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 Chapter 2 Chapter 3 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

3-1: 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

Binary Data Representation

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

Binary-Encoded Data Hello 11011001… 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…

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

3-2: Arithmetic with Binary Numbers Binary Arithmetic for Whole Numbers (Integers) (Counting Begins with 0, not 1) Integer 1 2 3 4 5 6 7 8 Binary 10 11 100 101 110 111 1000 “There are 10 kinds of people— those who understand binary and those who don’t”

3-2: Arithmetic with Binary Numbers Binary Arithmetic for Binary Numbers Basic Rules 1 0 0 1 1 +1 +0 +1 +0 +1 +1 =0 =1 =1 =10 =11

3-2: Arithmetic with Binary Numbers Examples Binary Decimal 1000 8 +1 +1 =1001 =9 =1010 =10 =1011 =11 +1 +1 =1100 =12

Encoding Alternative Bits (N) Alternatives (2N) 1 2 4 3 8 16 5 32 6 64 7 128 256 10 1,024 65,536 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

3-3: Binary Encoding for a Number of Alternatives Number of Bits in Field Number of Alternatives that Can Be Encoded1 Specific Bit Sequences Example 1 21 = 2 0, 1 Yes or No, Male or Female, etc. 2 22 = 4 00, 01, 10, 11 North, South, East, West 4 24 = 16 0000, 0001, 0010, … Top 10 security threats (6 values go unused) 8 28 = 256 00000000, 00000001, … ASCII text representation (128 values go unused) 1There are 2N alternatives with N bits

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?

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)

3-4: ASCII ASCII Sufficient for all keyboard characters Category Meaning ASCII Capital letters A 1000001 Lower-case letters a 1100001 Digits 3 0110011 Punctuation . 0101110 Special characters @ 1000000 Space 0100000 Printing control Carriage Return 0001101 Line feed 0001010

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

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

3-4: Extended ASCII and Calculators 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

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

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 stream We have already seen this Second, the 1s and 0s need to be converted into signals—disturbances that travel down the medium

Where is binary data encoding done? 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

Signaling

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

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

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

3-10: Four-State Digital Signaling 2 As you add more states/clock cycle, you can send more bits/clock cycle 2 states/clock cycle = 1 bit/clock cycle (binary) 4 states/clock cycle = 2 bits/clock cycle However, the states are closer together as you add more states This reduces immunity to error

3-10: Four-State Digital Signaling In a box but shown here The baud rate is the number of clock cycles per second The bit rate is the number of bits sent per second

Quiz Which Is Binary? Which Is Digital? 4. On/Off Switch 5. Number of Fingers 1. Calendar 3. Gender: Male or Female 2. Day of the Week

Box: Multistate Digital Signaling

3-11: Multistate Digital Signaling Box Digital Signaling Clock cycles Signal is held fixed during each clock cycle Binary signaling: two states Digital signaling: a few states (two or more) Up to about 256 states

3-11: Multistate Digital Signaling Box Why Use More than Two States? With more than two states, can send more than one bit per clock cycle Two states = 1 bit per clock cycle (1 or 0) Four states = 2 bits per clock cycle (00, 01, 10, 11) Eight states = 3 bits per clock cycle, etc. Each doubling of states gives one more bit per clock cycle

3-11: Multistate Digital Signaling Box Problem of Multiple States As the number of states increases, the difference between states decreases There is less tolerance for changes in the signal This is why there is a limit of a few states (256 maximum and usually much less)

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

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 …

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

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

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

Unshielded Twisted Pair wiring UTP Propagation Unshielded Twisted Pair wiring

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

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

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

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

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

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

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

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

Expressing Power Ratios in Decibels

3-15: Expressing Power Ratios in Decibels Power Ratios (Such as S/N Ratios) Are Encountered Frequently in Networking. Power Ratios for Attenuation The signal power ratio is P2/P1 P1 is the initial power and P2 is the received power The final received power is P2/P1 of the original power Example. Power starts (P1) at 200 milliwatts (mW) and falls to (P2) 100 mW P2/P1 = 100 / 200 = 0.5 = 50%

3-15: Power Ratios in Decibels Power Ratios Expressed in Decibels Power reduction ratios vary widely in attenuation When there is a wide range of values, engineers often express them in logarithms The equation for power ratios in decibels is dB = 10 log10 (P2/P1) Where P1 is the initial power and P2 is the final power after transmission If P2 is smaller than P1, then the answer will be negative In calculations, the Excel LOG10 function can be used

