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Computer Networks

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Basic Concepts

A computer network is a set of independentcomputers connected by telecommunicationlinks.

The individual computers are called nodes, or

hosts. The nodes are connected by some method of

carrying digitized signals.Wires

Light

Microwaves

Radio Waves

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Typical Network Uses

Resource sharing - sharing of physicalresources (such as printers, files, databases)

Information sharing - accessing scientific,legal, medical and commercial data files

stored anywhere in the world. Communication

E-mail

Chat

Bulletin Boards

Blogs

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Sharing Data

File server contains documents

used by other computers.

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More Network Uses

Electronic commerce (e-commerce) supportsthe paperless exchange of goods, informationand services. (eg. ATMs, electronic tickets,etc.)

Entertainment Digital cable TV, multi-player

distributed gaming, on-demand movies

Voice over IP

Phone communication over network wires

More??

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Voice Over IP

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Organization of a Machine

Data Bus

memoryinput/output

controlunit

arithmetic-logic unit

Central Processing

Unit (CPU)

registers

Network InterfaceCard (NIC)

Outside World

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Common Network Types

Local Area Network (LAN)

Contains printers, servers and computers

Systems are close to each other

Contained in one office or building

Organizations often have several LANS

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Common Network Types

Wide Area Networks (WAN)

Two or more LANs connected

Over a large geographic area

Typically use public or leased lines

Phone lines

SatelliteThe Internet is a WAN

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Hybrid Network Types

Campus Area Networks (CAN)

A LAN in one large geographic area

Resources related to the same organization

Each department shares the LAN

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Metropolitan Area Network (MAN)

Large network that connects different

organizationsShares regional resources

A network provider sells time

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Home Area Network (HAN)

Small scale network

Connects computers and entertainmentappliances

Found mainly in the home

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Personal Area Network (PAN)

Very small scale network

Range is less than 2 meters

Cell phones, PDAs, MP3 players

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How Networks Are Structured

Server based network

Node is any network device

Servers control what the node accesses

Users gain access by logging in

Server is the most important computer

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Client/Server network

Nodes and servers share data roles

Nodes are called clients

Servers are used to control access

Database software

Access to data controlled by serverServer is the most important computer

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Peer to peer networks (P2PN)

All nodes are equal

Nodes access resources on other nodes

Each node controls its own resources

Most modern OS allow P2PN

Distributing computing is a formKazaa

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Network Components

Physical Media

Interconnecting Devices

ComputersNetworking Software

Applications

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Network Media

Links that connect nodes

Choice impacts

Speed

Security

Size

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Wire Based Media

Twisted-pair cabling

Most common LAN

cable Called Cat5 or

100BaseT

Four pairs of copper

cable twistedMay be shielded from

interference

Speeds range from

1 Mbps to 1,000 Mbps

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Wire Based Media

Coaxial cable

Similar to cable TV wire

One wire runs through cable

Shielded from interference

Speeds up to 10 Mbps

Nearly obsolete

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Wire Based Media

Fiber-optic cable

Data is transmitted

with light pulsesGlass strand instead

of cable

Immune to

interference Very secure

Hard to work with

Speeds up to

100 Gbps

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Wireless Media

Data transmitted through the air

LANs use radio waves

WANs use microwave signals

Easy to setup

Difficult to secure

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Network Topology

The network topologydefines the way inwhich computers,printers, and otherdevices are connected.A network topologydescribes the layout ofthe wire and devices as

well as the paths usedby data transmissions.

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Network Topology

Linear Bus Tree

Star/

(Ring)

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Bus Topology

Commonly referred to asa linear bus, all thedevices on a bus topology

are connected by onesingle cable.Advantages

Easy to setupSmall amount of

wireDisadvantages

SlowEasy to crash

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Star & Tree Topology

The star topology is the mostcommonly used architecture inEthernet LANs.

When installed, the star topology

resembles spokes in a bicyclewheel.

Larger networks use the extendedstar topology also called treetopology. When used with network

devices that filter frames orpackets, like bridges, switches,and routers, this topologysignificantly reduces the traffic onthe wires by sending packets onlyto the wires of the destinationhost.

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Network Topologies

Star topology

All nodes connect to a hub

Packets sent to hub

Hub sends packet to destination Advantages

Easy to setup

One cable can not crash network

Disadvantages One hub crashing downs entire network

Uses lots of cable

Most common topology

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Star Topology

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Ring TopologyA frame travels around the ring,stopping at each node. If a nodewants to transmit data, it adds thedata as well as the destination

address to the frame.The frame then continues around thering until it finds the destinationnode, which takes the data out of theframe.

Single ring All the devices on thenetwork share a single cable

Dual ring The dual ring topologyallows data to be sent in bothdirections.

