Physical Layer Propagation

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. 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

3. BinaryData Representation

4. Binary-Encoded Data • Computers store and process data in binary representations • Binary means “two” • There are only ones and zeros • Called bits Non-Binary Data Must Be Encoded into Binary 1101010110001110101100111 Hello 11011001…

5. Encoding Alternative An N-bit field can represent 2N alternatives Each additional bit doubles the number of possibilities Start with one you know and double or halve until you have what you need E.g., if you know 8 is 256, 10 must be 4 times as large or 1,024

6. 3-3: Binary Encoding for a Number of Alternatives 1There are 2N alternatives with N bits

7. 3-3: Binary Encoding for a Number of Alternatives • Examples: 1. You have N bits. How many alternatives can you represent? 2. You have 4 bits. How many alternatives can your represent? 3. You need to represent 8 things. How many bits must you use? 4. You need to represent 6 things. How many bits must you use?

8. 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)

9. 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

10. 3-4: ASCII To send “Hello world!” (without the quotes), how many bytes will you have to transmit?

11. 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

12. 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

13. 3-11: Multistate Digital Signaling Box • Computing the Bit Rate Bit rate = Baud rate X Bits sent per clock cycle EX: • If baud rate is 10,000 baud • If two bits per clock cycle • Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps

14. 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 • States = 2Bits per clock cycle Bit rate = Baud rate X Bits sent per clock cycle EX • 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

15. 3-11: Multistate Digital Signaling Box • Computing the Required Number of States • Know the required bit rate and baud rate • Bits sent per clock cycle =Bit rate / Baud rate • Compute the required number of states • EX: • 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

16. Bit Rate versus Baud Rate Number of Possible States Bits per Clock Cycle If a Baud Rate is 1,200 Baud, Bit Rate is 2 (Binary) 1 1,200 bps 4 2 2,400 bps 8 3 3,600 bps 16 4 4,800 bps Each Doubling of States Gives One More Bit per Clock Cycle

17. Quiz: There are eight states. Each clock cycle is 1/8000 of a second. What is the baud rate? What is the bit rate?

18. UTP Propagation Unshielded Twisted Pair wiring

19. Transmission Media • Two main categories: • wires, cables • wireless transmission, e.g. radio, microwave, infrared, … • Wired • Twisted-Pair cables: • Coaxial cables • Fiber-optic cables

20. 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

21. 3-12: Unshielded Twisted Pair (UTP) Wiring • Cord Organization • A length of UTP wiring is a cord • Each cord has eight copper wires • 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

22. 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

23. 3-14: Attenuation and Noise Power 1. Signal 4. Noise Spike 3. Noise Floor (Average Noise level) 5. Error 6. 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

24. 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

25. 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.

26. 3-16: Electromagnetic Interference (EMI) and Twisting UTP is twisted to reduce EMI Electromagnetic Interference (EMI) Twisted Wire Interference on the Two Halves of a Twist Cancels Out

27. 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

28. 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

29. Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP

30. 3-20: Optical Fiber Transceiver and Strand An optical fiber strand has a thin glass core This core is 8.3, 50, or 62.5 microns in diameter This glass core is surrounded by a tubular glass cladding The outer diameter of the cladding is 125 microns, regardless of the core’s diameter The transceiver injects laser light into the core

31. 3-20: Optical Fiber Transceiver and Strand When a light wave ray hits the core/cladding boundary, there is perfect internal reflection. There is no signal loss

32. 3-21: Roles of UTP and Optical Fiber in LANs

33. Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors Cord A fiber cord has two-fiber strands for full-duplex (two-way) transmission Two Strands SC Connectors ST Connectors

34. Radio Propagation

35. Radio Propagation Radio signals also propagate as waves. Radio waves are measured in hertz (Hz), which is a measure of frequency. Radio usually operates in the MHz and GHz range. Hertz (Hz) is the term for cycles per second

36. 3-27: Omnidirectional and Dish Antennas

37. 3-28: Wireless Propagation Problems UTP and optical fiber propagation are fairly predictable. However, radio suffers from many propagation effects. This makes radio transmission difficult to manage. We will look at these problems one at a time.

38. 3-28: Wireless Propagation Problems The first propagation problem is electromagnetic interference (EMI) from nearby radio sources This includes other wireless devices It can include microwave ovens an other devices

39. 3-28: Wireless Propagation Problems Another problem is inverse square law attenuation. As a signal propagates, its energy spreads out over the Surface of an ever-expanding sphere.

40. 3-28: Wireless Propagation Problems Shadow Zone Laptop Comm. Tower No Signal

41. 3-28: Wireless Propagation Problems Multipath Interference Laptop Comm. Tower Signals Arriving by Different Paths May Cancel Out

42. Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines It is a physical layer concept

43. 3-29: Major Topologies The simplest topology is the point-to-point topology

44. 3-29: Major Topologies Ethernet uses a star topology Note that the switch does not have to be in the middle of the star

45. Mesh (Routers, Frame Relay, ATM) 3-29: Major Topologies A Path ABD B C D Path ACD In a mesh topology, there are many connections between switches or routers Consequently, there are many alternative routes between hosts

46. 3-29: Major Topologies In the ring topology, messages travel around a loop

47. 3-29: Major Topologies The bus topology uses broadcasting. The message receives each host at almost the same time. All wireless transmission uses a bus topology.