GSM SYSTEM.

GSM SYSTEM

The GSM Network Architecture
Time division multiple access-TDMA 124 radio carriers, inter carrier spacing 200khz. 890 to 915MHZ mobile to base - upLINK 935 to 960MHZ base to mobile - downLINK 8 channels/carrier GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand. kb/s per carrier GMSK with a time bandwidth product BT =0.3 Slow frequency hoping 217/hops/second. Synchronization compensation for up to 233micro seconds absolute delay Block and convolutional channel coding copuled with interleaving to combat channel perturbations- overall channel rate of 22.8 kb/s Full rate channel 13 kb/s voice coder rate using regular pulse excitation/linear predictive coding RPE/LPC, half rate channel 6.5 kb/s using Vector coder rate using vector sum excited linear predictivie coding VSELP Overall full rate channel bit rate of 22.8 kb/s. Each cell can have from 1 to 16 pairs of carriers.

GSM uses paired radio channels
UPLINK DOWNLINK 890MHz 915MHz 935MHz 960MHz 124 124

The GSM Radio Interface

Concept of TDMA Frames and Channels
GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

GSM System Architecture
PSTN ISDN PDN BSC MS BTS MSC GMSC BTS BSC VLR MS EIR BTS AUC MS HLR

Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book.[1] Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI) and Broadband ISDN (B-ISDN). ISDN is a circuit-switched telephone network system, which also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kilobit/s. A major market application for ISDN in some countries is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions. Channel bonding can achieve a greater data rate; typically the ISDN B-channels of 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.

A public data network is a network established and operated by a telecommunications administration, or a recognized private operating agency, for the specific purpose of providing data transmission services for the public. The public switched telephone network (PSTN) is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiberoptic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables all inter-connected by switching centers which allows any telephone in the world to communicate with any other. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones. The technical operation of the PSTN utilises standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. There is also a single global address space for telephone numbers based on the E.163 and E.164 standards. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.

Cellular Systems The geographic area is divided into cells
Each cell has a Base Station managing the communications A set of cells managed by a single MSC is called Location Area MSC VLR HLR land link land link VLR MSC Base Station Radio link MSC Mobile Switching Center VLR Visitor Location Register HLR Home Location Register

GSM ARCHITECTURE NSS Network and Switching Subsystem PLMN
EIR Equipment Identity Register AuC Authentication Center GMSC Gateway MSC BSS Base Station System BSC Base Station Controller BTS Base Transceiver Station MS Mobile Station SSP Service Switching Point PLMN Databases EIR VLR VLR HLR AuC PSTN MSC MSC GMSC SSP NSS Switches SSP BSS BSC BSS MS BTS Radio Systems

Network and switching subsystem
NSS is the main component of the public mobile network GSM switching, mobility management, interconnection to other networks, system control Components Mobile Services Switching Center (MSC) controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC Databases (important: scalability, high capacity, low delay) Home Location Register (HLR) central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs) Visitor Location Register (VLR) local database for a subset of user data, including data about all user currently in the domain of the VLR

Operation subsystem The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems Components Authentication Center (AUC) generates user specific authentication parameters on request of a VLR authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system Equipment Identity Register (EIR) registers GSM mobile stations and user rights stolen or malfunctioning mobile stations can be locked and sometimes even localized (سياه سفيد خاکستری) Operation and Maintenance Center (OMC) different control capabilities for the radio subsystem and the network subsystem

Mobile Handset Provides access to the GSM n/w Consists of
TEMPORARY DATA PERMANENT DATA - Temporary Subscriber Identity Permanent Subscriber Identity - Current Location Key/Algorithm for Authentication. - Ciphering Data Provides access to the GSM n/w Consists of Mobile equipment (ME) Subscriber Identity Module (SIM)

GSM System Architecture-I
Mobile Station (MS) Mobile Equipment (ME) Subscriber Identity Module (SIM) Base Station Subsystem (BSS) Base Transceiver Station (BTS) Base Station Controller (BSC) Network Switching Subsystem(NSS) Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AUC) Equipment Identity Register (EIR)

System Architecture Mobile Station (MS)
The Mobile Station is made up of two entities: Mobile Equipment (ME) 2. Subscriber Identity Module (SIM)

System Architecture Mobile Station (MS)
Mobile Equipment Portable,vehicle mounted, hand held device Uniquely identified by an IMEI (International Mobile Equipment Identity) Voice and data transmission Monitoring power and signal quality of surrounding cells for optimum handover Power level : 0.8W – 20 W 160 character long SMS.

