GSM ( Overview )

GSM Overview

GSM :Global System for Mobile Communications

‫تستخدم شبكة ‪  GSM‬الترددات حول ‪  900 MHz‬أو ‪ 1800MHz‬ و تقسم الى قنوات ‪  , 200KHz‬ على شكل موجات Digital 

*what is difference between Digital and analog ?

*how to convert analog to digital ?

تم تطوير نوع جديد من الشبكة وهو ما يسمى بـ ‪  UMTS‬وهو اختصار لـ‬ Universal Mobile Telecommunication System

‫‪‬هذا النظام يعمل على ‪ 2GHz‬ و يستخدم طريقة‬ اخرى للتشفير (‪ (Codec‬ والتي تسمح بنقل سريع و بكمية كبيرة للمعلومات ( ما يقارب ‪.( 2 MByte/s‬‬

مكونات الشبكة GSM 

‫( ‪Base Transceiver Station ( BTS

كل انتنا او برج توضع على بعد  53 كيلومتر في الأماكن الخالية الى ما يقارب عدة مئات من الأمتار داخل‬ المدينة

‫الانتينة تكون موجهة باتجاه معين حيث تقوم بتغطية المكان بزاوية 120 أو 60 درجة

يتم تغطية المدينة او المحافظة او البلد على شكل Cells على شكل خلايا نحل 

فيغطى البرج او الانتنا المنطقة المراد تغطيتها بالشكبة عن طريق الانتنا بزوايا بين 120 او 60 احيانا 

BTS 

هيا حلقة الوصل بين الموبيل و الشبكة وكيفية اجراء الاتصال 

بعض انواع ال BTS 

‫‪ : Regional ‬المناطق الكبيرة , عند عدد المستخدمين القليل , تسمي الانتينا ‪Macrocell‬‬ 

‫‪ : Suburban ‬أطراف المدن , تسمى الأنتينة ‪Satellite- Cell‬‬

‫‪Urban ‬‬ ‫: المدن , أماكن تواجد عدد كبير من المستخدمين , تسمى الأنتينا ‪Microcell‬‬ 

‫‪ : In-house‬البيوت ومحطات القطارات , أماكن تواجد عدد كبير جدا من المستخدمين , ‪Picocell‬‬

Function Of  (BTS): وظيفة ال 

• Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna.
• Transcoding and rate adaptation
• Time and frequency synchronizing
• Voice through full- or half-rate services
• Decoding, decrypting, and equalizing received signals
• Random access detection
• Timing advances
• Uplin

The Base Station Controller (BSC): وظيفة ال 

• Control of frequency hopping
• Performing traffic concentration to reduce the number of lines from the MSC
• Providing an interface to the Operations and Maintenance Center for the BSS
• Reallocation of frequencies among BTSs
• Time and frequency synchronization
• Power management
• Time-delay measurements of received signals from the MS

MS and the BSS communicate across the Um interface ( air interface or radio link )

The BSS communicates with the Network Service Switching center across the A interface.

GSM - Architecture

 BTS: one BTS covers one cell

 BSC :A group of cells form a Location Area. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs 

MSC: The area covered by one MSC is called the MSC/VLR service area.

PLMN :The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs 

Home Location Register (HLR)

a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription in the form of SIM then all the information about this subscription is registered in the HLR of that operator.

Mobile Services Switching Center (MSC)

The central component of the Network Subsystem is the MSC. The MSC performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others. Every MSC is identified by a unique ID.

Visitor Location Register (VLR)

The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Authentication Center (AUC)

The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel. The AUC protects network operators from different types of fraud found in today's cellular world.

Equipment Identity Register (EIR)

The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS. An IMEI is marked as invalid if it has been reported stolen or is not type approved.

The Operation Support Subsystem(OSS)

The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS).

Here are some of the OMC functions:

• Administration and commercial operation (subscription, end terminals, charging and statistics).
• Security Management.
• Network configuration, Operation and Performance Management.
• Maintenance Tasks.

The operation and Maintenance functions are based on the concepts of the Telecommunication Management Network (TMN) which is standardized in the ITU-T series M.30.

Following is the figure which shows how OMC system covers all the GSM elements.

OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Modulation

The GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method.

Why Modulation is Used?

a modulated signal requires less power to transmit and a smaller antenna due to the higher frequency of the carrier wave. Modulation also lets many different radio, TV, cell phone, etc. users send messages over the airwaves without interfering with each other.

 

 Modulation types
1-BPSK
Binary phase-shift keying
 

( 0-1  into   1-0 ) Change by 90 degree

2- QPSKQuadrature phase-shift keying
Change by 45 degree

Higher-order PSK   Constellation diagram for 8-PSK with Gray coding.

The MSK method. 

Has the advantage of reducing sideband power, which in turn reduces out-of-band interference between signal carriers in adjacent frequency channels    advantage of GMSK is that it can be amplified by a non-linear amplifier and remain undistorted This is because there are no elements of the signal that are carried as amplitude variations importance when using small portable transmitters, such as those required by cellular technology. Non-linear amplifiers are more efficient in terms of the DC power input from the power rails that they convert into a radio frequency signal. This means that the power consumption for a given output is much less, and this results in lower levels of battery consumption; a very important factor for cell phones.  A further advantage of GMSK modulation again arises from the fact that none of the information is carried as amplitude variations. This means that is immune to amplitude variations and therefore more resilient to noise, than some other forms of modulation, because most noise is mainly amplitude based.   Back  GSM 

Access Methods:

  GSM chose a combination of TDMA/FDMA as its method. The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth Because radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible.
GSM chose a combination of TDMA/FDMA as its method. The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth.
One or more carrier frequencies are then assigned to each BS. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception. They are separated in time so that the mobile unit does not receive and transmit at the same time.

 

Transmission Rate:

 

  

Sites : 
http://www.radio-electronics.com
http://www.tutorialspoint.com/gsm

http://en.wikipedia.org