introduction_to_gprs_and_edge.pdf

8/11/2019 Introduction_To_GPRS_and_EDGE.pdf

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Introduction To GPRS and EDGE:Technology, Operation, and Services

Lawrence Harte, Bryan Strange

Excerpted From:

Mobile Systems

With Updated Information ALTHOS Publishing

GPRS GSM Radio GPRS IP Encapsulation

GPRS NormalBurst Structure

GPRS and EDGEChannel Coding Schemes


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ALTHOS Publishing

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Copyright , 2005, ALTHOS, Inc


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About the Authors

Mr. Harte is the president of Althos, an expert informationprovider which researches, trains, and publishes on technolo-gy and business industries. He has over 29 years of technologyanalysis, development, implementation, and business manage-ment experience. Mr. Harte has worked for leading companiesincluding Ericsson/General Electric, Audiovox/Toshiba andWestinghouse and has consulted for hundreds of other compa-

nies. Mr. Harte continually researches, analyzes, and tests new communi-cation technologies, applications, and services. He has authored over 50books on telecommunications technologies and business systems coveringtopics such as mobile telephone systems, data communications, voice overdata networks, broadband, prepaid services, billing systems, sales, andInternet marketing. Mr. Harte holds many degrees and certificates includ-ing an Executive MBA from Wake Forest University (1995) and a BSETfrom the University of the State of New York, (1990).

Bryan Hashim Strange has performed a wide range of techni-cal and managerial roles for 12 years at Wray Castle. Thisbegan with four years lecturing on a variety of engineering andacademic modules for HND students, as a trainer and coursedeveloper for short courses based on European second and

third generation technologies. These courses include UMTSSystem Overview, UMTS Air Interface, UMTS Cell Planning,

GSM Air Interface, GSM Cell Planning, GSM Optimisation, GSM IndoorCoverage Planning, GSM Applied Cell Planning and GPRS. He has alsodeveloped and presented courses covering other global technologies such ascdmaOne, CDMA2000, TETRA, APCO 25, iDEN, WLAN and Bluetooth. Mr.Strange was educated in the United Kingdom at Clayesmore School (1976-1984) and Wray Castle College (1987-1990) and Lancaster University (1999-2000). He also specializes in HNC Electronics and CommunicationsEngineering and he has a Marine Radio General Certificate. Bryan Strangealso holds 3 'A' Levels; Maths, Physics, Engineering Drawing and Designplus 9 'O' Levels; inc. Maths, English, Physics.


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Table of Contents

INTRODUCTION TO GPRS AND EDGE . . . . . . . . . . . . . . . . . . 1

GLOBAL S YSTEM FOR M OBILE COMMUNICATION (GSM) . . . . . . . . . . . 3GENERAL P ACKET R ADIO S ERVICE (GPRS) . . . . . . . . . . . . . . . . . . . . 4

E NHANCED D ATA FOR GLOBAL E VOLUTION (EDGE) . . . . . . . . . . . . . 4E NHANCED GPRS (EGPRS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4COMPACT GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

UPGRADING GSM TO GPRS AND EDGE . . . . . . . . . . . . . . . . 5

GPRS AND EGPRS INDUSTRY SPECIFICATIONS . . . . . . . . 9

PACKET DATA SERVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

M OBILE S ERVICES (M-S ERVICES ) . . . . . . . . . . . . . . . . . . . . . . . . . . 11S TREAM P RIORITIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

E NHANCED M ESSAGING SERVICE (EMS) . . . . . . . . . . . . . . . . . . . . . 12M ULTICAST SERVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

A SYNCHRONOUS CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 A SYMMETRIC CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12QUALITY OF SERVICE (Q OS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Conversation Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Streaming Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Interactive Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Background Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 S HORT M ESSAGING SERVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14SMS AND GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15LOCATION B ASED S ERVICES (LBS) . . . . . . . . . . . . . . . . . . . . . . . . . 15


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P ACKET D ATA S ERVICE M EASUREMENT T YPES . . . . . . . . . . . . . . . . . 16 Data Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Packet Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 Jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

GPRS AND EGPRS DEVICES (MOBILE STATIONS) . . . . . . 17

M OBILE DEVICE CLASSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Class A Simultaneous Voice and Data . . . . . . . . . . . . . . . . . .17 Class B Automatic Transfer of Voice and Data . . . . . . . . . . .18

Class C Single System Selection . . . . . . . . . . . . . . . . . . . . . .18 Dual Transfer Mode (DTM) . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Multi-slot Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

S UBSCRIBER IDENTITY M ODULE (SIM) . . . . . . . . . . . . . . . . . . . . . . 19E XTERNAL M ODEMS (USB OR E THERNET ) . . . . . . . . . . . . . . . . . . . . 19PCMCIA A IR C ARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20E MBEDDED R ADIO M ODULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20M OBILE TELEPHONES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20DUAL M ODE C APABILITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

GPRS AND EGPRS RADIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

RF C HANNEL T YPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23F REQUENCY B ANDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26F REQUENCY REUSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26F REQUENCY H OPPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27RF P OWER CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27D YNAMIC T IME A LIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29CHANNEL S TRUCTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30M ULTI -FRAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30CODING SCHEMES (CS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31E NCRYPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33M ODULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34D ATA P ACKET E NCAPSULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34P ACKET D ATA CHANNEL SHARING . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

P HYSICAL CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Packet Data Channel (PDCH) . . . . . . . . . . . . . . . . . . . . . . . . . .37

LOGICAL CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38TRAFFIC CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38CONTROL CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39GSM L OGICAL CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Broadcast Channels (BCH) . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 Common Control Channels (CCCH) . . . . . . . . . . . . . . . . . . . . .41

Dedicated Control Channels . . . . . . . . . . . . . . . . . . . . . . . . . . .42 General Logical to Physical Channel Mapping for GSM . . . . .43

GPRS L OGICAL CHANNELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Packet Broadcast Control Channel (PBCCH) . . . . . . . . . . . . . .44 Packet Common Control Channels (PCCCH) . . . . . . . . . . . . . .44 Packet Data Traffic Channel (PDTCH) . . . . . . . . . . . . . . . . . .46 Packet Dedicated Control Channels (PDCCH) . . . . . . . . . . . . .46 Compact Mode of GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

GPRS AND EDGE/EGPRS NETWORK . . . . . . . . . . . . . . . . . . 47

B ASE S TATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Packet Control Unit (PCU) . . . . . . . . . . . . . . . . . . . . . . . . . . . .50 Communication Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

S WITCHING CENTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Mobile Switching Centre (MSC) . . . . . . . . . . . . . . . . . . . . . . . .51Serving General Packet Radio Service Support Node (SGSN)

52 Gateway GPRS Support Node (GGSN) . . . . . . . . . . . . . . . . . . .52

N ETWORK D ATABASES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Home Location Register (HLR) . . . . . . . . . . . . . . . . . . . . . . . . .52 Visitor Location Register (VLR) . . . . . . . . . . . . . . . . . . . . . . . .53 Equipment Identity Register (EIR) . . . . . . . . . . . . . . . . . . . . . .53

Charging Gateway (CG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 Billing Center (BC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

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Authentication Center (AuC) . . . . . . . . . . . . . . . . . . . . . . . . . . .54 SMS Service Center (SC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

W IRELESS N ETWORK S YSTEM INTERCONNECTION . . . . . . . . . . . . . . . 55 Public Switched Telephone Network (PSTN) . . . . . . . . . . . . . .56 Public Packet Data Network (PPDN) . . . . . . . . . . . . . . . . . . . .56 Interworking Function (IWF) . . . . . . . . . . . . . . . . . . . . . . . . . .56

IP B ACKBONE N ETWORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56GPRS Roaming Exchange (GRX) . . . . . . . . . . . . . . . . . . . . . . .57

