Wireless Data Communication Networks Lecture 19: Introduction to GSM Florida Institute of Technology ECE Department Outline Brief history GSM network architecture Elements of the air interface organization Florida Institute of Technology ECE Department Page 2 Global System for Mobile Communication Most popular cellular standard Started as Groupe Special Mobile 2nd generation global digital standard Important time stamps: o 1982 CEPT created Groupe Special Mobile o 1987 MoU – 13 European countries Official GSM logo o 1989 Transferred to ETSI o 1991 Publishing of Phase 1 specs o 1991 Radiolinia Finland – first GSM network o 1993 Telstra Australia – first non-European GSM network o 1995 US implements GSM in PCS band o 1996 Russia and China implement GSM o Since 1996 tremendous growth all over the world Florida Institute of Technology ECE Department Page 3 GSM in the World GSM currently holds about 80% of the market share worldwide Florida Institute of Technology ECE Department Page 4 GSM Network Layout MSC Area BSS BSS BTS Gateway MSC NSS BSC TRAU PSTN MSC VLR BTS BTS BSS HLR MSC Area MSC Area PLMN - Public Land Mobile Network GSM architecture is first standardized cellular system architecture GSM network can be divided into three main subsystems: Base Station Subsystem - BSS Mobile Station Subsystem - MSS Network Switching Subsystem - NSS Florida Institute of Technology ECE Department Page 5 GSM Network Components VLR BTS D B BSC HLR C MSC BTS AUC BSC EIR E BTS F MSC VLR BTS PSTN Um Interface B,C,D,E,F - MAP Interfaces Florida Institute of Technology A-Bis Interface A Interface BTS BSC MSC VLR HLR AUC EIR - Network has many functional components Components are integrated through a network protocol – MAP Majority of GSM interfaces are standardized – allows interworking of equipment from different vendors Major equipment vendors: Ericsson, Nokia-Siemens, Alcatel-Lucent, Huawei, Motorola, etc. Base Station Base Station Contoller Mobile Sw itching Center Visitor Location Register Home Location Register Authentification Center Equipment Identity Register ECE Department Page 6 Overview of GSM - Mobile Station Offered as a phone for voice services Data services will bring new devices to the market CONTROL SECTION ANTENNA ASSEMBLY TRANSCEIVER UNIT Receive Audio Signal Processing Display Channel Encoding Interleaving Message Generator Ciphering RF Processing Antenna RECEIVER Duplexer Two functional parts: HW/SW radio interface SIM SIM Control Keyboard Transmit Audio Signal Processing Channel Decoding Deinterleaving Message Regenerator Ciphering RF Processing TRANSMITTER GSM Mobile Architectural Diagram • Two types of SIM (Subscriber Identity Module) – Smart Card – Plug-in Florida Institute of Technology ECE Department Page 7 Overview of GSM - Base Transceiver Station BTS is a set of transceivers (TX/RX). GSM BTS can host up to 16 TX/RX. In GSM one TX/RX is shared by 8 users. The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side. Depending on the application, it can be configured as megacell, macrocell, microcell, pico and femto cell, omni, sectored, etc. BTS Typical BTS installations Florida Institute of Technology ECE Department Page 8 Overview of GSM - Base Station Controller Provides a small digital exchange with some mobility tasks Connects to one or several BTS on the Abis Interface Database Abis Interface Towards BTS Connects to the MSC on the A Interface Designed to offload most of the radio link related processes from the MSC Provides clock distribution to BTS Communicates with the OMC Florida Institute of Technology Switch Matrix A Interface Towards MSC Central Module OMC OMC - Operation and Maintenance Center In GSM, BSC handles majority RRM functions • Handoff • Power control • Channel assignment ECE Department Page 9 Overview of GSM - TRAU TRAU is responsible for trans-coding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec. It can physically reside on either BSC side or MSC side. • If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel. • TRAU, BSC and BTSs form Base Station Subsystem (BSS) Location of TRAU in GSM Florida Institute of Technology ECE Department Page 10 Overview of GSM - MSC • One of the MSC has an added functionality for communication with public network – Gateway MSC (GMSC) Responsible for connecting the mobile to the landline side GSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility support • All calls from the “outside networks” are routed through GMSC GSM Network can have more than one MSC BSC MSC GMSC BTS GSM MSC and Gateway MSC Florida Institute of Technology ECE Department Page 11 Introduction to GPRS Overview of GSM - HLR/AuC Database for permanent or semi-permanent data associated with the user Logically, there is only one HLR per network Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc. HLR is usually implemented as an integral part of MSC AUC is an integral part of HLR responsible for ciphering and encryption. GSM specifies elaborate encryption schemes. There are three levels of the encryption: A5/1 – Used by countries in Europe and USA A5/2 – Used by countries and the so called COCOM list No encryption – used by all other countries Florida Institute of Technology ECE Department