System Architecture Mobile Station (continue)
Subscriber Identity Module (SIM) Smart card contains the International Mobile Subscriber Identity (IMSI) Allows user to send and receive calls and receive other subscribed services Encoded network identification details - Key Ki,Kc and A3,A5 and A8 algorithms Protected by a password or PIN Can be moved from phone to phone – contains key information to activate the phone

A personal identification number (PIN, pronounced "pin") is a secret numeric password shared between a user and a system that can be used to authenticate the user to the system. Typically, the user is required to provide a non-confidential user identifier or token (the user ID) and a confidential PIN to gain access to the system. Upon receiving the user ID and PIN, the system looks up the PIN based upon the user ID and compares the looked-up PIN with the received PIN. The user is granted access only when the number entered matches with the number stored in the system. Hence, despite the name, a PIN does not personally identify the user.[1]

System Architecture Base Station Subsystem (BSS)
Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers Base Transceiver Station (BTS) Base Station Controller (BSC)

Base Transceiver Station (BTS):
Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna. Frequency hopping Communicates with Mobile station and BSC Consists of Transceivers (TRX) units

Base Station Controller (BSC)
Manages Radio resources for BTS Assigns Frequency and time slots for all MS’s in its area Handles call set up Transcoding and rate adaptation functionality Handover for each MS Radio Power control It communicates with MSC and BTS

System Architecture Network Switching Subsystem(NSS)
Mobile Switching Center (MSC) Heart of the network Manages communication between GSM and other networks Call setup function and basic switching Call routing Billing information and collection Mobility management - Registration - Location Updating - Inter BSS and inter MSC call handoff MSC does gateway function while its customer roams to other network by using HLR/VLR.

System Architecture Network Switching Subsystem …
Home Location Registers (HLR) - permanent database about mobile subscribers in a large service area(generally one per GSM network operator) database contains IMSI,MSISDN,prepaid/postpaid,roaming restrictions,supplementary services. Visitor Location Registers (VLR) Temporary database which updates whenever new MS enters its area, by HLR database Controls those mobiles roaming in its area Reduces number of queries to HLR Database contains IMSI,TMSI,MSISDN,MSRN,Location Area,authentication key

An International Mobile Subscriber Identity or IMSI ( /ˈɪmziː/) is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field in the SIM inside the phone and is sent by the phone to the network. It is also used for acquiring other details of the mobile in the Home Location Register (HLR) or as locally copied in the Visitor Location Register. To prevent eavesdroppers identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead. The IMSI is used in any mobile network that interconnects with other networks, in particular CDMA and EVDO networks as well as GSM networks. This number is provisioned in the phone directly or in the R-UIM card (a CDMA analogue equivalent to a SIM card in GSM). An IMSI is usually presented as a 15 digit long number, but can be shorter. For example MTN South Africa's old IMSIs that are still being used in the market are shown as 14 digits. The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (European standard) or 3 digits (North American standard). The remaining digits are the Mobile Subscription Identification Number (MSIN) within the network's customer base. The IMSI conforms to the ITU E.212 numbering standard.

TMSI The "Temporary Mobile Subscriber Identity" (TMSI) is the identity that is most commonly sent between the mobile and the network. TMSI is randomly assigned by the VLR to every mobile in the area, the moment it is switched on. The number is local to a location area, and so it has to be updated each time the mobile moves to a new geographical area. The network can also change the TMSI of the mobile at any time. And it normally does so, in order to avoid the subscriber from being identified, and tracked by eavesdroppers on the radio interface. This makes it difficult to trace which mobile is which, except briefly, when the mobile is just switched on, or when the data in the mobile becomes invalid for one reason or another. At that point, the global "international mobile subscriber identity" (IMSI) must be sent to the network. The IMSI is sent as rarely as possible, to avoid it being identified and tracked. A key use of the TMSI is in paging a mobile. "Paging" is the one-to-one communication between the mobile and the base station. The most important use of broadcast information is to set up channels for "paging". Every cellular system has a broadcast mechanism to distribute such information to a plurality of mobiles. Size of TMSI is 4 octet with full hex digits and can't be all 1 because the SIM uses 4 octets with all bits equal to 1 to indicate that no valid TMSI is available.[1]

MSISDN MSISDN is a number uniquely identifying a subscription in a GSM or a UMTS mobile network. Simply put, it is the telephone number of the SIM card in a mobile/cellular phone. This abbreviation has several interpretations, the most common one being "Mobile Subscriber Integrated Services Digital Network Number".[1] The MSISDN together with IMSI are two important numbers used for identifying a mobile subscriber. The latter identifies the SIM, i.e. the card inserted in to the mobile phone, while the former is used for routing calls to the subscriber. IMSI is often used as a key in the HLR ("subscriber database") and MSISDN is the number normally dialed to connect a call to the mobile phone. A SIM is uniquely associated to an IMSI, while the MSISDN can change in time (e.g. due to number portability), i.e. different MSISDNs can be associated to the SIM. The MSISDN follows the numbering plan defined in the ITU-T recommendation E.164.

