Networks University of Stirling CSCU9B1 Essential Skills for the Information Age. Content

Transcription

1 Networks 2 Lecture Networks 2/Slide 1 Content How is Internet connected? Internet backbone Internet service providers (ISP) How to connect a home computer to the Internet? 1. Phone modem 2. Digital subscriber line (DSL) 3. Cable modem How do networks work? Packet switching Open systems Protocols Lecture Networks 2/Slide 2 Page 1 1

2 World-WideWeb: nodes are web documents, connected with directed hyperlinks (URLs). Internet: nodes are the routers and computers, the edges are the wires and cables that physically connect them. Lecture Networks 2/Slide 3 Lecture Networks 2/Slide 4 Page 2 2

3 The Internet backbone A set of high capacity data routes that carry Internet traffic (high data transfer rates: Mbps, Gbps) Analogy with human backbone or spine Provided by various companies (per regions) such as US: AT&T, Verizon, NTT, etc. UK: British Telecom China: China Telecom, China Unicom Germany: Deutsche Telekom AG Use special optical cables Note: There is no really central network. Internet is redundant. Lecture Networks 2/Slide 5 Kbps, Mbps, Gbps The data rate of a computer network connection is normally measured in units of bits per second (bps) Network equipment makers rate their products using related, larger units of Kbps, Mbps and Gbps. Kilobit per second (Kbps): 1000 bps Megabit per second (Mbps): 1000 Kbps or one million bps ( bps) Gigabit per second (Gbps): 1000 Mbps or one million Kbps or one billion bps ( bps) What is a bit? Contraction of binary digit Basic unit of information in computing and digital communications One of two values (0,1) physically implemented with a two-state device Lecture Networks 2/Slide 6 Page 3 3

4 The internet backbone: BT Core topology of the BT IP Network within Europe, and its connectivity to the USA. BT also offers IP services globally through strategic supply arrangements. Lecture Networks 2/Slide 7 Internet backbone: NTT network Ranked among the top 5 networks worldwide World s largest transpacific network Lecture Networks 2/Slide 8 Page 4 4

5 Internet service providers (ISP) Company or organization that offers users access to the Internet and related services. Most ISP are telephone companies or other telecommunications providers ISPs connect directly to the Internet backbone, or connect to larger ISPs with a connection to the backbone Top ISP in the UK: Lecture Networks 2/Slide 9 How to connect a home computer to Internet? Three main technologies available to connect a home computer to the Internet 1. Phone modem converts computer data into an analog audio signal for transfer over a telephone line, and then a modem at the destination converts it back again into data 2. Digital subscriber line (DSL) uses regular copper phone lines to transfer digital data to and from the phone company s central office 3. Cable modem uses the same line that your cable TV signals come in on to transfer the data back and forth Lecture Networks 2/Slide 10 Page 5 5

6 Phone modem Use the telephone system, which is already in place Modem: stands modulator/demodulator A phone modem converts computer data into an analog audio signal for transfer over the telephone line, a modem at the end converts it back into data again Establish a telephone connection between your home computer and a computer that is permanently connected to Internet ISP provides the right to call to one or several (preferably local) computers set for this purpose Data transfer limited to that of analogue voice communication: 64 kbps Lecture Networks 2/Slide 11 Digital subscriber line (DSL) Still uses the telephone system (copper lines) without preventing preventing normal use of the telephone line BUT, treats data as digital rather than analogue High speed connection Most telephone companies today offer DSL The telephone company, may become your ISP, or sells the use of its lines to a third-party ISP. No need to dial in. An active connection is maintained between your home an a computer at the ISP. Your home should be close to the DSL central office. The farther away you get from the central office, the weaker the signal becomes. What is the difference between analogue and digital signals? Lecture Networks 2/Slide 12 Page 6 6

7 Cable modem Many people get their TV signal from cable television CATV provides a clearer picture and more channels CATV can also provide a high connection to the Internet Uses either Coaxial cables (radio frequency signals) Fibre-optic cables (light pulses) All nodes for cable subscribers in a neighbourhood connect to a cable company's central office, known as the "head end. The cable company then connects to the Internet using either: fibre optic cable or digital satellite and microwave transmission Like DSL, cable provides a continuous connection with an ISP Optical fibre cable Coaxial Cable Lecture Networks 2/Slide 13 Broadband A connection in which transfer speeds are faster than 1 Mbps (up to 100 Mbps or more) DSL connections and cable modems are broadband connections Debate between the DSL and cable modem communities continues, to have the larger market share The speed for downloads (getting data from the Internet to your home computer) may not be the same as uploads (sending data from your home computer to the Internet) Most traffic for home users consists of downloads. Therefore, companies devote more speed to downloads. Lecture Networks 2/Slide 14 Page 7 7

8 Computer network hierarchy POP (Point of Presence) : place for local users to access to the organization network (e.g. through a local phone or dedicated line) NAP (Network Access Point): centralized point where Internet traffic is exchanged or shared. Serve to tie ISPs Lecture Networks 2/Slide 15 How do networks work? Communication requires that devices agree on the format of the data Software is required to organise and structure users data Protocol: set of rules defining format of the data. Prepare information to be transmitted via hardware (encoding/decoding) Users Software Protocol software virtual links Users Software Protocol software Hardware physical link Hardware Lecture Networks 2/Slide 16 Page 8 8

9 Packet switching Messages are divided into fixed-sized, numbered packets; packets are individually routed to their destination, then reassembled Packet: A unit of data sent across a network Router: A network device that directs a packet between networks toward its final destination Lecture Networks 2/Slide 17 A logical progression... Open systems Proprietary system A system that uses technologies to kept private by a particular commercial vendor Interoperability The ability of software and hardware on multiple machines and from multiple commercial vendors to communicate Open systems Systems based on a common model of network architecture and a suite of protocols used in its implementation Lecture Networks 2/Slide 18 Page 9 9

10 Open systems Open Systems Interconnection Reference Model A seven-layer logical break down of network interaction to facilitate communication standards Each layer deals with a particular aspect of network communication The layers of the OSI Reference Model Lecture Networks 2/Slide 19 Summary How is Internet connected? Backbone, Internet providers How to connect a home computer to Internet How do networks work? Packet switching Open systems and protocols Lecture Networks 2/Slide 20 Page 10 10

11 Networks - References N. Dell & J. Lewis, Computer Science Illuminated, Chapters H.L Capron & J.A. Jhonson, Computers Tools for an information Age, Chapters 7-8 How Stuff Works - Internet Infrastructure: Lecture Networks 2/Slide 21 Page 11 11