Media for Transmitting Data Optical Communications • Description of optical communications: – – – • Advantages: – – – – • They use light as a carrier of information (as opposed to electrical signals) Light waves can be used for the direct transmission of signals. Use of Fibre Optics High bit rate – handles both speech and data Better quality service as it is less susceptible to interference Unaffected by electro magnetic disturbance (voltages, clicks, atmosphere) Difficult to tap into them - secure Disadvantages: – – Only economical when the bandwidth is fully utilised High cost of installation Examples of Optical Communications Computer networks – also utilise infra red Computer controlled telecommunications Transmitting signals in aircraft Measuring Instruments – devices that use laser to measure small distances • High voltage installations • • • • Fibre optic cable Summary • Sends pulses of light rather than electricity. • Expensive. • Runs over long distances. • Is very quick. • Can be used for: – video conferencing – interactive services. Wireless LANs Communications that take place without the use of wires or cables – Mobile phones – Networked computers – Television broadcasts etc – – – – satellite radio signals infrared light beams lasers Each workstation and file server has some sort of transceiver/ antenna to send and receive the data. Wireless LANs Advantages • • • • • • mobility and elimination of unsightly cables. It's fast (11 - 125Mbps) It has a long range (5,000 feet in open areas, 250 to 400 ft / 76 to 122 m in closed areas) It's easily integrated into existing wired-Ethernet networks. Are great for allowing laptop computers or remote computers to connect to the LAN. Also beneficial where it may be difficult to install cables. Wireless communication methods Disadvantages potential for radio interference due to • weather, • other wireless devices, • obstructions like walls. Communications media • Unshielded Twisted Pair (UTP) cable. used in telephone network and LANs – cheap to purchase – flexible and easy to install – used to provide telephone lines to offices – eases installation and management of the cabling – easy to upgrade (future networks will support this media) Other Communications media • Coaxial cable – high quality, well insulated cable • Communications satellite – in geosynchronous orbit CONNECTION OPTIONS FOR WANS Dial up, using analogue modems • Cheap option • Have to connect every time used • Low bandwidth (54 kilobits) CONNECTION OPTIONS FOR WANS Leased line • High bandwidth (can be gigabit with fibre optic cable) • Always on • High cost (several £1000s per annum) CONNECTION OPTIONS FOR WANS ADSL Asymmetric Digital Subscriber Line modem or cable modem • High bandwidth for price (e.g. 2 mb upload for £350 pa) • Always on • Not available in all areas ISDN Integrated Services Digital Network • The line is more expensive to rent than a standard line • There is no need for a modem as the signal is sent in digital form. • Uses existing telephone cables. Factors affecting rate of data transmission • The speed of the modem – Different modems vary in data transmission rates, typically from 9K to 56K bps (bits per second). • The nature of the transmission line – A digital line such as an ISDN line has a much higher transmission speed than an analogue line. • The type of cable used – Twisted pair has a transfer rate of 10Mbps; fibre optic cable is about 10 times as fast. • The type of transmission – synchronous or asynchronous. Definitions • asynchronous transmission – In modem communication, a form of data transmission in which data is sent intermittently, one character at a time, rather than in a steady stream with characters separated by fixed time intervals. Asynchronous transmission relies on the use of a start bit and stop bit(s), in addition to the bits representing the character (and an optional parity bit), to distinguish separate characters. • synchronous transmission – Data transfer in which information is transmitted in blocks (frames) of bits separated by equal time intervals.