GSM for Begineers

Q1. What is air interface?
A1. The air interface, also referred to as the Um interface is the interface between the Mobile station and the Base Transceiver Station. It basically is a frequency that is used for the communication between these entities. Also known as radio interface.
Q2. What is TDMA?
A2. TDMA is Time Division Multiple Access. In this type of access, the air interface frequencies are shared by users on a time basis. In other words, time-slots are defined and one user can use the entire frequency spectrum for the entire time-duration.
Q3. What is FDMA?
A3. FDMA is Frequency Division Multiple Access. In this type of access, one frequency is assigned to a user and the user can use it indefinately.
Q4. What are the disadvantages of TDMA?
A4. Since the sharing is done on a time-slot basis, the number of users gets restricted by the time-slots defined. This is inadequate usage of the radio resource.
Q5. What are the disadvantages of FDMA?
A5. Since the sharing is done on the frequecy basis, the number of users gets restricted by the number of frequencies in the frequency band. This is inadequate usage of the radio resource.
Q6. What is the access technique used for the air interface?
A6. The allocation is done using a combination of TDMA and FDMA. The frequency spectrum is divided into frequencies and timeslots are also defined. A user is allocated a frequency and a timeslot. So, one user can use that frequency only during that timeslot. That allows more number of users to use the precious radio interface. :)
Q7. How is TDMA and FDMA applied on the air interface?

A7. As per A6, both TDMA and FDMA are applied. The air interface gets divided into TDMA frames. Each of these TDMA frames consist of 8 timeslots(TS), 0-7. The frames keep repeating themselves continuously.A user is allocated a timeslot and a frequency for use.For example, a user may be allocated TS 2 of a frequency "x". In this case, the user would transmit on every TS 2 of each frame on frequency "x".

Q8.What is the hierarchy on the air interface?

A8. As per A6, both TDMA and FDMA are applied. The air interface gets divided into TDMA frames. Each of these TDMA frames consist of 8 timeslots, 0-7. The TDMA frames form the multiframes. There are 2 multiframes : 26 multiframe which is formed by 26 consecutive TDMA frames and 51-multiframe which is formed by 51 consecutive TDMA frames. These multiframes then form superframes (which may consist of 26, 51-multiframes or 51, 26-multiframes). The multiframes then form hyperframes, which are formed of 2048 superframes.