ics124 session 4 introduction to networking 1. objectives: by the end of this session, the student...

ICS124

Session 4

Introduction to Networking

1

Objectives:By the end of this session, the student will be able to:Describe how data is represented for transmissionDecode a bit pattern from transmitted dataList the two methods modems use to modulate a carrier waveDiagrammatically represent data streams using the two modulation typesDefine BandwidthRank various communications types in order of bandwidthList the two methods of modem synchronizationContrast the modem synchronization methods in relation to speed and contentList the three modes of data flowProvide an analogy of each of these methods to demonstrate how each method functionsList three ways users can interact with a computer systemDistinguish each of these through discussion of how the data is accessed, and where the data is processedList the three network topologiesDiagram each of the topologies and show where each will failList four components of a networkDescribe the purpose of each componentDefine the term protocolGive examples of protocolsUse TCP/IP utilities to map a networkDefine terms related to the Internet (Browser, URL, Frames, Plug-ins, Java, Search Engines)Differentiate between the types of search engines in order to more effectively search the Internet

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Introduction to Networking

BackgroundThere are various ways that users can interact with a computer system.

terminal to multi-user system PC to multi-user system PC on a network

The basics are the same for each of these methods. As the complexity of these methods increases, the user is provided more access to the data in the computer system.

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Introduction to Networking, continued

How data is representedComputers store data in binary form. We have looked at the storage of binary data in RAM and ROM as the states of switches:

a switch turned on represents a 1 bit a switch turned off represents a 0 bit

This binary form can also be represented using electrical pulses, where the presence of a voltage represents a 1 the absence of a voltage represents a 0

+5V- 0V-

In this waveform do we have:00101011001100? or0000110011001111000011110000? orsomething else?

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How data is represented, continuedThe only way to know is by measuring the waveform against time:10 seconds

IF we are transmitting at a rate of:1 bit per second, then the bit stream would be 001010110011002 bits per second, then the bit stream would be 0000110011001111000011110000

Serial portDuring the first week we briefly mentioned the serial port on a PC. Its function is to transmit or receive one bit at a time. The typical maximum rate of serial ports on PCs is 115,400 bits per second, or 115,400 baud

The serial port is only able to transmit data about 15.3m (50') before the electrical properties of the wire seriously degrade the signal (at 20Kbps).

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ModemsA means to send data over longer distances has been devised using older technologies (such as the phone) and newer technologies that are specifically designed for computer equipment.

A modem is a device that modulates a digital data onto an analog signal, and demodulates the analog signal, providing digital data.

An analog signal, called a carrier, can be modulated in two basic ways:amplitudefrequency

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Modems, continuedamplitude - two wave-forms with different amplitude

The crests and troughs line up.The height, amplitude, of the waves are different (1:2)

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Modems, continuedfrequency - two wave-forms with different frequency

The amplitude of the waves are the sameThe number of repetitions per unit time, frequency, are different (1:1.5)

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Modems, continuedBy altering the amplitude or the frequency of the carrier wave, information can be transmitted. These are how the two types are radio transmissions are classified. The AM radio uses Amplitude Modulation to transmit the audio, and the FM radio uses Frequency Modulation.

This is an example of how digital information may be transmitted on a carrier:

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BandwidthAlthough the PC can transmit data at very high rates, modems are restricted in their speed due to the limitations of the phone system.

The amount of data that can be transmitted per second using any particular medium is defines its bandwidth.

The greater the bandwidth, the more data that can be transmitted in time.

Other communications typesThere are other communications types that yield better bandwidth:

ISDN (Integrated Services Digital Network)ADSL (Asymmetric Digital Subscriber Line)cableT1

Each of these technologies require more expensive equipment to be installed.

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Other communications types, continuedThe approximate speed of each of these technologies is:

ISDN - 128Kbps1M DSL - 1Mbpscable - 1.2MbpsT1 - 1.5Mbps

SynchronizationWhen talking about the number of bits per second being transmitted, a means for each side of the communications link needs to be synchronized so that the bits are properly interpreted.

There are two schemes to transmit data that takes the synchronization into account:

asynchronous transmissionsynchronous transmission

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Asynchronous transmissionAsynchronous transmission is based on one character being sent at a time. The line remains idle until a start bit is sent. When the start bit is received, the receiver starts a timer to accept the bits at a predetermined speed. After the character sent, 1 or more stop bits are then sent:

idle............ S 0 0 1 10 1 0 1 S idle................................

In this example the word-size was defined to be 8-bits (ie. ASCII-8), with 1 stop bit.