[edit] Abbreviation Depending on source or standardization body, the abbreviation MSISDN can be written out in several different ways. These are today the most widespread and common in use. An MSISDN is limited to 15 digits, prefixes not included (e.g., 00 prefixes an international MSISDN when dialing from Sweden). MSISDN - Mobile Station International Subscriber Directory Number In GSM and its variant DCS 1800, MSISDN is built up as MSISDN = CC + NDC + SN CC = Country Code NDC = National Destination Code, identifies one or part of a PLMN SN = Subscriber Number In the GSM variant PCS 1900, MSISDN is built up as MSISDN = CC + NPA + SN CC = Country Code NPA = Number Planning Area SN = Subscriber Number [edit] Example MSISDN: CC 380 Ukraine NDC 56 Dnipropetrovsk SN Subscriber's number For further information on the MSISDN format, see the ITU-T specification E.164.

MSRN - Mobile Station Roaming Number
The Mobile Station Roaming Number is an E.214 defined telephone number used to route telephone calls in a mobile network from a GMSC (Gateway Mobile Switching Centre) to the target MSC (see Network Switching Subsystem). It can also be defined as a directory number temporarily assigned to a mobile for a mobile terminated call. A MSRN is assigned for every mobile terminated call, not only the calls where the terminating MS lives on a different MSC than the originating MS. Although this seems unnecessary since many vendors' VLR's are integrated with the MSC, the GSM specification indicates that the MSC and VLR (Visitor Location Register) do not need to reside on the same switch. They are considered two different nodes as they have their own routing addresses. i.e.the MSRN is one of the returned parameters into SRI_ACK message. In particular the MSRN is used into an MNP scenario (in this case it can be modified as 'RgN + MSISDN'). Another temporary address that hides the identity of a subscriber. The VLR generates this address on request from the MSC,and the address is also stored in the HLR. MSRN contains the current visitor country code(VCC), the visitor national destination code (VNDC), the identification of the current MSC together with the subscriber number. If we have all the MSC working as a GMSC like the latest technologies so what would be the states of the MSRN ? we can use it only for test to route the calls to a specific MSC otherwise we don't need it to use it.

System Architecture Network Switching Subsystem …
Authentication Center (AUC) Protects against intruders in air interface Maintains authentication keys and algorithms and provides security triplets ( RAND,SRES,Kc) Generally associated with HLR Equipment Identity Register (EIR) - Database that is used to track handsets using the IMEI (International Mobile Equipment Identity) Made up of three sub-classes: The White List, The Black List and the Gray List Only one EIR per PLMN

GSM Specifications-1 RF Spectrum GSM 900
Mobile to BTS (uplink): Mhz BTS to Mobile(downlink): Mhz Bandwidth : 2* 25 Mhz GSM 1800 Mobile to BTS (?uplink): Mhz BTS to Mobile(?downlink) Mhz Bandwidth : 2* 75 Mhz

GSM Specification-II Carrier Separation : 200 Khz
Duplex Distance : 45 Mhz No. of RF carriers : 124 Access Method : TDMA/FDMA Modulation Method : GMSK Modulation data rate : Kbps

GSM Operation Radio Interface Demodulation Speech decoding
Channel decoding De-interleaving Burst Formatting De-ciphering Demodulation Modulation Ciphering Interleaving Channel Coding Speech coding Radio Interface Speech 13 Kbps 22.8 Kbps 33.6 Kbps Kbps

*A brief Overview of the GSM Radio Interface, Thierry Turletti
Complete GSM system *A brief Overview of the GSM Radio Interface, Thierry Turletti

GSM - TDMA/FDMA higher GSM frame structures GSM TDMA frame 1 2 3 4 5 6
MHz 124 channels (200 kHz) downlink frequency MHz 124 channels (200 kHz) uplink higher GSM frame structures time GSM TDMA frame 1 2 3 4 5 6 7 8 4.615 ms Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below. Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes: Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors Class II 78 bits - least sensitive to bit errors Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps. To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples. Recall that each time-slot burst is transmitted at a gross bit rate of kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. GSM time-slot (normal burst) guard space guard space tail user data Training S 3 bits 57 bits 26 bits 1 3 546.5 µs 577 µs

GSM multiple access scheme = FDMA + TDMA
120ms multiframe burst 1 burst 2 burst 8 …… 200kHz 890MHz carrier 1 carrier 2 guard band 915MHz carrier 124 frame 1 frame 2 …... frame 3 frame 26 control frames Uplink spectrum 0.577ms *Wireless Communications, T. Rappaport, Prentice Hall Communications Engineering and Emerging Tech. Series

Normal Burst Structure in GSM
Total number of bits in one GSM burst= 2*(3+57+1)+26=148 bits Out of 148 bits, 114 bits are pure data bits; 114*(260/456)=65 bits are true information bits So, the transmission efficiency is 65/148=44% *Principles & Applications of GSM, Vijay K. Garg, Prentice Hall Communications Engineering and Emerging Tech. Series