ADDRESSING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

M OBILE DEVICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Mobile Station ISDN (MSISDN) . . . . . . . . . . . . . . . . . . . . . . .58 International Mobile Subscriber Identity (IMSI) . . . . . . . . . . .58 International Mobile Equipment Identifier (IMEI) . . . . . . . . . .58 Temporary Mobile Subscriber Identity (TMSI) . . . . . . . . . . . . .58 Temporary Block Flow (TBF) . . . . . . . . . . . . . . . . . . . . . . . . . .59 Temporary Flow Identifier (TFI) . . . . . . . . . . . . . . . . . . . . . . . .59

INFRASTRUCTURE A DDRESSING . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Access Point Naming (APN) . . . . . . . . . . . . . . . . . . . . . . . . . . .59 GPRS Tunneling Protocol (GTP) . . . . . . . . . . . . . . . . . . . . . . .60 Tunneling End Point Identifier (TEID) . . . . . . . . . . . . . . . . . .60

IP A DDRESSING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Static and Dynamic (DHCP) Addressing . . . . . . . . . . . . . . . . .61

GPRS AND EDGE/EGPRS SYSTEM OPERATION . . . . . . . . 63

INITIALIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63S TANDBY S TATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64INITIAL A SSIGNMENT /A CCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65READY S TATE (C ONNECTED M ODE ) . . . . . . . . . . . . . . . . . . . . . . . . . 67

Dynamic Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68 Extended Dynamic Allocation . . . . . . . . . . . . . . . . . . . . . . . . . .68 Fixed Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

Packet Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 A LWAYS -ON CONDITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70REGISTRATION A REA U PDATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

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GPRS R OAMING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 V OICE C ALL AND D ATA S ESSION TRANSFER OPERATION . . . . . . . . . . 71D ATA TO VOICE (GPRS TO GSM) . . . . . . . . . . . . . . . . . . . . . . . . . . 72TRANSFER BETWEEN GPRS AND IS-136 . . . . . . . . . . . . . . . . . . . . . 72

GPRS AND EGPRS RADIO LAYERS . . . . . . . . . . . . . . . . . . . . 73

P HYSICAL L AYER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73M EDIUM A CCESS CONTROL (MAC) L AYER . . . . . . . . . . . . . . . . . . . . 73R ADIO L INK CONTROL (RLC) L AYER . . . . . . . . . . . . . . . . . . . . . . . . 74LOGICAL L INK CONTROL (LLC) L AYER . . . . . . . . . . . . . . . . . . . . . . 74

S UBNETWORK DEPENDENT CONVERGENCE P ROTOCOL (SNDCP) . . . . 74FUTURE EVOLUTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

THIRD GENERATION GSM (3GSM) . . . . . . . . . . . . . . . . . . . . . . . . . 75

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-1-Copyright , 2005, ALTHOS, Inc

Introduction to GPRS and EDGE

The general packet radio service (GPRS) and enhanced data for global evo-lution (EDGE) systems are additions to the GSM mobile radio communica-tion system that provide for medium-speed packet data communication ser-vices. Upgrading to GPRS and EGPRS capability allows GSM serviceproviders to offer their customers wireless medium-speed packet based ser-vices. This involves upgrading one or more GSM radio channels on eachtower to the GPRS and/or EGPRS technology. The term Enhanced GPRS(EGPRS) is used to describe the combination of GPRS and EDGE.

Customers can access wireless Internet services through GPRS or EGPRS

capable handsets or external modems that connect to their desktop or lap-top computers. The GPRS and EGPRS radio channels are an always-onsystem that allows users to browse the Internet without complicated dialupconnections.

The GPRS and EGPRS systems are composed of mobile stations (mobiletelephones or data communication devices), base stations (forming the radioaccess network) and a packet data core network (potentially connected tothe Internet).


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Figure 1.1 shows a simplified diagram of a GPRS system. This diagramshows that the GPRS system includes mobile communication devices(mobile telephones or data terminals) that communicate through base sta-tions and a mobile switching center (MSC) or data routing networks to con-nect to other mobile telephones, public telephones, or to the Internet. Thisdiagram shows that the GPRS system contains both GPRS data radio chan-nels and GSM voice radio channels. GPRS devices can be data only (such asa computer browsing the Internet), they can be single mode (such as a GSMtelephone), or they can be dual mode (able to access either or both the GPRSand GSM channels). Data information is typically passed through data net-work (such as the Internet) and that voice information is typically connect-

ed to the public telephone network. This diagram also shows that it is pos-sible to send medium speed data through the public telephone.

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Figure 1.1, GPRS System Overview


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that hold 8 time slots. GSM was originally named Groupe Special Mobile,but the name was changed to reflect its global use. The GSM systemincludes mobile stations (mobile telephones), base stations (radio towers/cellsites), and the core network (interconnecting switching system).

General Packet Radio Service (GPRS)

General packet radio service is a packet data communication system thatadapts the Global System for Mobile communication (GSM) radio system forpacket radio transmission. The GPRS system modifies the GSM channelallocation and time slot control processes to allow for the dynamic assign-ment of time slots to individual users. GPRS provides a theoretical maxi-mum radio channel data transmission rate of 171.2 kbps.

Enhanced Data for Global Evolution (EDGE)

Enhanced Data for Global Evolution (EDGE) is a radio enhancement thatincreases the throughput in a GSM radio channel. If used in conjunctionwith GPRS it enables medium speed packet data communication. TheEDGE system modifies the GSM radio channel by adding new phase modu-lation and channel coding processes. EDGE uses 8 level phase shift keying(8PSK) to increase the net radio channel data transmission rate from 271kbps to 604.8 kbps (theoretical 473.6 kbps user rate).

Enhanced GPRS (EGPRS)

Enhanced general packet radio service is the combination of GPRS andEDGE in the same system. EGPRS combines packet control with new mod-ulation and coding techniques.

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Compact GPRS

Compact GPRS is a GPRS system that uses a limited (compact) frequencyplan. This allows the system operator to use a relatively small amount of fre-quency bandwidth or to convert a limited number of GSM channels to GPRScapability. Because compact GPRS channels will experience higher interfer-ence levels from nearby radio channels, the signaling processes between cellsites are coordinated differently than standard GPRS channels.

Upgrading GSM to GPRS and EDGE

The deployment of GPRS allows carriers to upgrade their systems hard-ware and software (such as replacing or adding channel cards and addingpacket switching nodes). Carriers can upgrade their GSM radio systems tooffer GPRS and EGPRS services by simply exchanging, upgrading, oradding one or more of their existing radio channels to offer packet data ser-vices.

Figure 1.2 shows how a GSM system can be upgraded to offer GPRS ser-vices. This diagram shows that an existing GSM channel is removed,replaced, or upgraded to have GPRS and EGPRS modulation and transmis-sion capability. This diagram shows that packet control unit (PCU) must beadded to the base station controller (BSC) and packet data switching nodesand gateways must also be added to allow data packets to be routed betweenmobile devices and data networks (e.g. the Internet or private corporatenetworks).




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Each GPRS system can be single mode (voice or data) or dual mode (dataand voice). To obtain dual mode voice and data operation, mobile devices

must be capable of processing voice and data signals at the same time. Thisallows the GPRS system to continue to transmit data when a user is on avoice call (such as web browsing while talking).

Figure 1.3 shows how the GPRS system can provide both voice and datainteroperation. This diagram shows how a voice call can be received while auser is transferring a file via a GPRS data session. In this example, theGPRS packet data channel sends a message to the mobile telephone alert-ing the user that an incoming call is waiting. If the user accepts the call, themobile telephone may add a voice channel on another time slot. Note thatonly some types of GPRS mobile devices are capable of simultaneous opera-tion.

