CSCI 465 D ata Communications and Networks Lecture 18 Martin van Bommel CSCI 465 Data Communications & Networks 1

CSCI 465Data Communications and Networks

Lecture 18

Martin van Bommel

CSCI 465Data Communications & Networks

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Wireless Phone NetworksPrior to cellular was • Mobile radio telephone service– Provided by one high-power transmitter/receiver– Support 25 channels– Effective radius of 80 km

Move to• Cellular radio network– Lower-power systems with shorter radius– Numerous transmitters/receivers


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Cellular Network Organization

• Area is divided into cells– uses tiling pattern to provide full coverage– each with own antenna (max 100 Watts)– each with own range of frequencies– served by base station

• consisting of transmitter, receiver, and control unit– adjacent cells use different frequencies to avoid crosstalk

• cells sufficiently distant can use same frequency band


3

Cellular Geometry

• Divide area into cells– Square cells easy, but distances not equal– Hexagonal pattern permits equidistant antennas– Distance between cell centers

– In reality, slight variation dueto topographical limitations,local signal propagation, andlimits on antennae


4

Frequency Reuse

• Objective– Share nearby (but not adjacent) cell frequencies

without interfering with each other• Power of base transceiver controlled

– Permit communications within cell on given frequency– Limit escaping power to adjacent cells

– Allow multiple simultaneous conversations in cell• Generally 10 to 50 frequencies assigned per cell


5

Frequency Reuse Patterns

• Pattern of N cells• K frequencies• Cell has K/N freqs.• AMPS– Advance Mobile

Phone SystemK = 395, N = 7< 57 freqs/cell

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Increasing Cellular Capacity• Add new channels– Add more frequencies to the system

• Frequency borrowing– Move frequencies into the congested cells from less

used adjacent cells– Dynamically assign frequencies

• Cell splitting– Non-uniform topography and traffic distribution– Use smaller cells in high use areas

• less power but more handoffs


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Operation of Cellular System


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Cellular System Channels

• Two types of channels between mobile unit and base station (BS)

• Traffic channels– Carry voice and data

• Control channels– Set up and maintain calls– Establish relationship between mobile unit and

nearest base station (BS)


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Typical Scenario

• Mobile unit initialization (power on)– Scan and select strongest control channel• Repeated periodically to account for mobility

– Handshake between unit and MTSO via BS• Identify user and register location

• Mobile-originated call– Unit checks if preselected channel is free– Unit sends number on channel– BS sends request to MTSO


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Typical Scenario (2)

• Paging– MTSO attempts to complete connection by

sending paging message to certain BSs– BS sends paging signal on setup channel

• Call accepted– Called unit recognizes its number– Responds to BS – passed on to MTSO– MTSO sets up circuit by assigning channels


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Typical Scenario (3)• Ongoing call– Two mobile units exchange voice or data by going

through BS and the MTSO

• Handoff– If mobile unit moves out of range of one cell,

finds new BS and establishes channel for it, switching call to new BS

• Hangup– Either unit hanging up sends signal to MTSO which

instructs BS channels to be released


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First Generation: Analog

• Original cellular telephone networks– Analog traffic channels

• Advanced Mobile Phone Service (AMPS)– Early 1980s in North America– Also common in South America, Australia, China

• Replaced by later generation systems– Still in use in some areas


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AMPS Parameters


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AMPS Operation

• AMPS-capable phone has read-only memory with numeric assignment module (NAM)– NAM contains • Telephone number of phone – assigned by provider• Serial number of phone – assigned by manufacturer

– When power on, transmit both to MTSO• MTSO has database of stolen units - blocked• MTSO uses phone number for billing


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AMPS Call Sequence• Subscriber initiates call keying in number• MTSO validates number and checks if user authorized to call• MTSO issues message indicating traffic channels to use• MTSO sends ringing signal to called party• When answered, MTSO– establishes circuit– initiates billing information

• When one party hangs up, MTSO– releases circuit,– frees radio channels, and– completes billing information


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AMPS Control Channels• 21 full-duplex 30-kHz control channels– Transmit digital data using FSK– Data transmitted in frames

• Control information can be transmitted over voice channel during conversation– Mobile unit or base station inserts burst of data• Turn off voice FM transmission for about 100 ms• Replace it with an FSK-encoded message

– Used to exchange urgent messages• Change power level• Handoff to another base station


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2nd Generation: GSM

• Global System for Mobile Communication• First appeared in 1991 in Europe• Similar to working of AMPS• Designed to support phone, data, and image• Rates up to 9.6 kbps• GSM transmission is encrypted using secret

keys


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GSM SIM

• Subscriber Identity Module• Smart card or plug-in module to activate unit• stores – subscriber’s identification number– networks subscriber is authorized to use– encryption keys

• Can use any unit anywhere with your SIM


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Second Generation: Digital

• Provide higher quality signals, higher data rates for support of digital services, with overall greater capacity

• Key differences– Digital traffic channels – data or digitized voice– Encryption – easy to encrypt digital traffic– Error detection and correction– Channel access – channels are shared in CDMA

• Time division multiple access (TDMA)• Code division multiple access (CDMA)


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CDMA

• Each cell allocated frequency bandwidth which is split in two– Half for reverse, half for forward– Uses direct-sequence spread spectrum (DS-SS)• Each bit encoded using pre-assigned code which

represents it using several bits• Orthogonal codes permit multiple access


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CDMA Advantages

• Frequency diversity – Spread over larger bandwidth– Noise bursts and fading have less effect

• Multipath resistance– Signal delay and echo does not interfere

• Privacy– Unique coding implies privacy

• Graceful degradation– More users means more noise and more errors– Leads to slow signal degradation until unacceptable


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CDMA Disadvantages

• Self-jamming– Some cross correlation between users– Arriving signals not perfectly aligned

• Near-far problems– Signals closer to receiver have less attenuation– Transmissions from remote units more likely to

fail


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Third Generation: 3G• high-speed wireless communications to support multimedia,

data, and video in addition to voice• 3G capabilities:

• voice quality comparable to PSTN• 144 kbps available to users over large areas• 384 kbps available to pedestrians over small areas• support for 2.048 Mbps for office use• symmetrical and asymmetrical data rates• packet-switched and circuit-switched services• adaptive interface to Internet• more efficient use of available spectrum• support for variety of mobile equipment• allow introduction of new services and technologies


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3-G Driving Forces

• trend toward universal personal telecommunications access– GSM (Global System for Mobile communications) telephony with

subscriber identity module– personal communications services (PCSs)– personal communication networks (PCNs)

• technology is digital – time division multiple access (TDMA) or– code-division multiple access (CDMA)

• PCS handsets low power, small, and lightweight


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Fourth Generation: 4G

• Rapid increase in data traffic on wireless– More terminals can access Internet– Permanent connections to e-mail– Multimedia services– Support for real-time services• Instant messaging

• Two standards– LTE – Long Term Evolution– WiMax 4G – IEEE 802.16 committee standard


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4G - OFDM

• Both standards based on OFDM– Orthogonal Frequency Division Multiplexing• Uses multiple carrier signals at different frequencies

– Sends some bits on each channel– All subcarriers dedicated to single data source

• Extends symbol period on each signal– Intersymbol interference higher at higher bit rates– Each subcarrier carries lower bit rate


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CSCI 465 D ata Communications and Networks Lecture 18 Martin van Bommel CSCI 465 Data Communications & Networks 1