Physical Channel

GSM-Frame Structure

GSM delays uplink TDMA frames
The start of the uplink TDMA is delayed of three time slots TDMA frame (4.615 ms) Downlink TDMA F1MHz R1 R2 R3 R5 R6 R7 R4 R8 Uplink TDMA Frame F1 + 45MHz T1 T2 T3 T5 T6 T7 T4 T8 The start of the uplink TDMA frame is delayed with respect to downlink by a fixed period of three timeslots. Why ? Staggering TDMA frames allows the same timeslot number to be used in both the down and uplink while avoiding the requirement for mobile to transmit and receive simultaneously. Between T and R the MS is in the IDLE mode, makes measurement of signal strength of neighboring cells. R T R T Fixed transmit Delay of three time-slots

DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms
3 57 1 26 8.25 NORMAL BURST - NB 3 142 8.25 FREQUENCY CORRECTION BURST - FB 3 8.25 39 64 SYNCHRONISATION BURST - SB Normal burst 148 bits guard bits Frequency correction burst 148 bits guard bits Synchronizing burst 148 bits guard bits Access burst 88 bits guard bits used to access a cell for the first time in case of a call set up or handover The data structure within a normal burst consists of 148 bits transmitted at a rate of kb/s. Each burst in GSM system modulates one of the carriers assigned to a particular cell using GMSK. 3 6 41 36 68.25 ACCESS BURST - AB TAIL BIT ENCRYPTION BIT GUARD PERIOD TRAINING BITS MIXED BITS SYNCHRONISATION BITS FIXED BITS FLAG BITS

HIERARCHY OF FRAMES 1 HYPER FRAME = 2048 SUPERFRAMES = TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS ) TRAFFIC CHANNELS 1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S ) LEFT (OR) RIGHT 1 SUPER FRAME = 51 MULTI FRAMES SIGNALLING CHANNELS 1 SUPER FRAME = 26 MULTI FRAMES 1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms ) 1 MULTI FRAME = 51 TDMA FRAMES ( ms ) Speech in GSM is digitally coded at a rate of 13 kbps, so-called full-rate speech coding. This is quite efficient compared with the standard ISDN rate of 64 kbps. One of the most important Phase 2 additions will be the introduction of a half-rate speech codec operating at around 7 kbps, effectively doubling the capacity of a network. This 13 kbps digital stream (260 bits every 20 ms) has forward error correction added by a convolutional encoder. The gross bit rate after channel coding is 22.8 kbps (or 456 bits every 20 ms). These 456 bits are divided into 8 57-bit blocks, and the result is interleaved amongst eight successive time slot bursts for protection against bursty transmission errors. Each time slot burst is bits and contains two 57-bit blocks, and a 26-bit training sequence used for equalization. A burst is transmitted in ms for a total bit rate of kbps, and is modulated using Gaussian Minimum Shift Keying (GMSK) onto the 200 kHz carrier frequency. The 26-bit training sequence is of a known pattern that is compared with the received pattern in the hope of being able to reconstruct the rest of the original signal. Forward error control and equalization contribute to the robustness of GSM radio signals against interference and multipath fading. The digital TDMA nature of the signal allows several processes intended to improve transmission quality, increase the mobile's battery life, and improve spectrum efficiency. These include discontinuous transmission, frequency hopping and discontinuous reception when monitoring the paging channel. Another feature used by GSM is power control, which attempts to minimize the radio transmission power of the mobiles and the BTS, and thus minimize the amount of co-channel interference generated. (4.615ms) TDMA FRAME NO. 1 1 TIME SLOT = BITS ( ms) (4.615 ms) 1 1 bit =36.9 micro sec

GSM Frame Full rate channel is idle in 25 SACCH is transmitted in frame 12 0 to 11 and 13 to 24 Are used for traffic data Frame duration = 120ms Frame duration = 60/13ms The full rate TCH uses 24 out of the 26 available in the multiframe The duration of the multiframe is therefore 26X60/13ms = 120ms At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications. Frame duration = 15/26ms 3 57 1 26 8.25

114 bits are available for data transmission.
The training sequence of 26 bits in the middle of the burst is used by the receiver to synchronize and compensate for time dispersion produced by multi-path propagation. 1 stealing bit for each information block (used for FACCH) The GSM System uses a frame structure where each frame consist of 8 time slots, and each time slot contains bits, and data is transmitted at kbps in the channel. Distinct training sequences will therefore be allocated to channels using the same frequencies in cells which are close enough to interfere with one another.

…Example Time duration of a bit Time duration of a slot
Time duration of a frame and How long must a user occupying a single slot must wait between two simultaneous transmissions?

Solution Time duration of a bit Time duration of a frame

Time duration of a Slot A user has to wait ms before next transmission

Example If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame efficiency Solution Time slots have /58 = bits. A frame has 8 * = 1250 bits / frame.