Synchronous transmissionSynchronous transmission is based on large messages being sent at a time. A message is preceded by a pattern of bits that allow the clocks/timers at both the sending and receiving sides to synchronize.

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Asynchronous/Synchronous transmissionsA representation of the two transmissions methods is:

Because there are no start and stop bits for each character in synchronous transmission, it is faster than asynchronous transmissions.

Asynchronous transmission increases the number of bits sent by a factor of 10/8 (1.25) assuming 8 bits, no parity, 1 stop bit. To send 1KB of data:

8192 data bits+2048 start and stop bits=====10240 total bits sent

The serial port on a PC is asynchronous.13


Data flow directionsThe data in a communications link can flow in one-way or two-way directions. This is defined by the terms:

SimplexHalf-duplexFull-duplex

SimplexThe data flows in one direction only. This is like listening to the radio in your car, all the information flows to you. You don't send information back through the radio.

Half-DuplexThe data flows in two directions, but only not simultaneously. This is like a polite conversion. The first person talks while the second person listens. When the first person stop talking the second person is then free to talk, while the first person listens.

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Full-DuplexThe data flows in both directions simultaneously. Each side of the conversation can listen and talk at the same time.

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Background, revisitedThere are various ways that users can interact with a computer system.

terminal to multi-user systemPC to multi-user systemPC on a network

Terminal to multi-user computerA terminal is a specialized piece of hardware that contains the monitor and keyboard and some means of data transmission.

The terminal has no processing or storage capabilities. It simply provides an interface to a multi-user, central computer system.

All processing and storage is on the central computer.

Predominate technology up to the mid to late-eighties.

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Terminal to multi-user computer

IBM

Central Computer

Terminal

Terminal

Terminal

Terminal

Front-End Processor(FEP)

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PC to multi-user computerOnce PCs made their appearance in the workplace, they provided a means for users to perform some local processing and storage. However, the mainframe or multi-user system still provided centralized processing and storage, the PC was just an adjunct.

Data is commonly transferred between the PC and the mainframe. To transfer from PC to mainframe is to upload, the reverse is to download.

File transfers can be accomplished using standardized protocols such as Xmodem, Ymodem, Zmodem and Kermit.

In this type of environment, the PC needs to run specialized software that emulates the older terminals. For UNIX systems the terminal type that is most commonly emulated is the VT100. For IBM mainframes, the 3270 terminal.

VT100 - can be emulated using HyperTerminal (distributed with Windows)

3270 - can be emulated using IBM Communications Manager (licensed software)

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PC to multi-user computer

IBM

Central Computer

Front-End Processor(FEP)

PC

PC

PC

PC

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PC on a networkAlthough mainframe and multi-user computers are still very much a part of business today as they still outperform the best PCs. However, by networking computers data can be shared and processed outside of the mainframe.

The following sections deal with the technology of networking.

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Network topologiesA computer can be connected to only one other computer, or it can be connected to a network of computers. The network can be laid out in different arrangements, or topologies.

The topologies that we will look at are:Star networkRing networkBus network

There are variations on these networks.

Star networkA central computer is responsible for controlling all activity on the network. The workstations communicate with the central computer for all requests.

The problem with this arrangement is if the central computer fails, the network fails. Also, the speed of the network will be restricted by the speed of the central computer.

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Ring networkThe computers are arranged in a ring, where a computer only talks to the computer beside it.

The problem with this arrangement is that if one computer fails, then the ring is broken and the network fails. The advantage of the ring network is that there is no risk that messages will collide with each other (see Bus Network).

Bus networkThe computers are all connected to a common line. Any computer can talk to any other computer in this network.

The advantage of the bus network is that if any computer fails, the network continues to operate. The problem with this network is if more than one machine sends a message at the same time (collision).

The problem of collisions is why the ring network can support more computers than the bus network.

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Network topologies, continuedThis is a diagram of each of the network topologies:

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Classification of networksA network can be classified as either:

Wide Area Network (WAN)Local Area Network (LAN)

A WAN refers to a network that spans a large geographic area, from the size of a city to a global network.

A LAN refers to a network that spans a small geographic area, from an office to complex of buildings such as a school campus.

Components of networksThere are various devices that are part of connecting computers and networks together. The short list discussed here is:

Network Interface cardBridgeGatewayRouter

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Network Interface CardThe Network Interface Card (NIC) is used to connect a computer to a particular type of network. An Ethernet NIC will not run on a Token Ring network, and vice versa.