3-15: Power Ratios in Decibels Example Over a transmission link, power drops by 63 percent Therefore it drops to 37% of its original value P2/P1 = 37% / 100% =.37 From Excel: LOG10(0.37) = -0.4318 10*LOG10(0.37) = -4.3 dB The negative indicates power reduction through attenuation

3-15: Power Ratios in Decibels You Can Always Use the Formula But There are Two Useful Sets of Figures -3 dB loss is a power ratio of 1/2 (precisely -3.0103) -6 dB is a power ratio of 1/4 (precisely -6.0206) -9 dB is a power ratio of 1/8 (precisely -9.0309)  -10 dB loss is a power ratio of 1/0 (exactly) -20 dB is a power ratio of 1/100 (exactly) -30 dB is a power ratio of 1/1000 (exactly) So a power ratio of .4 is about ….

3-15: Power Ratios in Decibels When to Use Ratios as Fractions or as Decibels Expressed in decibels only for human reading In equations, the power ratio fraction is used

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

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

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

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

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

Shielded Twisted Pair Wiring (STP) 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 be good enough for normal environments However, we will see that Cat 7 wiring uses STP New Not in Book

UTP Limitations Limit cords to 100 meters 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

3-18: Serial Versus Parallel Transmission

Figure 3-19: Wire Quality Standards Category Technology Maximum Speed Maximum Ethernet Distance at this Speed 1 UTP Never defined Not Applicable 2 3 10 Mbps 100 meters 4 5 1 Gbps 5e 6 10 Gbps 55 meters 6A 7 STP1 10 Gbps+ Category numbers indicate wire quality

Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP

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

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

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

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

3-22: Full-Duplex Optical Fiber Cord with SC and ST Connectors (bayonet connectors: push and click) SC Connector (push and click) In contrast to UTP, which always uses RJ-45 connectors, there are several optical fiber connector types SC and ST are the most popular

3-23: Frequency and Wavelength Light travels in waves The amplitude is the intensity of the wave In sound waves, amplitude is loudness Amplitude is a measure of power

3-23: Frequency and Wavelength Wavelength is the physical distance between comparable points on adjacent cycles (peak-to-peak, trough-to-trough, start-to-start, etc.) Wavelengths are measured in meters Light is measured in wavelength So optical fiber transmission is specified by wavelength

3-20 Optical Fiber Strand In optical fiber transmission, light is expressed in nanometers. The transceiver transmits at 850 nm, 1,310 nm, or 1,550 nm Shorter-wavelength (850 nm) transceivers are less expensive Longer-wavelength (1,310 or 1,550 nm) light travels farther for a given speed For LAN fiber, 850 nm provides sufficient distance and dominates

3-23: Frequency and Wavelength Waves can also be measured in frequency The frequency is the number of complete cycles per second Hertz (Hz) is the term for cycles per second Radio transmission is measured in frequency Radio transmission usually takes place in the MHz or GHz range

3-25: Multimode Fiber and Single-Mode Fiber Multimode fiber has a thick core (50 or 62.5 microns in diameter) Light can only enter the core at certain angles, called modes Modes traveling straight through arrive faster than modes that bounce against the cladding several times

3-25: Multimode Fiber and Single-Mode Fiber Modal dispersion is the difference in time it takes modes to propagate If modal dispersion is too large, light from adjacent clock cycles will overlap, producing errors Modal dispersion is the limiting distance factor for multimode fiber

3-25: Multimode Fiber and Single-Mode Fiber Modal dispersion can be reduced by having a graded index of refraction in the core—decreasing from the center to the cladding. All multimode fiber is graded index multimode fiber today. Modal dispersion is also reduced by better-quality multimode fiber. Modal bandwidth (measured as MHz-km) is the measure of multimode fiber quality. (In UTP, quality is expressed by Category number)

3-26: Wavelength, Core Diameters, Modal Bandwidth, and Maximum Propagation Distance for Ethernet 1000BASE-SX Wavelength Core Diameter Modal Bandwidth Maximum Propagation Distance 850 nm 62.5 microns 160 MHz.km 220 m 200 MHz.km 275 m 50 microns 500 MHz.km 550 m With 850 nm light, distance can be increased by using a smaller core diameter or using better-quality fiber with higher modal bandwidth

3-25: Multimode Fiber and Single-Mode Fiber Single-mode fiber has a core diameter that is so small (8.3 microns) that only one mode can propagate. Consequently, there is no modal dispersion. Single mode fiber transmission distance is limited only by absorptive attenuation, which is extremely low. Consequently, single-mode fiber can carry signals for kilometers. However, single-mode fiber is more expensive than multimode. It is rarely used in LANs It is almost always used in carrier transmission lines

3-24: LAN Fiber Versus Carrier WAN Fiber Required Distance Span 200 m to 300 m 1 to 40 kilometers Transceiver Wavelength 850 nm 1,310 nm (and sometimes 1,550 nm) Type of Fiber Multimode (thick core) Single mode (thin core) Core Diameter 50 microns or 62.5 microns 8.3 microns Primary Distance Limitation Modal dispersion Absorptive attenuation Quality Metric Modal bandwidth (MHz.km) NA LAN distance requirements are so short (200-300 m) that multimode fiber and 850 nm light are sufficient. Multimode fiber quality (modal bandwidth), however, is important.

