© Oxford University Press 2011 Computer Networks Bhushan Trivedi, Director, MCA Programme, at the GLS Institute of Computer Technology, Ahmadabad

© Oxford University Press 2011 Chapter 4 The Physical layer

© Oxford University Press 2011 Duties of physical layer Machine port level addressing Transferring bits Synchronizing the sender and receiver Multiplexing multiple data streams

© Oxford University Press 2011 Machine port level addressing

© Oxford University Press 2011 Transferring bits

© Oxford University Press 2011 Multiplexing data streams

© Oxford University Press 2011 Synchronizing

© Oxford University Press 2011 Self Synchronization in Manchester encoding

© Oxford University Press 2011 Speed mismatch

© Oxford University Press 2011 Time Division Multiplexing

© Oxford University Press 2011 Inappropriateness of FDM and TDM for bursty data: Trouble when only one sender

© Oxford University Press 2011 Best case

© Oxford University Press 2011 Frequency

© Oxford University Press 2011 Wavelength

© Oxford University Press 2011 The Electromagnetic Spectrum Radio Waves Microwaves Infrared and Millimeter Waves The ISM Bands The optical light and Free Space Optics

© Oxford University Press 2011 The spectrum

© Oxford University Press 2011 Radio Waves

© Oxford University Press 2011 Radio Waves 10 4 to 10 8 Hz Frequency is less and waves are long Travel in all directions (omnidirectional) Passing through obstacles Travels a long distance Poor candidates for data transmissions

© Oxford University Press 2011 Radio Waves Subdivided into VLF, LF, MF, HF, and VHF VLF, LF, and MF waves are known as ground waves HF and VHF travel in straight line HF and VHF refracted back by ionosphere

© Oxford University Press 2011 Microwaves 10 8 to Hz Travel straighter and not in all directions The line of sight (LoS) requirement. Get more and more focused as the wavelength decreases

© Oxford University Press 2011 Microwaves Parabolic antennas Do not penetrate through the walls; have a tendency to bounce off the obstacles Waves above 4 GHz absorbed by raindrops. multipath fading No wiring between the sender and receiver

© Oxford University Press 2011 Microwaves Relatively inexpensive Licensing is required FCC (Federal Communications Commission) in US does this job. In India, this is done by DoT

© Oxford University Press 2011 Other part of the spectrum Infrared (TV remote control) Millimeter waves ISM bands – 902 to 928 MHz – 2.4 to 2.48 GHz – to GHz Only the middle is available in India Free Space Optics using visible light

© Oxford University Press 2011 Wired Physical Layer The UTP cable Total Internal Reflection principle Fiber Optic Cables Design of fiber cables Sending and receiving devices Comparison between UTP and Fiber Optics Other cables

© Oxford University Press 2011 UTP advantages Twists help cancel out crosstalk Inexpensive Possible to bend the UTP Technology behind UTP is fairly matured There is less attenuation in UTP cables The technology is evolving

© Oxford University Press 2011 Total Internal Reflection

© Oxford University Press 2011 Dispersion in fiber optic

© Oxford University Press 2011 Multimode - 1

© Oxford University Press 2011 Multimode - 2

© Oxford University Press 2011 Single Mode

© Oxford University Press 2011 Fiber cable structure

© Oxford University Press 2011 Oscillation of light

© Oxford University Press 2011 Sending and receiving devices

© Oxford University Press 2011 Comparison: UTP vs. FO Thickness Weight Photons vs. electrons Attenuation Erosion

© Oxford University Press 2011 Comparison: UTP vs. FO Effect of EM interference Leaking Bandwidth Cost Need for skilled engineer

© Oxford University Press 2011 Wireless Physical Layer Two special cases: hidden and exposed station Solution Components of the Wireless system – Antennas – Access Points

© Oxford University Press 2011 Multiple senders and receivers acting in parallel

© Oxford University Press 2011 Exposed station Problem

© Oxford University Press 2011 Hidden Station Problem

© Oxford University Press 2011 Solution

© Oxford University Press 2011 Wireless LAN components Modes – Ad-hoc (DCF) – Infrastructure (PCF) Antennas – Omnidirectional – Focused (Parabolic usually) Access point

© Oxford University Press 2011 Access Point

© Oxford University Press Ad-hoc and Infrastructure modes physical layer b OFDM and the a g

© Oxford University Press b 2.4 GHz ISM band Direct Sequence Spread Spectrum 11 Mb Ad-hoc mode uses CSMA/CA Available bandwidth depends on distance Covers more distance than a

© Oxford University Press 2011 OFDMA and a 52 narrow frequencies Transmission is distributed to all these freq Better immunity to interference Different schemes for different ranges Maximum bandwidth of 54 Mb a came a little later than b.

© Oxford University Press g Uses the same OFDM Operating in the 2.4 GHz ISM band Network deployed with b can be upgraded to 11g without much hassle b/g cards In India, the g is more significant

© Oxford University Press physical layer d, the standard for Fixed Wireless Broadband OFDMA and e

© Oxford University Press WiMax in Slang Achieve a data rate of 30–75 Mb Distance of coverage between 3 to 10 km WiMax forum takes care of interoperability issues of devices 10–66 GHz originally but now much lower d for fixed, later version e provides both, fixed and mobile wireless

© Oxford University Press d OFDMA (multi-user version of OFDM) with 256 sub-carriers (DMT) It is connection oriented Each user has a specific slot allocated to it for sending as well as receiving. The receiving slot is bigger than the sending slot. Initial part of frame contains the sending part from the base station and then the receiving part by the base station

© Oxford University Press d The second part is smaller than the first one, as uploading is usually less than downloading Two different frequencies are used. Hamming code for forward error correction. Frames are sent like a continuous bit stream which improves the bandwidth utilization. Different quality of service (QoS) is provided

© Oxford University Press 2011 Base station and subscriber station

© Oxford University Press 2011 OFDMA Sub-carriers with the same number belong to same sub channel A customer has one channel allocated A channel with more noise use QPSK while less noise use QAM

© Oxford University Press 2011 Wireless communication using Satellite Three different orbits GEO is almost full. The LEO is near to earth satellite phones with few miliwatts of power. 1 to 7 msec esponse time LEO satellites are less expensive to launch Teledesic and GlobalStar

© Oxford University Press 2011 Physical Layer based on telephone line The telephone The xDSL Discrete Multitone: Standard for ADSL and VDSL

© Oxford University Press 2011 xDSL DSLs have a continuous connection to the Internet The phone line can be used while surfing the Web Most service providers today provide their billing on either the usage or the time We need a splitter at the subscriber’s end The splitter is attached to a phone at one end and to an ADSL device at the other end.

© Oxford University Press 2011 xDSL The ADSL modem is connected to the computer, usually by an Ethernet cable Network Interface Device is needed at customer’s premise. DSLAM connects to the DSL line and forwards the data to the ISP. The frequency band is divided in voice band, upstream band, and downstream band.

© Oxford University Press 2011 ADSL Home Set up

© Oxford University Press 2011 Discrete Multi tone

© Oxford University Press 2011 Other ranges Cable Internet Whitespaces

© Oxford University Press 2011