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Figure 1.2, GSM System Upgrade to GPRS


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GPRS product types include external radio modems, wireless PCMCIA cards, embedded radio modules, and mobile telephones. GPRS devices maybe single mode data or dual mode GSM voice and GPRS data.

Figure 1.4 shows the common types of GPRS products available to cus-tomers. This diagram shows that the product types available for GPRSinclude mobile telephones, integrated (embedded) radio modules, PCMCIA cards, and external radio modems. GPRS mobile telephones may be capableof operating on both the GSM voice and GPRS data radio channels simulta-neously or may only be capable of GPRS data or GSM voice service. Smallradio assemblies may be integrated (embedded) into other devices such asdigital security cameras or tablet PCs. PCMCIA cards may allow for bothdata and voice operations when inserted into portable communications

devices such as laptops or personal digital assistants (PDAs). External radiomodems may plug into standard interfaces such as USB or Ethernet RJ-45connectors.



Figure 1.3., GSM and GPRS Dual Mode Operation


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Each GPRS channel can directly communicate with up to 8 connected usersand can transparently provide service (always-on) to tens of users.However with this small amount of radio resource allocated to each user thedata rates experienced would be very low. Nevertheless, in this configura-tion a single radio channel can have up to 8 GPRS packet data channels anda cell site can have several GPRS radio channels in several sectors (focusedradio coverage areas).

GPRS uses the same modulation type as GSM and adds new signaling pro-tocols (software). EGPRS uses new modulation that will require a hardwarechange. Expanding from GSM to GPRS requires the addition of servicenodes (packet switches). Expanding from GPRS to EGPRS may requireadditional capacity for the service nodes.

The service provider will also need to modify or upgrade billing systems totrack and bill for packet data and information services.

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Figure 1.4., GPRS Product Types


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GPRS and EGPRS Industry Specifications

GPRS is a portion of the GSM specification that allows packet radio serviceon the GSM system. The GPRS system adds (defines) new packet controlchannels and gateways to the GSM system.

GPRS and EGPRS standards are a portion of the GSM industry standardcalled phase 2+. The phase 2+ specification is an enhancement to the secondphase of the GSM system. GPRS was developed to provide medium-speedpacket data access for the GSM system. EGPRS developed from contribu-tions of the universal wireless consortium 136 (UWC-136) [ i].

Figure 1.5 shows the evolution of GPRS and EDGE/EGPRS industry speci-fication. This diagram shows that the GSM standard was developed in phas-es. GSM phase 1 provided basic voice services data services. GSM phase 2brought enhancements to system operation and to circuit switched datatransmission. GPRS was added to GSM to provide packet data service in



Figure 1.5., GPRS and EDGE/EGPRS Standards Evolution


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phase 2+. EDGE specification was created to increase the data transmissionrate of the GPRS and is also applicable to IS-136 TDMA. EDGE was inte-grated with GPRS to produce the Enhanced GPRS (EGPRS).

The GSM association assists with the promotion, protection, and evolutionof GSM technology (including GPRS and EDGE) and products throughoutthe world. Information about the GSM association can be found atwww.gsmworld.com. GSM association members include mobile operators,manufacturers, and suppliers.

Originally the GSM development group was hosted by CEPT. GSM technol-

ogy basics were created in 1987 and in 1989 ETSI became the managingbody. In 1990, the first GSM specification was released (more than 6,000pages of specifications). In 1998, the Third generation Partnership Project(3GPP) group was formed to create the next evolution of mobile specifica-tion. 3GPP has now taken over the management of GSM and GPRS specifi-cations. GSM and GPRS specifications (and evolved versions of the specifi-cations) can be obtained at www.3gpp.org.

Packet Data Services

Packet switched data is the transfer of information between two pointsthrough the division of the data into small packets. The packets are routed(switched) through the network and reconnected at the other end to recre-ate the original data. Each data packet contains the address of its sourceand destination. In some circumstances this allows each packet to take a dif-ferent route through the network to reach its destination.

The basic service that GPRS and EGPRS provide is efficient medium-speedpacket data transmission. The maximum data transmission rates can beadjusted and capped for different types of users up to 171.2 kbps for GPRSand up to 473.6 kbps for EGPRS, but in practice they are much lower.Typical rates for GPRS would be in the region of 30 kbps with EGPRS per-haps providing rates in excess of 100 kbps.

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Packet data service can be data only or a mixture of voice and data services. An example of a data-only service would be wireless Internet service for adesktop that is provided by an external GPRS modem that plugs into theUSB or RJ-45 Ethernet plug.

GPRS can be used to provide an always-on type of service. When the GPRSdevice is initially turned on, it takes only a few seconds to obtain an IPaddress that is necessary to communicate with the network. Even when theGPRS or EGPRS device is inactive and placed in the dormant state, recon-nection is typically less than 1/2 a second.

The services that GPRS can provide include medium-rate data transmis-sion, streaming media, various levels of quality of service (QoS), multicastservices, and location based services. It is also possible for GPRS to providedifferent classes of these services for different types of users.

Mobile Services (M-Services)

Mobile services are a set of applications and their required characteristicsfor operation in a mobile environment that is defined by the GSM associa-tion. Some of the objectives for M-Services include a more uniform look and

feel of mobile applications on different types of mobile devices, a gradualevolution from text messaging to multimedia messaging services (MMS), touse established protocols and standards when possible, and to help ensuredigital rights management of electronic media.

Stream Prioritization

Stream prioritization service is the identification and management of infor-mation flow for different types of communication sessions. This allows thesystem to prioritize the delivery of packets for time critical applications suchas IP telephony or audio streaming and to delay packets for non-real timeapplications such as email or web browsing.




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Enhanced Messaging Service (EMS)

Enhanced messaging service is an evolution of short messaging service(SMS) that adds the capabilities of text formatting, animation, pictures, andsound to be transferred in short messages. The 3GPP standard for EMS ser-vices is TS 23.040.

Multicast Services

Multicast service is a one-to-many media delivery process that sends a sin-gle message or information transmission that contains an address (code)that is designated for several devices (nodes) in a network. Devices mustcontain the matching code to successfully receive or decode the message.GPRS multicast services can include news services or media (digital audio)broadcasts.

Asynchronous Channels

Asynchronous channels are dynamically adjusted channels that do not have

a fixed synchronization with some other reference signal. Communicationon an asynchronous channel is not sequential and may appear random orunbalanced in nature. This type of data can be carried over GPRS andEGPRS.

Asymmetric Channels

An asymmetric channel is a bi-directional channel in which different datarates are carried in each direction. The GPRS and EGPRS systems permitthe assignment of different data transmission rates for the forward andreverse directions.

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Quality of Service (QoS)

Quality of Service (QoS) is one or more measurements of desired perfor-mance and priorities of a communications system. QoS measures mayinclude service availability, maximum bit error rate (BER), minimum com-mitted bit rate (CBR) and other measurements that are used to ensure qual-ity communications service.

The GPRS and EGPRS system can offer different types of quality of service(QoS) for different types of customers. A key QoS attribute includes priorityaccess for different types of users. For example, priority access could applyto executive user or public safety user classes over lower cost consumerbased users.

QoS provision in the GPRS and EGPRS systems will depend on the specifi-cation release to which the network has been built. In current builds thecapability is quite limited, but in later releases four different service class-es are defined. These are known as Conversational, Streaming, Interactiveand Background.

Conversation Class

Conversation class is aimed at the provision of communication services (typ-ically voice) through a network with minimal delay in two directions. Whileconversation has stringent maximum time delay limits (typically tens of milliseconds), it may be acceptable to loose some data during transmissiondue to errors.

