Introduction To Computer Networks

Presented ByMd. Asadul Islam

Lecturer,Dept. Of CSE,KUET

1

Referances

1. Computer Networking By F.Kuross & W.Ross

2. Computer Networks By Tanenbaum

3. Data Communication and Networking By A.Forouzan

4. And WWW.

2

Network A network can be defined as a number of

autonomus device connected together in such a way that they can share resources.

The purpose of a network is to share resources A resource may be:

– A file– A folder– A printer– A disk drive– Or just about anything else that exists on a computer.

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Network(More defination) A network is simply a collection of computers or

other hardware devices that are connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate.

Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise working with networks and network technologies.

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Applications of Networks Resource Sharing

Hardware (computing resources, disks, printers) Software (application software)

Information Sharing Easy accessibility from anywhere (files, databases) Search Capability (WWW)

Connectivity and Communication Email Message broadcast

Remote computing Distributed processing (GRID Computing) or Performance

Enhancement and Balancing Internet Access Data Security and Management Entertainment 5

Fundamental Network Classifications Local Area Networks (LANs):

A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings

Metropolitan Area Network (MAN): Network in a City is call MAN (Metropolitan Area Network) Covered by even a large local area network (LAN) but smaller than

the area covered by a wide area network (WAN). It is also used to mean the interconnection of several local area

networks by bridging them with backbone lines. Wide Area Networks (WANs):

Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

A network that uses routers and public communications links The largest and most well-known example of a WAN is the

Internet. WANs are used to connect LANs and other types of networks

together. 6

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Packet Transmission Modes

• Unicast– Transmission to single specific receiver

• Broadcast– Transmission to all network nodes

• Multicast– Transmission to specific subset of nodes

• Anycast– Transmission to one of a specific subset of nodes

Network Topology Defines the way in which

computers, printers, and other devices are connected.

Describes the layout of the wire and devices as well as the paths used by data transmissions.

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Bus Topology A bus is the simplest physical topology. It consists of

a single cable that runs to every workstation This topology uses the least amount of cabling, but

also covers the shortest amount of distance. Each computer shares the same data and address

path. With a logical bus topology, messages pass through the trunk, and each workstation checks to see if the message is addressed to itself. If the address of the message matches the workstation’s address, the network adapter copies the message.

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Star & Tree Topology The star topology is the most

commonly used architecture in Ethernet LANs.

When installed, the star topology resembles spokes in a bicycle wheel.

Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.

Star Topology

Tree Topology10

Ring Topology A frame travels around the ring,

stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame.

The frame then continues around the ring until it finds the destination node, which takes the data out of the frame. Single ring – All the devices on the

network share a single cable Dual ring – The dual ring topology

allows data to be sent in both directions.

Ring Topology

Dual Ring Topology11

Mesh Topology

The mesh topology connects all devices (nodes) to each other for redundancy and fault tolerance.

It is used in WANs to interconnect LANs and for mission critical networks like those used by banks and financial institutions.

Implementing the mesh topology is expensive and difficult. Mesh Topology

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Topology(cont.)

Topology Advantages Disadvantages

Bus Cheap. Easy to install. Difficult to reconfigure.Break in bus disablesentire network.

Star Cheap. Easy to install.Easy to reconfigure.Fault tolerant.

More expensive than bus.

Ring Efficient. Easy to install. Reconfiguration difficult.Very expensive.

Mesh Simplest. Most fault tolerant. Reconfiguration extremely difficult. Extremely expensive.Very complex.

Advantages and Disadvantages of Network Topologies

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Intranet, Internet & Extranet Internet:

Is a worldwide system of computer networks The Internet is an open, public space.

Intranet: An intranet is a private network that is contained within an enterprise. It may consist of many interlinked local area networks and

also use leased lines in the wide area network. An intranet may be accessible from the Internet, but as a

rule it's protected by a password and accessible only to employees or other authorized users.

