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Physical Layer Propagation

Physical Layer Propagation. Chapter 3 Updated January 2009 Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book. Orientation. 1. Chapter 2 Data link, internet, transport, and application layers

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Physical Layer Propagation

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  1. Physical Layer Propagation Chapter 3Updated January 2009 Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book

  2. Orientation 1 • Chapter 2 • Data link, internet, transport, and application layers • Characterized by message exchanges • Chapter 3 • Physical layer (Layer 1) • There are no messages—bits are converted into signals • Concerned with transmission media, plugs, signaling methods, propagation effects • Chapter 3: Signaling, UTP, optical fiber, radio, and topologies

  3. 3-1: Signal and Propagation A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver If propagation effects are too large, the receiver will not be able to read the received signal

  4. BinaryData Representation

  5. Binary-Encoded Data • Computers store and process data in binary representations • Binary means “two” • There are only ones and zeros • Called bits 1101010110001110101100111

  6. Binary-Encoded Data • Non-Binary Data Must Be Encoded into Binary • Text • Integers (whole numbers) • Decimal numbers • Alternatives (North, South, East, or West, etc.) • Graphics • Human voice • etc. Hello 11011001…

  7. Binary-Encoded Data • Some data are inherently binary • 48-bit Ethernet addresses • 32-bit IP addresses • Need no further encoding

  8. 3-3: Binary Encoding for a Number of Alternatives • Examples: • 1. You have N bits. How many alternatives can you represent? • 1. You have 4 bits. How many alternatives can your represent? • 2. You need to represent 8 things. How many bits must you use? • 3. You need to represent 6 things. How many bits must you use?

  9. 3-4: ASCII • Purpose • To represent text (A, a, 3, $, etc.) as binary data for transmission • ASCII • Traditional code to represent text data in binary • Seven bits per character • 27 (128) characters possible • Sufficient for all keyboard characters (including shifted values)

  10. 3-4: ASCII • ASCII • Sufficient for all keyboard characters

  11. 3-4: ASCII Data Byte 1 0 1 0 0 1 1 1 ASCII Code for Character Unused. Value does not matter • Each ASCII Character is Sent in a Byte • 8th Bit in Data Bytes Normally Is Not Used

  12. 3-4: ASCII To send “Hello world!” (without the quotes), how many bytes will you have to transmit?

  13. 3-4: Extended ASCII and Calculators • Extended ASCII • Used on PCs • 8 bits per character • 2 8 (256) characters possible • Extra characters can represent formatting in word processing, etc. • Text-to-ASCII and Text-to-Extended ASCII Calculators • Readily available on the Internet

  14. 3-5: Graphics Image and Conversion to Binary 2 Example 2: Screen Resolution: 1000 x 500, so 500,000 pixels per screen If 24 bits/pixel, then 500,000 pixels/screen x 24 bits/pixel = 12,000,000 bits/screen or 1,500,000 bytes/screen Example 1: 8 bits per base color gives 256 levels per base color (28). Three base colors gives 2563 or over 16 million colors

  15. 3-6: Data Encoding and Signals We have just seen this We will now see this Before transmission, two things must happen First, data must be converted into a bit streamWe have already seen this Second, the 1s and 0s need to be converted intosignals—disturbances that travel down the medium

  16. Layering Perspective New: Not in the Book • Where is binary data encoding done? • It is done at the application layer, not at the physical layer. • Where is signaling done • It is done at the physical layer

  17. Signaling

  18. Figure 3-7: On/Off Signaling On/off signaling is used in optical fiber The light is turned on during a clock cycle for a 1 The light is turned off during a clock cycle for a 0 There are two signaling states—on and off This is called binary signaling This is a simple type of signaling

  19. Carrier Wave • There are three properties of a wave that can be modulated or altered: • Amplitude (Amplitude shift keying ASK) • Frequency (Frequency shift keying FSK) • Phase (Phase Shift Keying PSK)

  20. amplitude The peak amplitude of a signal is the absolute value of its highest intensity(strength), proportional to the energy it carries. For electric signals, amplitude is normally measured in volts.

  21. Amplitude Modulation • Each vertical lines separates opportunities to identify a 1 or 0 from another. • These timed opportunities are known as signaling events. • The proper name for one signaling event is a baud

  22. 3-8: Binary Voltage Signaling in 232 Serial Ports 1 The high state (0) is anything from +3 to +15 volts The low state (1) is anything from -3 to -15 volts

  23. 3-9: Relative Immunity to Errors in Binary Signaling Binary signaling gives some immunity to errors. This is one of its major attractions.

  24. Frequency Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. Frequency refers to the number of periods in 1 second. Frequency is measured in Hertz, cycles per second.

  25. Frequency Modulation • frequency shift keying or FSK

  26. Phase • Phase describes the position of the wavelength of the waveform relative to time 0. • Phase is measured in degrees, • a phase shift of 3600 corresponds to a shift of complete period; • a phase shift of 1800 corresponds to a shift of one-half of a period; • a phase shift of 900 corresponds to a shift of one-quarter of a period.

