GSM—Global System for Mobile

2 How does GSM handle multiple users The 1G cellular systems used FDMA. The first cellular standard adopting TDMA was GSM, a 2G system using digital technology. First deployment in 1992.

3 GSM is a TDMA/FDMA/FDD System A total of 124 carrier frequencies are available in the 25 MHz band in each direction (forward or reverse). This is FDMA. Each carrier frequency, occupying a frequency band of 200 kHz, has 8 time slots for 8 users. This is TDMA (TDMA overlaid on FDMA). The forward (downlink) and reverse (uplink) uses two different frequency bands: MHz for downlink and for uplink. This is FDD (Frequency Division Duplex).

4 Mobile station Mobile Equipment (ME): the phone including transmitter, receiver, etc. Subscriber Identity Module (SIM): a smart card that stores the subscriber’s ID number, the networks the subscriber is authorized to use, encryption keys, etc. Thus a subscriber needs only carry his or her SIM to use a wide variety of subscriber devices in many countries. The SIMs roam, not the phones. (The US is different from most other countries. In the US, service providers lock the phones because they subsidize them.)

5 Base Station Subsystem (BSS) Base Transceiver Station (BTS) for a single cell: an antenna, a transceiver (transmitter & receiver), and a link to a base station controller A GSM cell can have a radius of 100m-35km. Base Station Controller (BSC) may control multiple BTS units and hence multiple cells. It reserves frequencies, manages the handoff of a mobile unit from one cell to another within the BSS.

6 Network Subsystem (NS) Provides the link between the cellular network and the public switched telecommunications networks (PSTN). The central element of the NS is the Mobile Switching Center (MSC), which  Determines whether the subscriber is associated with the switching center  Stores locations of the subscribers currently in the region covered by the switching center  Stores authentication and encryption keys for all the subscribers in both the home and visitor location registers  Keeps track of the type of equipment that exists at the mobile stations

7 GSM Frames and Data Rate Each time slot has 114 bits of data Each frame has 8 time slots, one for a user Each multiframe (duration=120ms) has 24 data frames Data rate:

8 Speech Coding Speech coding is used to compress the data (reduce the data sent). This will increase the speed of transmission. It uses previous data samples to predict current sample. Analog voice is transformed into 13 kbps digital signal. Adding error-detection and error-correction coding brings the transmission rate to 22.8 kbps. Data rate: 9.6, 4.8 or 2.4 kbps.

9 Frequency Hopping A slow frequency hopping: 216 hop/s To compensate multipath (frequency selective) fading To reduce the cochannel interference