Postacademic Interuniversity Course in Information Technology – Module C1p1 Chapter 4 Communications, Theory and Media

Postacademic Interuniversity Course in Information Technology – Module C1p2 A three layers model. Applications Layer Internet & Transport Layer Networks Layer Connectivity Interoperability

Postacademic Interuniversity Course in Information Technology – Module C1p3 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

Postacademic Interuniversity Course in Information Technology – Module C1p4 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

Postacademic Interuniversity Course in Information Technology – Module C1p5 Parallel Transmission Disadvantages : Differences in propagation delay Cost of multiple channels Consequence : Restricted to very short distances Clock In computers, data is structured in bytes

Postacademic Interuniversity Course in Information Technology – Module C1p6 Serial Transmission Clock Serial Data b 0 b 1 b 2 b 3 b 4 b 5 b 6 b 7 b0b0 b7b7 Parallel in Serial out Serial in Parallel out Transmission rate expressed in bits/second

Postacademic Interuniversity Course in Information Technology – Module C1p7 Serial Transmission with clock/data multiplexing + ClockSerial Data

Postacademic Interuniversity Course in Information Technology – Module C1p8 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

Postacademic Interuniversity Course in Information Technology – Module C1p9 Digital Data Communications TX RX Analog communication channel Modem

Postacademic Interuniversity Course in Information Technology – Module C1p10 Encoding and Decoding Transmitter (Tx) –Input : stream of binary numbers –Output : stream of analog signals suitable for transmission over long distances Receiver (Rx) –Input : stream of analog signals generated by transmitter distorted by transmission channel –Compares each input signal with all signals which could have been transmitted and decides from which one the input is a distorted image. –Output : stream of binary numbers, preferably identical to the input of the transmitter

Postacademic Interuniversity Course in Information Technology – Module C1p11 Analog Transmission Channel Bandwidth –Difference between highest and lowest frequency of sine waves which can be transmitted –Number of possible state changes per second Signal to Noise ratio –S/N = (signal power) / (noise power) –S/N determines number of distinct states which can be distinguished within a given observation interval Characterized by : Frequency Received power B

Postacademic Interuniversity Course in Information Technology – Module C1p12 Binary vs. Multi-bit encoding Modulation rate = 1/t (in Baud) Data rate = (1/t) Log 2 n (in b/s) t 0001 t

Postacademic Interuniversity Course in Information Technology – Module C1p13 Shannon’s Theorem DataRate <= B.Log 2 (1+S/N) B : Channel Bandwidth (in Hertz) S/N : Signal to Noise ratio Example: Telephone channel, B = 3000 Hz, S/N = 1000 DataRate <= b/s