CSCD 433 Advanced Networks Winter 2019

1. CSCD 433Advanced Networks Winter 2019 Lecture 5a Physical Media, Digital Line Coding and other ... 1

2. Transmission Media

3. Twisted Pair Oldest transmission medium Historical use, Phone systems Two insulated Copper wires Wires twisted together Straight they would interfere To reduce electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Twisted pair cabling – Several varieties Category 5 – Two insulated wires – 4 pairs Encased in a protective plastic sheath Category 7 – Higher quality yet Has added shielding on individual twisted pairs Helps reduce external interference and crosstalk

4. Twisted Pair Bit Rates Twisted pairs can provide high bit rates at short distances Asymmetric Digital Subscriber Loop (ADSL) High-speed Internet Access Lower 3 kHz for voice Upper band for data 64 kbps outbound 640 kbps inbound Much higher rates possible at shorter distances Strategy for telephone companies is to bring fiber close to home & then twisted pair Higher-speed access + video Standard Data Rate Distance T-1 1.544 Mbps 18,000 feet, 5.5 km DS2 6.312 Mbps 12,000 feet, 3.7 km 1/4 STS-1 12.960 Mbps 4500 feet, 1.4 km 1/2 STS-1 25.920 Mbps 3000 feet, 0.9 km STS-1 51.840 Mbps 1000 feet, 300 m Data rates of 24-gauge twisted pair

5. Coaxial Cable Better shielding and greater bandwidth than unshielded twisted pairs So it can handle longer distance at higher speeds Coaxial cable consists of a stiff copper wire surrounded by insulation material Encased in conductor – woven mesh and finally a plastic sheath Cable has bandwidth up to a few GHz Has been replaced by fiber optics in Telephone systems

6. Coaxial Cable 35 0.7/2.9 mm 30 Attenuation (dB/km)‏ 25 1.2/4.4 mm 20 15 10 2.6/9.5 mm 5 100 0.1 10 1.0 f (MHz)‏ • Cylindrical braided outer conductor surrounds insulated inner wire conductor • High interference immunity • Higher bandwidth than twisted pair • Hundreds of MHz • Cable TV distribution • Long distance telephone transmission • Original Ethernet LAN medium

7. Fiber Optics Fiber consists of a light, transmission medium and detector Transmission medium is thin fiber of glass Detector generates a pulse when it detects a light So, way it works, attach a light at one end, detector to other end Accepts electrical signals, converts and transmits light pulses and converts back to signals at receiving end

8. Fiber Optics Consists of core of glass, very thin Surrounded by glass cladding to keep all light in the core Surrounded by plastic jacket

9. Optical Fiber Light sources (lasers, LEDs) generate pulses of light that are transmitted on optical fiber Very long distances (>1000 km)‏ Very high speeds (>40 Gbps/wavelength)‏ Nearly error-free Huge influence on network architecture Dominates long distance transmission Distance less of a cost factor in communications Plentiful bandwidth for new services Electrical signal Electrical signal Optical fiber Modulator Receiver Optical source

10. Transmission in Optical Fiber Very fine glass cylindrical core surrounded by concentric layer of glass (cladding)‏ Core has higher index of refraction than cladding Light rays incident at less than critical angle c is completely reflected back into the core Light Cladding Jacket Core c Geometry of optical fiber Total Internal Reflection in optical fiber

11. Optical Fiber Properties Advantages Very low attenuation Noise immunity Extremely high bandwidth Security: Very difficult to tap without breaking No corrosion More compact & lighter than copper wire Disadvantages New types of optical signal impairments & dispersion Difficult to splice Mechanical vibration becomes signal noise

12. Radios Radios work by Frequency Frequencies are Easy to generate Can travel long distances Penetrate buildings Widely used for communications, waves are omnidirectional Low frequencies pass through obstacles well, but power falls off sharply with distance from source

13. Radio Spectrum Frequency (Hz)‏ 106 1012 105 108 107 104 1011 109 1010 FM radio and TV Wireless cable AM radio Cellular and PCS Satellite and terrestrial microwave LF MF HF VHF UHF SHF EHF 10-1 1 102 10-3 10-2 101 104 103 Wavelength (meters)‏ Omni-directional applications Point-to-Point applications

14. More Complete Spectrum

15. Examples Cellular Phone Allocated spectrum First generation: 800, 900 MHz Initially analog voice Second generation: 1800-1900 MHz Digital voice, messaging WirelessLAN Unlicenced ISM spectrum Industrial, Scientific, Medical 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz IEEE 802.11 LAN standard 11-54 Mbps Point-to-MultipointSystems Directional antennas at microwave frequencies High-speed digital communications between sites High-speed Internet Access Radio backbone links for rural areas SatelliteCommunications Geostationary satellite @ 36000 km above equator Relays microwave signals from uplink frequency to downlink frequency Long distance telephone Satellite TV broadcast

16. Compare Wireless to Wired Media Wireless Media Signal energy propagates in space, limited directionality Interference possible, so spectrum regulated Limited bandwidth Simple infrastructure: antennas & transmitters No physical connection between network & user Users can move Wired Media Signal energy contained & guided within medium Spectrum can be re-used in separate media (wires or cables), more scalable Extremely high bandwidth Complex infrastructure: ducts, conduits, poles, right-of-way

17. Attenuation Wireless Wireless media has logarithmic dependence Received power at d meters proportional to d-n Attenuation measured by dB = n log d, where n is path loss exponent n=2 in free space Signal level maintained for much longer distances Space communications possible

