© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 1 Transmission Media Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya,

Medium and Physical Layer

Classification of Media Guided Unguided

Noise Noise  Undesirable signals added between the transmitter and the receiver Sources: Heat in cables Electromagnetic radiation

Widely-Used Copper Wiring Twisted pair Unshielded twisted pair (UTP) Shielded twisted pair (STP) Coaxial

Twisted Pair Copper Wiring

UTP Cables Usually consist of several pairs

Shielded Twisted Pair Cables

Coaxial Cables

Categories of Twisted Pair Cables

Optical Fibers LED or Laser Diode Photodiode

Types of Optical Fibers

Optical Fibers vs. Copper Wires Optical fibers Immune to electrical noise Less signal attenuation Higher bandwidth Copper wires Lower cost Less expertise and equipment needed Less easily broken

Infrared (IR) Communications IR is a form of electromagnetic radiation Similar to visible light But invisible to human eyes Used in TV remote controls Can reflect from smooth, hard surfaces

RF Communications RF – Radio Frequency Most common form of unguided transmission Can traverse long distance and penetrate objects such as walls and buildings

Electromagnetic Spectrum 1 kHz1 MHz1 GHz1 THzVisible light RF range

Satellite Communications Classified by orbit types Low Earth Orbit (LEO) Medium Earth Orbit (MEO) Geostationary Earth Orbit (GEO)

GEO Communication Satellites GEO satellites appear stationary on the sky Orbit distance is 35,785 km

GEO Sat: Example How long does it take a signal to travel from a ground station to another ground station via a GEO satellite? Given that light speed is 3  10 8 m/s

Measuring Transmission Media Two most important characteristics of a transmission medium are: Propagation delay Time required for a signal to traverse the medium Channel capacity (bandwidth) Maximum data rate that medium can support

Data Rate: Noiseless Channels Nyquist Theorem D – Data rate in bps B – Bandwidth in Hz K – number of signal levels D = 2Blog 2 K

265,000 = 2  20,000  log 2 K log 2 K = K = = 98.7 levels We need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need? Solution We can use the Nyquist formula as shown: Since this result is not a power of 2, we need to either increase the number of levels or reduce the bit rate. If we have 128 levels, the bit rate is 280 kbps. If we have 64 levels, the bit rate is 240 kbps. Example

Data Rate: Noisy Channels Shannon Capacity C – Capacity (maximum bit rate) in bps B – Bandwidth of the channel in Hz SNR – Signal-to-Noise Ratio C = Blog 2 (1+SNR)

A telephone line normally has a bandwidth of The signal-to-noise ratio is usually Calculate the theoretical highest bit rate of a regular telephone line. This means that the highest bit rate for a telephone line is kbps. If we want to send data faster than this, we can either increase the bandwidth of the line or improve the signal-to-noise ratio. Example

Measuring Power Levels Difference often measured in decibel (dB) Negative dB  Signal power gets attenuated (reduced) Positive dB  Signal power gets amplified

Summary Transmission media and noise Guided transmission Copper wiring: UTP, STP, coaxial Optical fibers Unguided transmission Infrared communications Radio frequency (RF) communications Satellite communications Characteristic of transmission media