Cellular Mobile Communications-III Introduction to GSM

1
Cellular Mobile Communications-IIIIntroduction
to GSM

• Dr. Nasir D. Gohar

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AN INTRODUCTION TO GSM

• Second Generation Cellular Mobile Radio
  Systems-Digital Cellular Telephony
• Main Advantages of Digital Cellular Networks
• Higher Spectrum Efficiency
• Speech Encoding
• Better Access Technologies TDMA and CDMA
• Higher System Capacity
• Aggressive Frequency Reuse
• Low Infra-Structure and Terminal Cost
• Application of VLSI/VVLSI Chips
• Many Traffic Channels per Carrier Channel
  Transceivers are shared among several users)
• Low S/I Ratio (SIR) Allowing Smarter Equipment
• Better Integration with Digital PSTN ISDN
• New Services such as Data Communications, Fax,
  and SMS
• Better Privacy due to Encryption

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AN INTRODUCTION TO GSM

• Digital Cellular Standards
• Why So Many Standards?

4
AN INTRODUCTION TO GSM

• Digital Cellular Standards

5
AN INTRODUCTION TO GSM

• Overview of GSM Milestones
• 1982 GSM Group, set up by CEPT, started
  Investigation to reserve a
• Spectrum in 900 MHz band
  for Pan-European Cellular Digital
• Telephony
• 1986 Two 25 MHz frequency Blocks 890-915 MHz,
  935-960 MHz
• reserved by EC, July 01,
  1991 set as Introduction date.
• Choice of Digital and Narrow band Approx. 200
  KHz TDMA made.
• 1987 13 Operators from 12 European Countries
  signed MOU to Support GSM
• 1989 ETSI established which took over GSM from
  CEPT
• 1992 Introduction of first Commercial GSM Network
• 1994 51 Commercial Networks Established
• 1996 More Than 20 Million Subscribers in 191
  Networks, More than Double of all
  Subscribers of other Digital Systems D-AMPS,
  
• PDC, IS-95 CDMA
• 2006 More than 2 billion subscribers (30 of
  world population and
• 82 of cellular market)

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AN INTRODUCTION TO GSM

• Implementation Phases of GSM
• Phase-1 Most Essential Services Voice, Data,
  SMS Implemented. Mostly all
  Current GSM Systems belong to this Phase.
• Phase-2 Half-Rate Speech Encoding, Doubling the
  System Capacity, Several Encryption
  Algorithms, Conference Call Facility up to 5
• Participants ,
• Call Waiting Facility,
• Information on Call Charges, Calling or
  Called Part etc.,
• Cell-Broadcast Point to Multi-Point SMS.
• Many of the Phase-2 Facilities have already been
  Implemented by some Manufacturers.
• Phase-2 GPRSGeneral Packet radio Service for
  Packet Switched
• Data Transmission to Support
  LAN and Internet Traffic.
• Considerably Delayed due to
  Lack of Interest on part of Network
• Operators due to Large
  Investment required for Infra-Structure and MS
• Modifications
• GSM 3G..

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AN INTRODUCTION TO GSM

• 2G EVOLUTION TO 3G

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AN INTRODUCTION TO GSM

• GSM EVOLUTION

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AN INTRODUCTION TO GSM

• GSM SYSTEM ARCHITECTURE

• MS
• SIM Card
• EIN/ESN
• Base Station Subsystem (BSS)
• Several Dozens of BTS under one BSC
• Each BTS has 3-5 Carrier Channels
• Network Switching Subsystem
• MTX up to 1 M users
• Call set up , all Other Functions

• GMTX An Interface to Other Networks
• HLR Keeps Record of Systems Own Users
• VLR Keeps Temporary Record of Visitors / Roamers
  SID, ST,SVCES
• Au C/EIR Manages the Sub. Authentication and
  Encryption Data

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AN INTRODUCTION TO GSM

• GSM SYSTEM ARCHITECTURE-2

• Management Subsystem Operation Subsystem (OSS)
• Directly or Indirectly Connected to all Other
  Subsystems
• Fault Management
• System Configuration
• Performance Management
• Cost Management
• Security Management

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AN INTRODUCTION TO GSM

• GSM SYSTEM ARCHITECTURE-3

• GSM Interfaces
• Radio Interface Describes Data Interchange
  between MS and BSS
• Abis Interface Describes Data Communications
  between BTS and BSC, allows Various Manufacturers
  Equipment
• A Interface Describes Data Interchange between
  BSS and NSS