…Example bOH = 8(6) + 8(8.25) + 8(26) = 322 bits
The number of overhead bits per frame is given by bOH = 8(6) + 8(8.25) + 8(26) = 322 bits Frame efficiency = (1250 – 322 ) / = %

Logical Channels Half rate 11.4kbps Speech TCH (traffic)
Full rate 22.8kbps 2.4 kbps Data 4.8 kbps 9.6 kbps FCCH(Frequency correction) BCH SCH(Synchronization) PCH(Paging) CCCH RACH(Random Access) CCH (control) AGCH(Access Grant) SDCCH(Stand Alone) Dedicated SACCH(Slow-associated) FACCH(Fast-associated)

LOGICAL CHANNELS TRAFFIC SIGNALLING FULL RATE Bm 22.8 Kb/S HALF RATE
Lm Kb/S BROADCAST COMMON CONTROL DEDICATED CONTROL FCCH SCH BCCH RACH PCH AGCH FCCH -- FREQUENCY CORRECTION CHANNEL SCH SYNCHRONISATION CHANNEL BCCH -- BROADCAST CONTROL CHANNEL PCH PAGING CHANNEL RACH -- RANDOM ACCESS CHANNEL AGCH -- ACCESS GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL SDCCH SACCH FACCH DOWN LINK ONLY BOTH UP & DOWNLINKS UPLINK ONLY

Broadcast Channel - BCH
Broadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cells Frequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronization Synchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver

Common Control Channel - CCH
Paging channel (PCH) is a base to mobile channel used to alert a mobile to a call originating from the network Random access channel (RACH) is a mobile to base channel used to request for dedicated resources Access grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)

Dedicated Control Channel - DCCH
Stand-alone dedicated control channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information

Dedicated Control Channel - DCCH
Slow associated control channel (SACCH) is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channel Fast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH

Um logical channels Um logical channel types are outlined in GSM Broadly speaking, non-GPRS Um logical channels fall into three categories: traffic channels, dedicated control channels and non-dedicated control channels. [edit] Traffic channels (TCH) These point-to-point channels correspond to the ISDN B channel and are referred to as Bm channels. Traffic channels use 8-burst diagonal interleaving with a new block starting on every fourth burst and any given burst containing bits from two different traffic frames. This interleaving pattern makes the TCH robust against single-burst fades since the loss of a single burst destroys only 1/8 of the frame's channel bits. The coding of a traffic channel is dependent on the traffic or vocoder type employed, with most coders capable of overcoming single-burst losses. All traffic channels use a 26-multiframe TDMA structure. [edit] Full-rate channels (TCH/F) A GSM full rate channel uses 24 frames out of a 26-multiframe. The channel bit rate of a full-rate GSM channel is 22.7 kbit/s, although the actual payload data rate is kbit/s, depending on the channel coding. This channel is normally used with the GSM Full Rate, GSM Enhanced Full Rate or GSM Adaptive Multi-Rate speech codec. It can also be used for fax and Circuit Switched Data.

[edit] Half-rate channels (TCH/H)
A GSM half rate channel uses 12 frames out of a 26-multiframe. The channel bit rate of a half-rate GSM channel is 11.4 kbit/s, although the actual data capacity is kbit/s, depending on the channel coding. This channel is normally used with the GSM Half Rate or GSM Adaptive Multi-Rate speech codec. [edit] Dedicated Control Channels (DCCHs) These point-to-point channels correspond to the ISDN D channel and are referred to as Dm channels. [edit] Standalone Dedicated Control Channel (SDCCH) The SDCCH is used for most short transactions, including initial call setup step, registration and SMS transfer. It has a payload data rate of 0.8 kbit/s. Up to eight SDCCHs can be time-multiplexed onto a single physical channel. The SDCCH uses 4-burst block interleaving in a 51-multiframe. [edit] Fast Associated Control Channel (FACCH) The FACCH is always paired with a traffic channel. The FACCH is a blank-and-burst channel that operates by stealing bursts from its associated traffic channel. Bursts that carry FACCH data are distinguished from traffic bursts by stealing bits at each end of the midamble. The FACCH is used for in-call signaling, including call disconnect, handover and the later stages of call setup. It has a payload data rate of 9.2 kbit/s when paired with a full-rate channel (FACCH/F) and 4.6 kbit/s when paired with a half-rate channel (FACCH/H). The FACCH uses the same interleaving and multiframe structure as its host TCH.

[edit] Slow Associated Control Channel (SACCH)
Every SDCCH or FACCH also has an associated SACCH. Its normal function is to carry system information messages 5 and 6 on the downlink, carry receiver measurement reports on the uplink and to perform closed-loop power and timing control. Closed loop timing and power control are performed with a physical header at the start of each L1 frame. This 16-bit physical header carries actual power and timing advance settings in the uplink and ordered power and timing values in the downlink. The SACCH can also be used for in-call delivery of SMS. It has a payload data rate of kbit/s, depending on the channel with which it is associated. The SACCH uses 4-burst block interleaving and the same multiframe type as its host TCH or SDCCH. [edit] Common Control Channels (CCCHs) These are unicast and broadcast channels that do not have analogs in ISDN. These channels are used almost exclusively for radio resource management. The AGCH and RACH together form the medium access mechanism for Um. [edit] Broadcast Control Channel (BCCH) The BCCH carries a repeating pattern of system information messages that describe the identity, configuration and available features of the BTS. BCCH brings the measurement reports it bring the information about LAI And CGI BCCH frequency are fixed in BTS