3-24: LAN Fiber Versus Carrier WAN Fiber Required Distance Span 200 m to 300 m 1 to 40 kilometers Transceiver Wavelength 850 nm 1,310 nm (and sometimes 1,550 nm) Type of Fiber Multimode (thick core) Single mode (thin core) Core Diameter 50 microns or 62.5 microns 8.3 microns Primary Distance Limitation Modal dispersion Absorptive attenuation Quality Metric Modal bandwidth (MHz.km) NA Carrier distances are so long (1 to 40 km) that carrier fiber is single-mode fiber, and wavelengths are long (1,310 or 1,550 nm). This is very expensive.

Radio Propagation

Radio Propagation Radio signals also propagate as waves. As noted earlier, radio waves are measured in hertz (Hz), which is a measure of frequency. Radio usually operates in the MHz and GHz range.

3-27: Omnidirectional and Dish Antennas

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.

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

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.

3-28: Wireless Propagation Problems An Example of Inverse Square Law Attenuation P1 = Power at Point A. P2 = Power at Point B (which is farther from A). r1 = Distance to Point A. r2 = Distance 2Point B (which is farther from A). P2 = P1 * (r1/r2)2 If the power is 400 mW (milliwatts) at 100 meters What is the power at 200 meters? P2 = 400 mW * (100/200)2 P2 = 400 mW * (1/2)2 = 400 mW * 1/4 = 100 mW

3-28: Wireless Propagation Problems Another Example of Inverse Square Law Attenuation P1 = Power at Point A. P2 = Power at Point B (which is farther from A). r1 = Distance to Point A. r2 = Distance to Point B (which is farther from A). P2 = P1 * (r1/r2)2 If the power is 900 mW (milliwatts) at 10 meters What is the power at 30 meters?

3-28: Wireless Propagation Problems Confusingly, wireless propagation suffers from two forms of attenuation. We have just seen inverse square law attenuation. There is also absorptive attenuation, which is attenuation because power is absorbed by water molecules along the way. Absorptive attenuation increases with frequency.

3-28: Wireless Propagation Problems When radio waves hit thick objects, they may not be able to penetrate. This creates shadow zones, which are also called dead spots. Shadow zones get worse as frequency increases.

3-28: Wireless Propagation Problems Multipath interference is the oddest propagation problem for radio. It is also the most important at wireless LAN frequencies. Sometimes, a reflected signal arrives just slightly after the direct signal. The direct and reflected signals will add together. If one signal is at its peak and the other is at its trough, then they may partially or completely cancel out.

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

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

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

3-29: Major Topologies Larger Ethernet LANs use an extended star topology This is better called a hierarchical topology

3-29: Major Topologies In a mesh topology, there are many connections between switches or routers Consequently, there are many alternative routes between hosts

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

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.

Topics Covered

Topics Covered Binary Data Representation Signaling Must first convert data into bits For instance, keyboard characters are represented with ASCII Signaling Then, bit streams must be converted into signals Binary versus digital signaling 98

Topics Covered UTP wiring Limit cords to 100 meters to make both noise and attenuation negligible problems Limit the untwisting of wires at the ends to 1.25 cm (a half inch) to reduce terminal crosstalk interference to a negligible problem Category number specifies UTP wiring quality Serial versus parallel transmission

Topics Covered Optical Fiber Typically, fiber for trunk lines, UTP for access lines Usually, a cord uses two strands for full-duplex transmission LANs use multimode fiber with a large core Modal bandwidth is the measure of multimode fiber quality LANs usually use 850 nm light, which is inexpensive but will carry signals far enough for LANs

Topics Covered Wireless Transmission Freedom of mobility Dish versus omnidirectional antennas Many propagation effects EMI Inverse square law attenuation Absorptive attenuation Shadow zones (dead spots) Multipath interference Absorptive attenuation and shadow zones get worse as frequency increases

Topics Covered Topology The arrangement of transmission lines, hosts, switches, and routers A physical layer concept Ethernet uses a star or extended star topology Wireless transmission uses a bus (broadcast) topology

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