Chapter 4: Physical Layer

Transcription

1 What are the Duties of physical layer? Machine port level addressing Transferring bits Synchronizing the sender and receiver Multiplexing multiple data streams Chapter 4: Physical Layer Machine port level addressing: Every machine that is networked contains a network interface card. Card can be one or many. Physical layer works with multiple cards. Different cards are associated with different ports of motherboard. These ports are also known as Interface. For every machine with multiple connections has multiple interfaces which is taken care by physical layer and ensured that data is passed only to correct interface. Transferring bits: Two ways of transferring bits: Analog and Digital. Sub: FON, bhargavigoswami@gmail.com Page 1

2 The mechanisms to find out standard to represent ones and zeros is popularly known as coding mechanism. Eg. Manchester Encoding Technique. CDMA: Code Division Multiple Access is used by third and fourth generation mobile phones. OFDM: Orthogonal Frequency Division Multiplexing used with and networks. Multiplexing data streams: Synchronizing: Self Synchronization in Manchester encoding Sub: FON, Page 2

3 Speed mismatch Time Division Multiplexing: Sub: FON, Page 3

4 Inappropriateness of FDM and TDM for bursty data: Trouble when only one sender Best case: Q. The Electromagnetic Spectrum: Electromagnetic energy travelling from sender to receiver. Different waves have different properties. Three Characteristics: Frequency, wavelength and power. Frequency: No. of times waves oscillates in unit time. More the oscillations, higher the frequency. Wavelength: Distance between two amplitudes or distance between two consecutive cycles of same wave. Wavelength is inversely proportional to frequency. Power: Every wave has energy in the form of power which degrades by covering distance. Entire electromagnetic spectrum indicates different bands of spectrum and frequencies used by cables. Eg. Fiber Optics has very high frequency compared to UTP cables. At high frequencies, data encoded per MHz is much more. o At lower end, 1MHz have 1 to 2 Mb data. o At upper end, 1MHz have 4 to 8 Mb data. Sub: FON, bhargavigoswami@gmail.com Page 4

5 Distribution: o Radio Waves o Microwaves o Infrared and Millimeter Waves o The ISM Bands o The optical light and Free Space Optics Sub: FON, bhargavigoswami@gmail.com Page 5

6 Radio Waves: Speed of light in vacuum: λf=c Where λ is wavelength, f is frequency and c is the speed of light. Even though equation is for vacuum, it remains same for other mediums to 10 8 Hz is the range of frequency. Frequency is less and waves are long Travel in all directions (omni-directional). Tend to scatter everywhere in surroundings. (unlike TV, no need of line of sight with satellite) Passing through obstacles, makes us easily listen radio indoor. Travels a long distance, as high frequency with small wavelength. Poor candidates for data transmissions. Prone to interference. If 2 users start transmission at a time on same frequency, signal destroys. That s y each radio station is allocated its own frequency like 92.7, 98.3, etc Sub: FON, bhargavigoswami@gmail.com Page 6

7 V-very, L-low,F-frequency, h-high,m-medium. 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, but how to use in earth curvature, so need to bounce off. Solution? HF and VHF refracted back by ionosphere (natural solution by charged atmospheric layer ionosphere) What is FM then? FM(Frequency Modulation) broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 to MHz is used (92.7, 98.3, etc) Microwaves: 10 8 to Hz, come in gigahertz range n more. Travel straighter and not in all directions The line of sight (LoS) requirement. Eg. Communication of conference failed due to day temperature defocused the line to sight of two buildings. Get more and more focused as the wavelength decreases. 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, solution? Reserved bandwidth. No wiring between the sender and receiver, just have high antennas between two large distance and communication starts. Relatively inexpensive Licensing is required FCC (Federal Communications Commission) in US does this job. In India, this is done by DoT(Department of Telecommunication) Support 50 to 60 km away communication. Govn control, but politics affect, eg. 3G case. Countries to (ITU-R International Telecommunication Unit-Radio) recommended 2MB spectrum to be used for communication, so that companies can produce devices useful worldwide, but no country followed it with the feeling of insecurity, only china followed it. Multipath fading: Some waves may be refracted off low-lying atmospheric layers and may take slightly longer to arrive than the direct waves. The delayed waves may arrive out of phase with the direct wave and thus cancel the signal. This effect is called multipath fading and is often a serious problem. It is weather (rain) and frequency dependent. Solution? 1. Some operators keep 10 percent of their channels idle as spares to switch on when multipath fading wipes out some frequency band temporarily. 2. Shut off links that are being rained on and route around them. Infrared and Milli-meter waves Widely used for short range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. Directional, Cheap and easy to build. Sub: FON, bhargavigoswami@gmail.com Page 7