[edit] Synchronization Channel (SCH)
The SCH transmits a Base station identity code and the current value of the TDMA clock. SCH repeats on every 1st, 11th, 21st, 31st and 41st frames of the 51 frame multi frame. So there are 5 SCH frames in a 51 frame multiframe. [edit] Frequency Correction Channel (FCCH) The FCCH generates a tone on the radio channel that is used by the mobile station to discipline its local oscillator. FCCH will repeat on every 0th, 10th, 20th, 30th and 40th frames of the 51 frame multiframe. So there are 5 FCCH frames in a 51 frame multiframe. [edit] Paging Channel (PCH) The PCH carries service notifications (pages) to specific mobiles sent by the network. A mobile station that is camped to a BTS monitors the PCH for these notifications sent by the network. [edit] Access Grant Channel (AGCH) The AGCH carries BTS responses to channel requests sent by mobile stations via the Random Access Channel. [edit] Random Access Channel (RACH) The RACH is the uplink counterpart to the AGCH. The RACH is a shared channel on which the mobile stations transmit random access bursts to request channel assignments from the BTS.

[edit] Allowed channel combinations
The multiplexing rules of GSM allow only certain combinations of logical channels to share a physical channel. The allowed combinations for single-slot systems are listed in GSM Section Additionally, only certain of these combinations are allowed on certain timeslots or carriers and only certain sets of combinations can coexist in a given BTS. These restrictions are intended to exclude non-sensical BTS configurations and are described in GSM Section 6.5. The most common combinations are: Combination I: TCH/F + FACCH/F + SACCH. This combination is used for full rate traffic. It can be used anywhere but C0T0. Combination II: TCH/H + FACCH/H + SACCH. This combination is used for half rate traffic when only one channel is needed. It can be used anywhere but C0T0. Combination III: 2 TCH/H + 2 FACCH/H + 2 SACCH. This combination is used for half rate traffic. It can be used anywhere but C0T0. Combination IV: FCCH + SCH + BCCH + CCCH. This is the standard C0T0 combination for medium and large cells. It can be used only on C0T0. Combination V: FCCH + SCH + BCCH + CCCH + 4 SDCCH + 2 SACCH. [(5x1)+(5x1)+(1x4)+(3x4)+(4x4)+(2x4)+1idle=51frame multiframe] This is the typical C0T0 combination for small cells, which allows the BTS to trade unnecessary CCCH capacity for a pool of 4 SDCCHs. It can be used only on C0T0. Combination VI: BCCH + CCCH. This combination is used to provide additional CCCH capacity in large cells. It can be used on C0T2, C0T4 or C0T6. Combination VII: 8 SDCCH + 4 SACCH.[(8x4)+(4x4)+3idle=51frame multiframe] This combination is used to provide additional SDCCH capacity in medium and large cells. It can be used anywhere but C0T0.

Call Routing Call Originating from MS Call termination to MS

Outgoing Call MS sends dialled number to BSS
BSS sends dialled number to MSC 3,4 MSC checks VLR if MS is allowed the requested service.If so,MSC asks BSS to allocate resources for call. MSC routes the call to GMSC GMSC routes the call to local exchange of called user 7, 8, 9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS

Incoming Call Calling a GSM subscribers Forwarding call to GSMC
Signal Setup to HLR 5. Request MSRN from VLR Forward responsible MSC to GMSC Forward Call to current MSC 9. Get current status of MS 11. Paging of MS 13. MS answers 15. Security checks 17. Set up connection

Handovers Between 1 and 2 – Inter BTS / Intra BSC Between 1 and 3 –
Inter BSC/ Intra MSC Between 1 and 4 – Inter MSC

LOGICAL CHANNELS TRAFFIC SIGNALLING FULL RATE Bm 22.8 Kb/S HALF RATE
Lm Kb/S BROADCAST COMMON CONTROL DEDICATED CONTROL FCCH SCH BCCH RACH PCH AGCH FCCH -- FREQUENCY CORRECTION CHANNEL SCH SYNCHRONISATION CHANNEL BCCH -- BROADCAST CONTROL CHANNEL PCH PAGING CHANNEL RACH -- RANDOM ACCESS CHANNEL AGCH -- ACCESS GRANTED CHANNEL SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL SACCH -- SLOW ASSOCIATED CONTROL CHANNEL FACCH -- FAST ASSOCIATED CONTROL CHANNEL SDCCH SACCH FACCH DOWN LINK ONLY BOTH UP & DOWNLINKS UPLINK ONLY

