1 Physical Layer Propagation Chapter 3 (Revised August 2002) Copyright 2003 Prentice-Hall Panko’s Business Data Networks and Telecommunications, 4 th edition

2 The Physical Layer Chapter 2 – Standards Standards above the physical layer Chapter 3 – The Physical Layer Real connections between machines No messages Propagation effects that change the signal as it propagates over the transmission medium Perspective

3 Figure 3.1: Signal and Propagation Sender Transmitted Signal Transmission Medium Received Signal (Attenuated & Distorted) Because of Propagation Effects Receiver Propagation

4 Analog, Binary, and Digital Analog and binary data and analog, binary, and digital signals

5 Figure 3.2: Analog and Binary Data Binary Data Analog Data Smoothly changing among an infinite number of states (loudness levels, etc.) Two states: One state represents 1 The other state represents 0

6 Quiz Which is Analog? Which is Binary? Gender Clock On/Off Switch Thermometer

7 Figure 3.3: Binary Data and Binary Signal 15 Volts (0) 0 Volts -15 Volts (1) Transmitted Signal 00 1 There are two states (in this case, voltage levels). One, (high) represents a 0. The other (low) represents a 1.

8 Figure 3.3: Binary Data and Binary Signal 15 Volts (0) Clock Cycle 0 Volts -15 Volts (1) Transmitted Signal 00 1 Time is divided into clock cycles The State is held constant within each clock cycle. It can jump abruptly at the end of each cycle. One bit is sent per clock cycle.

9 Figure 3.4: Binary Data and Digital Signal Client PC Server In binary transmission, there are two states. In digital transmission, there are few states (in this case, four). With four states, two information bits can be sent per clock cycle. 00, 01, 10, and 11 Binary transmission is a special case of digital transmission.

10 Quiz Which is Analog? Which is Digital? Calendar Clock Number Of Fingers Audio CD On/Off Switch

11 Figure 3.4: Baud Rates for Digital Signals Client PC Server Suppose that the clock cycle is 1/10,000 second. Then the baud rate is 10,000 baud (10 kbaud). The bit rate will be 20 kbps (two bits/clock cycle times 10,000 clock cycles per second). (The bit rate gives the number of information bits per second.) Baud Rate = # of Clock Cycles/Second

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

13 Perspective Analog Data Smooth changes among an infinite number of states—like hands going around an analog clock Digital Data Few states In a digital clock, each position can be in one of ten states (the digits 0 through 9) Binary Data Two states (a special case of digital)

14 Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line Computer Modem Telephone PSTN Modulated Analog Signal 1011 Amplitude (Loudness or Intensity) Modulation Binary Data 1011 becomes loud-soft-loud-loud

15 Figure 3.5: Using a Modem to Send Binary Data Over an Analog Transmission Line Computer Modem Telephone PSTN Modulated Analog Signal 1011 Amplitude (Loudness or Intensity) Modulation Demodulated Binary Data Loud-soft-loud-loud becomes 1011

16 Figure 3.6: Sending Analog Data Over a Digital Line Analog Data Source Digital Transmission Line Encoding Decoding Digital Signal (Binary Example) Digital Signal (Binary Example) Many Time Periods So Fairly Smooth Codec Analog Data

17 Data and Signals: Modems Vs. Codecs Analog Line Signal Digital Line Signal Analog dataCodec Digital data (including binary data) Modem

18 UTP Media and Propagation Effects

19 Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector Single Twisted Pair Jacket Four pairs (each pair is twisted) There is insulation around each wire.

20 Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector A length of UTP is called a cord. There is no metal shielding around The individual pairs or around the entire Cord. Hence the name unshielded UTP UTP Cord

21 Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector The cord terminates in an 8-pin RJ-45 connector, which plugs into an RJ-45 jack in the NIC, switch Or wall jack. Pin 1 on left of Jack RJ-45 Jack 8-Pin RJ-45 Connector

22 Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector RJ-45 Connector UTP Cord

23 Figure 3.7: 4-Pair Unshielded Twisted Pair Cable with RJ-45 Connector With RJ-45 Connector 4 Pairs Separated Pen

24 Figure 3.8: Noise and Attenuation Distance Signal Noise Floor (average) Noise Power

25 Figure 3.8: Noise and Attenuation Distance Signal Noise Spike Noise Power Damage Noise Floor (average)

26 Figure 3.8: Noise and Attenuation Distance Signal Noise Floor (average) Noise Signal- to-Noise Ratio (SNR) Power SNR = Signal Power / Noise Power If SNR is high, noise errors are rare As signal travels, it attenuates, and noise errors increase

27 Noise and Attenuation The TIA/EIA-568 standard recommends that UTP runs be kept to 100 meters If this distance limit is observed, problems with noise and attenuation usually are minor Low-tech solution, but it works well.

28 Figure 3.9: Twisting Wire Paris to Reduce Electromagnetic Interference (EMI) Interference On the Two Halves of a Twist Cancels Out Interference Twisted Wire

29 Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference Signal Crosstalk Interference

30 Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference

31 Figure 3.10: Crosstalk Electromagnetic Interference (EMI) and Terminal Crosstalk Interference EMI is any interference Signals in adjacent pairs interfere with one another (crosstalk interference) 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. Solution: untwist wires for connector no more than 1.25 cm (0.5 in). Does not eliminate terminal cross-talk interference but makes it negligible

32 Limiting UTP Propagation Problems Two simple things can limit UTP propagation problems Limit cord distances to 100 meters to control attenuation and noise effects Limit the untwisting of wires at the connectors to 1.25 cm (0.5 inch) to control terminal crosstalk interference. If these rules are followed strictly, propagation problems should be negligible

33 Figure 3.11: Serial versus Parallel Transmission Serial Transmission (1 bit per clock cycle) Parallel Transmission (1 bit per clock cycle per wire pair) 4 bits per clock cycle on 4 pairs

34 Figure 3.11: Serial versus Parallel Transmission Serial Transmission: one bit per clock cycle if binary transmission Parallel Transmission with N wire pairs: N bits per clock cycle if binary transmission Not limited to four wire pairs (can be 2, 8, 100, etc.) The advantage of parallel transmission is that it is faster than serial transmission Only works over very short distances.

