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Transmission Media Chapter 4

Transmission Media Chapter 4. Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. Types of media: Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber.

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Transmission Media Chapter 4

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  1. Transmission Media Chapter 4 • Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. • Types of media: • Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber. • Unguided: “wireless” transmission (atmosphere, outer space). University of Delaware CPEG 419

  2. Guided Media: Examples 1 • Twisted Pair: • 2 insulated copper wires arranged in regular spiral. Typically, several of these pairs are bundled into a cable. (What happens if the twist is not regular? Reflection?) • Cheapest and most widely used; limited in distance, bandwidth, and data rate. • Applications: telephone system (from home to local exchange connection). • Unshielded and shielded twisted pair. • What is a differential amplifier? University of Delaware CPEG 419

  3. Guided Media: Examples 1 • Twisted pair – continued • Category 3: Unshielded twisted pair (UTP) up to 16MHz. • Cat 5: UTP to 100 MHz. • Table 4.2. Suppose Cat 5 at 200m (the limit of 100Mbps ethernet is 300m). • The dB attenuation at 100m is 22.0. So at 200m, the attenuation is ???. Suppose we transmit at –80dBW. Then the received signal has energy of ????. • The near-end crosstalk gain is 32dB per 100m. So the crosstalk energy is ???? • The SNR is ????? (neglecting thermal noise). 44 –124dBW –144dBW 20dB University of Delaware CPEG 419

  4. Examples 2 • Coaxial Cable • Hollow outer cylinder conductor surrounding inner wire conductor; dielectric (non-conducting) material in the middle. • Less capacitance than twisted pair, so less loss at high frequencies. Also, Coaxial has more uniform impedance. • Applications: cable TV, long-distance telephone system, LANs. • Repeaters are required every few kilometers at 500MHz. • +’s: Higher data rates and frequencies, better interference and crosstalk immunity. • -’s: Attenuation at high frequency (up to 2 GHz is OK) and thermal noise. University of Delaware CPEG 419

  5. Examples 3 • Optical Fiber • Thin, flexible cable that conducts optical waves. • Applications: long-distance telecommunications, LANs (repeaters every 40km at 370THz!). • +’s: greater capacity, smaller and lighter, lower attenuation, better isolation, • -’s: Not currently installed in subscriber loop. Easier to make use to current cables than install fiber. University of Delaware CPEG 419

  6. Examples 3 – types of fiber lower index of refraction • Step-index multimode shorter path longer path absorbed total internal reflection higher index of refraction Since the signal can take many different paths, the arrival the received signal is smeared. Input Pulse Output Pulse University of Delaware CPEG 419

  7. Examples 3 – types of fiber • Single mode If the fiber core is on the order of a wavelength, then only one mode can pass. Wavelengths are 850nm, 1300nm and 1550nm (visible spectrum is 400-700nm). 1550nm is the best for highest and long distances. Attenuation: -0.2dB/km to -0.8dB/km (if the ocean was made of this glass you could see the floor like you can see the ground from an airplane) University of Delaware CPEG 419

  8. Input Pulse Output Pulse Examples 3 – types of fiber Even for single mode fiber, a pulse gets smeared. Solitons are a particular wave pulse that does not disperse. University of Delaware CPEG 419

  9. Fiber Repeaters : Two Approaches • Convert the signal to analog. Convert to digital and then send a transmit received signal. • Optical repeater. A nonlinear optical amplifier shapes and amplifies the pulse. A single repeater works for all data rates! (more about optical networks later) University of Delaware CPEG 419

  10. Wavelength-division multiplexing (WDM) • Wavelength-division multiplexing • Multiple colors are transmitted. • Each color corresponds to a different channel. • In 1997, Bell Labs had 100 colors each at 10Gbps (1Tbps). • Commercial products have 80 colors at 10Gbps. University of Delaware CPEG 419

  11. Fiber vs. Cable • Fiber is light and flexible. • Fiber has very high bandwidth. • Fiber is difficult to install (I can’t do it). • Fiber interfaces are more expensive than cable (?) University of Delaware CPEG 419