8 Disadvantage: Do not pass thru solid objects. Advantage: will not interfere with a similar system in adjacent rooms or buildings. U can not control yr neighbour s tv. Hehehhe. Because of above properties.. No eavesdropping.. Hence secure.. 1 more Advantage: No government license is required. Limited use, it is not a major player in the communication game. Other part of the spectrum Infrared (TV remote control) Millimeter waves ISM bands o 902 to 928 MHz o 2.4 to 2.48 GHz o to GHz Only the middle is available in India Free Space Optics using visible light ISM: Industrial, Scientific & Medical Use ISM, Industrial, Scientific and Medical bands which is unlicensed bands. Low power, hence short range so that no interference from each other For unlicensed usage : o Garage door openers, o Cordless phones, o Radio-controlled toys, o Wireless mouse, o And numerous other wireless household devices use the ISM band Bluetooth and some of the wireless LANs operate in this band. The 5.7-GHz band is new and relatively undeveloped, so equipment for it is expensive, but since a uses it, it will quickly become more popular Classification of Computer Networks Transmission medium: 1. Guided (Wired) 2. Unguided (Wireless) S.No. Guided Media Unguided Media 1 The signal energy propagates within the guided media.i.e. through wires. 2 It is mainly suited for point to point line configurations. 3 The signal propagates in the form of voltage, current or photons. The signal energy propagates through air. It is mainly used for broadcasting purpose. The signal propagates in the form of electromagnetic waves. 4 Examples of guided media are:- Examples are:- =>Twisted Pair Cable =>Microwave or Radio Links =>Co-axial Cable =>Infrared =>Optical Fiber Cable Sub: FON, bhargavigoswami@gmail.com Page 8

9 Q. Wired Physical Layer Topic List: 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 UTP Cables: Unshielded Twisted Pair (UTP) Ethernet installation Wires are twisted in helical fashion like DNA. Why twisted? Twists help cancel out crosstalk, more twist, more immunity Inexpensive (2Rs to 5Rs for 1m wire) Possible to bend the UTP(unlike coax, fiber) Technology behind UTP is fairly matured, easy to find engineers who can install and maintain UTP cables. There is less attenuation in UTP cables The technology is evolving, Cat 3 support 25mb, Cat 5 support 125mb, Cat 6 offer 250mb, Cat 7 will offer 650mb 3,5,6,7 are set based on twist per centimeter. Sub: FON, bhargavigoswami@gmail.com Page 9

10 Q. Fiber Optics: Total Internal Reflection Principle: Consider two different transparent media. Ray is travelling from medium with higher refractive index (pure glass) to another medium with lower refractive index (ordinary glass). The light ray tends to reflect upon transition. At a particular angle of light ray refracts from the boundary between two media, travels along the boundary. This angle is called critical angle. Critical angle refracts back into the medium and does not leak outside. This phenomenon is known as total internal reflection. Fiber Optics Cable Fiber optic cable is made up on pure glass. Extremely transparent, easily can be bent. Offers least possible resistance to the light passing through it. Higher frequency than radio and microwaves. Frequency range: Hz. Bandwidth offered, 50 Terabytes. Detector is used to generate and detect light pulse signal. Three optical fibers with attenuation: 0.85 micron with 0.8 decibels/km attenuation 1.30 micron with 0.2 decibels/km attenuation 1.55 micron with 0.2 decibels/km attenuation All three ranges are to GHz. Problem of Dispersion. Solution: keep distance between two rays. Sub: FON, bhargavigoswami@gmail.com Page 10