Extranet: Is a portion of an organization's Intranet accessible to authorized outside users without full access to

an entire organization's intranet. 14

1-15

What’s the Internet: “nuts and bolts” view

millions of connected computing devices: hosts = end systems – running network apps

Home network

Institutional network

Mobile networkGlobal ISP

Regional ISP

router

PC

server

wirelesslaptopcellular handheld

wiredlinks

access points

communication links fiber, copper, radio,

satellite transmission rate =

bandwidth

routers: forward packets (chunks of data)

Introduction 1-16

What’s the Internet: “nuts and bolts” view

protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, Skype,

Ethernet Internet: “network of

networks” loosely hierarchical public Internet versus private

intranet Internet standards

RFC: Request for comments IETF: Internet Engineering Task

Force

Home network

Institutional network

Mobile networkGlobal ISP

Regional ISP

Introduction 1-17

What’s the Internet: a service view communication infrastructure

enables distributed applications: Web, VoIP, email, games, e-

commerce, file sharing communication services

provided to apps: reliable data delivery from

source to destination “best effort” (unreliable) data

delivery

Introduction 1-18

What’s a protocol?Protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission,

receipt a human protocol and a computer network protocol:

Q: Other human protocols?

Hi

Hi

Got thetime?2:00

TCP connection requestTCP connectionresponseGet http://www.awl.com/kurose-ross

<file>time

Introduction 1-19

A closer look at network structure

• network edge: applications and hosts

access networks, physical media: wired, wireless communication links network core:

interconnected routers network of networks

Introduction 1-20

The network edge:• end systems (hosts):

– run application programs– e.g. Web, email– at “edge of network”

client/server

peer-peer client/server model

client host requests, receives service from always-on server

e.g. Web browser/server; email client/server peer-peer model:

minimal (or no) use of dedicated servers

e.g. Skype, BitTorrent

Introduction 1-21

Physical Media

• Bit: propagates betweentransmitter/rcvr pairs

• physical link: what lies between transmitter & receiver

• guided media: – signals propagate in solid media:

copper, fiber, coax

• unguided media: – signals propagate freely, e.g.,

radio

Twisted Pair (TP)• two insulated copper

wires– Category 3: traditional

phone wires, 10 Mbps Ethernet

– Category 5: 100Mbps Ethernet

Introduction 1-22

Physical Media: coax, fiber

Coaxial cable:• two concentric copper

conductors• bidirectional• baseband:

– single channel on cable– legacy Ethernet

• broadband:– multiple channels on cable– HFC

Fiber optic cable: glass fiber carrying light pulses,

each pulse a bit high-speed operation:

high-speed point-to-point transmission (e.g., 10’s-100’s Gps)

low error rate: repeaters spaced far apart ; immune to electromagnetic noise

Introduction 1-23

Physical media: radio

• signal carried in electromagnetic spectrum

• no physical “wire”• bidirectional• propagation environment

effects:– reflection – obstruction by objects– interference

Radio link types: terrestrial microwave

e.g. up to 45 Mbps channels

LAN (e.g., Wifi) 11Mbps, 54 Mbps

wide-area (e.g., cellular) 3G cellular: ~ 1 Mbps

satellite Kbps to 45Mbps channel

(or multiple smaller channels)

270 msec end-end delay geosynchronous versus low

altitude

Introduction 1-24

The Network Core

• mesh of interconnected routers

• the fundamental question: how is data transferred through net?– circuit switching:

dedicated circuit per call: telephone net

– packet-switching: data sent thru net in discrete “chunks”

Introduction 1-25

Network Core: Circuit Switching

End-end resources reserved for “call”

• link bandwidth, switch capacity

• dedicated resources: no sharing

• circuit-like (guaranteed) performance

• call setup required

Introduction 1-26

Network Core: Circuit Switchingnetwork resources (e.g., bandwidth) divided into “pieces”• pieces allocated to calls• resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces”

frequency division: total frequency bands are divided into several users eg : television broad casting

time division: total available time is divided into several user eg: telephone system

wdm: Total wave lengnth is divided in to number of users eg: optical networking

Introduction 1-27

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

Introduction 1-28

Numerical example

• How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?– All links are 1.536 Mbps– Each link uses TDM with 24 slots/sec– 500 msec to establish end-to-end circuit

Let’s work it out!