Page 12 GSM frequency bands 14 bands defined Most dominant o GSM-900 (P-GSM+ E-GSM) o DCS-1800 o GSM-850 (US) o PCS-1900 (US) It is common that a phone is capable of multiband operation o Dual band - 2 bands o Quad band - 4 bands Florida Institute of Technology ECE Department Page 13 GSM – Frequency <-> ARFCN calculations ARFCN – Absolute Radio Frequency Channel Number 2 2 Example – US PCS allocation Florida Institute of Technology ECE Department Page 14 Overview of GSM - Air Interface USER 1, ARFCN1 GSM is a FDMA/TDMA based technology USER 2, ARFCN1 Transmissions are discontinuous USER 1 Each user is assigned a timeslot .... USER 8 ARFCN1 USER 8, ARFCN1 Each frequency is divided into eight timeslots USER 2 USER 6 USER 7 USER 8 USER 1 ARFCN2 Each channel has a 200 kHz bandwidth USER 9, ARFCN2 USER 10, ARFCN2 Overhead signaling is required for coordination and control USER 16, ARFCN2 Information is sent in bursts BTS Several types of bursts GSM as a FDMA/TDMA Interface Florida Institute of Technology ECE Department Page 15 Overview of GSM - Burst Types 3 Tail 57 Traffic/Signaling 1 26 1 57 3 Flag Training Sequence Flag Traffic/Signaling Tail Normal Burst Used to carry information on both control and traffic channels Mixture of data and overhead GSM defines 8 training sequences assigned in color code mode Both on the forward and reverse link 3 39 64 39 3 Tail Synchronization Training Sequence Synchronization Tail Synchronization Burst Facilitates the synchronization of the MS to the network at the base band Commonly referred to as S-burst Only on the forward link The same sync sequence is used in all GSM networks Florida Institute of Technology ECE Department Page 16 Overview of GSM - Burst Types 8 41 Tail Synchronization 36 3 Access Bits Tail Access Burst Used when the MS is accessing the system Shorter in length – burst collision avoidance • GSM mobiles use slotted ALOHA to access the system • In the case of collision – a hashing algorithm is provided Extended synchronization sequence Used only on the reverse link 3 142 3 Tail Predefined Bit Sequence Tail Dummy Burst • Supports MAHO • Used to ensure constant power level of the broadcast control channel • Only on the forward link Florida Institute of Technology ECE Department Page 17 Overview of GSM - Burst Types Sometimes referred to as the F-burst Power Spectrum Density Provides mobile with precise reference to the frequency of the broadcast control channel Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier Only on the forward link BW = 200KHz fc fc+67.7 KHz frequency • Spectral characteristics of the control channel. • The peak in the spectrum allows for easier MS network acquisition Frequency synchronization burst 3 142 3 Tail Fixed Bit Sequence (All zeros) Tail Florida Institute of Technology ECE Department Page 18 Overview of GSM - Physical Channels A GSM physical channel can carry several different types of logical channels Can be divided into two categories: traffic and signaling Signaling channels can be further categorized as: Broadcast Common Control Dedicated Control Group Name Function BCH BCCH FCCH SCH PCH AGCH RACH SDCCH SAACH FACCH TCH Broadcast control Frequency correction Synchronization Paging Access Grant Random Access Standalone dedicated control Slow-associated control Fast-associated control Traffic data and voice CCCH DCCH TCH Florida Institute of Technology ECE Department Fwd. Link Yes Yes Yes Yes Yes No Yes Yes Yes Yes Rev. Link No No No No No Yes Yes Yes Yes Yes Page 19 Overview of GSM - Frame Hierarchy Different organization on the superframe level for different logical channels Hyperframe 3 h 28 min 53 s 760 ms 0 1 2 3 4 5 6 7 2043 2044 2045 2046 2047 48 49 50 51 x 26 Superframe or 26 x 51 Superframe 6s 120 ms 0 1 2 3 4 5 6 7 46 47 48 49 50 0 1 2 3 4 5 6 7 21 22 23 24 25 26 Multiframe 120 ms 0 1 2 3 4 51 Multiframe 235.4 ms 23 24 25 0 1 2 0 3 4 5 1 6 2 3 4 7 1 TDMA Frame 4.615 ms Florida Institute of Technology ECE Department Page 20 GSM Time Division Duplex Communication on the forward and reverse link does not happen simultaneously Delay of three slots between TX and RX Time division duplexing avoids RF duplexer at the RF stage o Reduces the cost of mobile o Saves battery Forward Link - BTS Transmits 0 1 2 3 4 5 6 7 0 3 4 5 Reverse Link - MS Transmits 5 Florida Institute of Technology 6 7 0 1 2 ECE Department Page 21 Review questions What are the elements of GSM system? What is a BTS? What is a BSC? What is a MSC? What are four most prevalent frequency bands for GSM deployment? Explain GSM FDMA/TDMA access? What is a HLR? How many time slots are on one GSM ARFCN? What is a VLR? What is an ARFCN? What is TRAU? Name 2 basic channel types in GSM? What is a SIM card? What is the Abis? Florida Institute of Technology Name five burst types used in GSM? ECE Department Page 22 Review questions Draw GSM TDMA hierarchy for traffic channel. Draw GSM TDMA hierarchy for control channel. What is the bandwidth of a GSM channel? What is the difference between DCS-1800 and PCS-1800? What is a quad-band phone? Do GSM mobiles use RF duplexers? Please explain. Florida Institute of Technology ECE Department Page 23
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