GSM and CDMA
To know what GSM and CDMA mean, we must first know how a cell phone works. The mobile phone was devised by combining Alexander Graham Bell’s telephone and the radio invented by Nikola Tesla.
Help you make and receive calls
Primitive mobile communication technology consisted of CB radios and walkie-talkies. A CB radio system consists of ‘central antenna with around 20-30 channels and the phone transmits data on a particular channel to the tower which then retransmits it and can be received by any one logged on to that channel. A walkie-talkie a is also used for communication but it has a single channel and no tower to retransmit its signal, hence it has a limited range. Both walkie-talkies and CB radio are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. That’s why you always hear soldiers saying “over” after they finish their sentence so that the other person can start talking. Also limited number of conversations can take place simultaneously because of the limited number of channels.
On the other hand, a cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at the same time. The cellular system divides the city into small cells (25 sq km) so thousands of people can use their cell phones simultaneously. A cell phone also has considerable large number of channels to communicate. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system. and controls all of the base stations in the region.
Now that we have a fair idea about the setup of a cellular network, let’s see how a call is placed.
When you turn on the phone. the phone tries to communicate with a specific channel called the control channel. The phone receives a unique identity number called System Identification Code (SID. specific to each network operator) and compares it to the SID programmed into the phone. If they match, it knows that it’s using the home network, if it doesn’t, it’s on roaming, If the phone can’t find any control channels to listen to, it knows it’s out of range and displays a no service” message.
It also sends a registration request. This way, the IVITS0 can keep a track of your location in its database, it looks for you in the database when you get a call.
The MTSO then selects a frequency pair on which you’ll communicate and using the control channel tells your phonic to use these frequencies. Once your phone and the tower switch on t hose frequencies, the call is connected.
If you’re travelling you’re bound to change cells. As you move closer to the edge of a cell, your signal strength decreases, meanwhile the tower in the other cell (the one you’re moving closer to) notices that your phone :signal strength is increasing, These two towers coordinate with the MTSO and it records that you’ve switched cells.
There are three technologies that are used by network operators to carry out this communication, Although the names look scary, you can break it down to simple parts. The first word tells you what the access method is. The second word, division. lets you know that it splits calls based on that access method. The last part says multiple access which means that more than one user CD it utilise each cell.
- Frequency division multiple access (FDMA): FMDA puts each call on a different frequency. It separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. It’s used for analog transmission but is not considered an efficient method for digital transmission.
- Time division multiple access (TDMA): In TMDA, a narrow band (channel) that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. This technology is used for GSM. GSM systems use encryption to make phone calls more secure.
- Code division multiple access (CDMA): In CDMA, the data is digitized and spread out over the entire available bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. In simple words, data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range. At the receiver, that same unique code is used to recover the signal. What does this mean for you? In terms of connectivity and speeds, there’s not much difference between them. However, in CDMA, there’s no SIM (subscriber identification module) card and so changing your network provider can be a little difficult. In a GSM phone, you can switch networks by simply changing your SIM card. To overcome this draw-back, CDMA network providers try to provide better rates and plans.