1. Telecommunication & Networking
An Introduction
MIS 524 Winter 2004

2. Agenda Definitions Communication Model The Telecommunications Problem
Networking
Internetworking
Technical Basics
MIS 524 Winter 2004

3. Definitions
Communication: The act of coordinating behavior to some end.
Requirements:
Source
Destination
Message
Medium
Implications
MIS 524 Winter 2004

4. Communication Model Meaning-2 Meaning-1 Expression Interpretation
Sender
Channel
Receiver
Encoding
Decoding
M e s s a g e
Challenges:
1. Various processes 2. Will meanings match? 3. Why encode? 4. Purpose? Intention?
MIS 524 Winter 2004

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)
MIS 524 Winter 2004

6. The Telecommunications Problem
Sender
Channel
Receiver
Encoding
Decoding
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
MIS 524 Winter 2004

7. Solutions to the problems
Sender
Channel
Receiver
Encoding
Decoding
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
MIS 524 Winter 2004

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
MIS 524 Winter 2004

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)
MIS 524 Winter 2004

10. Components – 3 Other ISPs (internet service providers) Node services
Network services
MIS 524 Winter 2004

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
MIS 524 Winter 2004

12. 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.
MIS 524 Winter 2004

13. What Is a Signal? A communication event Has a definite start and stop
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 1 1 1 1
MIS 524 Winter 2004

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 (1’s in color)
Beginning of byte has special “bit” called a start bit
Ending of byte has special “bit” called a stop bit
MIS 524 Winter 2004

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).
MIS 524 Winter 2004

16. Amplification Noise intrudes Over distance, signal weakens
“On” threashold
Noise intrudes
Over distance, signal weakens
Original 0-1
…and then amplified
Signal is “clipped” at threashold level
…and sent on its way again
MIS 524 Winter 2004

17. Channel Terminology Directionality Modulation/Keying Bandwidth
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
MIS 524 Winter 2004

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
MIS 524 Winter 2004

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.
MIS 524 Winter 2004

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.
MIS 524 Winter 2004

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)
MIS 524 Winter 2004

22. 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.
MIS 524 Winter 2004

23. Networking
Node
Node Code Mode
MIS 524 Winter 2004

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
MIS 524 Winter 2004

25. Internetworking Working across networks Challenges:??? Gateway
MIS 524 Winter 2004

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
MIS 524 Winter 2004

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
MIS 524 Winter 2004

28. 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
MIS 524 Winter 2004

29. 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.
MIS 524 Winter 2004

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 network Packet handling Smaller units Chance of error More even use of n/w Retransmissions Higher traffic
Packet creation
MIS 524 Winter 2004