Streaming Class

Streaming class is the delivering of audio or video signals through a network

by establishing and managing of a continuous flow (a stream) of informa-tion. Upon request of streaming class of service, a server system (informa-tion source) will deliver a stream of audio and/or video (usually compressed)to a client. The client will receive the data stream and (after a short buffer-




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ing delay) decode the audio and play it to a user. Internet audio streamingsystems are used for delivering audio from 2 kbps (for low-quality speech)up to hundreds of kbps (for audiophile-quality music).

Streaming class provides a continuous stream of information that is com-monly used for the delivery of audio and video content with minimal delay(e.g. real-time). Streaming signals are usually compressed and error pro-tected to allow the receiver to buffer, decompress, and time sequence infor-mation before it is displayed in its original format.

Interactive Class

The interactive class provides for data and control information transmissionthrough a network with minimal delays and with very few data errors. Theinteractive class allows a user or system to interact with a software appli-cation (typically a web host) in near-real time (limited transmission delays).The interactive class allows the communication channel to be shared byother users during periods of inactivity (such as when the user is thinkingabout a response to a web page question.)

Background Class

Background class is the process of providing information transfer serviceson a best-effort basis. Background class is used for non-time critical services(such as Internet web browsing).

Short Messaging Services

Short message service (SMS) gives mobile phone subscribers the ability tosend and receive text or data messages. GSM mobile device can send short

messages or it can be sent by other systems (such as an email or web pagelink).


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The GSM system limits the short message to 160 alphanumeric characters(7 bits each), 140 data elements (8 bits each), or 70 two-type characters (16bits each). SMS messages can be received while the mobile telephone is instandby (idle) or while it is in use (conversation). While the mobile telephoneis communicating both voice and message information, short message trans-fer takes slightly longer than it does while the mobile telephone is in stand-by. Short messages can be concatenated together to produce longer mes-sages. Short messages are received, stored, and forwarded through the useof a SMS service center (SC).

Short messages that are received by a mobile telephone are typically stored

in the SIM card. This allows the user to keep all their messages on a singleSIM card regardless on which mobile telephone they use with the SIM card.

SMS and GPRS

Although SMS messages are a form of packet data they are not carried inthe GPRS part of the system. This is because their application in GSM wasspecified before GPRS was defined. Instead they are carried as a separatedata flow in dedicated signaling channels associated with a mobile station.

Location Based Services (LBS)

Location based services are information or advertising services that varybased on the location of the user. The GSM system permits the use of dif-ferent types of location information sources including the system itself orthrough the use of the Global Positioning System (GPS).

Although the location functionality is not part of GPRS, there are manyexamples of services accessed using the GPRS bearer that may make use of information about a users location.


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Packet Data Service Measurement Types

Packet data service can be characterized by data throughput, packet loss,latency, and jitter.

Data Throughput

Data throughput is the amount of data information that can be transferredthrough a communication channel or transferred through a point on a com-

munication system.

Packet Loss

Packet loss is a ratio of the number of data packets that have been lost intransmission compared to the total number of packets that have been trans-mitted.

Latency

Latency is the amount of time delay between the initiation of a servicerequest for data transmission or when data is initially received for retrans-mission to the time when the data transmission service request is grantedor when the retransmission of data begins.

Jitter

Jitter is a short-term variation of transmission delay time for data packetsthat usually results from varying time delays in transmission due to differ-ent paths or routing processes used in a packet communication network.

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GPRS and EGPRS Devices (Mobile Stations)

GPRS and EDGE/EGPRS devices (also called mobile stations) are datainput and output devices that are used to communicate with a nearby basestation. All GPRS devices include a removable Subscriber Identity Module(SIM) that holds a variety of information including service subscriptioninformation. The common available types of GPRS devices include externalradio modems, PCMCIA cards, radio modules, and mobile telephones.

Mobile Device Classes

GPRS mobile device class is a parameter that indicates the capabilities of adevice. GPRS device class capabilities may include maximum and minimumtransmitter power levels, available modulation and coding types, and whichservices can be supported on the device.

One of the ways in which GPRS mobile devices are categorized relates to thenumber of simultaneous timeslots that they can use in the uplink and down-link directions. This capability affects the attainable bit rate since all eighttimeslots must be used to achieve the highest theoretical bit rates. It mayalso have an impact on whether simultaneous GSM and GPRS operation ispossible. This is known as its Multi-slot Class. A typical capability would befour slots in one direction and two in the other.

Some mobiles may also be multimode, being able to access other technolo-gies, including for example, IS-136 TDMA.

A GPRS mobile devices ability to access circuit switched (voice) services inparallel with GPRS is described with a capability class that may be A, B orC. Note that almost all current devices are class B.

Class A Simultaneous Voice and Data

GPRS class A devices are capable of performing GSM voice and GPRS datasimultaneously.




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Class B Automatic Transfer of Voice and Data

GPRS class B devices are capable of performing GSM voice or GPRS packetdata transactions, but not simultaneously. The mobile can be registered onand monitoring establishing connections over either as required.

Class C Single System Selection

GPRS class C devices are capable of performing either GSM voice or GPRSdata service. The selection of which service to use is performed manuallyand once applied the mobile cannot use the other system.

Dual Transfer Mode (DTM)

Dual transfer mode is a class of GPRS service that allows a mobile device tooffer simultaneous voice and packet data service without having therequirement of transmitting and receiving at the same time. DTM is a sub-set of GPRS classes A capability.

Multi-slot Class

GPRS multi-slot class represents the capabilities of the mobile device toreceive/transmit and process multiple time slots per frame.

The maximum capability in any one direction is eight time slots. Note thatwhen four or more time slots are assigned, the mobile telephone must becapable of full duplex transmission

Figure 1.6 shows the different multi-slot classes available for GPRS mobiledevices. This table shows that the multi-slot class ranges from 1 through 12.

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Subscriber Identity Module (SIM)

The SIM is a mandatory part of a GSM/GPRS/EGPRS mobile. It is smallinformation card that contains a variety of information including servicesubscription, identity and personal information. Other information relatesto the mobiles operation, the processes required for security procedures anda small amount of user specific data (such as feature preferences, a phonebook and short messages). This information is being stored in the cardrather than programming this information into the phone itself. This intel-ligent card, either credit card-sized (ISO format), or the size of a postage-stamp (Plug-In format), must be present in the phone before it will operate.

External Modems (USB or Ethernet)

External radio modems allow the customer simply to plug in their GPRSdevice to their USB or Ethernet data port to their desktop or laptop com-puter. An external modem behaves as if it were a self-contained modula-tor/demodulator (MODEM) that can be connected to a computer or other



Figure 1.6., GPRS Multislot Classes


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data processing equipment. External modems are commonly connected tocomputers via standard connections such as universal serial bus (USB) orRJ-45 Ethernet connections.

PCMCIA Air Cards

GPRS PCMCIA cards can be added to most laptop computers.

The PCMCIA card uses a standard physical and electrical interface that isused to connect memory and communication devices to computers, typicallylaptops. The physical card sizes are similar to the size of a credit card 2.126inches (51.46 mm) by 3.37 inches (69.2 mm) long. There are 4 different cardthickness dimensions: 3.3 (type 1), 5.0 (type 2), 10.5 (type 3), and 16 mm(type 4).

Embedded Radio Modules

Embedded radio modules are self-contained electronic assemblies that maybe inserted or attached to other electronic devices or systems. Embeddedradio modules may be installed in computing devices such as personal digi-tal assistants (PDAs), laptop computers, and other types of computingdevices that can benefit from wireless data and/or voice connections.

Mobile Telephones

Mobile telephones are radio transceivers (a combined transmitter andreceiver) that convert signals between users (typically people, but notalways) and radio signals. Mobile telephones can vary from simple voiceunits to advanced multimedia personal digital assistants (PDAs).

