Gsm Specification & Air Interface

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WHAT IS GSM????

GSM: GSM (Global System for Mobile Communications: originally from Groupe Spécial Mobile) is the most popular standard for mobile telephone systems in the world. The GSM Association, its promoting industry trade organization of mobile phone carriers and manufacturers, estimates that 80% of the global mobile market uses the standard.

GSM Specifications-I

RF Spectrum

GSM 900

1. Mobile to BTS (uplink): 890-915 MHz

2. BTS to Mobile (downlink):935-960 MHz

3. Bandwidth: 2* 25 MHz

GSM 1800

1. Mobile to BTS (uplink): 1710-1785 MHz

2. BTS to Mobile (downlink) 1805-1880 MHz

3. Bandwidth: 2* 75 MHz

GSM Specification-II

1. Carrier Separation : 200 KHz

2. Duplex Distance : 45 MHz

3. No. of RF carriers : 124

4. Access Method : TDMA/FDMA

5. Modulation Method: GMSK

6. Modulation data rate: 270.833 Kbps

ADVANTAGES OF GSM:

1. GSM uses radio frequencies efficiently & due to the digital radio path the system tolerates more intercellular disturbances.

2. The average quality of speech achieved is better than in existing analogue systems.

3. Data transmission is supported throughout the system.

4. Speech is encrypted & subscriber information security is guaranteed.

5. Due to the ISDN compatibility, new services are offered compared to analogue systems.

6. International roaming is technically possible within all the countries concerned.

7. The large market toughens the competition & lowers the prices both for investments & usages.

GSM SPECIFICATIONS

The main idea behind the GSM specifications is to define several open interfaces which then are limiting certain parts of the GSM system. Because of this interface, openness the operator maintaining the network may obtain different parts of the network from different GSM network suppliers. Also when an interface is open it defines strictly what is happening through the interface & this in turn strictly defines what kind of actions/procedures/functions should ne implemented between the interfaces.

STRUCTURE OF GSM:

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GSM Specifications:

Before looking at the GSM specifications, it is important to understand the following basic terms:

* Bandwidth —the range of a channel's limits; the broader the bandwidth, the faster data can be sent

* Bits per second (bps) —a single on-off pulse of data; eight bits are equivalent to one byte

* Frequency —the number of cycles per unit of time; frequency is measured in hertz (Hz)

* Kilo (k) —kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second

* Megahertz (MHz) —1,000,000 hertz (cycles per second)

* Milliseconds (ms) —one-thousandth of a second

* Watt (W) —a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specifications and characteristics for GSM.

* Frequency band — the frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).

* Duplex distance —The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.

* Channel separation —The separation between adjacent carrier frequencies. In GSM, this is 200 kHz.

* Modulation —Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).

* Transmission rate —GSM is a digital system with an over-the-air bit rate of 270 kbps.

* Access method —GSM...
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