Introduction to Computer Communications

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Introduction to

Computer Communications

Prepared by :Swapan PurkaitDirectorNettech Private [email protected]+ 91 93315 90003

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Introduction to Networking

• Technology focus

– 18th Century: Industrial Revolution...

– 19th Century: Steam Engine, Telegraph, ...

– 20th Century: Telephone, Radio, Television,computers and data networks, Internet....

– 21st Century: Wireless and MobileCommunication, Personal Area Networks,Video conference, ....




Brief History

• 1837: Invention of Telegraph by SamuelMorse

• 1944: Howard A. Aiken of Harvard

University develops MARK I computerbased on Charles Babbage's calculator

• 1969: Advanced Research ProjectAgency's ARPANET becomes operational

• 1990s- Emergence of Internet andWWW




What is Computer DataCommunication?

• Computer data communication is

– the exchange of data between twocomputers via some transmission medium

such as wire.




LAN, MAN, WAN

InternetPlanet10,000km

WANContinent1000km

WANCountry100km

MANCity10km

LANCampus1km

LANBuilding100m

LANRoom10m

Multicomputer System1m

Multiprocessor Circuit Board0.1m

NameProcessor Locationin Same

Interprocessor Distance




Standards Organizations

• ISO: International StandardsOrganization

– Members are national standards

organizations – Nearly 200 technical committees

• ITU: International TelecommunicationUnion (Formerly known as CCITT)

– ITU-T sector responsible for networkingstandards




Protocol Concepts

• A protocol is a set of rules that governsthe communication between twodevices.

• Communication protocols have beenstandardized by ISO

– The goal of the standardization is to achievean open system,

– the devices manufactured by differentmanufacturers can communicate easily




What’s a Network Protocol?

• All communication activity in anetwork are governed by protocols

protocols define format, order of messages sent and received among

network entities, and actions taken

on message transmission, receipt




A Human Protocol and a Computer

Network Protocol:

Hi

HiGot thetime?

2:00

TCP connectionreq.

TCP connectionreply.

Get http://nettech.in/index.html

<file>

time




ISO/OSI Protocol Hierarchy

Layer 7

Layer 6

Layer 2

Layer 5

Layer 4

Layer 3

Layer 7

Layer 6

Layer 2

Layer 5

Layer 4

Layer 3

Bit

Frame

Packet




Protocol Layers

• Each layer offers certain services tohigher layers

– Shields the details of how the services are

actually implemented – Hides the complexity of the lower layers,

helps to easily understand the functionality,makes debugging easier.

• Layer functions should minimizeinformation flow across boundaries

• Protocol stack: List of protocols




Layer 7: Application Layer

• Deals with file transfer, e-mail, networkmanagement, etc.




Layer 6: Presentation Layer

• Responsible for presenting data in aformat that the user understands

– Two computers may use different numeric

and character formats – Translation of one representation to the

other

– Provides security measure throughencryption and decryption




Layer 5: Session Layer

• Responsible for establishing a session orlogical connection between twoapplications




Layer 4: Transport Layer

• Responsible to ensure that data is sent(transported) reliably from source todestination.

– Provides end-to-end error recovery and

– flow control.




Layer 3: Network Layer

• Deals with routing strategies




Layer 2: Data link Layer

• Provides for transfer of frames across atransmission link.

• Data are normally transmitted in frames – Organized in a specified format

• Responsible for transfer across a link byadopting flow control, error detection andcorrection techniques.

• Functions: framing, error control, flow control, and medium access (in shared networks).




Layer 1: Physical Layer

• Sending raw bits across a transmissionmedium (“the wire”)

– Conversion of bits into electrical signals andtheir transmission.

• Issues:

– What’s being transmitted.

– Transmission medium.

– How it’s being transmitted.




Physical Media

• physical link:transmitted databit propagatesacross link

• guided media: – signals propagate

in solid media:copper, fiber

•unguided media: – signals propagate

freely, e.g., radio

Twisted Pair (TP)

• two insulatedcopper wires

– Category 3:

traditional phonewires, 10 Mbpsethernet

– Category 5 TP:100Mbps ethernet




Physical Media: Coax, Fiber

Coaxial cable:• wire (signal

carrier) within awire (shield)

– baseband: singlechannel on cable

– broadband: multiplechannels on cable

•

bidirectional• common use in

10Mbps Ethernet

Fiber optic cable:

• glass fiber carrying

light pulses

• high-speed operation:

– 100Mbps Ethernet

– high-speed point-to-

point transmission

(e.g., 5 Gbps)

• low error rate




Twisted Pair

• EIA (Electronic Industries Association)has developed standards for UTP(Unshielded Twisted Pairs)

– Cat 3: Gently twisted

– Cat 5: More twists per cm

• Better quality signals

• Higher speed communication




Optical Fiber

• Single mode

– Much smaller diameter (8 to 10 microns)

• Multi mode

– Larger diameter (50 microns)




Physical Media: Radio

• signal carried inelectromagneticspectrum

• no physical “wire”

• bidirectional

• propagationenvironment effects:

– reflection

– obstruction by objects

– interference

Radio link types:

• microwave – e.g. up to 45 Mbps channels

• LAN (e.g., wireless LAN)

– 2Mbps, 11Mbps

• wide-area (e.g., cellular)

– 10’s Kbps

• satellite – up to 50Mbps channel (or multiple

smaller channels)

– 270 Msec end-end delay




LAN

• Nodes often share a single cable

– Low delay

– Broadcast transmissions

• Speed: 10 Mbit/Sec to 10Gbits/Sec

• Topology: bus, ring, tree

• Examples of LAN: Ethernet, Token ring,Token bus




Bus Topology

• Failure or removal of a device does notcause the network to fail.




Star Topology

• Failure of the hub can bring down anentire network.




Ring Topology




MAN

• May be a single network such as a cableTV network or made up of many LANs

• The high speed links between individual

LANs and MAN is made possible byusing either of

– a fiber optic conection

– A satellite conection

• Technologies: FDDI, DQDB




WAN

• Spans a country or a continent

• Made up of thousands of LANs andMANs

– Subnets connected through routers

• The most well known WAN is theInternet

• Store and forward (i.e. packet switched)

network




Packet Switchingversus Circuit Switching

• 1 Mbit link

• each user:

– 100Kbps when “active”

– active 10% of time

• circuit-switching:

– 10 users

• packet switching:

– with 35 users,

probability > 10 activeless that .004

Packet switching allows more users to usenetwork!

N users

1 Mbps link




Two Ways to Share

• Circuit switching (isochronous)

• Packet switching (asynchronous)




Circuit Switching

DATA

Caller Callee

BostonSwitch LASwitch

propagationdelay

between

caller and

and Boston

switch

processing delay at switch

• It’s the methodused by thetelephone network

• A call has threephases:

1. Establish circuitfrom end-to-end(“dialing”),

2. Communicate,

3. Close circuit (“teardown”).

• If circuit notavailable: “busysignal”

(1)

(2)

(3)




Switch

Circuit Switching:Multiplexing/Demultiplexing

• Time divided into frames and frames divided into slots• Relative slot position inside a frame determines which

conversation the data belongs to – E.g., slot 0 belongs to the red conversation

• Need synchronization between sender and receiver

Frames

0 1 2 3 4 5 0 1 2 3 4 5Slots =

One way for sharing a circuit is TDM:

Lecture notes use the word “frame” for slot




Circuit Switching

• Assume link capacity is C bits/sec• Each communication requires R bits/sec

• #slots = C/R

• Maximum number of concurrent communications isC/R

• What happens if we have more than C/Rcommunications?

• What happens if the a communication sendsless/more than R bits/sec?

Design is unsuitable for computer networks wheretransfers have variable rate (bursty)




Internet Traffic Is Bursty

Daily traffic at a router

Max In:12.2Mb/s Avg. In: 2.5Mb/s

Max Out: 12.8Mb/s Avg. Out: 3.4 Mb/s






Router

Packet Switching:Multiplexing/Demultiplexing

• Multiplex using a queue – Routers need memory/buffer

• Demultiplex using information in packetheader

– Header has destination – Router has a routing table that contains

information about which link to use to reach adestination

Queue




Queues introduce

• Variable Delay

– Delay = Queuing delay + propagation delay+ transmission delay + processing delay

• Losses – When packets arrive to a full queue/buffer

they are dropped

Packet Switching




Packet SwitchingMay Reorder Packets

Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6Node 7

Packets in a flow may not follow the same path (depends

on routing as we will see later)

packets may bereordered




Signal

• Signal: electro-magnetic wave carryinginformation.

• Time domain: signal as a function of time.

– Analog signal: signal’s amplitude variescontinuously over time, ie, no discontinuities.

– Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).

• Frequency domain:

– Signal spectrum: signal’s frequencycomponents.