Location update from the mobile
Mobile looks for BCCH after switching on RACH send channel request AGCH receive SDCCH SDCCH request for location updating SDCCH authenticate SDCCH authenticate response SDCCH switch to cipher mode SDCCH cipher mode acknowledge When a mobile station is first switched on it is necessary to read the BCCH in order to determine its orientation within the network. The mobile must first synchronize in frequency and then in time. The FCCH, SCH and BCCH are all transmitted on the same carrier frequency which has a higher power density than any of the other channels in a cell because steps are taken to ensure that it is transmitted information at all times. The mobile scans around the available frequencies, picks the strongest and then selects the FCCH. Fc+67.7kHz SDCCH allocate TMSI SDCCH acknowledge new TMSI SDCCH switch idle update mode

Call establishment from a mobile
Mobile looks for BCCH after switching on RACH send channel request AGCH receive SDCCH SDCCH send call establishment request SDCCH do the authentication and TMSI allocation SDCCH send the setup message and desired number SDCCH require traffic channel assignment FACCH switch to traffic channel and send ack (steal bits) FACCH receive alert signal ringing sound FACCH receive connect message FACCH acknowledge connect message and use TCH TCH conversation continues

Call establishment to a mobile
Mobile looks for BCCH after switching on Mobile receives paging message on PCH Generate Channel Request on RACH Receive signaling channel SDCCH on AGCH Answer paging message on SDCCH Receive authentication request on SDCCH Authenticate on SDCCH Receive setup message on SDCCH Receive traffic channel assignment on SDCCH FACCH switch to traffic channel and send ack (steal bits) Receive alert signal and generate ringing on FACCH Receive connect message on FACCH FACCH acknowledge connect message and switch to TCH

Security in GSM On air interface, GSM uses encryption and TMSI instead of IMSI. SIM is provided 4-8 digit PIN to validate the ownership of SIM 3 algorithms are specified : - A3 algorithm for authentication - A5 algorithm for encryption - A8 algorithm for key generation

Authentication in GSM

Key generation and Encryption

9. GPRS General Packet Radio Service: allocate more than one slot
Rx 935MHz Tx 890MHz Time TDMA frame Half duplex: emit then transmit (Class 1-12) Full duplex: simulateous emit/transmit (Class 19-29) Much more complex mobile phone Improves data rate up to 4x10kbit/s

9. GPRS GSM radio compatibility (Low cost)
Reliability Classes : Loss probability Delay classes: <0,5s, <5s, <50s, no limit Data rate classes 1 to 18 (0,22bit/s up to 111Kb/s) Question: GSM data rate with 8 slots: only 80kb/s 4 different coding scheme 4 different data rate Strong protection Weak protection

And after? 9. GPRS Extended GSM life following 2001 telecom crash
Unexpected SMS success 2004: GPRS proposed in some towns for high speed data exchange (Pictures) Rising importance of Wi-Fi for data exchange (2.45GHz) GPRS could still be used for low resolution video GSM/GPRS network could survive until 2010

Characteristics of GSM Standard
Fully digital system using 900,1800 MHz frequency band. TDMA over radio carriers(200 KHz carrier spacing. 8 full rate or 16 half rate TDMA channels per carrier. User/terminal authentication for fraud control. Encryption of speech and data transmission over the radio path. Full international roaming capability. Low speed data services (upto 9.6 Kb/s). Compatibility with ISDN. Support of Short Message Service (SMS).

Advantages of GSM over Analog system
Capacity increases Reduced RF transmission power and longer battery life. International roaming capability. Better security against fraud (through terminal validation and user authentication). Encryption capability for information security and privacy. Compatibility with ISDN,leading to wider range of services

Disadvantages of GSM No full ISDN bandwidth of 64 kbit/s to the user
Reduced concentration while driving Electromagnetic radiation Abuse of private data possible High complexity of the system Several incompatibilities within the GSM standards

Future Of GSM 2nd Generation GSM -9.6 Kbps (data rate)
2.5 Generation ( Future of GSM) HSCSD (High Speed ckt Switched data) Data rate : 76.8 Kbps (9.6 x 8 kbps) GPRS (General Packet Radio service) Data rate: Kbps EDGE (Enhanced data rate for GSM Evolution) Data rate: Kbps (max) 3 Generation WCDMA(Wide band CDMA) Data rate : – 2.0 Mbps

Bearer Services Unified Messaging Services(UMS)
Include various data services for information transfer between GSM and other networks like PSTN, ISDN etc at rates from 300 to 9600 bps Short Message Service (SMS) up to 160 character alphanumeric data transmission to/from the mobile terminal Unified Messaging Services(UMS) Group 3 fax Voice mailbox Electronic mail

Supplementary Services
Call related services : Call Waiting- Notification of an incoming call while on the handset Call Hold- Put a caller on hold to take another call Call Barring- All calls, outgoing calls, or incoming calls Call Forwarding- Calls can be sent to various numbers defined by the user Multi Party Call Conferencing - Link multiple calls together CLIP – Caller line identification presentation CLIR – Caller line identification restriction CUG – Closed user group

GSM speech coding

Transmit Path To Channel Coder 13Kbps To Channel Coder 13Kbps
BS Side 8 bit A-Law to 13 bit Uniform RPE/LTP speech Encoder 8 K sps To Channel Coder 13Kbps MS Side RPE/LTP speech Encoder 8 K sps, LPF A/D To Channel Coder 13Kbps Sampling Rate - 8K Encoding - 13 bit Encoding (104 Kbps) RPE/LTP - Regular Pulse Excitation/Long Term Prediction RPE/LTP converts the 104 Kbps stream to 13 Kbps

GSM Speech Coding GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employed by current telephone systems for multiplexing voice lines over high speed trunks and is pulse coded modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link.

