itc242 – introduction to data communications week 8 topic 13 wireless wans reading 2
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ITC242 – Introduction to Data Communications Week 8 Topic 13 Wireless WANS Reading 2. Topic 12 – Circuit/Packet switching. Learning Objectives Define and describe the characteristics of: Circuit switched network Packet switched network - PowerPoint PPT PresentationTRANSCRIPT
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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 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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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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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