Intro to GSM:• GSM (Global System for Mobile Communication)• It is a standard set developed by the ETSI.• It describe protocols for 2G digital cellular networks.• Full duplex.• GPRS & EDGE facilities.
GSM ArchitectureWhat so ever the variants are the basic architecture of GSM network remains the same as elaborated below:• BSS (Base Station Subsystem)• NSS ( Network Switching Subsystem )• NMS ( Network Management Subsystem )
Handset:• is a device that can make and receive telephone calls• over a radio link• Can moving around a wide geographic area.
SIM:Subscriber identification moduleIt contains• IMSI ( international mobile subscriber identity )• PIN (personal identification number )• PUK ( personal un blocking code )• ICCID ( integrated circuit card identification )
• IMSI ( used by operators to identify user)• IMSI = MCC+MNC+MSINi. MCC ( mobile country code)ii. MNC ( mobile network code)iii. MSIN ( mobile user identification number )• ICCID (SIM is internationally recognized by this number )• Usually printed on SIM
Base Station Subsystem:• The Base Station Subsystem is responsible for managing the radio network, and it is controlled by an MSC.• A BSS itself may cover a considerably large geographical area consisting of many cells (a cell refers to an area covered by one or more frequency resources).
BSS consists of three element• BTS ( Base transceiver station)• BSC ( Base station controller )• TC ( Tran coder)
Base transceiver station: • Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna. • Time and frequency synchronizing • Decoding, decrypting, and equalizing received signals • Random access detection • Uplink channel measurements
Base station controller :• The BSC manages the radio resources for one or more BTSs. It handles radio channel setup, frequency hopping, and handovers.• The BSC is the connection between the mobile and the MSC.• Control of frequency hopping• Performing traffic concentration to reduce the number of lines from the MSC• Reallocation of frequencies among BTSs• Time and frequency synchronization• Power management
Transcoder:• In the air interface (between MS and BTS), the media carrying the traffic is a radio frequency.• To enable an efficient transmission of digital speech information over the air interface, the digital speech signal is compressed.• We must however also be able to communicate with and through the fixed network, where the speech compression format is different.• Somewhere between the BTS and the fixed network, we therefore have to convert from one speech compression format to another, and this is where the Transcoder comes in.
• For transmission over the air interface, the speech signal is compressed by the mobile station to• 13 k bit/s (Full Rate and Enhanced Full Rate),• 5.6 k bit/s (Half Rate),• 12.2 k bit/s (Enhanced Full Rate).• AMR (Adaptive Multirate Coding) which is more flexible since it produces speech with bitrates similar to older solutions but adapted to link conditions.• However, the standard bit rate for speech in the PSTN is 64 Kbits/s. The modulation technique is called "Pulse Code Modulation" (PCM). This requires the GSM network to perform bit rate adaptation of speech.
Network switching subsystem (NSS)NSS typically consists of following elements• MSC (Mobile switching center)• HLR (Home location register )• VLR ( visitor location register )• AUC (Authentication center )• EIR ( Equipment identity register)• GMSC ( Gate way mobile switching center )
HLR:• centralized network database• stores and manages all mobile subscriptions belonging to a specific operator.• a permanent store for subscriber until that subscription is canceled.It stores1. Subscriber identity2. Subscriber supplementary services3. Subscriber location information4. Subscriber authentication information
VLR :• It is a temporary database .• Stores only information of subscriber those are in that MSC.• Every MSC has its own VLR• When a subscriber roams into a new MSC service area, the VLR connected to that MSC requests information about the subscriber from the subscribers HLR.
EIR :• Data base used to store mobile identity• Help us blocking calls from stolen MS
AUC:• AUC is a database connected to the HLR• Function is to authenticate the subscribers attempting to use a network.
MSC & GMSC:• MSC performs the telephony switching functions for the mobile network.• controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN).• In GMSC Gateway functionality enables an MSC to interrogate a networks HLR in order to route a call to a Mobile Station (MS).• Any MSC can be GMSC
NMS:NMS consists of two element1. OMC ( Operation and maintenance center )2. NMC (Network management center )
OMC & NMC• OMC a computerized monitoring & maintenance center which is connected MSCs and BSCs.• OMC is local.• In NMC centralized control of a network is done.• One NMC per network to control many OMCs.• OMC and NMC functionality can be combined in the same physical network node.
GSM GEOGRAPHICAL NETWORKSTRUCTURELocal area (LA)• LA is as a group of cells.• Within the network, a subscriber’s location is known by the LA.• The identity of the LA in which an MS is currently located is stored in the VLR.
MSC SERVICE AREA• An MSC service area is made up of a number of LAs and• MSC use LA to route a call to MS
GMSK:• Gaussian Minimum Shift Keying.• Gaussian Minimum Shift Keying (GMSK) is a form of continuous-phase FSK in which the phase change is changed between symbols to provide a constant envelope. Consequently it is a popular alternative to QPSK.• The RF bandwidth is controlled by the Gaussian low-pass filter.
continuous-phase :continuous-phase• In CPM the carrier phase is modulated in a continuous manner in contrast to QPSK .
• CPM is typically implemented as a constant- envelope waveform.• QPSK requires a relatively large percentage of the power to occur outside of the intended band.• CPM is attractive because the phase continuity yields high spectral efficiency.• The primary drawback is the high implementation complexity required for an optimal receiver.
MSK:• Minimum shifting keying .• It is similar to OQPSK• OQPSK gives us square wave which is difficult to handle.• MSK gives us half cycle sinusoidal wave for a symbol
GMSK:• Gaussian minimum shifting keying• MSK with Gaussian filter.•
Gaussian filter• Gaussian filters are designed to give no overshoot to a step function input while minimizing the rise and fall time.• Gaussian filter has the minimum possible time delay
signal is integrated over time to obtain a continuouswaveform which captures the bit transition information
• Send this wave to IQ modulator .• Integrate out put and send it
ADVANTAGES DISADVANTAGES• High spectral efficiency. • Higher power level than• Reducing sideband QPSK. power. • Requiring more complex• Excellent power channel equalization efficiency due to constant algorithms such as an envelope. adaptive equalizer at the• Good choice for voice receiver. modulation. • Inter symbol interference
Interleaving• Interleaving rearranges a group of bits in a parti cular way.• Improve the performance of the error correction mechanisms• The interleaving decreases the possibility of losing whole bursts during the transmission.