MIS 524 Winter 2004 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

Sender Channel ReceiverEncoding Decoding

M e s s a g e

Meaning-1

Meaning-2

Challenges:

1. Various processes2. Will meanings match?3. Why encode?4. Purpose? Intention?

Expression Interpretation

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 ReceiverEncoding Decoding

Distance: Sender and Receiver are not in direct contactEquivocation: Message loses power over distanceNoise: Channel introduces unwanted messageCoordination: It’s not clear what a message event is

MIS 524 Winter 2004 7

Solutions to the problems

Sender Channel ReceiverEncoding Decoding

Distance: Long “wires” of various typesEquivocation: Boosting of power (introduces noise)Noise: Special encoding schemesCoordination: 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

ANALOG signal: strength

is proportional to “content”

1

What Is a Signal?

• A communication event

• Has a definite start and stop

• Carries information (which is NOT the signal)

0

DIGITAL signal: strength is fixed at either 0 or a

constant

1 1 11 0 0 0

MIS 524 Winter 2004 14

Inside a Digital Signal

Beginning of byte has special “bit” called a start bit

Ending of byte has special “bit” called a

stop bit

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)

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

Original 0-1Over distance, signal weakens

“On” threashold

Noise intrudes

Signal is “clipped” at threashold level

…and then amplified

…and sent on its way again

MIS 524 Winter 2004 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

MIS 524 Winter 2004 18

Directionality

Full-duplex: Essentially two

simplex signals, one in each

direction

Simplex: In one direction only

Half-duplex: Alternating

directions (first one way, then

the other)

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

NodeCodeMode

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

Gateway

Challenges:???

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

B

A

C

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”

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

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

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

223

456

P4

P3P2

P1

P3

P2P4

P1

Packet creation

Packet reassembly

Costs Benefits

Packet creation Better use of networkPacket handling Smaller unitsChance of error More even use of n/wRetransmissions Higher traffic

Transmission: each packet has its own path through the network