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Chapter 2

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  1. Chapter 2 The Physical Layer Samir Chettri

  2. Transmission of light through fiber Attenuation depends on wavelength CS481 Samir Chettri Samir Chettri Optic fibers

  3. Transmission of light through fiber • Attenuation in dB= 10log (transmitted power/recd. Power) • 3 bands for comm. 0.85, 1.3, 1.55 micron • dispersion: Light pulses sent down a fiber spread out in a manner that is wavelength dependent. Solution: 1) separate pulses 2) or raised hyperbolic cosines - solitons can propagate 1000 or more km Samir Chettri Optic fibers

  4. Fibre Cables Samir Chettri Optic fibers

  5. Humor • Terrestrial fiber sheaths are normally laid in the ground within a meter of the surface, where they are occasionally subject to attacks by backhoes or gophers. Near the shore, transoceanic fiber sheaths are buried in trenches by a kind of seaplow. In deep water, they just lie on the bottom, where they can be snagged by fishing trawlers or eaten by sharks. Samir Chettri Optic fibers

  6. Fiber connections • Method 1: Terminate in connectors and end up in fiber sockets. Loss = upto 20% of light • Method 2: Spliced mechanically. Use a sleeve and align and calibrate. • Method 3: Fuse 2 ends of fibre into solid connection. Samir Chettri Optic fibers

  7. Fibre Optic networks (ring) Optic fiber networks Samir Chettri

  8. Fibre Optic networks (star - broadcast) Silica cylinder Optic fiber networks Samir Chettri

  9. Fibre optic vs Copper wire Fiber Copper 30km 5km Repeaters External events (power surges) Unaffected Affected Size (weight) Light Heavy Security Difficult to tap Easier Expense $$$ $ Optic fiber Samir Chettri

  10. Wireless/Electromagnetic spectrum Electromagnetic spectrum Samir Chettri

  11. Wireless Transmission/Radio • Advantage: Radio waves are easy to generate, can travel long distances, penetrate material objects easily and are omnidirectional. • Disadvantage: 1/r^3 behavior. Absorbed by rain. Interference. Wireless/Radio Samir Chettri

  12. Wireless Transmission/Radio Wireless/Radio Samir Chettri

  13. Wireless/Microwave trans. • > 100MHz waves travel in straight lines. • Repeaters are needed. High towers are constructed. • Do not pass through buildings • Multipath fading (late arriving waves are out of phase with original wave, therefore fading). • Above 8GHz absorption by H2O occurs. • Some bands are free (e.g., 2.4-2.484 GHz) Samir Chettri Wireless/Micro

  14. Wireless/IR and mm waves • Used in VCR’s stereos • Candidates for indoor wireless LAN’s e.g., portable computers with IR capability can be on the local LAN without a physical connection. • Don’t pass through walls therefore security is good. • Can’t use outdoors - sunlight washes it out. Samir Chettri Wireless/IR + mm

  15. Lightwave transmission Samir Chettri

  16. Signal Transmission Samir Chettri Signal transmission

  17. Signal Transmission • Square waves (digital signaling) have a wide spectrum. • Attenuation is frequency dependent therefore a large range of frequencies is undesirable for long distance • Baseband (DC) is not suitable for long distance transmission • Discovery - a continuous oscillating signal propagates further. Samir Chettri

  18. Modems • Input serial bit stream is converted to a modulated carrier (and vice-versa) by MODEM (modulator-demodulator) • Need to increase the # of bits / sample (per baud) 4 bits/baud on 2400 baud line Samir Chettri

  19. Modems • Patterns (phase and amplitude diagrams) like shown in previous slide are called constellation patterns. • 6 bits/baud on a 2400 baud line (14,400 bps) is called V.32 bis. V.34 runs at 28,800 bps. • Constellation pattern is complicated and small noise leads to large error. Samir Chettri Modems

  20. Modems • Some modems have compression built in - thereby increasing the effective data rate. • Popular compression scheme is MNP5 which uses run length encoding. • Another coding scheme is V.42 bis which uses a Ziv-Lempel algorithm. Samir Chettri Modems

  21. Multiplexing • Economies of scale.Building a trunk line is expensive, the cost of optic fibre is not (relatively speaking). • Therefore there are many schemes for multiplexing many conversations over a single physical trunk. • Two categories: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). Samir Chettri Multiplexing

  22. Multiplexing • Radio: Each station is given a frequency and it broadcasts only on that frequency. Frequencies of radio stations are widely separated to minimize interference (FDM). • Radio II: Radio sends ads followed by music (TDM). • Two categories: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). Samir Chettri Multiplexing

  23. FDM Overlap 4000Hz Samir Chettri Multiplexing

  24. FDM • A standard is 12, 4000Hz voice channels (3000Hz + 2, 500Hz guard bands) multiplexed into the 60-108kHz band. This is called a group. • 5 groups (60 voice channels) are multiplexed to form a super-group. • 5-10 super-groups form a master-group. Samir Chettri Multiplexing

  25. FDM (WDM) • A variation on FDM for fibre-optic channels is wavelength division multiplexing. Samir Chettri Multiplexing

  26. TDM Samir Chettri Multiplexing

  27. TDM • Codec: Samples at 8000 samples/second (125 microsec/sample) (Nyquist thm. States that we need sampling rate of 2 x max frequency). • The above technique is called Pulse Code Modulation. • Consider the T1 carrier (next slide). Samir Chettri Multiplexing