11 Dispersion in Fiber Optics: Types of Fibers: 1. Multi mode Multiple signals travel thru same fiber at a time Angle remains independent of each other Ray bounce off different angle greater than critical angle Distance covered is few km Uses LED (Light Emitting Diodes) 2. Single mode Extremely thin so that signal don t bounce off Travels in straight line without bounce off More expensive Speed of 50Gbps and up to 100 km without amplification Uses laser device. Multimode : Multimode Subdivided into two types: 1. Single Step Index Multimode Fiber Cladding has less refractive index than core Core has consistent index throughout its width 2. Graded Index Multimode Fiber Higher refractive index at centre and reduces towards cladding Light ray travelling thru inner half of cable moves slowly in straight line due to higher refractive index of material. Light travel faster on outer side but they bend (refract) helically. Sub: FON, bhargavigoswami@gmail.com Page 11

12 Single Mode: Fiber cable structure: Comparison: UTP vs. FO Thickness (FO thicker than UTP) Weight (FO 1/100 th of UTP) Photons vs. electrons (Electrons feel more resistance, temperature, hence cross talk. Photons immune to them) Attenuation (FO need amplification after 60km, UTP needs amplification every 5km) Erosion (Copper erode faster, Glass has less environmental effects. Effect of EM interference (High on UTP and less on FO) Leaking (High risk of eavesdropping and tapping attack in UTP then FO) Bandwidth (Far more in FO then UTP) Cost (FO far more expensive than UTP) Need for skilled engineer (in FO and not in UTP) Technology Complexity (High in FO and easy in UTP) Flexibility ( High in UTP and low in FO) Sub: FON, bhargavigoswami@gmail.com Page 12

13 Q. Wireless Physical Medium: Two special cases: hidden and exposed station Solution Components of the Wireless system Antennas Access Points Multiple senders and receivers acting in parallel Chapter 4: Physical Layer Wireless sender usually has a limited range and all machines beyond that range do not receive broadcast. Limit of range is also boon when we want multiple transmissions between multiple senders and receivers. A is sending to B and E is sending to F. Range of A and E is defined with dotted lines. Both ranges do not overlap. Even if it overlaps, it will not receive one transmission of many if not the intersection node. If D is receiver, and receiving from two nodes, summation of both signals result to garbage. Solution? CDMA (Code Division Multiple Access) technique Exposed station Problem: B is sending to A D is interested to send to E When D senses channel, it finds channel busy, so don t send. E does not receive signal from B and it is safe if D sends data to E. Signal would not be garbled if sent now, but D has no way to know that. B, is unfortunately exposed to D, hence named Exposed Station Problem Hidden Station Problem At times sender starts transmitting completely unaware of fact that receiver is busy at that point of time, results to garbage. Sub: FON, bhargavigoswami@gmail.com Page 13

14 B has started sending to D. E wants to send to D. When E senses channel, it cannot listen to transmission and concludes that channel is free and E sends to D. But D is already receiving from B. Here, B is sender and is hidden from E, thus named Hidden Station Problem. Solution: Actually sender is sensing his own surrounding channel. But, actually he should sense receiver s surrounding channel instead of his own. We can solve this problem if we deploy a sensing mechanism at receiver s end. Base: Send short frame to recipient and invoking response from it. B sends RTS(Request to Send) to D. D replies with CTS(Clear to Send) to B. All other stations do not sent till length indicated by CTS and RTS is not transmitted. So, all knows who is sending and who is receiving. Problem: if two or more senders send RTS at a time. Here, no response comes from receiver, so sender tries after some random time. Min chance of same situation repetition as it follows binary exponential back off algorithm. Drawbacks: Overhead, Lack of bandwidth, impose delay. Only ad-hoc mode has this problem. Infrastructure mode has no such problem. Sub: FON, bhargavigoswami@gmail.com Page 14

15 Wireless LAN components Modes Ad-hoc (DCF) Infrastructure (PCF) Antennas Omni-directional Focused (Parabolic usually) Q. Explain Antennas: Chapter 4: Physical Layer Antenna types: o Omnidirectional o Focused (Parabolic usually Antenna is a device to send and receive wireless signals. Hidden antennas are there in cable tv and mobile radios, but not visible. Dish deployed by cable tv operators on roof of our home is example of focused parabolic antennas. Antennas are important components for sending and receiving data in wireless mode. Scientifically, antennas are the components that radiates or collects electromagnetic rays and energy in the form of data. Antennas used in radios are omnidirectional. No need of aligning omnidirectional antennas. Range of omnidirectional antennas is 30MHz to 1GHz. Antennas used in mobile phones relay towers are not parabolic in shape, but rectangular in shape. Transmission in parabolic focused antennas is also known as line of sight transmission is used in satellite communication for DTH. Focused antenna provides point to point transmission. Q. What is FSO? Explain in detail. An interesting idea is to use optical light for data communication without using any physical medium. This is known as Free Space Optics (FSO). The communication is wireless here. Like, touch used to send a signal. Can be implemented where wires are tough to be implemented. So, FSO is optical communication without fibers. FSO can transmit data upto 1Gbps speed upto 2 Km. Eg. Communication in spacecraft. Dis-advantage? o Communication heavily depends on atmospheric conditions. Rain, Fog, Lightening, Heat, Dust, Fog affects communication adversely. o Dispersion: The tendency of light to spread in all direction. o Sunlight can affect the communication adversely. Sub: FON, bhargavigoswami@gmail.com Page 15

