Networks

Learning Objectives Explain the client-server model of networked computers. Give examples of applications which use the client-server model. Describe what is meant by the World Wide Web (WWW) and the Internet Explain how hardware is used to support the Internet: networks, routers, gateways, servers. Explain how communication systems are used to support the Internet: The Public Service Telephone Network (PSTN), dedicated lines, cell phone network Explain the benefits and drawbacks of using copper cable, fibre- optic cabling, radio waves, microwaves, satellites. Show understanding of bit streaming (both real-time and on- demand) Show understanding of the importance of bit rates/broadband speed on bit streaming.

Stand-alone machines A computer: cannot communicate with and is not connected to other computers. with all devices connected directly to it.

Disadvantages of stand-alone machines Users cannot easily share information Every computer needs to be connected to a printer. With these problems in mind, it was realised at an early stage that connecting these computers together would overcome many of these disadvantages.

Servers and Workstations / Terminals Workstations / Terminals: A computer connected to other computers. Servers: A computer that 'serves‘ and store files for the workstations. They are often more powerful than a standard computer because they need to store a vast amount of data.

Communications software Necessary to allow computers to communicate.

Computer Networks Advantages A user can access their files from any workstation / terminal. Files and software can be shared by users. Also some software can installed on all workstations from the server which makes installation quicker and easier. Users can communicate with others on the network. Individual workstations do not need a printer, one high quality printer can now be shared by everyone, thus cutting costs. Networks provide security: A user must have the correct Password and User ID in order to be able to access the information on the network.

Computer Networks Disadvantages Networks can be expensive to set up: They often involve taking up floors and ceilings to lay hundreds of metres of cables. The Server needs to be a powerful computer, which often means that it is expensive. Networks are vulnerable to security problems: Hackers, disgruntled employees or even competitors might try to break into the system to read or damage crucial information. Much effort is spent preventing unauthorised access to data and software.

Computer Networks Disadvantages If the main Server breaks down, then the whole system becomes useless and no-one can carry on working. Because networks are often complicated, they need expensive expert staff to look after them. As the number of users increase on the network, the performance of the system can be affected and things start to slow down.

World Wide Web / Internet The Internet is worldwide and intended to be accessed by anybody. A global network connecting millions of computers. The best “known” network. More than 100 countries are linked into exchanges of data, news and opinions. The World Wide Web, or just "the Web," as ordinary people call it, is a subset of the Internet. The Web consists of pages that can be accessed using a Web browser. The Internet is the actual network of networks where all the information resides. Things like Telnet, FTP, Internet gaming, Internet Relay Chat (IRC), and are all part of the Internet, but are not part of the World Wide Web. Things like Telnet, FTP, Internet gaming, Internet Relay Chat (IRC), and are all part of the Internet, but are not part of the World Wide Web.

Advantages Convenience: accessible at home, work, or through a wireless device accessible at home, work, or through a wireless device Easy to contact other people no matter where they are. Access to other cultures Access to a vast amount of information Can be used Night or Day - it never stops Can be used to learn (or teach) Makes it possible to Telework Possible to buy and sell things online

Advantages Can present their work more neatly Pupils can share ideas Less waste of paper

Disadvantages Must have a home computer Transmission of viruses Must have Internet / Modem Must remember passwords / usernames Pupils can copy off others

Disadvantages Undesirable sites may be accessed Have to pay for internet access May receive unsolicited s from undesirable characters Health reasons Demand on equipment Can waste time exploring unreliable sites Can get distracted playing games

Transmission Media (cables) Presented in order of performance (increasing expense, speed of communication, level of traffic that can be managed, difficulty and usable length of installation and decreasing susceptibility to interference)

Copper - UTP & Shielded TP Unshielded Twisted Pair (UTP) Two conductors, enclosed in a plastic sheath, twisted together. Two conductors, enclosed in a plastic sheath, twisted together. Shielded Twisted Pair Has a grounded screen around the conductors to shield the cable from extraneous electromagnetic radiation. Has a grounded screen around the conductors to shield the cable from extraneous electromagnetic radiation. Used in pre-existing telephone networks. Changing current denotes different signals.

Copper - Coaxial cable Baseband coaxial: Single copper braid shield. Broadband coaxial: Two shields 1 - aluminium & 1 - copper. Central conductor Plastic insulators Copper screen conductor A central conductor enclosed in a plastic sheath surrounded by a copper sheath which is surrounded by a plastic coating. Used in pre-existing telephone networks. Changing current denotes different signals.

Optical Fiber - Used as backbones (fast route) for LANs or in WANs Contains many optical fibers: Glass or plastic fibers designed to guide light along them. Glass or plastic fibers designed to guide light along them. Signals transmitted as light pulses so travel at the speed of light with the highest bandwidth currently possible. Signals transmitted as light pulses so travel at the speed of light with the highest bandwidth currently possible. Separate fibres can be used for separate signals. Separate fibres can be used for separate signals.

Wireless Communication Radio signals: Satellite communications. Microwave signals: Communication must be a straight line from sender to receiver. Wireless communication allows for mobile communication. what-when-how.com/data-communications-and- networking/communication-media-data-communications- and-networking

Routers LAN Router Public network Router LAN

Routers Connect different types of network together. Stores logical IP addresses and can deal with complex networks where there is more than one possible path to a destination by calculating the optimum route at the time. Stores logical IP addresses and can deal with complex networks where there is more than one possible path to a destination by calculating the optimum route at the time. i.e. Directs packets across a packet switched network. Is software based as it uses its own operating system to make its decisions. Is software based as it uses its own operating system to make its decisions.