  12. Wireless Transmission University of Delaware CPEG 419

  13. 1022 1016 UV Gamma -ray X-ray Electromagnetic Spectrum Cell phones put out 0.6 – 3 watts. Light bulbs put out 100 watts. University of Delaware CPEG 419

  14. Wireless Transmission • Omni-directional – the signal is transmitted uniformly in all directions. • Directional – the signal is transmitted only in one direction. This is only possible for high frequency signals. University of Delaware CPEG 419

  15. Terrestrial Microwave • Parabolic dish on a tower or top of a building. • Directional. • Line of sight. • With antennas 100m high, they can be 82 km (50 miles). • Use 2 – 40 GHz. • 2 GHz: bandwidth 7MHz, data rate 12 Mbps • 11 GHz: bandwidth 220MHz, data rate 274 Mbps The M in MCI is for microwave University of Delaware CPEG 419

  16. Satellite Microwave • Satellites are repeaters. • 1 – 10 GHz. Above 10 GHz, the atmosphere (like rain) attenuates the signal, and below 1 GHz there is too much noise. • Typically, 5.925 to 6.425 GHz for earth to satellite and 4.2 to 4.7 GHz for satellite to earth. (Why different frequencies?) • A stationary satellite must be 35,784 km (22000 miles) above the earth. • The round-trip delay is about ½ a second. University of Delaware CPEG 419

  17. Low-Earth Orbit Satellites (LEO) • Iridium The idea of some executive’s wife while vacationing in the tropics and her cell phone didn’t work.. • Cost 5 billion dollars. • Went out of business in 1999. Sold for $25 million and is still operational. • Provides phone, fax, paging, data and navigation WORLD WIDE! (jungle, Afghanistan (both sides), etc.) • 66 low orbit satellites. Low Orbit, so they move out of range fast • Cool thing. The calls go hop from satellite to satellite before returning to the destination. So they have to track every user. • Globalstar 48 LEOs. The call goes to the ground as soon as possible and uses a terrestrial network. So they are simpler. Also, the satellites relay the analog signal. On the ground is a large, sensitive antenna to pick up the weak phone signal. • Teledesic. 30 satellites. Data network 100Mbps to 720Mbps. Planned for 2005. Bill Gates and Craig McCaw founders. University of Delaware CPEG 419

  18. Other • Cell phones – Omni-directional. GSM-900 uses 900MHz, GSM-1800 and GSM-1900 (PCS). Typical data rate seems to be around 40kbps. But the protocol is specified to 171kbps. • 802.11 wireless LANs • Omni-directional • 802.11b 2.4 GHz (where microwave ovens and cordless phones are) up to 11Mbps • 802.11a 5 GHz up to 54Mbps • Infrared – Line of sight, short distances. University of Delaware CPEG 419

  19. Spectrum Allocation • Some bands are allocated for unlicensed usage (ISM) • 900 MHz – cell phones, cordless phones. Is not available in all countries. Bandwidth is 26MHz. • 2.4 GHz – cordless phones, 801.11b, Bluetooth, microwave ovens. Is available in most countries. Bandwidth is 83.5 MHz. • 5.7 GHz – 802.11a. Is new and relatively uncrowded (so far) but a bit expensive. Bandwidth is 125MHz. (Why can 802.11a transmit at a high data rate?) • These are actually several bands. University of Delaware CPEG 419

  20. Spectrum Allocation University of Delaware CPEG 419

  21. Types of Connections • Long-haul – about 1500km (1000 miles) undersea, between major cites, etc. High capacity: 20000-60000 voice channels. Twisted pair, coaxial, fiber and microwave are used here. Microwave and fiber are still being installed. • Metropolitan trunks – 12km (7.5 miles) 100,000 voice channels. Link long-haul to city and within a city. Large area of growth. Mostly coaxial, twisted pair and fiber are used here. • Rural exchange trunks – 40-160km link towns. Twisted pair, coaxial, fiber and microwave are used here. • Subscriber loop – run from a central exchange to a subscriber. This connection uses twisted pair, and will likely stay that way for a long time. Cable uses coaxial and is a type of subscriber loop (it goes from central office to homes). But a large number of people share the same cable. • Local area networks (LAN) – typically under 300m. Sizes range from a single floor, a whole building, or an entire campus. While some use fiber, most use twisted pair as twisted pair is already installed in most buildings. Wireless (802.11) is also being used for LAN. University of Delaware CPEG 419