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Some GPRS mobile telephones include both GSM (voice and low speed data)and GPRS (high-speed packet data) capability. Because the signal processand channel coding is different for GPRS radio channels, existing GSMmobile telephones can only access the voice and medium-speed data capa-bility of GPRS systems.

Dual Mode Capability

Dual mode (multi-mode operation) capability is the ability of a device or sys-tem to operate in two different modes (not necessarily at the same time). Forwireless systems, it refers to mobile devices that can operate on two differ-ent system types such as analog and digital or different forms or digital.

GPRS and EGPRS Radio

There are two types of radio channels used in the GSM/GPRS system; onefor packet data (GPRS) and another for voice and circuit switched data ser-vices (GSM). The characteristics of GPRS radio channels include frequencybands, frequency reuse, channel multiplexing techniques, RF power control,and channel structure.

The GPRS system dynamically assigns time slots, as data transmission isneeded. GPRS is more efficient than circuit switched data for communica-tion processes that have bursty types of transmission such as browsing theInternet.

The GPRS system uses the existing GSM radio channel structure. It definesnew logical packet data and packet control channels.

The Packet Control Unit (PCU) manages radio transmission for packetsthrough coordinates with the base station and the Serving GPRS SupportNode (SGSN). The SGSN is responsible for maintaining the connection withGPRS mobile devices that are operating within its area. The SGSN providesa packet data switching function that is similar to that of an MSC that isresponsible for managing a communication session with mobile telephones.




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Communication between the base station and the SGSN uses Base StationSubsystem GPRS Protocol (BSSGP). BSSGP was designed to allow for effi-cient communication between the base station and SGSN. The SGSNs com-munication with GSM and GPRS network databases including HLR, VLR,

AuC to authenticate, manages, and determines what services the users areauthorized to utilize.

RF Channel Types

GPRS radio channels use the same time division multiplexing structure forthe radio channel as the GSM system. Time division multiplexing (TDM)allows up to 8 users to share a single transmission path by assigning thepath sequentially to each signal, each assignment being for a discrete timeinterval.



Figure 1.7., GPRS IP Datagram Encapsulation


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Figure 1.8 shows the structure of the normal burst used in the GPRS sys-tem. This example shows that a normal burst is 577 sec long and it con-tains 156.25 bit periods. The information field included in the normal burstconsists of initial tail bits (TB), data bits (D), a training sequence (T), andfinal tail bits (TB). A guard period (GP) is included at the end of the normalburst time period to help ensure that transmitted bursts from one mobiledevice does not overlap with transmitted bursts from another mobile device.The RF power structure of the transmitted burst includes a ramp up andramp down time to reduce unwanted radio emissions that occur from rapid-ly changing signals.

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Figure 1.8., GPRS Normal Burst Slot Structure


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The EGPRS system uses 8PSK modulation that provides more bits per timeslot.

To increase the efficiency of packet data transmission, one timeslot numberin four consecutive frames are grouped together to form a radio block. Eachradio block contains a header, data part, and error check parts. The headercontains the device address along with radio link control information. Thedata part may be user data or a control message. The error check partensures the radio block was successfully received without errors.

Some of the data in the radio blocks may be used for packet data control

channels. This would then include a packet broadcast channel, packet ran-dom access channel, a packet common control channel, and a packet controlchannel.

Figure 1.9 shows how one timeslot over four frames is grouped together toform a radio block. This diagram shows that the radio block structures arecomposed of four occurrences of one time slot. Each radio block structurecontains a header that identifies the mobile device(s) that should receive theblock, radio link control, the data or control information that is being trans-ferred, and a radio block error check field. The EGPRS radio block alsoincludes a separate error check field for the header.




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Frequency Bands

Frequency allocation is the amount of radio spectrum (frequency bands)that is assigned (allocated) by a regulatory agency for use for specific typesof radio services. The GPRS system uses the same frequency bands used byGSM mobile systems.

Frequency Reuse

Frequency reuse is the process of using the same radio frequencies on radiotransmitter sites within a geographic area such that they are separated bysufficient distance to minimize interference between them. Efficient fre-quency reuse allows for a dramatic increase in the number of customers that

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Figure 1.9., GPRS Radio Block Structure


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can be served (capacity) within a geographic area on a limited amount of radio spectrum (limited number of radio channels). The ability to reuse fre-quencies depends on various factors that include the ability of channels tooperate in with a limited amount of interference.

Frequency Hopping

Frequency hopping is a radio transmission process where a message or voicecommunications is sent on a radio channel that regularly changes frequen-cy (hops) according to a predetermined sequence. The receiver of the mes-sage or voice information must also receive on the same frequencies usingthe same frequency hopping sequence.

GSM and GPRS can use frequency hopping to reduce the effects of interfer-ence to and from other GSM and GPRS cell sites. This interference averag-ing increases the overall data throughput and reliability of the GPRS sys-tem.

RF Power Control

RF power control is a process of adjusting the power level of a mobile radioas it moves closer to and further away from a transmitter. RF power controlis typically accomplished by sensing the received signal strength level andthe relaying of power control messages from a transmitter to the mobiledevice with commands that are used to increase or decrease the mobiledevices output power level. However, in GPRS the mobile assesses its ownrequired power level based on measurements made on the base station.

Mobile station classification determines the maximum RF transmitterpower and the slotting capabilities. For the GPRS system on the frequencyband and the country the mobile device is operating in. However, in practiceonly one class is used in each band.




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Dynamic Time Alignment

Dynamic time alignment is a technique that allows a base radio station(tower site) to receive transmitted signals from mobile radios at varying dis-tances in an exact time slot. Time alignment keeps different mobile radiostransmit bursts from overlapping. Dynamic time alignment is necessarybecause subscribers are moving, and their radio waves arrival time at thebase station depends on their changing distance from the base station. Thegreater the distance, the more delay in the signals arrival time.Transmission roundtrip delay is approximately 6 sec per km (or 10 sec permile). The base station measures the observed time delay and sends correc-tions, called Timing Advance (TA), to the mobile.

However, unlike GSM connections where under normal conditions a burst istransmitted in every frame, GPRS connections may not require transmittedbursts for extended periods of time. This limits the ability of the base sta-tion to constantly track the necessary time alignment correction. To over-come this challenge, some packets may be sent to the mobile station for thepurpose of estimating and maintaining time alignment with the GPRS sys-tem.

The GPRS system uses a Packet Timing advance Control Channel (PTCCH)

for mobiles to send periodically transmit radio bursts that allows the GPRSsystem to determine the necessary transmit timing advance. This ensuresthat radio bursts do not overlap with transmission bursts in adjacent timeslots. The mobile device does not need to transmit these bursts while it istransmitting data communication as the system can use the transmittedbursts to determine its timing delay. A mobile device that is browsing theInternet may be transmitting data less than 10% of the connection time.




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Channel Structure

Channel structure covers many things, but relates to the standardized wayin which information is arranged as it is carried over the radio link. Thisincludes the division of information according to type, referred to as logicalchannels, and its subsequent organization into radio blocks, timeslots,frames and multi-frames.

Control information on the GPRS forward channel is performed by timemultiplexing data and control channels. This allows the GPRS device toreceive control messages while operating on a data channel.

Each GPRS packet is transmitted using one timeslot over four consecutivestandard GSM frames.

Multi-frame

Multi-frames are patterns of frames that allow for the multiplexing of dif-ferent information flows, known as logical channels, into a single radiochannel timeslot over a defined period of time. The GPRS system uses a newtype of multi-frame to mix packet traffic channels, control channels, and idleperiods. This 52-frame multi-frame is 240 msec in duration.

