Nets1

Standalone Computer Blues (Why network?): 

Standalone Computer Blues (Why network?) A standalone computer can be used: to write programs or play games to use packages Write letters Keep accounts and so on But all of these things are just standalone applications Sometimes we need more: We need to use large, expensive resources Banks of CD-ROMS Expensive printers Large data stores We may want to get (send) information from (to) other people in our organisation around the world

The Network: 

The Network In order to use such tools and to carry out these tasks, computers must be networked That means the machines can ‘talk’ to each other so that the programs running on them can communicate and even allow the users to communicate with each other Networks are widely used around the world They can be large or small From just 2 or three machines linked together... To many thousands And networks can themselves be networked...

Different Kinds of Network: 

Different Kinds of Network We will look at four different kinds of network LAN MAN WAN Internet Each kind of network is distinguished from the other by size technology

LANs: 

LANs LAN stands for Local Area Network These can be fairly small (e.g. in an office),or relatively large (in a building, or on a campus) They are found on one single site They are used in businesses and in Universities for connecting shared resources Or to allow communication between employees They are often linked together and can be quite complex (Like the 1A11 lab you use)

WANs & MANs: 

WANs & MANs A MAN is a Metropolitan Area Network A WAN is a Wide Area Network Both are much bigger than LANs Both are generally composed of many LANs networked together The benefits are: Access to more information and expertise Video Conferencing between sites Use of more expensive resources (supercomputers, for example) Ease of communication

The Internet: 

The Internet The Internet is a network of networks It started as a US Military network for communications in the late 1960’s/early 1970s – the ARPANET: a cold war spin-off Its growth has been very fast The Internet is a ‘multi-protocol’ network of networks That means there are lots of different computers All talking different languages across their networks Which are all connected together via the Internet And using different protocols (‘languages’) to do different things To send electronic mail To read bulletin boards To browse the World Wide Web To transfer files between machines To log on to remote machines

Internet Connectivity: 

Internet Connectivity

How do networks work?: 

How do networks work? Networks consist of a number of computers connected to each other Software (called protocols) uses this connection to allow communication between software running on different machines

Protocols: 

Protocols A Protocol is a set of rules for communicating between machines In fact, humans have their own protocols too... Computers need stricter protocols than humans, because they’re less flexible The Internet uses a base protocol for machines to talk to each other It’s called TCP/IP Which stands for: Transmission Control Protocol/Internet Protocol TCP/IP is actually very powerful, but relatively simple Many different systems can communicate using it Other protocols (for specific tasks) are built on top of it. For example: electronic mail the World Wide Web file transfer Usenet News

ftp – File Transfer Protocol: 

ftp – File Transfer Protocol The Internet has a vast store of information All of this information is in files A file contains data interpretable as pictures,(even movies), sound, text, or programs... The simplest way of getting hold of this information is by using ftp ftp provides a means of connecting to remote sites to grab files If you have authorisation, you can grab files from a site But many sites also offer anonymous ftp, which allows anyone access to all the files in particular (public) directories The program you can use for ftp is called Fetch It presents you with a better interface than command line ftp...

telnet: 

telnet Telnet is an application which allows you to ‘log on’ to machines anywhere in the world All you have to know is their ‘address’ (which we’ll talk about in the next lecture) Oh, and often a username and password... Telnet is in fact used for a wide range of services Access to library catalogues e.g. Stirling library is library.stir.ac.uk Library of Congress at ... (exercise) Tools, such as online dictionaries remote use of your own site’s machines Here you do need a username (uid) and password So you can log on to your machine wherever you are in the world Telnet provides the base tool for using other machines. More sophisticated tools can still not provide all this

Usenet News/Bulletin Boards: 

Usenet News/Bulletin Boards This is a global news service It has been around for a long time (since the late seventies) It covers many topics, divided into rough categories Each topic has its own newsgroup For example: comp.ai for AI related information stirling.cs.3131 (guess...) alt.fan.terry.pratchett (guess again...) The number of different topics is very large, and there’s something for everyone The topics range from being recreational to work-related Simply posting a query can result in hundreds of answers

The World Wide Web: 

The World Wide Web This is a global information service running on top of the Internet It was developed at the European Centre for Particle Physics (CERN) in 1990 It is very widely used now The World Wide Web (Web, or WWW) delivers information from a host server to a client machine at the same or different sites The information is in HyperText format This means that it contains information which points at other information These pieces of information are called links The links may be to other documents at the same site They may be to documents at other sites around the globe (and these may have more links ...) The effect is a web-like structure of information

Browsing the Web: 

Browsing the Web In order to use the Web, you need a piece of software called a Browser Browsers are available for several machines, including Macintosh, Unix and the various flavours of MS Windows Here’s how they work: The browser contacts the place you want information from It is given the information (document) you requested The connection is closed The browser displays the information in HyperText format on your screen This is a client server architecture

Client Server systems: 

Client Server systems Many Internet tools use a client server architecture (including the WWW) So what exactly is a client-server architecture? The general idea is that information (or facility) is stored somewhere else and you need to get and display that information (use that facility) Client server architectures separate holding information from information display (the facility from its users) The server has the information (facility) The client does not but wants to use it.

Client Server Systems: 

Client Server Systems The client you use (for example, the web client) asks the server for the information you want The server sends it back Then the client displays the information to you This means that servers only need to be able to respond to requests for information The clients need only know about how to display the information they receive on the system they are on This specialisation makes it easier for the clients to be very user friendly

Browsing the Web (continued): 

Browsing the Web (continued) The information can be in many different formats: Text (with or without links) Graphics (photos, pictures, drawings, possibly with links) Sound Movies The browser itself cannot show all of these types of information It knows their type, and starts up ‘helper applications’ which can display them e.g. on the Mac: SoundMachine for sounds, Sparkle for movies When you click on a hyperlink, the process starts again There is a lot of ‘network traffic’ here, so sometimes the process can be very slow

The Web Protocol: 

The Web Protocol The World Wide Web uses a fairly simple protocol for transferring information It is the HyperText Transfer Protocol (HTTP) This sits ‘on top of’ the TCP/IP protocol the Internet uses In fact, web broswers can also use ftp, news and mail protocols too HTTP is a simple ‘client-server’ protocol The client is the web browser you use on the local machine The server is the Web Server, on this or another site

Web Browsing: addresses: 

Web Browsing: addresses Each document on the World Wide Web has a unique name the URL (Uniform Resource Locator) This is the address of the machine, with an extension for a file name So: http://www.cs.stir.ac.uk/~ces Is the address of a page (my ‘Home Page’) The first part (after the //) is the name of the machine The second part (after the single /) is the address of the file on that machine The program you use to browse the Web is called Netscape

Netiquette: 

Netiquette One of the “issues” tutorials will be about the Computer Misuse Act Some of that Act is relevant to network use When using the Internet (or any network) it is important to remember: Others are using it too Doing silly things may slow it down for all Saying silly things may upset a lot of people It pays to behave sensibly when using the Internet Because at the extreme, you can be removed from it There are various resources around the Internet concerned with proper social behaviour One of these is Phil Agre’s ‘Networking on the Net’ paper, which can be seen on the Web at: http://www.cs.stir.ac.uk/~chh/networking/ (The latest version can be dragged from rre-request@ucsd.edu)