GSM is an acronym for “Groupe Special Mobile”, which has now been changed to “Global System for Mobile Communication’. CDMA refers to Code Division Multiple Access. The workings of GSM and CDMA are very different.
How GSM works.
GSM is a ‘cellular’ technology, that is, the entire coverage area is divided into various hexagonal-shaped cells. Every cell has a corresponding network tower, which serves the mobile phones in that cellular area.
That is the entire frequency band is divided into chunks, and each such chunk is divided into timeslots, and each such portion is made available to a user.
How CDMA works.
In CDMA, there are many devices that use the same spread spectrum (hence multiple access). There is one physical channel and a special code for every device in the coverage network. Using this code, the signal from the device is multiplexed, and the same physical channel is used to send the signal (the codes may or may not change).
That is, in simple terms, the entire frequency band is available to the user. So, in order to differentiate, the transmission from each user is “spread” or coded using a unique code given to the individual user. At the receiving end, the spread information is decoded.
For comparison, imagine a cocktail party where couples are talking to each other in a single room. The room represents the available bandwidth. In GSM, a speaker takes turns talking to a listener. The speaker talks for a short time and then stops to let another pair talk. There is never more than one speaker talking in the room, and no one has to worry about two conversations mixing. In CDMA, any speaker can talk at any time; however, each uses a different language. Each listener can only understand the language of their partner. As more and more couples talk, the background noise gets louder, but because of the difference in languages, the conversations do not mix.