Analog Technology

• Analog devices maintain exact physical analogof information

– E.g., microphone: the voltage at the output of the mic is proportional to the sound pressure

• Early telephones were all analog

• Problems with analog signals:

– Difficult to store (e.g.: audio tapes, videotapes)

– Must be processed by analog systems whichoften add distortion

– Noise always adds to the signal




Digital Technology

• It uses numbers to record and processinformation

– Inside a computer, all information is

represented by numbers

– Analog-to-digital conversion: ADC – Digital-to-analog conversion: DAC

• All signals (including multimedia) can be

encoded in digital form

• Digital information does not get distortedwhile being stored, copied or communicated




2 Levels Are Sufficient

• Computers encode numbers using only twolevels: 0 and 1

• A bit is a digit that can only assume the

values 0 and 1 (it is a binary digit)

• A word is a number formed by several bits – Example: ASCII standard for encoding text

• A = 1000001; B = 1000010; …

• A byte is a word with 8 bits




Definitions

• 1 byte = 8 bits

• 1 KB = 1 kilobyte = 1,024 bytes = 8*1,024 bits

• 210 = 1,024 is powr of 2 closest to 1,000.

• [also 1,000 bytes]

• 1 MB = 1 megabyte = 1,000 KB• 1 GB = 1 gigabyte = 1,000 MB

• 1 TB = 1 terabyte = 1,000 GB




Digitization

• Digitization is the process that allows us toconvert analog to digital (implemented by ADC)

• Analog signals: x(t)

– Defined on continuum (e.g. time)

– Can take on any real value• Digital signals: q(n)

– Sequence of numbers (samples) defined in a

discrete set (e.g., integers)




Digitization - Example

1.35 1.355 1.36 1.365 1.37 1.375-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

x ( t )

1.35 1.355 1.36 1.365 1.37 1.375-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

q ( n )

Analog signal x(t) Digitized signal q(n)




Some Definitions

• Interval of time between two samples: – Sampling Interval (T)

• Sampling frequency F=1/T

• E.g.: if the sampling interval is 0.1 seconds,

then the sampling frequency is 1/0.1=10

– Measured in samples/second or Hertz

• Each sample is defined using a word of B

bits

– E.g.: we may use 8 bits (1 byte) per sample.






Example of Digitization

Time (seconds)0 1 2

F=4 samples/second

10101110010100110011010000110100

B=4 bits/sample

Bit rate=BF=16 bits/second




Bit Rate - Example 1

• What is the bit-rate of digitized audio?

– Sampling rate: F= 44.1 KHz

– Quantization with B=16 bits

– Bit-rate = BF= 705.6 Kb/s

– Example: 1 minute of uncompressed

stereo music takes more than 10 MB!




Bit Rate - Example 2

• What is the bit-rate of digitized speech?

– Sampling rate: F = 8 KHz

– Quantization with B = 16 bits

– Bit-rate = BF = 128 Kb/s






Data Transmission

• Analog and digital transmission.

– Example of analog data: voice andvideo.

– Example of digital data: character

strings• Use of codes to represent characters as

sequence of bits (e.g., ASCII).

• Historically, communication infrastructure

for analog transmission. – E.g., telephone network.




Digital Transmission

• Current trend: digital transmission.

– Cost efficient: advances in digital circuitry(VLSI).

• Advantages:

– Data integrity: better noise immunity.

– Security: easier to integrate encryptionalgorithms.

– Channel utilization: higher degree of

multiplexing (time-division mux’ing).



M d




Modems

• MODEM = Modulator/Demodulator – Converts digital to analog before transmittingover analog channel (e.g., telephone networks).

• To transmit data: DAC (digital-to-analogconverter)

• To receive data: ADC (analog-to-digitalconverter)

• 2-way communication: needs twomodems.

– Each modem contains circuitry to encodeoutgoing data and decode incoming data.




Modems

continued…

• Modems contain complex circuitry to: – Modulate/demodulate the analog signal.

• Allows for the transmission of moderately high

bit-rate over the telephone line.

– Compress the data.• Reduces the amount of bits to be transmitted

– Detect bit errors due to transmission.

• Achievable bit rates.

– 14.4 - 56 Kb/s

S




Summary

• LAN, MAN, and WAN

• Protocol concepts

• ISO/OSI 7 layer protocol• Circuit Switching versus Packet

Switching

• Analog versus Digital signals



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Introduction to Computer Communications