  28. TDM Samir Chettri Multiplexing

  29. TDM • Analog signals from modems etc. are sampled in round robin fashion and then fed to the codec (rather than having 24 different codecs) • Each of the 24 channels puts 8 bits into the output stream. 7 are data bits, 1 is a control bit. (7x8000 + 1x8000) = 64Kbps per channel. • One frame = 24x8 + 1framing bit = 193bits Samir Chettri Multiplexing

  30. TDM • There are 193 bits / 125microsec. = 1.544Mbps • 193rd bit has alternating zeros and ones 01010101010101010….. and is used for frame synchronization. • Receiver checks 193rd bit to see if it is in sync • 24th channel also has a special sync pattern Samir Chettri Multiplexing

  31. TDM • Variations: E1 - 32 channels (8 bit data samples) in a 125 frame. • 30 channels are used for info. Two for signaling. • 4 frames together provide 2x8x4 signaling bits Samir Chettri Multiplexing

  32. Multiplexing T1 streams (TDM) Samir Chettri Multiplexing

  33. TDM (statistical techniques) • Signals have characteristics that make them amenable to compression through statistical techniques. • DPCM (Differential Pulse Code Modulation) - the amplitude is not output but the difference between current value & prev. one. Large jumps are not likely so perhaps 5 bits should work. Wild jumps lead to error. • Var: Modulation, predictive encoding Samir Chettri

  34. TDM (statistical techniques) Variation on Differential Pulse Code Modulation is called DELTA MODULATION Delta modulation One bit is transmitted telling whether the new sample is below or above the previous one. Samir Chettri Multiplexing

  35. Telephone networks • LAN’s are ok for computers in close proximity. For longer distances companies prefer to use existing telecommunication facilities. • Public Switched Telephone Network (PSTN) is therefore worth studying especially since they are going towards digital. • PSTN was designed for voice communic. Samir Chettri Telephone networks

  36. Telephone networks • Voice communication is quite tolerant of transmission errors but computer-computer communic. needs much less error. (Read example on page 102 of text) • STRUCTURE (next slide) Samir Chettri Telephone networks

  37. Structure of the tel. network Samir Chettri Telephone networks

  38. Samir Chettri

  39. Struct. of telephone network • Calls from caller in end office 1 to caller in end office 1 go through end office 1 • Calls from end office 1 to end office 2 go through toll offices. In a tree there is only one minimal route. • Some routes are busier than others (e.g., DC to NY) so direct trunks are created. Therefore many paths exist. Samir Chettri Telephone networks

  40. Fiber to the curb (FTTC) Samir Chettri Telephone networks

  41. Politics of telephone system • Please read this section on your own section 2.4.2 Samir Chettri Telephone networks

  42. SONET/SDH • Synchronous Optical NETwork and Synchronous Digital Hierarchy. • Goals of SONET • Different carriers (companies) need to work together • Unify US, Japanese and European models • Multiplex several digital channels together (i.e., the T hierarchy - T1, T2 etc to gigabit/s lines) • Provide support for operations, admin. & maint. Samir Chettri SONET/SDH

  43. SONET/SDH • SONET is a traditional TDM system. • Everything is controlled by a highly accurate clock (1x10E-9 accuracy) bits are sent out under clock control. • SONET consists of switches, multiplexers and repeaters. • Fibre from one device to another is a section. • Between two multiplexers is a line. • Between source and destination is a path. Samir Chettri SONET/SDH

  44. SONET/SDH Samir Chettri SONET/SDH

  45. SONET/SDH • SONET puts out a frame of 810 bytes every 125microseconds. The 8 frames/sec matches PCM channels used in all digital telephony systems. • Each frame is described as a rectangle of bytes 90 columns wide and 9 rows high. • SONET is synchronous since frames are emitted whether or not there is any data to send. Samir Chettri SONET/SDH

  46. System management info SONET/SDH Synchronous payload envelope starts any- where in frame Between two devices Between muxes Synchronous Payload Envelope Samir Chettri SONET/SDH

  47. SONET/SDH • First 3 columns are for system mgt. info. • First 3 rows (of first 3 cols) contain section overhead • Next 6 contain line overhead • Synchronous Payload Envelope (SPE) - the user data can start anywhere in the SONET frame.Useful when 1) a dummy frame is being constructed. 2) payload does not fit into a frame (we will study this in ATM) Samir Chettri SONET/SDH

  48. SONET/SDH • Section, Line and path overheads contain bytes for operations, admin. and maint.The fields are described in • Bellamy, J. Digital Telephony, NY, JohnWiley • Multiplexing of SONET streams are called tributaries. • Low speed input streams are converted to the basic STS-1 (Synchronous Transport Signal-1) SONET rate much like muxing T1 lines. Samir Chettri SONET/SDH

  49. SONET/SDH Multiplexing is done byte by byte, i.e., a byte from first tributary is used then a byte from second and a byte from third in round-robin fashion. This is true for all levels of the hierarchy. Samir Chettri SONET/SDH

  50. Switching • The act of choosing a “physical copper path” connection from transmitter to receiver is called circuitswitching. • In modern times the “physical copper” paths may well be microwave links. • An end to end path needs to be set up before any data can be sent. For long distance communication long setup times (10-20sec) occur. Computer apps. don’t like this. Samir Chettri