18. Microwave Transmission Above 100 MHz, waves travel in nearly straight lines Uses transmitting and receiving antennas Before fiber optics, for decades microwaves formed heart of long-distance telephone transmission system MCI – Built system with microwave communications – stands for Microwave Communication Incorporated

19. Infrared Transmission Unguided infrared waves Used for short range communication Remote controls for TV, VCR and Stereos Cheap, easy to build but has a major drawback What is it? Can't pass through solid walls Advantage – No interference in other rooms Don't need a government license

20. Examples of Channels Channel Bandwidth Bit Rates Telephone voice channel 3 kHz 33 kbps Copper pair 1 MHz 1-6 Mbps Coaxial cable 500 MHz (6 MHz channels)‏ 30 Mbps/ channel 5 GHz radio (IEEE 802.11)‏ 300 MHz (11 channels)‏ 54 Mbps / channel Optical fiber Many TeraHertz 40 Gbps / wavelength

21. Politics National and International agreements FCC regulates spectrum for United States AM/FM radio, TV and mobile phones They regulate some frequencies of the spectrum Unregulated frequencies ISM – Industrial, Scientific and Medical unlicensed bands Garage door openers, cordless phones, radio controlled toys and wireless mice FCC mandates all devices limit power in this unlicensed band

22. Politics In the US, 900 Hz was used for early versions of 802.11 It was crowded Baby monitors, garage door openers, cordless phones 2.4 GHz band is available in most countries for 802.11 b/g/n and Bluetooth 5 GHz is partly used for 802.11a/n

23. Politics of the Electromagnetic Spectrum The ISM bands in the United States.

24. Digital Line Coding

25. Digital Line Coding In order to be transmitted over digital communications system, an information signal must first be formatted so that it is represented by digital symbols (usually binary digits or bits). Next, these binary representations must be converted into electrical waveforms that are transmitted over the communications channel In baseband digital transmission, electrical waveforms used are pulses and this conversion from digital data to digital waveforms is known as line coding

26. Digital Line Coding Selecting coding technique involves several considerations Previously we said ... Wanted to maximize bit rate over channels with limited bandwidth Yet, LAN's have other concerns Ease of bit timing recovery from signal So, receiving sample clock can maintain its synchronization with transmitting clock Some methods better at noise and interference than others

27. Clock Synchronization In communication of digital data, clock recovery is the process of extracting timing information from a serial data stream to allow receiving circuit to decode the transmitted symbols When a digital communication channel does not transmit clock signal along with the data stream, Clock must be regenerated at receiver using timing information from data stream. Clock recovery is common component of systems communicating over wires, optical fibers, or by radio

28. Line Coding Variables Line Coding Formats: The various line coding waveforms can be categorized in terms of the following The duration of the pulses. The way in which voltage levels are assigned to the pulses.

29. Line Coding Types Pulse Duration: There are two classes used Non return-to-zero (NRZ) where the pulse or symbol duration T s = the bit period T b Return-to-zero (RZ) where the pulse or symbol duration T s < the bit period T b Usually T s = 0.5T b The pulse duration will usually have an effect on synchronization properties of line code (i.e. it determines the presence or absence of a frequency component at the clock frequency)

30. Linde Coding Voltage Levels Pulse Voltage Levels There are many voltage level formats: – Unipolar – Polar – Dipolar – Bipolar – High Density Bipolar substitution (HDBn) – Coded Mark Inversion (CMI)

31. Unipolar Signalling Unipolar signalling is where a binary 1 is represented by a high positive level (+A volts) and a binary 0 is represented by a zero level (0 volts) This is sometimes known as on-off keying (OOK). There are two variations possible: – Unipolar NRZ – Unipolar RZ

32. Unipolar NRZ and RZ Unipolar NRZ has the following features: – Narrow bandwidth – Significant dc component – No clock component – Easy to generate • Unipolar RZ has the following features: – Large bandwidth – Significant dc component – Clock component present – More difficult to generate • In both cases, there is no error detection capability and the codes are not transparent

33. Polar NRZ and RZ Polar NRZ has the following features: – Similar spectrum to unipolar NRZ (narrow bandwidth) – Significant dc component – No clock component Polar RZ has the following features: – Similar spectrum to unipolar RZ (large bandwidth) – Significant dc component – No clock component present, but clock extraction possible using rectification. In both cases, there is no error detection capability and the codes are not transparent.

34. Bipolar RZ / AMI Bipolar RZ or Alternate Mark Inversion (AMI) uses three voltage levels to represent the binary 1’s and 0’s. A binary 0 is represented by a zero level A binary 1 is represented by alternating positive and negative pulses (i.e. the alternating mark rule) This alternating pulse polarity gives bipolar signaling an error detection capability and also produces a spectral null at 0 Hz There is no clock component present but clock extraction is possible through rectification

35. Line Coding Schemes Unipolar: Uses one voltage level Polar: Uses two voltage levels Bipolar: Uses three or more voltage levels

36. Note In unipolar encoding, we use only one voltage level, positive

37. Unipolar Encoding

38. Note In polar encoding, we use two voltage levels: positive & negative

39. Polar: NRZ-L and NRZ-I Encoding

40. Note In NRZ-L, level of voltage determines value of the bit In NRZ-I, inversion or lack of inversion determines value of the bit

41. Polar: RZ Encoding

42. Polar: Manchester Encoding

43. Note In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization.

44. Manchester Coding

45. Note In bipolar encoding, we use three levels: positive, zero, and negative.

46. Bipolar: AMI (Alternative Mark Inversion) Encoding

47. Summary

48. Summary Looked at digital data over digital channels Theoretical maximum limits of transmitting bits in presence of noise and without Line encoding makes it possible to send more data as efficiency of coding increases

49. Work on Assignment 2 49