  27. Phases The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

  28. 3-11: Multistate Digital Signaling Box • Concepts • Bit rate: Number of bits sent per second • Baud rate: Number of clock cycles per second • If 1,000 clock cycles per second, 1 kbaud • If each clock cycle is 1/1,000 second = 1,000 clock cycles/second = 1 kbaud

  29. 3-11: Multistate Digital Signaling Box • Concepts • Bits per baud: Number of bits that can be sent per clock cycle • 1 if two states • 2 if four states • …

  30. 3-11: Multistate Digital Signaling Box • Computing the Bit Rate • Know the baud rate and the number of bits per baud • Multiply them • If baud rate is 10,000 baud (not bauds) • If two bits per clock cycle • Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps

  31. 3-11: Multistate Digital Signaling Box • Computing the Bit Rate • Know the baud rate and the number of states • Compute the number of bits from the number of states • Multiply the bits per clock cycle (per baud) • If baud rate is 10,000 baud (not bauds) • If four states, can send 2 bits per clock cycle • Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps

  32. 3-11: Multistate Digital Signaling Box • Computing the Required Number of States • Know the required bit rate and baud rate • Divide the bit rate by the baud rate to get the bits per baud • Compute the required number of states • Required bit rate is 4 Mbps • Baud rate is 1 Mbaud • Bit rate / baud rate = 4 bits per clock cycle • 4 bits per clock cycle are required

  33. UTP Propagation Unshielded Twisted Pair wiring

  34. 3-12: Unshielded Twisted Pair (UTP) Wiring • UTP Characteristics • Inexpensive and to purchase and install • Dominates media for access links between computers and the nearest switch

  35. 3-12: Unshielded Twisted Pair (UTP) Wiring • Standards • The TIA/EIA-568 standard governs UTP wiring in the United States • In Europe, the comparable standard is ISO/IEC 11801

  36. 3-13: 4-Pair UTP Cord with RJ45 Connector 3. 8-pin RJ-45 Connector 1. UTP cord Industry standard pen 2. 8 Wires organized as 4 twisted pairs UTP cord

  37. 3-12: Unshielded Twisted Pair (UTP) Wiring • Cord Organization • A length of UTP wiring is a cord • Each cord has eight copper wires • Each wire is covered with dielectric (nonconducting) insulation • The wires are organized as four pairs • Each pair’s two wires are twisted around each other several times per inch • There is an outer plastic jacket that encloses the four pairs

  38. 3-12: Unshielded Twisted Pair (UTP) Wiring RJ-45 Jack RJ-45 Jack 8-pin RJ-45 connectors • Connector • RJ-45 connector is the standard connector • Plugs into an RJ-45 jack in a NIC, switch, or wall jack

  39. 3-12: Unshielded Twisted Pair (UTP) Wiring • Characteristics • Inexpensive and easy to purchase and install • Rugged: Can be run over with chairs, etc. • Dominates media for access links • Connections to the workgroup switch

  40. 3-14: Attenuation and Noise Power 1. Signal 4. Noise Spike 3. Noise Floor (Average Noise level) 5. Error 2. Signal- to-Noise Ratio (SNR) 2. Noise Distance • The signal attenuates (falls in power) as it propagates • There is noise (random energy) in the wire that adds to the signal • The average noise level is called the noise floor • Noise is random. Occasionally, there will be large noise spikes • Noise spikes as large as the signal cause errors • You want to keep the signal-to-noise ratio high

  41. Limiting UTP Cord Length • Limit UTP cord length to 100 meters • This keeps the signal-to-noise ration (SNR) high • This makes attenuation and noise problems negligible • Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length

  42. UTP Wiring • Electromagnetic Interference (EMI) • Electromagnetic interference is electromagnetic energy from outside sources that adds to the signal • From fluorescent lights, electrical motors, microwave ovens, etc. • The problem is that UTP cords are like long radio antennas • They pick up EMI energy nicely • When they carry signals, they also send EMI energy out from themselves

  43. 3-16: Electromagnetic Interference (EMI) and Twisting UTP is twisted dpecifically to reduce EMI Electromagnetic Interference (EMI) Twisted Wire Interference on the Two Halves of a Twist Cancels Out

  44. 3-16: Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Crosstalk Interference Terminal crosstalk interference normally is the biggest EMI problem for UTP Terminal Crosstalk Interference

  45. Interference Hierarchy • EMI is any interference • Signals in adjacent pairs interfere with one another (crosstalk interference). This is a specific type of EMI • Crosstalk interference is worst at the ends, where the wires are untwisted. This is terminal crosstalk interference—a specific type of crosstalk EMI EMI Crosstalk Interference Terminal Crosstalk Interference

  46. Terminal Crosstalk Interference • Terminal crosstalk interference dominates interference in UTP • Terminal crosstalk interference is limited to an acceptable level by not untwisting wires more than a half inch (1.25 cm) at each end of the cord to fit into the RJ-45 connector • This reduces terminal crosstalk interference to a negligible level. 1.25 cm or 0.5 inches

  47. Shielded Twisted Pair Wiring (STP) New Not in Book • We have been talking about unshielded twisted pair wiring. • Is there a shielded twisted pair wiring? • Yes. It has a metal mesh shield around each pair to reduce cross-talk interference • It also has a metal mesh shield around the four pairs to reduce external EMI • It is no longer used extensively because UTP, which is much less expensive, was found to begood enough for normal environments • However, we will see that Cat 7 wiring uses STP

  48. UTP Limitations 2 • Limit cords to 100 meters • Limits BOTH noise AND attenuation problems to an acceptable level • Do not untwist wires more than 1.25 cm (a half inch) when placing them in RJ-45 connectors • Limits terminal crosstalk interference to an acceptable level • Neither completely eliminates the problems but they usually reduce the problems to negligible levels

  49. 3-18: Serial Versus Parallel Transmission

  50. Figure 3-19: Wire Quality Standards Category numbers indicate wire quality

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