MIS 524 Winter 20041 Telecommunication & Networking An Introduction

MIS 524 Winter 2004 1

Telecommunication & Networking

An Introduction


Agenda

• Definitions

• Communication Model

• The Telecommunications Problem

• Networking

• Internetworking

• Technical Basics


Definitions

• Communication: The act of coordinating behavior to some end.

• Requirements:– Source– Destination– Message– Medium

• Implications


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


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)


The Telecommunications Problem


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


Solutions to the problems


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

Notice: Nothing about meaning, intention


Components – 1 Hardware

• Cabling (or radio or light, etc.)

• Cards for interfaces

• Routers

• Splitters

• Network servers

• Multiplexors

• These may handle some of the challenges


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)


Components – 3 Other

• ISPs (internet service providers)

• Node services

• Network services


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


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.


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


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)


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).


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


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


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)


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.


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.


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)


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.


Networking

Node

NodeCodeMode


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


Internetworking

• Working across networks

Gateway

Challenges:???


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


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


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


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.


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

Documents

MIS 524 Winter 20041 Telecommunication & Networking An Introduction