Friday, April 22, 2011

2G Second generation network - GSM

2G Second generation network - GSM

Mobile communication of second generation 2G - standard GSM.

2G started in 1991 with a speed for data transmission 9.6 kbit/s. In 1999, in the GSM standard is incorporated, the standard GPRS (considered as intermediate generation - 2.5G), which provides mobile Internet with greater speed.

GSM (group name from Groupe Spécial Mobile, later renamed to Global System for Mobile Communications) is the global standard for digital mobile cellular connection with the TDMA separation of channel and a high degree of safety, thanks to an open encryption key. Developed under the auspices of the European Telecommunications Standards Institute (ETSI) in the 80 years of the twentieth century.

GSM refers to second generation networks (2nd Generation), although in 2006 it entered in a conditional phase 2,5 G and is the most popular standard for mobile cellular connection in the world. Mobile phones are manufactured for use in a range of four frequencies: 850 MHz, 900 MHz, 1800 MHz, 1900 MHz. The GSM standard uses GMSK modulation.

Network Architecture

Base station (BS) is connecting your cell phone with a cellular network. Each base station is called cell, because it has a specific coverage area. All base stations are linked, so you can get a reliable connection when you move from one cell to another. This process is called "transfer" (hand-over). With base stations included in the set of Base Stations Controller (BSC).

The mobile station (terminal) from your cell phone and SIM-module is one type of electronic signature (which includes your subscriber number) to be sent to the nearest cell as an application that you want as a subscriber access to the system. BSC submit the application to the heart of the cellular network - Mobile Switching Center (MSC). MSC takes care of providing a connection to route incoming and outgoing calls to and from fixed or mobile networks. MSC contains a critical component called HLR (Home Location Register) - Internal register (register of inland locations), providing administrative information, identifying you as an individual subscriber. Once receiving a request from the cellular phone, HLR instantly compare specific signature in this application, the data for the subscriber. If your subscription is correct, MSC sent back, via cell phone to your message that you are admitted to network. Usually at this point on the display is written the name of the cellular network. From here onwards can receive and make calls. On receipt of a call, MSC first checks HLR to see your location. At any time, cell phones will send a message to the nearest cell to inform the network where you are. This process is known as polling.

Each base station uses a purely digital technology to support a massive number of subscribers simultaneously connected to a cellular network, but also enable them to make and receive calls. This technique is called multiplexing.

The network is able to redirect your calls to the base station in whose coverage is your phone, even if other subscribers at the same time are connected to it. When you switch to another cell (eg. while driving), HLR information is automatically updated to make accurate routing of signal for your calls.

When you make an outgoing call, another module of the MSC is activated called the VLR (Visitor Location Register) - register which verify that you are actually allowed to make calls. (For example, your customer number may be banned for international calls). VLR response is forwarded back to your phone. Voicemail and the Centre for short messages (SMS) are other services that provide MSC. SMS messages are transmitted on a separate channel of the conversations, and you can receive short messages even when talking.

Radio interfaces of GSM digital cellular network

Interfaces are of two types:
  1. Interface for connection to external networks (signaling system No.7 /SS7/)
    • Abis - interface between BS and BSC;
    • A - interface between BSC and MSC;
    • B - interface between VLR and MSC;
    • C - interface between MSC and HLR;
    • D - interface between VLR and HLR;
    • E - interface between MSC and EIR;
    • F - interface between two stations;
    • Um - Radio Interface between MS and BS.
  2. Species series (burst):
    • Normal (normal burst);
    • To adjust the frequency;
    • For Synchronization;
    • For Access series (shortened series);
    • Blank (deaf serie).
Logical channels in 2G mobile communication - GSM

Logical channelDirection of transmissionType of transmitted information
AGCHMS→BSQuery for outgoing call
RACHBS←MSInformation for the independent channel (channel number, time interval)
SDCCHMS→BSSend the number of the called subscriber
SDCCHBS→MSApplication authentication
SDCCHMS→BSResponses of checking the authenticity
SDCCHBS→MSPermission to access the services of mobile network, appointing a number and time interval for the speech
SDCCHMS→BSDischarging channel
TCHMS↔BSTransmitting Speech
After the call
BSCHBS→MSFrequency correction and synchronization
CCCHMS←BSExchange of business information for control