  22. Data Encoding (Chap. 5) • Transforming original signal just before transmission. • Both analog and digital data can be encoded into either analog or digital signals. University of Delaware CPEG 419

  23. Digital Transmission Terminology • Data element: bit. • Signaling element: encoding of data element for transmission. • Unipolar signaling: signaling elements have same polarization (all + or all -). • Polar signaling: different polarization for different elements. University of Delaware CPEG 419

  24. More Terminology • Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec. • Modulation rate = baud rate (rate at which signal levels change). University of Delaware CPEG 419

  25. Switch when a 1 occurs Approach 1: NRZ • But how do you know when to sample? • Phase-locked-loop (PLL) – measures the difference when transitions occur on the wire and when they occur on a local adjustable oscillator, and then make adjustments accordingly. • YOU MUST HAVE TRANSISTIONS TO LOCK ON TO. University of Delaware CPEG 419

  26. Multilevel Binary opposite direction Pros: No DC component. Can be used to force transitions (to help PLL). Cons: We are using 3 levels and could send ?? bits instead of 1 University of Delaware CPEG 419

  27. Scrambling – to help the PLL • If there are not enough transitions, the PLL may have problems. • So we force extra transitions when there are not enough. • Approach 1 – Use special coding so that long strings of zeros (or ones) don’t occur. University of Delaware CPEG 419

  28. Scrambling – to help the PLL • Approach 2 – Use multilevel binary and set illegal transitions to long strings of zeros. • Here, if an octet of zeros occurs, send a special illegal sequence. • The receiver must be able to interpret this special sequence. used in long-distance transmission University of Delaware CPEG 419

  29. Biphase – Differential Manchester(Self-Clocking) A transition always occurs in the middle of the period. A zero is represented by a transition occurring at the beginning of the period. A one is represented by no transition at the beginning of the period. 0 0 1 1 always a transition in the middle Used in CD players and Ethernet University of Delaware CPEG 419

  30. Methods to Encode Digital Signals • NRZ • Multilevel binary • Manchester • Issues: • DC? • Self Clocking? • How big is the spectrum? University of Delaware CPEG 419

  31. Sending Digital Signals over Analog (e.g. Modem) • Amplitude shift keying (ASK) (Amplitude Modulation) • Frequency shift keying (FSK) (Frequency modulation) • Phase shift keying (PK) (Phase Modulation) • Modems use phase and amplitude of them. University of Delaware CPEG 419

  32. Modulation Techniques ASK FSK PSK University of Delaware CPEG 419

  33. Phase-shift Keying • Quadrature phase-shift keying (QPSK) - send 2 bits. 90 0 180 270 University of Delaware CPEG 419

  34. QAM - Quadrature Amplitude Modulation constellation diagrams 90 90 0 180 0 180 270 270 QAM-16 (16 levels, how many bits) QAM - 64 University of Delaware CPEG 419

  35. V32 128 bits: 6 data and 1 parity (error correction) University of Delaware CPEG 419

  36. Use 2400 sample each way - duplex Definition: a duplex connection means that we can send data in both directions at the same time. A simplex or half-duplex connection only sends data in one direction at a time. How fast is V32? The phone system transmits 300 to 3400 Hz So what bandwidth can we use. How fast can we send symbols? So 2400 * 6 = 14400 bps What is the baud rate? V.34 2400 baud - with 12 data bits/symbol V.34 2400 baud – with 14 data bits/symbol That’s the fastest there is! To get 56K you send at 4000 baud (if the phone system can handle it) University of Delaware CPEG 419