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AdvantagesTime to send data is knownNo data collisions

Disadvantages

SlowLots of cable

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Mesh Topology

The mesh topologyconnects all devices(nodes) to each other.

It is used in WANs tointerconnect LANs andfor mission criticalnetworks like thoseused by banks andfinancial institutions.

Implementing the meshtopology is expensiveand difficult.

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Mesh Topology

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Network Hardware

Network interface cards

Network adapter

Connects node to the media

Unique Machine Access Code (MAC)

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Network Hardware

Network linking devices

Connect nodes in the network

Cable runs from node to device

Crossover cable connects two computers

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Network Hardware

Hubs

Center of a star network

All nodes receive transmitted packetsSlow and insecure

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Network Hardware

Switches

Replacement for hubs

Only intended node receives transmissionFast and secure

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Network Hardware

Bridge

Connects two or more LANs together

Packets sent to remote LAN cross Other packets do not cross

Segments the network on MAC addresses

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Network Hardware RouterConnects two or more LANs together

Packets sent to remote LAN cross

Network is segmented by IP addressConnect internal networks to the Internet

Need configured before installation

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Network Hardware

Gateway

Connects two dissimilar networks

Connects coax to twisted pairMost gateways contained in other devices

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Network Cabling

Cabling specifications

Bandwidth measures cable speed

Typically measured in Mbps

Maximum cable length

Connector describes the type of plug

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Network Cabling

Ethernet

Very popular cabling technology

10 Base T, 10Base2, 10Base5

Maximum bandwidth 10 Mbps

Maximum distances100 to 500 meters

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Network Cabling

Fast Ethernet

Newer version of Ethernet

Bandwidth is 100 MbpsUses Cat5 or greater cable

Sometimes called 100Base T

Requires a switch

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Network Cabling

Gigabit Ethernet

High bandwidth version of Ethernet

1 to 10 GbpsCat 5 or fiber optic cable

Video applications

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Network Cabling

Token ring

Uses shielded twisted pair cabling

Bandwidth between 10 and 25 MbpsUses a multiple access unit (MAU)

Popular in manufacturing and finance

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Network Hardware

Repeater

Bridge

SwitchRouter

Network InterfaceCard

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Client-Server Model

Very common means of distributinginformation and services.

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Computers: Clients and ServersIn a client/servernetwork arrangement,network services arelocated in a dedicatedcomputer whose only

function is to respondto the requests ofclients.

The server contains the

file, print, application,security, and otherservices in a centralcomputer that iscontinuously availableto respond to clientrequests.

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Network Protocols

Language of the network

Rules of communication

Error resolutionDefines collision and collision recovery

Size of packet

Naming rules for computers

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Networking Protocol: TCP/IP

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Network Communication

Communication protocols (or rules) Ethernet uses contention based

transmission users compete for the same line and broadcast

a message; if two or more send at the same time there is a

collision and everyone must back off and wait arandom time before resending;

control is distributed and each computer makes

its own decision.

Token ring - user must obtain a token tosend- no collisions, fair but tokens can getlost.

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Figure 7.6

An Ethernet LAN Implemented Using a Hub

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Data transmission in Local Area Networks LANS & Wide Area Networks WANS

A LAN connects computers that aregeographically close- (same building,

campus). Each computer has its own network

address.

A LAN is a private network and owned an

operated by the company or institution.

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WANs extend across town, country or oceansacross public areas and use purchased point-to-point lines.

Uses store-and-forward packet- switching

technique (unlike LAN which just broadcastsmessage to all). Unit called a packethopsfrom one node to another until it reaches itsdestination.

Packet is a fixed size block of information withan address field for its destination.

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Internet Addresses Addressing scheme

32 bit IP address for each computer (for example192.207.177.133) Static IP

Dynamic Host Configuration Protocol (DHCP)

232possible addresses. Were running out! IPv6 comingsoon.

Domain Name Server -directory of machines withindomain Each domain is responsible for providing a name server

Contains mnemonic address and corresponding numericInternet address

Maps IP address to name of computer e.g.viking.cs.utexas.edu = 128.83.143.1

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Packet Transmission

Large amounts of data must be broken intosmaller packets.

Then it is given its source and destinationaddress.

It is transmitted to an adjacent node, whoseaddress is appended and anacknowledgment ( ACK) is sent ( by a router).

Routing algorithms try to determine the

shortest path. If the ACK does not arrive the packet is

resent.

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Complete Packet Packet contains TCP port information, IP addressing

information and application data. Imagine sending a novel to a friend by putting each

paragraph on a postcard, numbering them, and thensending them in the mail. They would arrive numbered, but out of order.

They would arrive at different times.