The wiring of the network is connected to the NIC.

BridgeA bridge is a means to connect two similar networks. If a message is destined for a computer on another network that is also connected to bridge, the bridge will forward the message onto the second network

GatewayA gateway is used to connect two dissimilar networks. Some form of protocol conversion may take place.

RouterA router is very much like a bridge, and can even incorporate gateway functions. However, it is more complex in that it looks for ways to send traffic through a network of networks, and find the best route for the message.

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Sample networkThis is a diagram of the various components of the network

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ProtocolsA protocol is a set of rules by which communications are controlled. If a set of computers use the same protocol, then they will be able to communicate with each other.

If the protocol is standardized internationally, then regardless of the computer system, and provided the protocol was properly implemented then the computer system will be able to communicate with any other computer system that implements the same protocol.

The protocol for the Internet is TCP/IP (Transmission Control Protocol / Internet Protocol).

Some other protocols are IPX/SPX, NetBios, IEEE802.2, DECnet, AppleTalk

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Protocols, continuedThere are protocols that allow for the TCP/IP protocol to be used over a serial connection (a modem). Two are mentioned here:

SLIPPPP

SLIPIs an acronym for Serial Line Interface Protocol. This is an older protocol that supports TCP/IP only. The IP address of the PC as well as the IP address of the server that you are dialling into must be known. (more on IP Addresses later).

PPPIs an acronym for Point-to-Point Protocol. This newer protocol supports TCP/IP as well as other protocols. Furthermore, multiple protocols can be running simultaneously through PPP (the term ‘protocol stack’ refers to the protocols that are available/running on a machine). PPP will automatically configure itself during the connection to determine what IP address to use for the PC as well as the server.

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TCP/IP ApplicationsThere are two TCP/IP applications that are commonly used. It is important to note that these applications are specific to TCP/IP networks. They are not defined for other protocols.

The applications are:telnetftp

TelnetTelnet is a terminal emulation package. This utility will be used in our labs when connecting to the RS/6000 (UNIX). Telnet will be configured to emulate a VT100 terminal.

FTPFTP is an acronym for File Transfer Protocol. This application will also be used in our labs to transfer data between the PC and the RS/6000.

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IP AddressesIn order to determine where a packet needs to be sent, or where to send a response, addresses are used.

A TCP/IP address is normally displayed in 'dotted-decimal' notation. The address consists of 4 numbers (each between 1 and 254) separated by a period. For example: 142.204.57.135

(the 0 and 255 values have special meaning that will not be discussed here)

The following is an Instructor-led in-class exercise to see these addresses in action:

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IP Addresses, continued1.Determine what the IP address of your PC is2.Send a 'test' message to the Instructor's PC3.Send a 'test' message to 142.204.57.1354.Determine the route that your message took going to the Instructor's PC5.Determine the route that your message took going to 142.204.57.135

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The InternetThis is a diagram of the Internet (www.lumeta.com), colour coded by IP address

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Browsers and the Internet

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Browsers and the Internet

BackgroundA browser is an application that facilitates the displaying of information retrieved from the Internet.

Although many people have surfed the web, not everybody has learned some of the basic definitions used in the web.

This will be a general discussion of terminology and structure of browsers, as well as a brief look at search engines.

I am making an assumption that everybody is familiar with a GUI interface, and know how to use scroll bars, pull-down menus and buttons.

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Browsers and the Internet, continued

BrowsersThis is a diagram of a browser with the parts labelled:

Welcome Banner

Pull-down menus

Buttons

Address window

Browser controlpanel

Browser displaywindow

Status line

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Browsers, continuedThere are terms used in the Internet, usually in conjunction with browsers.

URLFramesPlug-insJavaSearch engine

These terms will be defined in the next few pages.

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URLURL is an acronym for Uniform Resource Locator. It is the name given to a particular site or page within a site.

The URL is composed of:

The domain is structured as a hierarchy of nodes, like a company's organizational chart. At the top of the Internet is the root node (symbol is '.', but nobody uses it). The next level is the top-level domains such as .com .ca .net. The last levels are used to identify the company or name and then the server or partition.

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URL, continuedThe most common top level domains are:

The following slides show the progression from a file, to the top level domain of:

ilearn.senecac.on.ca/homepage/Travis.Mander/index.html

(this is representative only)

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URL, continuedilearn.senecac.on.ca/homepage/Travis.Mander/index.html

homepage

Travis.Mander

index.html

ilearn

39



ilearnwww

senecac

40



senecac

humberc

on

41



on

pq

mb

nb

ca42



caca

com

mil

edu

netorg

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FramesFrames are means to subdivide the browser display window into multiple, independently control windows. In this example, there are five frames in the browser display window.