A 52-frame multi-frame contains twelve radio blocks. Each radio block con-tains four time slots (one from each frame). The radio blocks are labeled B0through B11. The 52-frame multi-frame includes an idle period that allowsthe mobile phone to measure the radio signal quality on other radio chan-nels at least two times during each 52-frame multi-frame

Figure 1.11 shows the structure of a 52-frame multi-frame that is used inthe GPRS system. This diagram shows that some frames are used for data(D), some frames are used for packet timing control channel (T), and some

frames are used as idle periods (I).

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sequence of new bit patterns including error detection and error protectionbits. The order in which bits from different coded block are sent may also berearranged, this is known as interleaving and makes the error correctionscheme more effective.

The GPRS system uses various types of channel error coding that maximizethe reliability of data transmission while minimizing the amount of over-head due to added error detection and correction bits. The error correctioncode rates range from 1/3 to 1 (no error protection). Code rates are the ratioof information bits to a coding process to the total number of bits created bythe coding process. A coding rate of 1/3 indicates for each information bit

into the coding process (such as 8 kbps) there will be 3 bits created for trans-mission (output of 24 kbps). In general, the higher the code rate the moreeffective the protection and the higher the percentage of error detection/cor-rection overhead.

Punctured codes are linear error correction codes that remove one or moresymbols from the code word. This can significantly reduce the data trans-mission overhead (additional bits for error detection/correction) without sig-nificantly reducing the error detection/correction performance.

The error correction rates on the GPRS and EGPRS system vary from 1 (noerror protection) to 1/3 (66% of the bits are for error protection).

Figure 1.12 shows the different channel coding schemes that can be used inGPRS and EGPRS systems. This table shows that the GPRS system uses 4different code schemes CS-1 through CS-4 and EGPRS uses 9 different codeschemes MCS-1 through MCS-9 and that the type of modulation used onthese channels can be either GMSK or 8PSK. Because of the different typesof channel coding and modulation, the data transmission rate can vary upto 473.6 kbps. This table shows that the maximum theoretical data transferrate available for GPRS is 171.2 kbps and 473.6 kbps for EGPRS. However,is is very unusual to see anything other than CS-1 and CS-2 being used inGPRS, and similarly it is expected that only the lower numbered coding

schemes will see widespread use in EGPRS.

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Encryption

Encryption is a process of a protecting voice or data information from eaves-

dropping by unauthorized third parties. Encryption involves the use of adata processing algorithm (formula program) that uses one or more secretkeys that both the sender and receiver of the information use to encrypt anddecrypt the information. Without the encryption algorithm and key(s),unauthorized listeners cannot decode the message. When the encryptionand decryption keys are the same, the encryption process is known as sym-metrical encryption. When different encryption and decryption keys areused (such as in a public encryption system), the process is known as asym-metrical encryption.

The GPRS system uses encryption between the SGSN and the mobiledevice. The use of encryption prohibits other mobile devices or unauthorizedthird parties from capturing and decoding packets that are not intended forthem.



Figure 1.12., GPRS and EGPRS Channel Coding Schemes


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Modulation

Modulation is the process of changing the amplitude, frequency, or phase of a radio frequency carrier signal (a carrier) to reflect the information signal(such as voice or data). The GPRS system uses the same form of modulationas GSM (GMSK), but a new modulation process is available for EGPRS(8PSK).

The modulation may vary from time slot to time slot. When an existing GSMmobile telephone receives a time slot with 8PSK modulation, it will not beable to decode this new type of EGPRS modulation.

While 8PSK modulation can provide for higher data transmission rates,these high data transmission rates are only possible when radio channelconditions are good. This typically occurs when the user is near a base sta-tion or they are moving relatively slowly.

As the radio channel quality begins to degrade, more robust channel codingtypes are used (lower data transfer rates).

Data Packet Encapsulation

Data packet encapsulation is the process of inserting the entire contents of one packet (header, control, and data) into the payload of another packet.Packet encapsulation is used to transfer packets transparently from onepacket communication system by placing them inside the packets of anoth-er communications system.

Figure 1.13 shows an example of how IP packets (datagrams) can be encap-sulated in the payload (data portion) of other types of data packets to trans-parently transfer IP packets through different types of networks. This dia-gram shows a data file being sent from a laptop computer via a radio link toa destination computer that has a specific Internet protocol (IP) address.

The wireless laptop computer divides the data file into small IP packets.Each IP packet has its destination IP address. The laptop computer sendsthese IP packets via the radio link to a wireless access point. Because the

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radio packet payload size is relatively small, the IP datagram is furtherdivided into smaller portions that can be directly inserted into the payloadof the radio packet. These portions are received by the access point where(the end of link 1) they are reassembled into the original IP datagram. Thisexample shows that the IP datagrams are then sent through a frame relaynetwork. Because the frame relay packet payload is much larger, several IPdatagrams can be encapsulated into one frame relay packet. When theframe relay packets reach their destination (end of link 2), the packets areextracted so they can be sent on the next network. The IP datagrams arethen encapsulated in the data portion of the Ethernet packets. When theEthernet packets reach their destination, they are extracted and provided to

the file transfer application on the destination computer. This exampleshows that the use of data encapsulation allows the file transfer applica-tions in the laptop and destination to be unaware of the different types of networks that the IP packets travel through to reach their destination.



Figure 1.13., Data Packet Encapsulation


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Processes similar to this are used in GPRS, but in that case both encapsu-lation in frame relay and encapsulation in IP are used. Frame relay is usedas part of the access network and IP is used within the core network. Bothtechnologies encapsulate end-to-end IP packets being carried on GPRS.

Packet Data Channel Sharing

Up to 8 GPRS connected devices can share a single packet data channelthrough the use of a 3-bit uplink state flag (USF). GPRS and EDGE mobiledevices are assigned a temporary 3-bit USF code when they are connectedto the system.

Figure 1.14 shows how a single packet data channel can be shared by up to8 simultaneous users. This diagram shows that each GPRS or EGPRSmobile device is identified by a 3-bit USF code. The system relates (maps)the USF code to a specific IP address it has assigned for each device.

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Figure 1.14., GPRS USF Channel Sharing


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Channels

Channel is a general term describing a communication link carrying databetween two points. In GSM/GPRS the word channel is used in three veryspecific ways. Firstly, it can be used to describe the radio frequency used onthe link between the mobile and the base station. In this case the term radiochannel would be used. The radio channel is then divided into eight times-lots. Each timeslot is referred to as a physical channel, thus there are eightphysical channels in a radio channel. Finally, different types of informationflow are described as logical channels. Multiple logical channels can be mul-tiplexed into a single physical channel.

Physical Channels

Physical channels are timeslots on radio channels. In the GSM/GPRS sys-tem different names the general functions given to physical channels. Forexample they may be described as signaling or as traffic channels. Note thatthis designation is not absolute in that some types of signaling may be sentin a traffic channel and vice versa. However, for the GPRS system itself there is only one type of physical channel.

Packet Data Channel (PDCH)

A packet data channel is a physical channel (a time slot) in a GPRS systemthat is assigned for packet data communication. One or more PDCHs (timeslots) may be combined to provide higher data transmission rates to or froma particular user.

If EGPRS is in use the modulation type of the PDCH may change betweentime slots. Older mobile devices that cannot decode the new modulation willsimply ignore them.




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Logical Channels

Logical channel is a term used to describe a type of data flow (signaling ortraffic) that is being carried in a physical channel. A single physical channelmay carry more than one logical channel type multiplexed over a number of frames. The basic GSM system has two main categories of logical channel;traffic channels and control channels.

Logical channels can be shared by multiple users (common channels) or thecan be used for one-to-one communication (dedicated channels).

Traffic Channels

The term traffic channel is usually used to describe a timeslot (physicalchannel) that is configured to carry user traffic of some kind. In GSM thiswill be voice or circuit switched data, in GPRS the traffic will be packetswitched data. Configuration of the physical channel for traffic transferinvolves the mapping of traffic logical channels into it. There are severaltypes of traffic logical channel in GSM/GPRS.

