- History of GSM Technology
GSM is a European digital communication system; an application of TDMA mobile telecommunication technology.
For unification of various analogue mobile telephone systems in the region, EU organized GSM (Group Special Mobile) to develop a Pan-Europe mobile telecommunication system at CEPT (Conference of European Posts and Telegraphs) in 1982. GSM aimed to satisfy the following criteria:
◇ high voice quality
◇ low service rate
◇ supports international roaming service
◇ supports new services
◇ compatible with ISDN
In 1989, GSM was transferred to ETSI (European Telecommunication Standards Institute), then GSM Phase I was announced in 1990. Commercial service was started in the middle of 1991. By 1993, 36 GSM networks were set up in 22 countries world-wide. GSM, DCS1800 and PCS1900 have been produced in GSM, now stands for Global System for Mobile. GSM is now stands for Global System for Mobile.
- GSM Technologies
GSM technology provides the basis of GSM900, DCS1800 and PCS1900 systems. The basis of these technologies is TDMA.
PCS1900 is the system adopted in the North America. DCS1800 is very similar to GSM900, except using different frequency spectrum. DCS1800 ((Digital Cellular System 1800) system is using 1.8GHz band, while GSM900 is 900㎒.
- GSM Network Architecture
A GSM network is consists of Mobile Stations (MS), Base Station Subsystems (BSS) and Network Subsystems. Network Subsystem takes switchboard function, while BBS and MSC exchange signals via A Interface. MS and BSS are connected via Air Interface.
A. Mobile Stations (MS)
An MS (Mobile Station) is consists of a terminal device and SIM (Subscriber Identity Module), a smart card chip. To use any terminal device, users only need to insert SIM into it. This allows users mobility. Every mobile terminal has its own IMEI (International Mobile Equipment Identity). IMEI is encoded in SIM to be used in connecting to the network system. In other words, IMEI and IMSI are for ensuring users' mobility. SIM Card is protected by PIN (Personal Identity Number). PIN is consists of 4 digit code to connect to a GSM network.
B. Base Station Subsystem (BSS)
BBS is consists of BTS (Base Transceiver Station) and BSC (Base Station Controller). BTS and BSC exchange data using Abis Interface. BSC manages multiple BTS and the process of wireless channel setup, Frequency Hopping and Handover. BSC can be regarded as the equipment connecting BTS and MSC.
C. Network Subsystem
The core part of Network Subsystem is MSC (Mobile Switching Center). MSC is similar to a switchboard and take charge of mobility management, location registration and management, authentication, handover and roaming functions. With MSC, HLR (Home Location Register) and VLR (Visitor Location Register) provide call routing and roaming functions.
HLR has subscribers and terminal device location information. Terminal location information is included in Signaling Address of VLR. One HLR is installed on a network as a distributed database. VLR saves a part (current terminal device location) of HLR information, which is related with number control and services. Implementing VLR in MSC is the current trend, though it can be independent.
Another Network Subsystem is EIR (Equipment Identity Register)와 AuC (Authentication Center). EIR is a database storing IMEI information, and verifies each terminal device. If a terminal device is marked stolen, it is not allowed a connection to the network.
AuC, performing authentication and encryption, is a database storing authentication keys on SIM Cards.
- GSM Wireless Link
For GSM900, 890~915㎒ band is allocated to uplink and 935~960㎒ band is to downlink. Data transmission from a terminal device to a base transceiver station is called uplink, and the opposite direction is called downlink. For GSM900, added together, 2 * 25㎒ frequency spectrum band is allocated. For GSM1800, 1710~1785㎒ band is allocated to uplink and 1805~1880㎒ band is to downlink
A. Multi Access and Channel Structure
Since the radio spectrum is a limited resource and has to be shared by a great number of users, GSM chose TDMA as its Multi Access technology. The FDMA part involves the division by frequency of the maximum 25 ㎒ bandwidth into 124 carrier frequencies spaced 200 kHz apart. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a Time Slot (also called a burst period) and it lasts for 15/26 ms. Eight time slots are grouped into a TDMA frame and one logical channel corresponds to one time slot.
In TDMA, channel patters repeat every three hours. The channels used for transmitting voice and data are called Traffic Channels (TCH). TCHs can be defined as 26-frame multi-frame or 26 TDMA Frame Group. The length of 26-frame multi-frame is 120ms. 24 frames out of 26 are for traffic, one is for SACCH (Slow Associated Control Channel), and the one is not used. Since uplink and downlink TCHs are divided by 3 time slots, terminal devices do not need to do send and receive simultaneously, which means simpler electronic design for terminal devices. Full rate TCH and Half race TCH are defined, and latter enables the system doubling its capacity.
B. Power Control
GSM cell phones are classified by its maximum transmission power in 5 classes: 20, 8, 5, 2 and 0.5 watt. A 20W device is called Class 1 and 08 is Class 5. To minimize Co-Channel Interference, base transceiver stations and cell phones are operated with minimum power level of maintaining voice quality. The power level up and down by 2db at a time and adjusted to minimum 13dBm, or 20mW. Cell phones decide when to control the power level by measuring signal strength using BER (Bit Error Rate).