janakpuri, B- block cell of my Vodafone network.
In GSM, there are four kinds of network, macro, micro, pico and femto.
A macro network is typically used in rural areas or hilly and less populated regions.
Micro is used in typical urban areas like Delhi. So right now you are using a micro network.
Pico cells have a small range in meters and are usually used indoors, like in a multinational company.
Femto cells are even smaller are used in high tech and sensitive environments such as the famous silicon chip factory in USA.

The maximum distance a GSM cell can cover is 35km radially. So if you calculate the area, pi*(35)^2 ~ 4000m sq.

So what is modulation?
To understand it, let's see what happens when a device sends a message to another remote device.
If you said "hi" on your phone to a friend on the other side. Your phone won't just send a "hi" over the network. It will code it into a stronger wave so that our message is not affected by other waves zooming 24X7 in our environment.
So the way in which you superimpose or code "hi" into the carrier wave is called modulation. There can be many ways of making the carrier wave understand that "Ok listen, I'm trying to say "hi", take the msg and drop it to my friend's phone ", like amplitude modulation and frequency modulation.
In GSM, the kind of modulation used is GMSK (Gaussian Minimum Shift Keying).

Have you ever been in a situation when your phone was kept near a speaker, or TV and suddenly, there was a sound like dit dit-di-dit dit-di-dit-dit, and then you realize that there is an incoming call or an unread sms in your phone.
This is because the GSM signals are so strong that even the solid state amplifiers in the speakers detect it.

GSM also supports 3G technology. But what exactly is a 3G technolgy?
3G is the better known version of IMT 2000 (International Mobile Telecommunications 2000) is a family of standards (a set of rules of how communication is to be done) for mobile communications.It's services include wide-area wireless voice telephone, video calls, and wireless data, all in a mobile environment. Compared to 2G and 2.5G services, 3G allows simultaneous use of speech and data services and higher data rates . Basically this is possible because, 3G uses higher frequencies and hence larger bandwidths are possible.

I'll give you a short summation of how it all evolved:

1. 2G services were mainly meant for voice services and slow data transmission
2. 2.5G came into existence with the introduction of GPRS and hence MMS and internet was now available on mobiles
3. 2.75G was the upgradation of 2.5G with the introduction of EDGE over GPRS and it meant three fold increase in the speed of data transmission over GPRS
4. EDGE can be considered a 3G radio technology. WiMAX and CDMA2000 belong to the 3G standards.

Ok, here comes another question. What is a SIM?
Of course everybody knows, it's that small card that goes inside our mobiles.
But what is it and how is it used?
Ok, so let's start with some full-forms. A SIM stands for Subscriber Identity Module.
It has a memory which usually stores the subscriber's info and his/her phonebook. GSM networks keep a track of the user using this SIM card.
Usually if I remove my Vodafone SIM from my mobile and replace it with an airtel SIM, I should be able to use the airtel number…..right?

But some of operators apply a feature known as SIM locking, where a person can use a phone only with the SIM that the operators have provided. I think RELAINCE does that. Even you must have seen Ads of TATA phones and VODAFONE magic box.
But hey!!... remember that this activity is a crime in some countries.

But we are also engineers….right? Now-a-days there are many websites and softwares available to unlock the phone from the 'claws of the SIM card'. You can also contact the operator to do that. They won’t decline you.





CDMA

Stands for Code Division Multiple Access.
It's a channel access method and should not be confused as a category of mobile phones. The mobiles which are referred to as CDMA phones actually employ a special method of transferring data.
Suppose you are sitting in a room with many other people and you want to say something to a friend of yours who is also sitting in the same room. Now there are many methods by which you can communicate with that person with all the other persons speaking simultaneously. This is called multiplexing.

One method is that, time periods are assigned to different people to talk (known as time division multiple access) TDMA. Another way is that people speak with different pitches (frequencies). This is known as frequency division multiple access. (FDMA)
Also it is possible if people speak in different languages. This way people who speak the same language will only be able to understand each other. This is known as Code Division Multiple Access(CDMA).

So that's how CDMA mobile phones communicate with each other.


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