Transcript Telecommunication & Networking An Introduction MIS 524 Winter 2004

Telecommunication &
Networking
An Introduction
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Agenda
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Definitions
Communication Model
The Telecommunications Problem
Networking
Internetworking
Technical Basics
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Definitions
• Communication: The act of coordinating
behavior to some end.
• Requirements:
– Source
– Destination
– Message
– Medium
• Implications
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Communication Model
Expression
Meaning1
Sender
Encoding
Interpretation
Channel
Decoding
Meaning2
Receiver
Challenges:
1. Various processes
2. Will meanings match?
3. Why encode?
4. Purpose? Intention?
M e s s ag e
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Characteristics of Communication
• Encoding/decoding scheme
• Speed of transmission (baud)
• Directionality (one-way, bidirectional,
switchable)
• Noise
• Equivocation (loss of signal)
• Ambiguity (loss of meaning)
• Turntaking (protocol)
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The Telecommunications Problem
Sender
Encoding
Channel
Decoding
Receiver
Distance: Sender and Receiver are not in direct contact
Equivocation: Message loses power over distance
Noise: Channel introduces unwanted message
Coordination: It’s not clear what a message event is
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Solutions to the problems
Sender
Encoding
Channel
Decoding
Receiver
Distance: Long “wires” of various types
Equivocation: Boosting of power (introduces noise)
Noise: Special encoding schemes
Coordination: Coordination messages (protocols)
Notice: Nothing about meaning, intention
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Components – 1
Hardware
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Cabling (or radio or light, etc.)
Cards for interfaces
Routers
Splitters
Network servers
Multiplexors
These may handle some of the challenges
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Components – 2
Software
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Applications
Sessions (bundles of connections)
Connection (between interactors)
Operating Systems (across resource sets)
Transport (across physical links)
Physical (across physical media)
Internetworking (across networks)
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Components – 3
Other
• ISPs (internet service providers)
• Node services
• Network services
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Technical Basics
• Complex, electronic
• Interesting; almost all of the basics are
based on human communication
• Remember the basic problems in
communication:
– Distance
– Signal Loss
– Noise
– Turntaking
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Basic Economics
• Sources aren’t “on” all the time
• Sources make mistakes; repetition is dangerous
and costly
• Channels are usually relatively expensive
• Sharing channels is a good use of an expensive
resource; sharing is costly
• All channels are error-prone; the way to
compensate is redundancy
• The more complex the scheme, the higher the
cost and the more likely is failure or error.
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What Is a Signal?
ANALOG
signal: strength
is proportional to
“content”
• A communication event
• Has a definite start and stop
• Carries information (which is NOT the
signal)
DIGITAL signal:
strength is fixed
at either 0 or a
constant
1
0
1
1
1
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0
1
0
0
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Inside a Digital Signal
The bits that form part of the byte
may be ones (at or above a certain
level) or zero (below this level). This
byte is 1011 0110 (1’s in color)
Ending of byte has
special “bit” called a
stop bit
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Beginning of byte
has special “bit”
called a start bit
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What Is the Advantage of Digital
Signalling?
• First, simplicity, only two signal levels
• Second, resistance to noise
• Third, amplification can work without
amplifying noise
• Fourth, potential to add check bits to
reconstruct byte in the event of errors (for
example, parity checking).
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Amplification
Original 0-1
Signal is “clipped”
at threashold level
Over
distance,
signal
weakens
…and then
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amplified
“On” threashold
Noise
intrudes
…and sent on its way again
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Channel Terminology
• Directionality
– Simplex, (Half-)Duplex, Full Duplex
• Modulation/Keying
– Amplitude Shift, Frequency Shift, Phase Shift
• Bandwidth
– Number of signals per second
– Each signal can carry multiple bits (see next
Slide)
• Multiplexing
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Directionality
Simplex: In one
direction only
Half-duplex:
Alternating
directions (first
one way, then
the other)
Full-duplex:
Essentially two
simplex signals,
one in each
direction
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Modulation/Keying
• Value of signal (1 or 0) depends on either
• Amplitude (above/below a certain level)
– Frequency (above/below a certain level)
– Phase (mathematical quality above/below a
certain level)
• These can be combined (or multiplied) to key
many bits in a given signal. For example 4
values of amplitude x 4 levels of frequency x 2
levels of phase = 32 combinations or five bits
per signal. This increases complexity of
hardware, but raises “bandwidth” considerably.
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Bandwidth
• Generally limited by attenuation
(equivocation), noise, signal speed
• Increased by higher frequencies, better
amplification, more complex keying
schemes, more reliable channels with less
noise and less attenuation.
• Highest bandwidth: fiber optic cables
• Lowest bandwidth: signal lights,
semaphore, string.
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Multiplexing
• Previous slides concentrated on SINGLE
communication paths.
• It is possible to ShARE the path.
• This is called “multiplexing”
• Multiplexing may be done through
– Sharing TIME (time division multiplexing)
– Sharing FREQUENCIES (frequency division
m-xing)
– Sharing SPACE (space division multiplexing)
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What’s Good about Multiplexing
• Not all sources are maximally operational at all
times.
• This wastes a valuable resource (channel time)
• Any sharing is complex and comes at a cost,
usually equipment
• Where communication is bursty, multiplexing is
good.
• Where communication is continuous,
multiplexing is just an expensive overhead.
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Networking
Node
Node
Code
Mode
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Networking
• A generalization of the communication model.
• Each participant can send or receive or both
• New Challenges:
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Whose turn is it?
Communicating across nodes (transport)
Switching
Specialized nodes (servers)
Sharing resources
Common codes
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Internetworking
• Working across networks
Challenges:???
Gateway
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Networking Challenges
• Getting a message from one sender to
one receiver across a network
• This requires “addressing” and routing
• Routing is called “switching” in
telecommunications
• There are many switching schemes; all
are additional expenses; but there is a
savings in not having to connect all points.
• They are based on unique identifiers
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Switching Problem
To avoid
switching
altogether
requires
that all
points be
connecte
d together
B
One solution is to
route messages
around in a circle
or ring
A
C
A general solution is for each node
to know how to route messages to
a destination, although it may take
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several
“hops”
get a message
through
Another
solution is to
have one node
(or a new one)
be a central
“switch”
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Transmission Problems
• Most nodes are silent most of the time
• Hence most channels aren’t being used
• But channels can’t really be hogged by
senders and receivers for long periods of
time
• Solution is “packet” switching
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Packet Switching
• Sender’s message is broken into
(generally short, fixed-length) packets
• Each packet is numbered and sent “into”
the network
• The network transmits the packets
• The node assembles the packets in order
(not an easy task)
• The receiver gets the message from the
node.
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Example of Packet Switching
456
Message
FROM: Node 223
TO: Node 456
P3
Count: 4
This is packet 1
This is packet 2
This is packet 3
P2
P4
223
P4
P3
P2
P1
P1
Packet reassembly
Transmission: each
packet has its own path
through the network
This is packet 4
Costs
Packet creation
Benefits
Packet creation Better use of network
Packet handling Smaller units
Chance of error More even use of n/w
Retransmissions Higher traffic
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