For GSM the basic Traffic logical channel is called a TCH. This is then sub-divided into more specific types according to the type of traffic being carried.For example, there are several different types of voice coder that may beused in GSM. Thus a TCH carrying enhanced full rate coded voice is calledTCH/EFR, one carrying half rate-coded voice is called TCH/HR.

In GPRS there is only one type of traffic carrying logical channel, simplycalled a Packet Data Traffic Channel (PDTCH)

For both GSM and GPRS there is always a need to carry some signalinginformation to support the traffic connection. Associated control logicalchannels provide this function. These will be multiplexed into traffic physi-

cal channels alongside traffic logical channels.

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Control Channels

The term control channel is usually used to describe a physical channel thatis configured to carry signaling of some kind. The type of signaling may varyand for both GSM and GPRS different types of signaling logical channelscan be multiplexed into a single control physical channel. These differenttypes of signaling include paging (alerting), access control (channel assign-ment) and system broadcast information (access parameters and systemidentification).

There is a set of signaling logical channel defined specifically for GPRS, butbecause the GPRS is an upgrade to the existing GSM system, control mes-sages can be sent on either the GSM control channels or on the GPRS con-trol channels. Therefore it is common practice in most current GPRS sys-tems to use the existing GSM control channels for common control func-tions.

GSM Logical Channels

When the GPRS system does not have any packet control channels in oper-ation, existing GSM control channels (such as the paging channel and accesschannels) can be used for initial setup of GPRS data communication ses-sions.

Broadcast Channels (BCH)

The broadcast channels are transmitted in the forward direction (downlink)by the base station across the cell area. The information in them is intend-ed to be received by any mobile in the cell area.




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Broadcast Control Channel (BCCH)

The broadcast control channel is used for the broadcasting of system infor-mation. This includes a wide variety of information describing the networkand cell identities along with their respective configurations. This systeminformation also contains information about features and services support-ed and any access restriction. The mobile will also find parameters relatingto its activity whilst monitoring the system such as the neighbor cell cellsthat it should be measuring.

Each cell contains a one broadcast control channel. Mobile devices usually

monitor the radio signal strength of cell site broadcast channels to deter-mine which cell site may best provide it with service.

Frequency Correction Channel (FCCH)

The frequency correction channel provides frequency reference informationthat allows the mobile device to adjust its frequency so it can better decodethe received signals. The presence of FCCH also indicates the presence of BCCH since they are always broadcast on the same radio channel frequen-cy on a cell. The frequency correction channel transmission burst occursbefore the timing synchronization burst.

Synchronization Channel (SCH)

The synchronization channel a signaling channel that provides the systemtiming information that a mobile device needs to adjust its timing so that itcan better align, decode, and measure other communication channels. It alsocarries some basic cell identification information.

Cell Broadcast Channel (CBCH)

A cell broadcast channel is an optional channel carries short messages onthe broadcast channel. This is a special (and very rarely used) form of theShort Message Service (SMS) that can carry locally specific text based infor-mation. Each CBCH can transfer up to 80 octets/91 characters (in one mes-sage) every 2 seconds [ 1]. If the CBCH is included, it replaces one of the ded-icated signaling channels.

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Common Control Channels (CCCH)

The common control channels are used to coordinate the control of mobiledevices operating within its cell radio coverage area. Any mobile in the cellarea may receive (or transmit) the information in these channels, but it hasrelevance to only one mobile. The common control channels include theRandom Access Channel (RACH), Paging Channel (PCH), and Access GrantChannel (AGCH).

Random Access Channel (RACH)

The random access channel is a reverse (uplink) signaling channel that isused by mobile devices to initiate requests for access to the communicationsystem. Responses to service requests that are sent on a RACH channel areprovided on the downlink AGCH.

Because the distance between the mobile device and the cell site is notknown when the mobile first accesses the system, the access request isattempted using a shortened transmission burst. This prevents potentialoverlap of the transmission burst with adjacent time slots for the same cellsite.

Paging Channel (PCH)

The paging channel is used to send messages (paging messages) that alert amobile device of an incoming call. The paging message will contain an iden-tity for the intended mobile. This identity may be the users InternationalMobile Subscriber Identity (IMSI) or, if one has been allocated, a TemporaryMobile Subscriber Identity (TMSI).

In addition to sending paging messages, the paging channel is also used toprovide information about discontinuous reception (DRX). This featureallows the mobile device to turn off its receiver (sleep) during periodsbetween paging groups.




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Access Grant Channel (AGCH)

The access grant channel is used to assign mobile device to a channel whereit can begin to communicate with the system. In some cases, the AGCH mayassign the mobile device may be directly assigned to a traffic channel or itmay be assigned to an interim control channel where it can communicatewith the system before being assigned to a traffic channel.

Dedicated Control Channels

Some control channels are established to carry signaling messages to andfrom one specific mobile. These are described as dedicated control channels.In general these are not used in conjunction with GPRS operation. However,all GPRS mobiles will use them when supporting circuit switched services.

Stand Alone Dedicated Control Channel (SDCCH)

The stand alone dedicated control channel is a signaling channel that can beused to coordinate the radio channel assignment of a mobile device after ithas successfully competed for access. The SDCCH channel is used for off aircall setup (OACSU) to allow the mobile device to authenticate and complete

other control processes without being assigned to a dedicated traffic chan-nel.

Traffic Channel Signaling

Signaling on the traffic channel is divided into two channels, the Fast Associated Control Channel (FACCH) and the Slow Associated ControlChannel (SACCH). The FACCH channel replaces speech with signal data.The SACCH channel uses dedicated (scheduled) frames within each burst.

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General Logical to Physical Channel Mapping for GSM

Figure 1.15 shows the basic logical channels used in the GSM system. Thisdiagram shows that the TDMA physical channel is divided into a controlchannel (time slot 0) and a traffic channel (time slot 4 in this example). Theforward logical control channels include the frequency correction channel,synchronization channel, broadcast channel, paging channel, and accessgrant channel and the reverse logical control channel includes an accessrequest channel. The traffic channel carries user data in both directions.This example shows that while on the traffic channel, fast control channelmessages (FACCH) and slow control channel messages (SACCH) can besent.



Figure 1.15., Logical Channels Used in GSM Systems .


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GPRS Logical Channels

GPRS logical channels include packet data control channels (for signalingcontrol) and the packet data traffic channel (for user data).

When a GPRS system is first started (very few data users), GPRS controlfunctions related to access can be handled with standard GSM channels.However, they may come a point as the system develops where more sig-naling capacity is required. Therefore a set of GPRS specific control chan-nels is defined. These would be implemented on a radio frequency channelthat is dedicated for GPRS operation.

When GPRS mobiles are receiving to the packet data broadcast channel,they will also receive GSM system broadcast information.

Packet Broadcast Control Channel (PBCCH)

The packet broadcast control channel is a signaling channel that is used toconstantly transfer parameters needed by mobile devices to help them iden-tify and gain access to a communication system. A mobile device that is lis-tening on the PBCCH will receive both the GSM system information and

GPRS system information messages.

Packet Common Control Channels (PCCCH)

A packet common control channel carries one or more logical channels thatare used control packet data mobile devices on a shared basis. To controlspecific devices on this channel, control messages are addressed to specificmobile devices or groups of mobile devices.

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Packet Radio Access Channel (PRACH)

The packet radio access channel is a signaling channel that is used to trans-fer access request messages from mobile devices to the system.

Because the distance from the radio tower is not exactly known, the PRACHchannel uses a shortened transmission burst to help ensure transmittedpackets do not overlap with packets in an adjacent time slot do to the trans-mission delay.