16 Access point: Wireless device that manages nodes, controls incoming and outgoing transmission. Allocates time and bandwidth to nodes. Avoid possibility of collision. For easy access, mounted on ceiling. Can connect to other access points and wired network also. In that case, RTS and CTS have highest priority, access point comes next, ad-hoc transmission has lowest priority. IEEE 802 Standards: The 802 working groups. The important ones are marked with *. The ones marked with are hibernating. The one marked with gave up. Q. Wireless LAN WiFi (Wireless Fidelity) and are two different things but very similar is specification of Wireless networks. WiFi is name of alliance of device vendors. IEEE has given standard. WiFi is alliance is designed to solve interpretability issues between devices like interface cards, access points, etc. Two modes: Ad-hoc and Infrastructure modes physical layer flavors: b OFDM and the a g Is connectionless architecture. Next: WiMax or which is connection oriented b: 2.4 GHz ISM band High Rate Direct Sequence Spread Spectrum (HD-DSSS) 11 Mb No licensing Sub: FON, Page 16

17 Reduced Cost Ad-hoc mode uses CSMA/CA (carrier sense multiple access collision avoidance) with RTS and CTS. Available bandwidth depends on distance Covers more distance than a (7x more) so need less access points hence economical. Broadcast mechanism, point to multipoint case. OFDM and a OFDM (Orthogonal Frequency Division Multiplexing) is sibling of FDM. Disadvantage of FDM: We need guard band for multipath fading solution. Large part of frequency remains unused because of this. Solution? OFDM OFDM: Orthogonal to each other. So, minimum interference. Frequency nearer, offer more bandwidth and more frequency is utilized. Has concept of sub-channels. Signals are subdivided into small parts. 52 narrow frequencies (4 for sync and 48 for data) Transmission is distributed to all these freq (carried simultaneously) Better immunity to interference Different schemes for different ranges (Short: QAM, Long: QPSK) Maximum bandwidth of 54 Mb (shared by all devices) a came a little later than b 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 available in market. Intension: backward compatibility In India, the g is more significant, as 5GHz band of a is not available as free ISM. Y g give more bandwidth than b? Because of OFDM orthogonality Sub: FON, bhargavigoswami@gmail.com Page 17

18 Q. WiMax : WiMax (Worldwide Interoperability for Microwave Access) Achieve a data rate of Mb Distance of coverage between 3 to 10 km WiMax forum takes care of interoperability issues of devices GHz originally but now much lower d was developed for fixed wireless, later version e provides both, fixed and mobile wireless d: For fixed wireless/wimax/internet. OFDMA (Orthogonal Frequency Division Multiple Access) multi-user version of OFDM with 256 sub-carriers. 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 The second part is smaller than the first one, as uploading is usually less than downloading Two different frequencies are used for uplink and downlink, called frequency duplexing scheme. 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 as connection oriented communication is provided. See next figure. Base station and subscriber station OFDMA and e 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 Sub: FON, bhargavigoswami@gmail.com Page 18

