Telecommunication

1
Telecommunication Networking

• An Introduction

2
Agenda

• Definitions
• Communication Model
• The Telecommunications Problem
• Networking
• Internetworking
• Technical Basics

3
Definitions

• Communication The act of coordinating behavior
  to some end.
• Requirements
• Source
• Destination
• Message
• Medium
• Implications

4
Communication Model
Meaning-2
Meaning-1
Sender
Channel
Receiver
M e s s a g e
Challenges 1. Various processes2. Will meanings
match?3. Why encode?4. Purpose? Intention?
5
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)

6
The Telecommunications Problem
Sender
Channel
Receiver
Encoding
Decoding
Distance Sender and Receiver are not in direct
contactEquivocation Message loses power over
distanceNoise Channel introduces unwanted
messageCoordination Its not clear what a
message event is
7
Solutions to the problems
Sender
Channel
Receiver
Encoding
Decoding
Distance Long wires of various
typesEquivocation Boosting of power (introduces
noise)Noise Special encoding schemesCoordinatio
n Coordination messages (protocols)
Notice Nothing about meaning, intention
8
Components 1 Hardware

• Cabling (or radio or light, etc.)
• Cards for interfaces
• Routers
• Splitters
• Network servers
• Multiplexors
• These may handle some of the challenges

9
Components 2 Software

• Applications
• Sessions (bundles of connections)
• Connection (between interactors)
• Operating Systems (across resource sets)
• Transport (across physical links)
• Physical (across physical media)
• Internetworking (across networks)

10
Components 3 Other

• ISPs (internet service providers)
• Node services
• Network services

11
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

12
Basic Economics

• Sources arent 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.

13
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
1
0
0
0
14
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 (1s in color)
Beginning of byte has special bit called a
start bit
Ending of byte has special bit called a stop bit
15
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).

16
Amplification
On threashold
Original 0-1
17
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

18
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
19
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.

20
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.

21
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)

22
Whats 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.

23
Networking
Node
NodeCodeMode
24
Networking

• A generalization of the communication model.
• Each participant can send or receive or both
• New Challenges
• Whose turn is it?
• Communicating across nodes (transport)
• Switching
• Specialized nodes (servers)
• Sharing resources
• Common codes

25
Internetworking

• Working across networks

Challenges???
Gateway
26
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

27
Switching Problem
To avoid switching altogether requires that all
points be connected together
One solution is to route messages around in a
circle or ring
Another solution is to have one node (or a new
one) be a central switch
B
A
A general solution is for each node to know how
to route messages to a destination, although it
may take several hops to get a message through
C
28
Transmission Problems

• Most nodes are silent most of the time
• Hence most channels arent being used
• But channels cant really be hogged by senders
  and receivers for long periods of time
• Solution is packet switching

29
Packet Switching

• Senders 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.

30
Example of Packet Switching
456
Message FROM Node 223 TO Node 456 Count 4 This
is packet 1 This is packet 2 This is packet
3 This is packet 4
P3
P2
223
P4
P4
P1
P3
P2
Packet reassembly
Transmission each packet has its own path
through the network
P1
Costs
Benefits Packet creation Better use of
networkPacket handling Smaller unitsChance of
error More even use of n/wRetransmissions
Higher traffic
Packet creation