Md Arafat Hossain

1. Lecture 1: Communication Networks and Services Md Arafat Hossain Department of Electrical and Electronic Engineering, Khulna University of Engineering and Technology, Khulna, BANGLADESH

2. Introduction • Communication Networks What has made possible in Telemedicine? Physicians can share ideas Surgeons anywhere in the world can perform a single operation together irrespective of where the operating theatre is. Nurses and paramedics can retrieve a patient’s record anytime anywhere

3. Communication basics What is a communication system? “Exchange of information” • Analog source of information Voice, video, are analog continuous time signals • Digital source of information Text, data files etc.

4. Modulation and Demodulation • Baseband signal or modulating signal (low f) & Carrier signal (high f) • Baseband signals: • Voice (0-4kHz) • TV (0-6 MHz) • A signal may be sent in its baseband format when a dedicated wired channel is available.Otherwise, it must be converted to passband. • Modulation: The process of shifting the baseband signal to passband range is called modulation. • Demodulation: The process of shifting the passband signal to baseband frequency range is called demodulation. • Why do we need modulation? • Simultaneous transmission of several signals • Practical Design of Antennas • Exchange of power and bandwidth

5. Modulation techniques - analog

6. What is analog communication? • Analog mapping of analog source directly into analog transmit signal • No intermediate mapping into a discrete finite set of symbols

7. What is digital communication? • Digital communications involves: • Mapping into a discrete finite set of modulated symbols • Modulated symbols are analog in nature and sent over noisy communication channel • Demodulation Involves: • Analog symbols back into discrete finite set • Providing error correction to decoded symbols

8. Digital vs. Analog • Presently moving away from analog towards all digital end to end networks and links • • Reasons • – More digital sources of information than analog. Computer networking, internet, voice communication at saturated level • – Potentially less bandwidth per unit of information (example voice encoding) • – Effective error correction coding, message control etc. • – Regenerate signal along path between Tx and Rx • – Analog circuitry is finicky and therefore expensive • – Advancement in DSP: cheaper to integrate, sophisticated algorithms

9. Components of a digital communication link Data file, voice, video, picture etc Generates the appropriate output. Compression and encryption Adds coding for error detection/correction, Also adds framing and synchronization bits checks/corrects any errors Analog upconversion to an RFcarrier etc. an analog downconvertor from an RFcarrier to baseband. signal becomes distorted and noisy before it appears at the receiver end

10. Communication channel • Channel is always analog continuous time in nature • Regardless of whether source is analog or digital • Challenge for communications engineer is: • – that channel resources are limited (power, bandwidth etc.) • – Interference noise in channel • – Distortion effects

11. Types of communication channel • Guided • – Twisted pair telephone cable, PC cables • – Coaxial cable, optical fiber, waveguide • Radiated • – Acoustic • – Point to point wireless • – Mobile wireless • – Satellite wireless • – Wireless modem e.g 802.11

12. Communication types: wired vs. wireless (cont.)

13. Communication types: wired vs. wireless

14. Noise and Interferrence • Wide bandwidth independent additive noise (e.g. thermal) • “Lower frequency ”interference from lightning, electrical machines etc • Other user interference from adjacent bands • Hostile jamming • Controlled interference from multiple users on the same channel e.g. CDMA • Receiver self jamming –malfunction, out of tune, not properly synchronizing etc. • Variable channel conditions, fading, shadowing etc. (mobile radio communications) • Ionosphere interference, time variable plasma effects

15. Wired channels: conducting vs. optical cable Twist pair cable: a certain voltage representslogic ‘1’ and another voltage level represents logic ‘0’……define by encoding mechanism An optical fiber is a waveguide for light Core: inner part where wave propagates Cladding: outer part used to keep wave in core • Buffer: protective coating SiO2 doped with GeO2 Jacket: outer protective shield n1 n2 Refractive index, n2 > n1

16. An optical fiber is a waveguide for light Core: inner part where wave propagates Cladding: outer part used to keep wave in core • Buffer: protective coating SiO2 doped with GeO2 Jacket: outer protective shield n1 n2 Refractive index, n2 > n1

17. Wired channels: optical fibre How does it work? n1 > n2 n2 Escapes from core Cladding n1 Core i Stuck in core c i i c = Critical Angle, i c for total internal reflection 3D-printed Optical fibre

18. Advantages of optical fiber communication Why optical fiber is the ultimate choice for communication backbone network? • Enormous potential bandwidth (Δf~ 2 x 1013 Hz; fc ~ 2 x 1014 Hz) • Low transmission losses: as low as 0.1 dB/km • Immunity to electromagnetic interference • Smaller size and weight than copper cables (<human hair) • Ruggedness and flexibility • Greater security • Falling cost • Long repeater spacing

19. Data Transmission Speed (cont.) Goal of a communication system: “Message signal is delivered to the user efficiently and reliably, subject to certain design constrains; allowable transmit power, available channel bandwidth and affordable cost of building the system”. Fixed for a given transmission medium, but it may be possible to increase the data transmission rate by stuffing more bits into one ‘baud’. “count of the number of changes of electronic states per second” (e.g. Cu cable 1K baud = changes voltage level 1000 times/s ) The number of bits n per baud has a simple relationship of: L = number of different signal levels

20. Data Transmission Speed Bandwidth: refers to the band of frequencies that an electronic signal occupies when transmitting data across the channel. Measured in Hz. Example: telephone channel that transmits voice data between 300 Hz (minimum frequency) and 3400 Hz (maximum frequency) has a bandwidth of 3.1 KHz Data transmission speed Channel bandwidth Nyquist theorem