19 Employs multiple closely spaced frequencies, but these frequencies are divided into groups. Frequencies making sub-channel need not be adjacent. With same time slot, mobile WiMax can allocate more transmission power to devices with lower SNR (Signal to Noise Ratio). Mobile stations are not at same distance and so power needed is more when at more distance. Standard for mobile wireless broadband or mobile WiMax. WiMax Cards from vendors Provides QoS. Scalable OFDMA is used, has more range of bandwidths and different spectrum working capacity. Q. Wireless communication using Satellite Three different orbits LEO, MEO and GEO. GEO is almost full. The LEO is near to earth satellite phones with few miliwatts of power. 1 to 7 msec response time LEO satellites are less expensive to launch Teledesic and GlobalStar are the examples. It is unwise to have geostationary satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference. With a spacing of 2 degrees, there can only be 360/2 = 180 of these satellites in the sky at once. However, each transponder can use multiple frequencies and polarizations to increase the available bandwidth. To prevent total chaos in the sky, orbit slot allocation is done by ITU (International Telecommunication Union). But, this process is highly political. No country has a legal right to the orbit slots above its territory. Modern satellites can be quite large, weighing up to 4000 kg and consuming several kilowatts of electric power produced by the solar panels. Satellites are relay operators. Satellites contain few transponders, capable of sending and receiving. To eliminate interference, they are kept separate for receiving and sending. Data communication satellites use wide spectrum for broadcasting(tv channels), but use narrow beams for data communication. Footprint: The area that satellite covers is known as footprint. Arthur C. Clarke said, at 35,800 km, satellite in circular motion orbit would remain motionless in sky. Total time taken for communication: 270msecs to and fro. Sub: FON, bhargavigoswami@gmail.com Page 19

20 Van Allen Belts: Two belts of highly charged particles at 5000 and km. Used for broadcasting TV channels and not used for real time communication. Has 3 orbits: See next fig. (LEO,MEO and GEO) Low, Medium and Geostationary Earth Orbit. LEO: Preferred for data communication. More #satellites needed to cover country. Near the earth. Less delay. Best for interactive applications. Need less power for placing the satellites. Narrow beams. MEO: Generally used for GPS(Global Positioning Satellite) Systems. Need more #satellites than GEO. Between two van allen belts. GEO: Preferred for TV channels. Almost its full as #channels is too much. #satellites needed is very less. Wide footprint is used to cover entire country. Q. Differentiate Between Satellite and Fiber Optics: Fiber Optics Satellite: Practically? Single fiber has, more potential With satellites, it is practical for a user to erect an bandwidth than all the satellites ever launched but, antenna on the roof of the building and completely this bandwidth is not available to most users bypass wired system to get high bandwidth. practically. Mobile Communication? Terrestrial fiber optic links Satellite links potentially are most suitable for are of no use to them. mobile communication. Broadcasting? Not suitable for broadcasting. Message sent by satellite can be received by Cost? Costly for communication in places with hostile terrain or a poorly developed terrestrial infrastructure. Right of Way? for laying fiber is difficult or unduly expensive. Rapid Deployment? Fiber optics is costly in terms of skilled engineers thousands of ground stations at once. Best suited. Launching one satellite was cheaper than stringing thousands of undersea cables among the 13,677 islands. Instead launching one satellite would be cheaper. When rapid deployment is critical, as in military communication systems in time of war, satellites win easily Sub: FON, bhargavigoswami@gmail.com Page 20

21 University Questions List: 1. Explain in brief: (1) Satellite Orbit (2) Period of Satellite (3) Bent Pipe (4) Footprint (5) Station Keeping (6)VSAT (7) Geostationary Satellites [7] 2. Explain Propagation mode of Fiber Optic cable. [4] 3. is the basic access method in wireless LANs; stations contend with each other to get access to channel. [1] 4. Explain hidden station and exposed station problem of wireless LAN protocol.[4] 5. What if handoff? Discuss soft handoff and hard handoff.[4] 6. What is the importance of RTS and CTS in the wireless transmission?[3] 7. Compare light sources, required for fiber optic communication.[4] 8. Give minimum three comparisons between fiber optic cable and UTP.[3] 9. Write a note on: UTP (Unshielded Twisted Pair) Cables[3] 10. Explain the working of fiber optic networks with diagram.[4] 11. Briefly explain the need and application of ISM band in wireless communications[2] 12. Compare and contrast satellite systems with fiber optic systems[4] 13. Explain a) Hidden Station Problem b) Exposed Station Problem with respect to wireless networks.[4] 14. Station Keeping[1] 15. Frequency division multiplexing and OFDM.[3] 16. Hidden station problem[2] 17. Name any two types of wireless networks prevalent in the market[3] 18. Write two reasons which favours fiber optic cables over copper cable[3] Sub: FON, bhargavigoswami@gmail.com Page 21