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AN INTRODUCTION TO GSM

• LOGICAL CHANNELS

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AN INTRODUCTION TO GSM

• Control Channels
• Control channels fall into three categories
• Broadcast BCCH, FCCH, SCH
• One way, from base to mobile
• Common Control RACH, AGCH, PCH
• One way, some from base to mobile and some from
  mobile to the base
• Dedicated SDCCH, SACCG, FACCH
• Two-way, stand-alone or embedded in the traffic
  channels
• All signaling channels share one carrier in a
  cell
• the dedicated control channels may be transmitted
  on traffic carriers

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AN INTRODUCTION TO GSM

• Broadcast Channels
• Frequency Correction Channel (FCCH)
• Carries information for frequency correction
• Synchronization Channel (SCH)
• Carries information for frame synchronization and
  for
• identification of the BTS
• Broadcast Control Channel (BCCH)
• Broadcasts general information on the BTS
• Broadcasts cell-specific information, e.g.
  Control channel organization, frequency hopping
  sequences, cell identification, etc.

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AN INTRODUCTION TO GSM

• Common Control Channels
• Paging Channel (PCH) - downlink only
• for paging purposes
• Random Access Channel (RACH) - uplink only
• used by any MS to request allocation of a
  signaling channel (SDCCH)
• a slotted Aloha protocol is used, so collisions
  among MSs may happen
• Access Grant Channel (AGCH) - downlink only
• used to allocate a SDCCH or a TCH
• Notification Channel (NCH) - downlink only
• notify MS of voice group and voice broadcast call
  (ASCI feature)

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AN INTRODUCTION TO GSM

• Dedicated Control Channels
• Stand Alone Dedicated Control Channel (SDCCH)
• used for call setup (authentication, signaling,,
  traffic channel assignment), location updates and
  SMS
• Slow Associated Control Channel (SACCH)
• always coupled with a SDCCH or TCH
• for communicating measurement data and control
  parameters
• Fast Associated Control Channel (FACCH)
• to respond to increased signaling demand, e.g.
  during handover
• bandwidth (bit slots) are stolen from the
  associated TCH (traffic data are preempted)

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AN INTRODUCTION TO GSM

• Traffic Channels
• GSM support two types of traffic channels
• full rate (TCH/F) 22.8 kbps
• half rate (TCH/H) 11.4 kbps
• Mapping to physical channel
• full rate traffic channel - 1 timeslot
• half rate traffic channel - 1 timeslot in
  alternating frames
• Full rate channel may carry
• 13 kbps speech or data at 2.4, 4.8 or 9.6 kbps
• Half rate channel may carry
• 6.5 kbps speech or data at 2.4 or 9.6 kbps

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AN INTRODUCTION TO GSM

• Channel Usage
• MS Terminating Calls

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AN INTRODUCTION TO GSM

• GSM Call Setup Procedure
• Locating the Subscriber
• HLR keeps record of the MSC area which the
  Subscriber last registered
• VLR keeps record of the Location AreaLA in
  which Subscriber last registered
• Location Area A Group of Neighboring Cells
  having the same LAC
• LAC Each Cell in the Area Broadcasts this Code
• Each Mobile itself periodically registers itself
  with MSC
• Paging and Random Access Procedure RAP
• GMSC interrogates HLR for MSC the Subscriber last
  registered
• Call is Switched to that MSC
• VLR tells about the LA the user last registered
• MSC arranges a Paging in all the Cells in the LA

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AN INTRODUCTION TO GSM

• GSM Call Setup Procedure-2
• Paging and Random Access ProcedureRAP-Contd
• The Mobiles listen to the Paging Channels for
  their Number
• If received a Paging Message, the Mobile starts a
  RAP
• It sends a Random Number RN Brief Guide to
  describe the Purpose
• The System MSC responds to it by repeating the
  RN and providing the decided Channel
• The Mobile listens to this RN, if it is the same,
  it gets hold of the allocated Channel
• Reconfirmation is done by the System by sending
  again the Mobile Number in case some other
  Mobile has also sent the same RN at the same
  time
• If the Mobile hears its own number, it responds
  to the System and thus it gets connected to the
  incoming Call,
• If it was a wrong number other than its own
  number it must leave that Channel.
• In case, a Mobile listens no same RN from the
  System, it may repeat RAP at some time later
  randomly.

