# 4 - Long-Distance Communication(carriers and modems Sending...

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Unformatted text preview: Long-Distance Communication (carriers and modems) Sending signals over long distances Modems Leased serial data circuits Optical, radio and dialup modems Carrier frequencies and multiplexing Time division multiplexing Signalling across long distances Resistance in wires => signal loss => current Resistance cannot be propagated over long distances cannot A continuous oscillating signal will propagate continuous further than other signals further Modulation Send an oscillating carrier wave and then Send modulate it in some way modulate Technique originated with radio and TV (stations Technique use different carrier frequencies) use Transmitter generates carrier and modulates Transmitter according to data, receiver discards carrier according Two approaches from radio are frequency Two modulation (FM) and amplitude modulation (AM) modulation Amplitude modulation Change amplitude of the carrier according Change to the data to Frequency modulation Slightly change frequency of the carrier Slightly according to the data according Phase shift modulation FM and AM require at least one wave cycle FM to send a bit to Phase shift changes the timing of the carrier Phase and can send several bits per cycle and Note: Amount of phase shift can be measured How much of sine wave is "skipped" Example shows 1/4, 1/2 and 3/4 cycle Each phase shift can be used to carry more than one bit. In example: 00 - no shift 01 - 1/4 phase 10 - 1/2 phase 11 - 3/4 phase Thus, each phase shift carries 2 bits Modems Hardware that takes bits and applies Hardware modulation is a modulator modulator Hardware that takes a modulated wave and Hardware extracts bits is a demodulator demodulator Full duplex communication requires a Full combined modulator-demodulator (MODEM) at both ends (MODEM) Leased serial data circuits Long distance four wire circuits can be Long leased from a phone company (spare circuits are often included in trunk cables for expansion purposes) for Often called a serial line or serial data Often circuit circuit Example modem connection Optical, radio and dialup modems Modems also used with optical fibre, radio and Modems conventional phone connections conventional Dial-up modems work with the existing phone Dial-up system system mimic telephones use a carrier that is an audible tone use a single voice channel (2 wire circuit) and use co-ordinate to achieve full duplex communication communication Dial-up modem configuration Analogue and Digital Data And Signalling Signal Analogue Analogue Digital Telephone CODEC MODEM Digital Transmitter Data Digital Multiplexing Two or more signals with different carrier Two frequencies transmitted over one medium frequencies Several logical connections share a single Several physical connection - Frequency Division Multiplexing (FDM) Multiplexing Minimum frequency separation => requires Minimum high bandwidth connection high Spread spectrum - use of multiple carriers Spread to improve reliability to Also, single logical channel may Also, simultaneously use multiple carriers to improve performance improve Time division multiplexing TDM is an alternative to FDM where the TDM sources sharing the medium take turns (e.g., round robin) round Example 1 How fast data can be sent across a voice telephone system? Telephone system: Bandwidth = 3000 Hz S/N = 30dB decibels (db) = 10log10S/N Maximum data rate on a noisy medium is: D = B log2 (1 + S/N) (1 D =3000 x 10 ~30000bps Example 2 Exam question from last year: http://www.cs.nott.ac.uk/~mvr/1-ccn.doc Summary Signals degrade over distance Oscillating waves propagate further Modulation - FM, AM and phase shift Modems, including dial-up modems Multiplexing - FDM and TDM Summary of Part 1: data transmission Transmission media Local asynchronous communication and Local RS-232 RS-232 Long distance communication ...
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