ITC242 – Introduction to
Data Communications
Week 8
Topic 13 Wireless WANS
Reading 2
1
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.
2
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”
3
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
4
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
5
Circuit Switching: FDM and TDM
Example:
FDM
4 users
frequency
time
TDM
frequency
time
6
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
7
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
Bandwidth division into
“pieces”
Dedicated allocation
Resource reservation
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
8
packet before forwarding
Packet-switching: store-andforward
L
R
R
R
• store and forward: entire packet must
arrive at router before it can be
transmitted on next link
9
Delay and loss in packetswitched networks
packets queue in router buffers
• packet arrival rate to link exceeds output link
capacity
• packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
10
Four sources of packet delay
• 1. nodal processing:
• 2. queueing
– check bit errors
– determine output link
transmission
A
– time waiting at output
link for transmission
– depends on
congestion level of
router
propagation
B
nodal
processing
queueing
11
Delay in packet-switched
networks
3. Transmission delay:
• R=link bandwidth (bps)
• L=packet length (bits)
• time to send bits into
link = L/R
transmission
A
4. Propagation delay:
• d = length of physical link
• s = propagation speed in
medium (~2x108 m/sec)
• propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
nodal
processing
queueing
12
Caravan analogy
100 km
ten-car
caravan
toll
booth
• 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?
100 km
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
13
Topic 13 – Wireless WANs
Learning Objectives
• Describe the properties and applications of
the different types of satellite
communications.
14
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
15
Satellite Network
16
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
17
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
18
Problems with
Geostationary 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
19
LEO and MEO Orbits
• Alternatives to geostationary orbits
• LEO: Low earth orbiting
• MEO: Medium earth orbiting
20
Satellite Orbits
21
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.
22
Satellite Network Applications
•
•
•
•
Television distribution
Long-distance telephone transmission
Private business networks
Military applications
23
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25
Reading 2 – Wide Area and LargeScale 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
26
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
27
WAN Transmission Technologies
Three primary technologies are used to
transmit communications between LANs
across WAN links:
• Analogue
• Digital
• Packet switching
28
Analogue Connectivity
• PSTN – Public Switched Telephone
Network
• POTS – Plain Old Telephone System
29
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.
30
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
31
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
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