Packet Paging Channel (PPCH)

The packet paging channel is used to send messages (paging messages) thatalert mobile devices of an incoming communication session request. Thisincludes both circuit switched connections (in which case the mobile willmove to GSM mode) and packet switched sessions. However, in currentGPRS systems there is no mechanism for the support of mobile terminatedpacket sessions. The paging channel is also used to provide informationabout discontinuous reception (DRX) which allows the mobile device to turnoff its receiver (sleep) during periods between paging groups.

Packet Notification Channel (PNCH)

The packet notification channel is similar to the PPCH except that it is usedto inform mobile devices group (one-to-many) communication sessions.

Packet Access Grant Channel (PAGCH)

The packet access grant channel is used to assign mobile device to a chan-nel where it can begin to communicate with the system after initial accessin the PRACH . In some cases, the PAGCH may assign the mobile devicemay be directly assigned to a traffic channel or it may be assigned to aninterim control channel where it can communicate with the system beforebeing assigned to a traffic channel.




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Packet Data Traffic Channel (PDTCH)

A packet data traffic channel is an uplink or downlink communication chan-nel that is used to transfer user data. One or more PDTCHs may be tem-porarily allocated to a mobile for traffic transfer on a statistically multi-plexed basis. This statistical multiplexing allows up to eight mobiles to beallocated to one physical channel at the same time, each one having accessto the resource only when there is data to send.

Packet Dedicated Control Channels (PDCCH)

There are several dedicated control channels used for GPRS operation. Themain one is the Packet Associated Control Channel (PACCH). A packetassociated control channel is signaling channel that is used to communicatewith a specific (associated) mobile device. The PACCH is associated with thepacket data traffic channel (PDTCH). There are also associated logical con-trol channels for the assessment and control of timing advance.

Compact Mode of GPRS

Compact mode defines a specialized way of operating GPRS such that it canbe used efficiently in small radio bandwidths. This is mainly aimed at North

American operators, particularly those in the PCS1900 bands using the IS-136 TDMA technology. This frequency-efficient mode of operation isachieved by time-sharing between synchronized neighboring cell sites. Thisresults in compromise which limits maximum data throughput in a cell. Thestandards define a complete set of control and traffic logical channels forcompact mode.

Compact Packet Broadcast Control Channel (CPBCCH)

The compact packet broadcast control channel carries system information inthe downlink direction in a compact GPRS system.

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Compact Packet Radio Access Channel (CPRACH)

The compact packet radio access channel is a signaling channel that is usedto transfer access request messages from GPRS and EGPRS mobile devicesto the system when using a compact frequency plan.

Compact Packet Paging Channel (CPPCH)

The compact packet-paging channel is used to send messages (paging mes-sages) that alert mobile devices of an incoming communication sessionrequest on a compact GPRS system.

Compact Packet Notification Channel (CPNCH)

The compact packet notification channel is similar to the CPPCH exceptthat it is used to inform mobile devices group (one-to-many) communicationsessions on a compact GPRS system.

Compact Packet Access Grant Channel (CPAGCH)

The compact packet access grant channel is used to assign mobile device toa channel where it can begin to communicate with the system on a compact

GPRS system.

GPRS and EDGE/EGPRS Network

GSM networks consist of base stations (cell site radio towers), communica-tion links, switching center(s) and network databases. All of these are usedto create links to public telephone and data networks (e.g. the Internet).

The GPRS system adds packet radio and packet data switching parts to aGSM system. In the GSM part, the MSC coordinates the overall allocationand routing of calls throughout the wireless system. Inter-system connec-tions can link different wireless network systems to allow wireless tele-




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phones to move from cell site to cell site and system to system. In the GPRSpart, a Packet Control Unit (PCU) coordinates the assignment of radio pack-ets and GPRS Support Nodes (GSNs) receive and forward data packetstoward their destination.

The GPRS system can be divided into a base station subsystem (BSS), a net-work and switching system (NSS), and an operation and maintenance sub-system (OMS). The radio parts of the GPRS network are contained in theBSS. The switching, databases, and interconnection parts are contained inthe NSS. The OMS contains the necessary system to monitor and diagnosesystem operation.

Figure 1.16 shows a simplified functional diagram of a GPRS network. Thisdiagram shows that the GPRS network is an addition to the circuit switchedGSM system. However, this voice and a packet switched data systems thatshare a common radio access network. This diagram shows that the BaseStation (BTS) contains a radio transceiver (radio and transmitter) that con-verts the radio signal into a data signal (data and digital voice) that cantransfer through the network. The BTS is connected to a base station con-troller (BSC) that coordinates the radio channel assignments. In this exam-ple (there are other possible configurations), the packet data at theBSC/PCU is routed to a serving GPRS service node (SGSN) and the SGSNis connected to a gateway GPRS service node (GGSN). The circuit switcheddigital data at the BSC is routed to the mobile switching center (MSC) forconnection to the public telephone network or to a data network through aninter-working function (IWF).

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Base Stations

Base stations are composed of an antenna system (typically a radio tower),building, and base station radio equipment. Base station radio equipmentconsists of RF equipment (transceivers and antenna interface equipment),controllers, and power supplies.

The radio transceiver section is divided into transmitter and receiver assem-blies. The transmitter section converts a data or voice signal to RF for trans-mission to mobile devices and the receiver section converts RF from themobile devices to voice or data signals routed to the MSC or packet switch-ing network. The controller section commands insertion and extraction of signaling information.



Figure 1.16., GPRS Network Parts


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Packet Control Unit (PCU)

A packet control unit is a system or process in a GPRS system that managesthe channel setup and transfer of radio packets between the BTS and theserving GPRS support node (SGSN).

Communication Links

Communication links carry both data and voice information between theMSC, GSNs, BSCs and the base stations. Options for the physical connec-tions include wire, microwave, or fiber optic links. Alternate communicationlinks are sometimes provided to prevent a single communication link failurefrom disabling communication [ 2]. Some terrain conditions may prohibit theuse of one type of communication link. For example, microwave systems arenot usually used in extremely earthquake-prone areas because they requireprecise line-of-sight connection. Small shifts in the earth can miss-alignmicrowave transceivers to break communications.

Regardless of the physical type of communication link, the channel formatis usually the same. Communication links are typically digitally time-mul-tiplexed to increase the efficiency of the communication line. The standardformat for time-multiplexing communication channels between cell sites inNorth America is the 24-channel T1 line, or multiple T1 channels. The stan-dard format outside of North America is the 32-channel (30 useable chan-nels) E1 line.

Switching Centers

A switching center coordinates all communication channels and processes.There are two types of switches used in the GSM/GPRS system; a MobileSwitching Center (MSC) for circuit switched traffic and a GPRS Support

Node (GSN) for packet switched traffic.

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Mobile Switching Centre (MSC)

The mobile switching centre (MSC) processes requests for service frommobile devices and landline callers, and routes calls between the base sta-tions and the public switched telephone network (PSTN). The MSC receivesthe dialed digits, creates and interprets call-processing tones, and routes thecall paths.

The basic components of an MSC include system and communication con-trollers, switching assembly, operator terminals, primary and backup powersupplies and wireless telephone database registers.

A system controller coordinates the MSCs operations. A communicationscontroller adapts voice signals and controls the communication links. Theswitching assembly connects the links between the base station and PSTN.Operator terminals are used to enter commands and display system infor-mation. Power supplies and backup energy sources power the equipment.

The GSM system defines two types of MSC; the serving Mobile SwitchingCenter (MSC) and the Gateway Mobile Switching Center (GMSC). This isthe logical separation of the MSC for directly controlling the mobile tele-phone and providing a bridge to other networks.

The serving mobile switching center (SMSC) is the switch that is connectedto the RNC that is providing service directly to the mobile telephone. TheSMSC is responsible for coordinating the transfer of calls between differentBSCs. When the call is transferred to BSCs that are connected

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