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Plug-insPlug-ins are additional software modules that can be hooked into the browser to enhance its functionality.

Some common plugins are RealPlayer, Adobe Acrobat Reader, Quicktime.

To find out what plugins are installed in a browser enter the following command in the address window:

about:plugins

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JavaJava is a programming language developed by Sun Microsystems. It is very popular on the Internet because it is platform independent.

Platform independentWhen a program is written, it is usually targeted to run on a specific operating system. For example if you take an application that was written for the Mac and try to run it on a Windows95 PC, it will not run.

Java is different, in that the Java code can run on any machine IF it has a Java Virtual Machine installed.

The JVM is written for a specific operating system. The Java code is written to run on the JVM. Therefore any Java programs can run any operating system.

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Search enginesThe Internet is a vast landscape (cyberscape?) of computers with useful and useless data / information / misinformation, etc.

To try and find anything on the Internet is an impossible task without some means to sift through the sites and categorize the data.

A search engine provides this functionality (assuming that you can find a site for a search engine).

Knowing how some of the search engines behave will help you make intelligent decisions as to which engine to use for which purpose.

Some engines try to catalogue all the words it finds on the sites, others are research based where people sift though the sites and catalogue the sites using predefined criteria.

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Search engine typesIn general there are three classes of 'Search Engines':

DirectoryHuman researchers organize sites by content categoriesSearch engineSoftware that resides at a site and uses a set of rules to automatically catalogue internet sitesMetasearch siteSoftware that searches 'search engines'

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Search engines - Directories

Well-organized categories let user switch from browsing to searching in a certain area. Finds only keywords (not any word on any site).

Reviews entries before including, links to articles written for NetGuideNetGuide

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Search engines - Search Engines

Very fast. Indexes every word of every page of every site.

Fast. Unique search options let you restrict searches. Very comprehensive. Excels at finding current news

Powerful and well organized. Groups results by subject, type, source and language

Numerous search options. Comprehensive directory. Good returns on simple searches

Searches newsgroups, FAQs and e-mail addresses. Extras such as foreign language searches, and searches by geographic regions

Good returns for simple searches. Provides related hints and an array of extra content. Provides directory as well.

Results ranked by algorithm based on number of links from other pages

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Search engines - Metasearch Engines

Accepts search terms and submits to several popular search engines; eliminates duplicates and ranks by relevancy

Well-designed, easy-to-use interface

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OSI Reference ModelFOR INTEREST ONLY

OSI Reference Model

BackgroundThe OSI Reference Model (Open Systems Interconnection) was developed by the

International Standards Organization (ISO).

Protocols can be defined using this model. There are 7 layers to the model. Each layer solves a small portion of the problem of getting data from one location to another, independent of what technology exists at each end of the communications link:

7 - Application Layer6 - Presentation Layer5 - Session Layer4 - Transport Layer3 - Network Layer2 - Data Link Layer1 - Physical Layer

It is not carved in stone, it is a model to work towards when creating a protocol. For example, TCP/IP has only 4 layers. Some of the OSI layers are combined into one TCP/IP layer (1+2 and 5+6+7).

OSI Reference Model, continued

1 - Physical Layer

2 - Data Link Layer

3 - Network Layer

4 - Transport Layer

5 - Session Layer

6 - Presentation Layer

7 - Application Layer

1 - Physical Layer

2 - Data Link Layer

3 - Network Layer

4 - Transport Layer

5 - Session Layer

6 - Presentation Layer

7 - Application Layer

abc

abc

abc

abc

abcSending computer

Receiving computer


Application LayerUsers interact with the application layer.

Presentation LayerSyntax and representation of data is handled here (ASCII, EBCDIC), encryption/decryption, compression/decompression of data.

Session LayerControl mechanisms to establish, maintain and synchronize communication. Control of Full Duplex / Half Duplex sessions is an example.

Transport LayerProvides end-to-end accountability through use of sequence numbers and acknowledgements


Network LayerAddresses are managed at this layer, as well as routing and congestion control.

Data Link LayerData packets are encoded/decoded into bits. The bits are transmitted in groups of bits called frames.

Physical LayerThis layer maintains the physical circuits between two devices. It provides the means to send/receive data on a carrier.

ics124 session 4 introduction to networking 1. objectives: by the end of this session, the student...

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