Routers Directs messages down different routes according to their desired destinations. Use network addresses and addresses of other routers to create a route between two networks so must keep tables of addresses. Use network addresses and addresses of other routers to create a route between two networks so must keep tables of addresses. These tables are copied between routers using routing information protocol (RIP). These tables are copied between routers using routing information protocol (RIP). Often used to connects LANs & WANs e.g. connect a LAN to the internet using one IP address. Often used to connects LANs & WANs e.g. connect a LAN to the internet using one IP address. Operates at all layers up to the transport layer of OSI.

Routing Algorithm 1.Router receives a network level packet. 2.Network destination address is checked. 3.Packet is sent to the appropriate network.

Gateway A gateway is a network point/router that acts as an entrance to another network. On the Internet, a node/router or stopping point can be either a gateway node or a host (end-point) node/router. Both the computers of Internet users and the computers that serve pages to users are host nodes. The computers that control traffic within your company's network or at your local Internet service provider (ISP) are gateway nodes/routers. In the network for an enterprise, a computer server acting as a gateway node is often also acting as a proxy server and a firewall server

Proxy Server A proxy server is a computer setup to share a resource, usually an Internet connection. Other computers can request a web page via the proxy server. The proxy server will then get the page using its Internet connection, and pass it back to the computer who asked for it. Proxy servers are often used instead of router since additional software can be easily installed on the computer such as anti-virus, web filtering etc.

Bit / Baud Rate The speed at which a given amount of data can be transmitted. The number of bits that can be sent in one second is known as the BIT RATE. The units used to measure the bit rate are BAUD. 1 baud = 1 bit per second.

Transmitting Pictures / Graphics Pictures take up a lot of memory because each pixel must be defined. High resolution (quality) means more pixels and more colours. There is a limit to the amount of information that can be transmitted in a given time down the phone line. Choice 1: Have as much detail as possible and accept that it will take a long time to transmit it. Choice 2: Limit the resolution (quality) of the picture, meaning that there is less information to send so that the message will be sent more quickly.

Transmitting Text Text files contain a relatively small amount of data (compared to pictures / graphics / video) because each character only takes one byte. Text files are not time sensitive. They can still be read even if the bit / baud rate is slow and they get sent pretty quickly anyway due to their relatively small size.

Transmitting Video Large amount of data which must be transmitted in a standard time frame. A video cannot be run (watched) without a high bit / baud rate.

Importance of Bit / Baud Rate More important the more data there is. Due to the increasing time required for transfer. Different volumes per second mean that different transfer rates are appropriate. Videos demand a high bit / baud rate Contain a large amount of data. Must be transmitted in a standard time frame. Pictures take a long time without a high bit / baud rate. Large amount of data because each pixel must be defined. Text doesn’t need a high bit / baud rate. Small amount of data. Not time sensitive.

Bit Streaming Multimedia that is constantly received by and presented to an end-user while being delivered by a provider. Video on demand: Systems which allow users to select and watch/listen to video or audio content when they choose to, rather than having to watch at a specific broadcast time. Real-time: Refers to content delivered live over the Internet. Requires a form of source media (e.g. a video camera, an audio interface, screen capture software) and an encoder to digitize the content, a media publisher, and a content delivery network to distribute and deliver the content.

Plenary What are the advantages and disadvantages of using the Internet?

Advantages Allows communication. Pupils can share ideas. Large amounts of educational material. Allows students to access information to help with studies and to enjoy their learning. Allows student to learn at own speed / at own times.

Disadvantages Many sites do not give accurate information. Many sites give access to inappropriate material (downloading music / ordering drugs / watching pornography). Health and Safety problems. Sites available to buy coursework / helps to cheat in examinations. May allow hacker into your system / viruses prevalent.

Disadvantages Undesirable sites may be accessed Have to pay for internet access May receive unsolicited s from undesirable characters Health reasons Demand on equipment Can waste time exploring unreliable sites Can get distracted playing games

Plenary Explain the part played in network systems by: Switches Switches Routers Routers Bridges Bridges Modems Modems

Plenary Switches A device that allows many terminals all to use the same communications line at different times. A device that allows many terminals all to use the same communications line at different times.Routers Connects different types of network together. Connects different types of network together. Software at a node (on the network). Software at a node (on the network). Which directs messages down different routes. Which directs messages down different routes. According to their desired destination. According to their desired destination.

Plenary Bridges Links two LANs (which may or may not be similar). Links two LANs (which may or may not be similar). Uses address information in packets. Uses address information in packets. Has the ability to learn the layouts of the networks. Has the ability to learn the layouts of the networks. Can control access from one part of the network to the other. Can control access from one part of the network to the other.Modems Necessary if communication link is analogue. Necessary if communication link is analogue. Converts digital signals to analogue for transmission. Converts digital signals to analogue for transmission.

Plenary Explain the importance of bit rates to communication.

Importance of Bit / Baud Rate More important the more data there is. Due to the increasing time required for transfer. Different volumes per second mean that different transfer rates are appropriate. Videos demand a high bit / baud rate Contain a large amount of data. Must be transmitted in a standard time frame. Pictures take a long time without a high bit / baud rate. Large amount of data because each pixel must be defined. Text doesn’t need a high bit / baud rate. Small amount of data. Not time sensitive.