Introduction 1-29

Network Core: Packet Switching

each end-end data stream divided into packets

• user A, B packets share network resources

• each packet uses full link bandwidth

• resources used as needed

resource contention: aggregate resource demand

can exceed amount available

congestion: packets queue, wait for link use

store and forward: packets move one hop at a time Node receives complete

packet before forwarding

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Introduction 1-30

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame.

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

Introduction 1-31

Packet-switching: store-and-forward

• takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

• store and forward: entire packet must arrive at router before it can be transmitted on next link

• delay = 3L/R (assuming zero propagation delay)

Example:• L = 7.5 Mbits• R = 1.5 Mbps• transmission delay = 15

sec

R R RL

more on delay shortly …

Introduction 1-32

Packet switching versus circuit switching

• 1 Mb/s link• each user:

– 100 kb/s when “active”– active 10% of time

• circuit-switching: – 10 users

• packet switching: – with 35 users, probability >

10 active at same time is less than .0004

Packet switching allows more users to use network!

N users1 Mbps link

Q: how did we get value 0.0004?

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Network Core: Packet Switching

Packet-switching: Pipelining Discard error packet Carry packet header

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Differences Between Circuit & Packet Switching

Circuit-switching Packet-SwitchingGuaranteed capacity No guarantees (best

effort)Capacity is wasted if data is bursty

More efficient

Before sending data establishes a path

Send data immediately

All data in a single flow follow one path

Different packets might follow different paths

No reordering; constant delay; no pkt drops

Packets may be reordered, delayed, or dropped

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Types of ISPs • Tier-1 ISPs: Backbone

networks• Tier-2 ISPs: National

coverage• Tier-3 ISPs: Directly

attached to customers

Tier3

Tier2

Tier1

PP

PP

P

P

PP

P

P

P

ISPInterconnection of ISP

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Delay in packet-switched networks

packets experience delay on end-to-end path

• four sources of delay at each hop

• nodal processing: – check bit errors– determine output link

• queueing– time waiting at output link for

transmission – depends on congestion level of

router

A

B

propagationtransmission

nodalprocessing queueing

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Delay in packet-switched networks

Transmission delay:• R=link bandwidth (bps)• L=packet length (bits)• time to send bits into link = L/R

Propagation delay:• d = length of physical link• s = propagation speed in

medium (~2x108 m/sec)• propagation delay = d/s

A

B

propagationtransmission

nodalprocessing queueing

Note: s and R are very different quantitites!

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Queueing delay (revisited)

• R=link bandwidth (bps)• L=packet length (bits)• a=average packet arrival

rate

traffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay

infinite!

Introduction 1-39

Why layering?Dealing with complex systems:• explicit structure allows identification, relationship of

complex system’s pieces– layered reference model for discussion

• modularization eases maintenance, updating of system– change of implementation of layer’s service

transparent to rest of system– e.g., change in gate procedure doesn’t affect

rest of system• layering considered harmful?

Introduction 1-40

Internet protocol stack• application: supporting network

applications– FTP, SMTP, HTTP

• transport: process-process data transfer– TCP, UDP

• network: routing of datagrams from source to destination– IP, routing protocols

• link: data transfer between neighboring network elements– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical

Introduction 1-41

ISO/OSI reference model• presentation: allow applications to

interpret meaning of data, e.g., encryption, compression, machine-specific conventions

• session: synchronization, checkpointing, recovery of data exchange

• Internet stack “missing” these layers!– these services, if needed, must be

implemented in application– needed?

Application

Presentation

sessionTransport

Network

linkphysical

TCP/IP Referances Model

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Introduction 1-43

source

application

transportnetwork

linkphysical

HtHn Msegment Ht

datagram

destinationapplicatio

ntransportnetwork

linkphysical

HtHnHl MHtHn MHt M

Mnetwork

linkphysical

linkphysical

HtHnHl MHtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage MHt M

Hnframe