Explain GSM call routing

Explain GSM call routing. OR Call routing in GSM with block diagram.

Human interface is analog. However, the advancement in digital technology makes it very convenient to handle information in digital way.

Digitizer and source coding: The user speech is digitized at 8 KHz sampling rate using Regular Pulse Excited–Linear Predictive Coder (RPE–LPC) with a Long Term Predictor loop where information from previous samples is used to predict the current sample.

Each sample is then represented in signed 13-bit linear PCM value.

This digitized data is passed to the coder with frames of 160 samples where encoder compresses these 160 samples into 260-bits GSM frames resulting in one second of speech compressed into 1625 bytes and achieving a rate of 13 Kbits/sec.

Channel coding: This introduces redundancy into the data for error detection and possible error correction where the gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms).

These 456 bits are divided into eight 57-bit blocks and the result is interleaved amongst eight successive time slot bursts for protection against burst transmission errors.

Interleaving: This step rearranges a group of bits in a particular way to improve the performance of the error-correction mechanisms.

The interleaving decreases the possibility of losing whole bursts during the transmission by dispersing the errors.

Ciphering: This encrypts blocks of user data using a symmetric key shared by the mobile station and the BTS.

Burst formatting: It adds some binary information to the ciphered block for use in synchronization and equalization of the received data.

Modulation: The modulation technique chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK) where binary data is converted back into analog signal to fit the frequency and time requirements for the multiple access rules.

This signal is then radiated as radio wave over the air.

Multipath and equalization: An equalizer is in charge of extracting the ‘right’ signal from the received signal while estimating the channel impulse response of the GSM system and then it constructs an inverse filter.

The received signal is then passed through the inverse filter.

Synchronization: For successful operation of a mobile radio system, time and frequency synchronization are needed.

Frequency synchronization is necessary so that the transmitter and receiver frequency  match (in FDMA) while Time synchronization is necessary to identify the frame boundary and the bits within the frame (in TDMA).

To avoid collisions of burst transmitted by MS with the adjacent timeslot such collisions, the Timing Advance technique is used where frame is advanced in time so that this offsets the delay due to greater distance.

Fig.From speech to radio waves

Using this technique and the triangulation of the intersection cell sites, the location of a mobile station can be determined from within the network.

Example:

The MSISDN number of a subscriber in Bangalore associated with Airtel network is

+919845XYYYYY which is a unique number and understood from anywhere in the world.

Here, + means prefix for international dialing, 91 is the country code for India and 45 is the network operator’s code (Airtel in this case).

X is the level number managed by the network operator ranging from 0 to 9 while YYYYY is the subscriber code which, too, is managed by the operator.

The call first goes to the local PSTN exchange where PSTN exchange looks at the routing table and determines that it is a call to a mobile network.

PSTN forwards the call to the Gateway MSC (GMSC) of the mobile network.

MSC enquires the HLR to determine the status of the subscriber. It will decide whether the call is to be routed or not. If MSC finds that the call can be processed, it will find out the address of the VLR where the mobile is expected to be present.

If VLR is that of a different PLMN, it will forward the call to the foreign PLMN through the Gateway MSC. If the VLR is in the home network, it will determine the Location Area (LA).

Fig. Call routing for a mobile terminating call

 

Within the LA, it will page and locate the phone and connect the call.