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AN INTRODUCTION TO GSM

• CALL PROCESSING
• MOBILE TERMINATING CALLS

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AN INTRODUCTION TO GSM

• CALL PROCESSING
• MOBILE ORIGINATING

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AN INTRODUCTION TO GSM

• GSM Handover and Routing
• Handover
• The Mobile keeps updating MSC about the RSS
  levels from the neighboring Cells.
• MSC, while looking at the quality of the existing
  link, decides whether a Handoff/ Handover is
  necessary or not.
• If yes, it asks the Candidate Cell to prepare the
  Channel GSM uses Fixed Channel Assignment
• When Channel is allocated, it tells the Mobile to
  move to that Channel
• When Mobile has moved, it deactivates the old
  Channel
• Routing
• GMSC interrogates HLR to locate the Subscriber by
  telling the MSC where the Mobile last registered.
• PROBLEM Call originated from PSTN in CANADA for
  a US Mobile roaming in CANADA will result in two
  International Calls

TROMBONE PROBLEM ???? Dont Worry, Be Happy,
Other People will Take Care of this Problem
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AN INTRODUCTION TO GSM

• HANDOVER TYPES

• Intra-BSC
• Old and new BTSs are
• controlled by the same BSC
• The MSC is not involved
• Intra-MSC
• Old and new BTSs are
• attached to different BSCs
• The BSCs are attached to the
• same MSC
• Inter-MSC
• Handover to a new MSC
• Serving MSC becomes anchor
• MSC
• IMT (Inter Machine Trunk) is
• required

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AN INTRODUCTION TO GSM

• GSM Numbering Scheme and Spectrum Efficiency
• GSM Numbering Scheme
• Each GSM Mobile has at least three Numbers
• MSISDN Mobile Station Integrated Services
  Digital Network stored in SIM Card
• SIM Card holds Subscriber IDMSISDN, IMSI, some
  Extra Memory to store phone numbers and
  Encryption Algorithms
• IMSIInternational Mobile Subscriber Identity
  not known to the User, HLR does translation
  between MSISDN to IMSI and vice versa.
• ESN or IMEIPermanently stored/wired in the
  Mobile Station
• Spectrum Efficiency
• TDMA, 200 KHz Channel BW, 8-Time Slots per
  Carrier Channel
• Radio Spectrum 2 x 25 MHz bands can support
  125124 Duplex Carrier Channels and 1000 Traffic
  Channels 1000 Simultaneous Calls.
• A typical GSM System Cluster Size 12, so each
  Cell have Approx. 10 Carrier Channels and a
  Capacity of 80 Simultaneous Call Traffic
  Channels
• In case of half-rate Coder, Spectrum Efficiency
  will be Doubled.

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AN INTRODUCTION TO GSM

• Technical Realization of GSM
• GSM Speech Communications
• User Speech is Digitized using Adaptive PCM
• Speech Encoding using Linear Predictive Coding
  LPC
• User data flow 13 kbps 6.5 kbps in case of
  half-rate coding
• Adding an overhead of 9.8 kbpsfor error
  detection, error correction, and
  synchronization, we get an over all user data
  flow of 22.8 kbps.
• This user data flow is subdivided into short data
  blocks each of 456 bits, which is divided into 8
  sub-blocks, each of 57 bits, 2 such sub-blocks
  are used to make a burst to fit into a time
  slot of 0.58 ms. See frame structure.
• These bursts from a user are interleaved over 8
  time slots spread over 8 frames.
• TDMA/FDMA Mechanism
• Each Carrier Channels carries data burst of eight
  (8) users, each in its allocated time slot in the
  frame.
• Time slots are transmitted on several Channels
  Carrier Channel or Frequency
• Each Carrier Channel transmits the data of 8
  users at 271 kbps.

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM Speech Communications-2
• GSM Carrier Frequencies are numbered 1-124
  ChannelsDuplex
• Forward Channel 935.2 MHz (n-1) 200 KHz
• Reverse Channel 890.2 MHz (n-1) 200 KHz
• Due to slightly more than 200 KHz bandwidth of
  the modulated signal, Consecutive Frequencies are
  not used in the same System
• Also, 1 and 124 Channels are generally not used.
• The Spectrum of 124 Channels is generally not
  allotted to one Service Provider.
• Separation between two Duplex paired Channels is
  45 MHz.
• Each users bursts are transmitted every 4.6ms
  apart.