35 Optical Fiber Media and Propagation Effects

36 Figure 3.12: Optical Fiber Cabling Light Source (LED or Laser) Cladding Core Light Ray Reflection at Core/Cladding Boundary

37 Figure 3.13: Wavelength Division Multiplexing (WDM) in Optical Fiber Light Source 2 Light Source 1 Optical Fiber Core Multiple Light Sources Transmit on Different Wavelengths Each Carries a Separate Signal More Capacity Per Fiber

38 Figure 3.14:Full-Duplex Optical Fiber Cord Switch Router Fiber Cord A pair of fibers is needed for full-duplex (simultaneous 2-way) transmission. Each fiber carries a signal in only one direction. SC, ST, or other connector

39 Optical Fiber Cabling ST Connectors (Popular) SC Connectors (Recommended) Two fiber cords for full-duplex (two- way) transmission

40 Figure 3.15: Multimode & Single-Mode Fiber Light Source Core Cladding Multimode Fiber Modes Light only travels in one of several allowed modes Light travels faster at the edges to speed modes going the farthest Multimode fiber must keep its distance short or limit modal distortion Multimode fiber goes a few hundred meters and is inexpensive to lay It is dominant in LANs

41 Figure 3.15: Multimode & Single-Mode Fiber Light Source Core Cladding Graded Index of Refraction (Decreasing from Center) Graded Index Multimode Fiber Modes Signals Travel Fastest On Outside of Core

42 Figure 3.15: Multimode & Single-Mode Fiber Light Source Single Mode Fiber Cladding Core Single Mode Core is so thin that only one mode can propagate. No modal dispersion, so can span long distances without distortion. Expensive, so not widely used in LANs. Popular in WANs

43 Multimode and Single-Mode Fiber Multimode Limited distance (a few hundred meters) Inexpensive to install Dominates fiber use in LANs Single-Mode Fiber Longer distances: tens of kilometers Expensive to install Commonly used by WANs and telecoms carriers

44 Radio Transmission and Propagation Effects

45 Figure 3.16: Omnidirectional and Dish Antennas Dish Antenna Concentrates incoming and outgoing signals Signals can travel far Omnidirectional Antenna No need to point to sender or receiver Rapid attenuation with distance

46 Figure 3.17: Radio Wave Amplitude Wavelength Frequency Measured in Hertz (Cycles per Second) 2 Cycles in one Second, so 2 Hz Wavelength * Frequency = Speed of Propagation

47 Figure 3.18: The Frequency Spectrum, Service Bands, and Channels Channel 4 Channel 3 Channel 5 Channel 2 Channel 1 Frequency Spectrum (0 Hz to infinity) Service Band 0 Hz A service band has a specific purpose, such as FM radio or cellular telephony. Service bands are divided into channels. Signals sent in different channels do not interfere with one another. Channels with wider bandwidths can carry signals faster.

48 Shannon’s Law Here W = maximum possible speed in channel B = bandwidth (highest frequency minus lowest frequency) S/N = signal to noise ratio Wide bandwidth (broadband) gives high speed Small bandwidth (narrowband) gives low speeds W = B * Log 2 (1 + S/N) Figure 3.18: The Frequency Spectrum, Service Bands, and Channels

49 Laptop Comm. Tower Figure 3.19: Wireless Propagation Problems Inverse Square Law Attenuation Very Rapid Attenuation with Distance Compared to Wires and Fiber

50 Laptop Comm. Tower Shadow Zone: No Signal Figure 3.19: Wireless Propagation Problems Multipath Interference Signals Arriving at Slightly Different Times Can Interfere

51 Golden Zone At lower frequencies, there is little total bandwidth. At very high frequencies, propagation is poor. Mobile devices tend to work in the “golden zone” from the high megahertz to the low gigahertz range. Frequencies in the golden zone are limited and in high demand.

52 Topology Transmit

53 Figure 3.20: Major Topologies A network technology’s topology is the order in which stations are connected to one another via media. Point-to-Point The Simplest Topology

54 Figure 3.20: Major Topologies Star (Modern Ethernet)Extended Star or Hierarchy (Modern Ethernet) Root Switch Only one possible path between two stations Switch

55 Figure 3.20: Major Topologies Mesh (Routers, Frame Relay, ATM) Multiple alternative paths between two stations A B C D Path ABD Path ACD

56 Figure 3.20: Major Topologies Ring (802.5, FDDI, SONET/SDH) Only one possible path between two stations

57 Figure 3.20: Major Topologies Daisy Chain Bus (Ethernet 10Base2) Multidrop Line Bus (Ethernet 10Base5) All stations hear each transmission Only one possible path between two stations Transmit

58 Recap Analog, Binary, and Digital: Data and Signals Transmission Media UTP (limit distance and wire untwisting) Optical Fiber (multimode for most LAN use) Radio (freedom but weird propagation and limited spectrum) General Concepts Propagation effects Full duplex Serial versus parallel transmission