GSM Speech Coding Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. Regular pulse excited -- linear predictive coder (RPE--LPC) with a long term predictor loop is the speech coding algorithm. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop. Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding. Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate.

The 260 bits are divided into three classes:
Class Ia 50 bits - most sensitive to bit errors. Class Ib 132 bits - moderately sensitive to bit errors. Class II 78 bits - least sensitive to bit errors. Class Ia bits have a 3 bit cyclic redundancy code added for error detection = 50+3 bits. 132 class Ib bits with 4 bit tail sequence = = 136. Class Ia + class Ib = =189, input into a 1/2 rate convolution encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolution encoder thus outputs 378 bits, to which are added the 78 remaining class II bits. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolution encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples. 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1 3 57 bits 26 1

Bearer Services Telecommunication services to transfer data between access points Specification of services up to the terminal interface (OSI layers 1-3) Different data rates for voice and data (original standard) Data service Synchronous: 2.4, 4.8 or 9.6 kbit/s Asynchronous: bit/s

Tele Services Telecommunication services that enable voice communication via mobile phones. All these basic services have to obey cellular functions, security measurements etc. Offered services. Mobile telephony primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz. Emergency number common number throughout Europe (112); Mandatory for all service providers; Free of charge; Connection with the highest priority (preemption of other connections possible). Multinumbering several ISDN phone numbers per user possible.

Performance characteristics of GSM
Communication mobile, wireless communication; support for voice and data services Total mobility international access, chip-card enables use of access points of different providers Worldwide connectivity one number, the network handles localization High capacity better frequency efficiency, smaller cells, more customers per cell High transmission quality high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains) Security functions access control, authentication via chip-card and PIN

Analog to Digital converter
4. Voice Sampling 104 Kbit/s Amplifier Gain control 8000Hz Micro Serial data Analog to Digital converter 13bit Filter 300Hz-3KHz Gain Clock No need to shout, their is an automatic gain control High data rate not compatible with GSM

4. Voice Sampling Data rate higher than allowed by GSM
Voltage 125µs Sound 0.7 FF 0.6 80 0.5 00 Sampled data Time Data rate higher than allowed by GSM Needs for data compression

5. Voice Compression Short term prediction (8 parameters, 36 bits)
Long Term Prediction (8 parameters, 26 bits) F0 Regular Pulse Excitation (60 parameters, 188 bits) 2080 bits Parameter computing 260 bits Compression 1/10 20 ms of sampled signal

5. Voice Compression Short term prediction 20ms
Linear Predictive Coder for short term (20ms) prediction of the sound

5. Voice Compression Short term prediction
Fréquence (Hz) 1000 2000 Energie Filtre 1/(1-a0z-1-a1z-2) Filtre 1/(1-a0z-1-a1z-2- a2z-3-a3z a7z-8) In GSM: 8th order polynom for short term prediction

5. Voice Compression Filter 1/(1-bz-N) F0
Long term prediction: accounts for the variations of F0 N on 7 bits b on 2 bits Filter 1/(1-bz-N) F0 Updated 4 times in the 20ms period (4x9=36 bits)

5. Voice Compression Encryption 260 bits 50 b 132 b 132 b 50 b 3 132 b
filter coefs block amplitude LTP params RPE pointers RPE pulses 2nd LTP params 2nd RPE pulses 2nd filter params 50 b 3 132 b 4 132 b convolution 114 b 114 b 114 b 114 b

5. Voice Compression Is HR a real high resolution? Still 13 bits
Still Hz filtering Still 8000Hz sampling but… Enhanced full rate Specific hardware at base station to improve error recovery

Speech Encoder LPC Analysis LPF Decimation Conversion to bits Status
Voice Status C algorithm compiled using MS Visual C References

Interleaving T=Output Burst number {1-4} t=the bit number in T’th burst {1-114} B= tx_encoded number {1-2} b= the bit number in B’th tx_encoded {1-456} *Principles & Applications of GSM, Vijay K. Garg, Prentice Hall Communications Engineering and Emerging Tech. Series

Input : 260 bits Output: 456 bits
Channel Encoding in GSM Input : 260 bits Output: 456 bits Parity Encoder Generator Polynomial: G(x)=x3+x+1  G={ } Convolution Encoder : Rate=1/2 Constrained length=5 C2k=bk bk bk-4 C2k+1=bk bk bk bk-4 =Mod 2 addition k {0,1,2,3,…..189} and bk=0 for k<0

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