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AN INTRODUCTION TO GSM

• SPEECH PROCESSING

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AN INTRODUCTION TO GSM

• SPEECH CODING

• SPEECH ENCODER

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AN INTRODUCTION TO GSM

• SPEECH DATA PROCESSING

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AN INTRODUCTION TO GSM

• CHANNEL ENCODING

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AN INTRODUCTION TO GSM

• INTERLEAVING

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AN INTRODUCTION TO GSM

• VOICE TRANSMISSION PATH

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM Framing Structure

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AN INTRODUCTION TO GSM

• TIME-SLOT STRUCTURE

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AN INTRODUCTION TO GSM

• FRAME HIERARCHY

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AN INTRODUCTION TO GSM

• BURST

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AN INTRODUCTION TO GSM

• TYPES OF BURST

• Five different types of bursts
• Normal burst
• Traffic and control payload
• Frequency correction burst
• All zeroes sequence
• Synchronization burst
• A special fixed sequence
• Random access burst
• Extended guard period of 68.25 bitts (252 µs)
• Dummy burst

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AN INTRODUCTION TO GSM

• BURST STRUCTURES

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM Modulation Scheme
• Gaussian Minimum Shift Keying GMSK (For Detail
  see Ch-5 of the Text Book)
• No Speech, No Transmission
• Saves Energy in MS, Reduces Ave. Interference
• Comfort Noise added for the Listeners Pleasure.
• GSM MS Power Classes
• Class 1 20 W Not (yet) Available
• Class 2 8 W Car Phone
• Class 3 5 W -
• Class 4 2 W Normal Pocket Phone
• Class 5 0.8 W Limited Coverage Phone Urban
  Areas Only
• Maximum Cell Size Depends on Max. Permitted
  Delay, 35 km.
• Maximum Mobile Speed 250 km/hr

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM SMS
• Individual Messages
• up to 160 Characters long can be sent and
  received by MS
• SMS uses Signaling Channel, thus, it can be
  received during current Communication Session
• Max. rate 600 bps
• Cell Broadcast
• up to 93 Character long message can be sent to
  all users in a given area Cells.
• Uses Communication Channel, so, cant be received
  during current Communication Session
• Neither Addressed Nor Encrypted
• Applications of SMS .Network Operator Messages,
  Third Party Messages, Public Interest Information
  Messages
• SMS Cost Differs from Operator to Operator and
  depends on type of Application

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM Security Aspects
• GSM provides security about the identity of MS
• Guards against Eavesdropping
• Implementation
• User Authentication A3 Algorithm 128 bit
  Secret Number, Ki, assigned to each User, stored
  in SIM Card as well as AuC, is used to create
  SRES using Ki and 128 bit RAND number from the
  network, if SRES of MS matches with SRES of the
  network, the MS is accepted.

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AN INTRODUCTION TO GSM

• Data Encryption A8A5 Algorithm
• Frame (RANDKi(Alg-A8))(Alg-A5) -gt 144 bit
  Code Train ? 144 bit user Data Train -gt Network
  ? Frame (RANDKi(Alg-A8))(Alg-A5) -gtOriginal
  Message

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AN INTRODUCTION TO GSM

• Technical Realization of GSM Contd
• GSM Inter-Connection with Other Networks
• PSTN/ISDN for Voice
• GSM Speech data is Digital and Compressed
• Converted into audio of 3.1 KHz BW
• PSTN/ISDN for Data
• For Data Interconnection, Modem Pools are used in
  GSM Network. Many Telephone Modems at GMSC/IWF
  supporting all the important telephone modem
  standards V.210.3 kbps, AS, V.221.2 kbps, AS
  and S, V.32 4.8 kbps, S etc.
• PSDN (Basic PAD or Dedicated PAD) for Data
• 300 bps to 9600 bps on AS/S links

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NEW DEVELOPMENTS IN GSM

• Frequency Shortage and Extended GSM Band
• In some European countries, up to half GSM band
  is occupied by Interim Systems NMT, ETACS etc.
• Additional frequencies adjacent to GSM band is
  recommended by ERO1996, new terminals for this
  extended band are not available in bulk
• New Encoding Techniques
• Half-Rate Speech Encoding
• Instead of 13 kbps only 6.5 kbps will be required
  for the same speech quality
• Standardized in early 1995, but, only few
  manufacturers have implemented
• lack of interest on part of system suppliers
• fear of investment decrease in network expansion
  uCell
• Enhanced Full Rate EFRDeveloped by GSM and
  DCS-1800 Suppliers
• Same 13 kbps speed, but, considerable improvement
  in speech quality
• More Immunity to transmission errors

