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ITC242 – Introduction to Data Communications

Week 8

Topic 13 Wireless WANSReading 2

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Topic 12 – Circuit/Packet switching

Learning Objectives• Define and describe the characteristics of:

– Circuit switched network– Packet switched network

• Describe the application of both circuit switching and packet switching networks

• Compare Circuit/packet switched networks describing the advantages and disadvantages of each.

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The Network Core

• mesh of interconnected routers

• the fundamental question: how is data transferred through net?– circuit switching:

dedicated circuit per call: telephone net

– packet-switching: data sent thru net in discrete “chunks”

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Network Core: Circuit Switching

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required

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Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

• pieces allocated to calls

• resource piece idle if not used by owning call (no sharing)

• dividing link bandwidth into “pieces”– frequency division– time division

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Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

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Circuit Switching Applications

• Public Telephone Network (PSTN)

• Private Automatic Branch Exchanges (PABX / PBX)

• Private Wide Area Networks (often used to interconnect PBXs in a single organization)

• Data Switch

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Network Core: Packet Switching

each end-end data stream divided into packets

• user A, B packets share network resources

• each packet uses full link bandwidth

• resources used as needed

resource contention: • aggregate resource

demand can exceed amount available

• congestion: packets queue, wait for link use

• store and forward: packets move one hop at a time– Node receives complete

packet before forwarding

Bandwidth division into “pieces”

Dedicated allocation

Resource reservation

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Packet-switching: store-and-forward

• store and forward: entire packet must arrive at router before it can be transmitted on next link

R R RL

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Delay and loss in packet-switched networks

packets queue in router buffers

• packet arrival rate to link exceeds output link capacity

• packets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

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Four sources of packet delay

• 1. nodal processing: – check bit errors– determine output link

A

B

propagation

transmission

nodalprocessing queueing

• 2. queueing– time waiting at output

link for transmission

– depends on congestion level of router

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Delay in packet-switched networks

3. Transmission delay:• R=link bandwidth (bps)• L=packet length (bits)• time to send bits into

link = L/R

4. Propagation delay:• d = length of physical link• s = propagation speed in

medium (~2x108 m/sec)• propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantities!

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Caravan analogy

• cars “propagate” at 100 km/hr

• toll booth takes 12 sec to service car (transmission time)

• car~bit; caravan ~ packet• Q: How long until caravan is

lined up before 2nd toll booth?

• Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec

• Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr

• A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km

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Topic 13 – Wireless WANs

Learning Objectives

• Describe the properties and applications of the different types of satellite communications.

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Satellite Communications

• Two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in space

• The antenna systems on or near the earth are referred to as earth stations

• Transmission from an earth station to the satellite is an uplink, from the satellite to the earth station is downlink

• The transponder in the satellite takes an uplink signal and converts it to a downlink signal

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Satellite Network

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Geostationary Satellites

• Circular orbit 35,838 km above the earth’s surface

• Rotates in the equatorial plane of the earth at exactly the same angular speed as the earth

• Remains above the same spot on the equator as the earth rotates

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Advantages of Geostationary Orbits

• Satellite is stationary relative to the earth, so no frequency changes due to the relative motion of the satellite and antennas on earth (Doppler effect).

• Tracking of the satellite by its earth stations is simplified.

• One satellite can communicate with roughly a fourth of the earth; three satellites separated by 120° cover most of the inhabited portions of the entire earth excluding only the areas near the north and south poles

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Problems withGeostationary Orbits

• Signal can weaken after traveling that distance

• Polar regions and the far northern and southern hemispheres are poorly served

• Even at speed of light, the delay in sending a signal 35,838 km each way to the satellite and back is substantial

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LEO and MEO Orbits

• Alternatives to geostationary orbits• LEO: Low earth orbiting• MEO: Medium earth orbiting

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Satellite Orbits

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LEO Advantages

• Reduced propagation delay • Received LEO signal is much stronger than that

of GEO signals for the same transmission power• LEO coverage can be better localized so that

spectrum can be better conserved. • On the other hand, to provide broad coverage

over 24 hours, many satellites are needed.

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Satellite Network Applications

• Television distribution

• Long-distance telephone transmission

• Private business networks

• Military applications

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Reading 2 – Wide Area and Large-Scale Networks

Learning Objectives

• Describe the basic concepts associated with wide area networks

• Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

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WAN Transmission Technologies

Some of the communication links employed to construct WANs include:

• Packet-switching networks

• Fibre-optic cable

• Microwave transmitters

• Satellite links

• Cable television coax systems

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WAN Transmission Technologies

Three primary technologies are used to transmit communications between LANs across WAN links:

• Analogue

• Digital

• Packet switching

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Analogue Connectivity

• PSTN – Public Switched Telephone Network

• POTS – Plain Old Telephone System

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Digital Connectivity

• DDS – Digital Data Service: point-to-point, low data rates

• E1 – high speed digital lines: 2.048Mbps = 30 x 64kbps voice channels + 2 x 64kbps signalling channels.

• X.25: an interface between public packet switched networks and customers.

• Frame Relay: point-to-point permanent virtual circuit technology.

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Digital Connectivity

ISDN – Integrated Services Digital Network:

• BRI: Basic Rate Interface: consists of 2 B channels (64kbps each) – bearer channels for data, and one D channel (16Kbps) for setup and control. 2B+D

• PRI: In Australia 30 B channels (64Kbps each) and 2 D channels (64Kbps each). 30B+2D

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Advanced WAN Technologies

• ATM: Asynchronous Transfer Mode: high speed, packet-switching. Uses fixed sized cells of 53 bytes. High levels of quality of service to allow for different data types.

• SONET: Synchronous Optical Network: high speed Fibre optic WAN technology