You may have to call and say Did you mail # 42?

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Recent Developments

Gigabit networks (speeds greaterthan 1 billion bits/second (Gbps).

Wireless Communication- using

radio,microwave and infrared signals. Mobile computing - delivering data

to the user, wherever he or she is.Problems - line of sight transmission

requires transmitters. Wireless mediaare affected by environment, weatherand are not secure.

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The Internet Backbone Packets move up a hierarchy of ISPs

and then back down the branches

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World Wide Web (WWW)

Client-Server model

Client requests a web page from the server by usingthe Hypertext Transfer Protocol (HTTP) via the TCPand a Universal Resource Locator (URL).

http://www.cnn.com/

Usually via a web browser.

Connectionless protocol.

Web pages are encoded in Hypertext MarkupLanguage (HTML)

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Uniform Resource Locator (URL)

http://www.cs.utexas.edu/users/rpriebe/cs302_032/index.html

http:// (protocol)

www.cs.utexas.edu (name of the computer) /users/rpriebe/cs302_032/ (directory path)

index.html (name of the file)

What does this URL mean?

ftp://photo1.si.edu/images/gif89a/

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HTML

A tag-based language that is interpreted by abrowser.

Test Page

Testing!!

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What happens when you click thelink?

The browser connects to a Domain Name Server and gets theIP address for the web server

The browser uses an HTTP request to ask the web server forthe page

If the server cant find the page you get the 404 error

If the server finds the page, the HTML is returned to your browserand interpreted

The connection is closed

If the page has additional elements such as graphics or video, a

new HTTP connection is made for each element on the page.

Remember that each part of the page may be hundreds of packets!

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File Transfer Protocol (FTP)

Simple way to transfer files betweencomputers

Some common names are SmartFTP orFetch.

Anonymous FTP

FTP via login

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E-mail

Simple Mail Transfer Protocol (SMTP)More complicated than HTTP

Maintains connection

Checks identity of client

Commonly used for sending e-mail Post Office Protocol (POP3)

Common protocol for internet e-mail clients suchas Eudora, Outlook Express, etc.

Commonly used for checking e-mail, but not forsending

E-mail stored on a mail server and the client eithercopies the messages from the server or removesthem.

Fi ll

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Firewall

A firewall is a mechanism used to protect a trustednetwork from an untrusted network, usually whilestill allowing traffic between the two.

Can be hardware or software

Filters packets Blocks port scans

Especially important for Broadband cable ISPs.

computers on same LAN.

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Data Communications

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Modem Communications

Plain Old Telephone System (POTS)

Standard phone line

Two-way voice communicationUses analog transmission techniques

Data communication is slow

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ModemsModulator/Demodulator

Modulator converts digital to analog

Speed measured in bits per second (bps) Current fastest speed is 56 Kbps

Quality of phone lines dictates speed

V.92 is the current modem standardSeveral modem types

Internal

External

Voice

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Modem usesConnection to the Internet

File transfer

Uploading

Downloading

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Digital Data Connections

Digital phone lines

Local telephone companies are upgrading

Service will faster and more reliableNew phones will be needed

Modems will need to be upgraded

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Broadband connection

Any data connection faster than 56 Kbps

Common in businessBecoming popular in home installations

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ISDN lines

Integrated Services Digital Network

Basic rate uses three channels Two data channels each support 64 Kbps

Error correction channel

Primary rate uses 24 data channels

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ISDN Connection

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T lines

High-capacity voice/data lines

Used to control phone and dataSeveral variants

T1 transmits at 1.544 Mbps

T3 transmits at 44.736 Mbps

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DSL technologies

Digital Subscriber Line

Popular with home usersSpeeds range from 100 Kbps to 30 Mbps

Asymmetrical DSL (ADSL)

Upload speed slower than download speedSymmetrical DSL (SDSL)

Requires a DSL modem

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Cable modem connections

Popular with home and office users

Speeds between 1 and 3 MbpsRequires a cable modem

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ATM

Asynchronous Transfer Mode

Efficient transfer of video and soundRequires a special NIC and hardware

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Wireless Networks

Benefits

No cable to pull

Mobile devices access network resourcesMobility and flexibility for office workers

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Wireless Networks

Wireless 802.11

Also called Wi-Fi

IEEE standard Institute of Electronic and Electrical Engineers

Several versions

802.11b connects up to 11Mbps

802.11g connects up to 56Mbps

Use the same type of devices

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Wireless Networks

Wireless Access Point

Center of a wireless network

WAPS combined cover a larger areaDistance to WAP determines bandwidth

Range is 50 to 150 meters

Extension points can extend range

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Wireless Networks

Wireless Adapters

Used by devices to connect

Includes signal strength software

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