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NEW DEVELOPMENTS IN GSM

• Any Cellular Phone may have many Speech Coders,
  including FR/HR/EFR, it must be able to switch,
  and at least support FR for roaming purpose.
• Data Communications over GSM
• General Packet Radio ServiceGPRS offers
  packet-switched data communications suitable for
  LAN and Internet ApplicationsPVC
• Requires GPRS enabled Handsets and Changes at
  BSSaddition of Packet Control Unit, PCU
• Option of Upgrading of BTS to support Enhanced
  Data rate for GSM Evolution, EDGE 8-PSK system
• Multi-band Terminals
• Cellular phones to support more than frequency
  bands GSM and DSC-1800 or GSM and PCS-1900 or
  GSM/DCS/PCS or many more combinations
• GSM Moving into 3G Mode
• GSM extends itself into 3G as Universal Mobile
  Telecomm. Services UMTS
• Radio Interface will use WCDMA technology UMTS
  Terrestrial Radio Access, UTRA in two different
  modes FDD two different frequencies for uplink
  and downlink or TDDsame frequency for both
  uplink and downlink but time-shared

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The Market Share of GSM Suppliers 1/97

• Networking Switching Subsystems NSS
• Ericsson 48 , Siemens 21 , Nokia 14 ,
  and Alcatel 10 93
• Others (Lucent, Motorola, Nortel, etc 7
• Basestation Sub-System BSS
• Nokia 22, Motorola 13, Alcatel 10 and
  Ericsson 7 52
• Others Italtel, Lucent, Matra, Philips etc
  48
• GSM / DCS-1800 / PCS-1900 Mobile Terminals
• Ericsson 25 , Nokia 24, MOTOROLA 20 ,
  SIEMENS 9 78
• Others Alcatel, Panasonic, Nortel, etc. 22
• Due to Licensing Problems, Unfair and Restricted
  Competition in GSM Markets

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GSM and Health Problems

• GSM Terminals Cause Interference with
• Hearing Aid Devices and
• Pacemakers Instruments used to regulate the
  heart functionality of Heart Patients, and
• Some Research show that GSM phones cause Brain
  Tumors
• Inherent in GSM TDMA setup to generate strongly
  pulsating transmission signals Continuously Tx
  is switched on and off that generates LF signals
  217, 434, 651 Hz
• Extensive Research required to prove the
  validity of GSM Cellular Phones being the cause
  of Brain Tumors or Cancer.

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GSM Derivative Systems DCS-1800 PCS-1900

• Digital Cellular SystemDCS-1800
• Originally started in 1990 as a separate
  system, but, later on became just a GSM variant
• Main modifications were made only in Air
  Interface
• Developed by ETSI particularly for densely
  populated urban areas
• 1.7 - 2.3 GHZ band 2 x 75 MHz spectrum, 1710-
  1785 1805-1888 MHz
• Duplex separation is 95 MHz, Channel BW is 200
  KHz, 374 duplex channels
• Much smaller cells cells within a building,
  lower power BTS and MS as compared to GSM
• handoff problems are much cleverly settled using
  hierarchical Cell structure
• Max Cell size 8 km with Class 1 MS 1W
• Max Cell size with Class II0.25W even smaller
  0.5 -4/5 km
• International as well as National roaming is
  possible Country to Country, Network to Network,
  and Intra-Network
• Half-rate speech coding is possible/EFR is more
  likely to prevail.

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GSM Derivative Systems DCS-1800 PCS-1900

• Personal Communication Services PCS-1900
• A GSM variant to adapt to US Market
• Frequency band 1900 MHz 1850-1890, 1930-1970
  MHz
• 2 x 40 MHz bands with Duplex Separation of 80 MHz
• Channel BW is 200 KHz, 200 Duplex Channels
• TDMA 8 time-slots
• EFR speech encoding is getting more Interest from
  US Service Providers

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GSM Facing Challenges

• Higher Costs incurred on Network Capacity
  Increase
• Business Market Saturation
• More Money on Advertisement and
  Subsidies/Customer Discounts
• Interconnection Costs to PSTN/ISDN are very
  high
• Leased Line Costs to Interconnect own
  Infra-structural elements are very high
• High License Costs
• With Implementation of De-regulation policies
  this will CHANGE.

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• REFERENCES
• www.utdallas.edu/nhutnn/cs6v81/LECTURE_06.pdf
• http//www.gsmworld.com
• Dr. Veselin Rakocevic http//www.staff.city.ac.u
  k/veselin/Wireless
• Comms