  37. Digital Subscriber Lines (DSL) • ADSL – A for asymmetric, faster down load speed than up. • The 56kbps or 33kbps is because of a filter installed at the end office. • If this filter is removed, then the full spectrum of the twisted pair is available. • But, if you are far from the office, then you can’t get a very high data rate because…? • The DSL standard goes up to 8 Mbps down and 1 Mbps up. University of Delaware CPEG 419

  38. DSL A total of 256 4kHz channels Upstream downstream empty 25kHz (channel 6) voice (channel 0) 250 parallel channels: Each data channel uses QAM 16 (with 1 parity bit). The quality of each channel is monitored and adjusted. So channels may transmit at different speeds What is the maximum data rate? University of Delaware CPEG 419

  39. Digital Transmission: Receiver-Side Issues • Clocking: determining the beginning and end of each bit. • Transmitting long sequences of 0’s or 1’s can cause synchronization problems. • Signal level: determining whether the signal represents the high (logic 1) or low (logic 0) levels. • S/N ratio is a factor. University of Delaware CPEG 419

  40. Comparing Digital Encoding Techniques • Signal spectrum: high frequency means high bandwidth required for transmission. • Clocking: transmitted signal should be self-clocking. • Error detection: built in the encoding scheme. • Noise immunity: low bit error rate. University of Delaware CPEG 419

  41. Digital-to-Analog Encoding • Transmission of digital data using analog signaling. • Example: data transmission of a PTN. • PTN: voice signals ranging from 300Hz to 3400 Hz. • Modems: convert digital data to analog signals and back. • Techniques: ASK, FSK, and PSK. University of Delaware CPEG 419

  42. Amplitude-Shift Keying • 2 binary values represented by 2 amplitudes. • Typically, “0” represented by absence of carrier and “1” by presence of carrier. • Prone to errors caused by amplitude changes. University of Delaware CPEG 419

  43. Frequency-Shift Keying • 2 binary values represented by 2 frequencies. • Frequencies f1 and f2 are offset fromcarrier frequency by same amount in opposite directions. • Less error prone than ASK. University of Delaware CPEG 419

  44. Phase-Shift Keying • Phase of carrier is shifted to represent data. • Example: 2-phase system. • Phase shift of 90o can represent more bits: aka, quadrature PSK. University of Delaware CPEG 419

  45. Analog-to-Digital Encoding • Analog data transmitted as digital signal, or digitization. • Codec: device used to encode and decode analog data into digital signal, and back. • 2 main techniques: • Pulse code modulation (PCM). • Delta modulation (DM). University of Delaware CPEG 419

  46. Pulse Code Modulation 1 • Based on Nyquist (or sampling) theorem: if f(t) sampled at rate > 2*signal’s highest frequency, then samples contain all the original signal’s information. • Example: if voice data is limited to 4000Hz, 8000 samples/sec are sufficient to reconstruct original signal. University of Delaware CPEG 419

  47. PCM 2 • Analog signal -> PAM -> PCM. • PAM: pulse amplitude modulation; samples of original analog signal. • PCM: quantization of PAM pulses; amplitude of PAM pulses approximated by n-bit integer; each pulse carries n bits. University of Delaware CPEG 419

  48. Delta Modulation (DM) • Analog signal approximated by staircase function moving up or down by 1 quantization level every sampling interval. • Bit stream produced based on derivative of analog signal (and not its amplitude): “1” if staircase goes up, “0” otherwise. • Parameters: sampling rate and step size. University of Delaware CPEG 419

  49. Analog-to-Analog Encoding • Combines input signal m(t) and carrier at fc producing s(t) centered at fc. • Why modulate analog data? • Shift signal’s frequency for effective transmission. • Allows channel multiplexing: frequency-division multiplexing. • Modulation techniques: AM, FM, and PM. University of Delaware CPEG 419

  50. Amplitude Modulation (AM) • Carrier serves as envelope to signal being modulated. • Signal m(t) is being modulated by carrier cos(2p fct). • Modulation index: ratio between amplitude of input signal to carrier. University of Delaware CPEG 419

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