© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-2 Orientation 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 1

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Binary Data Representation

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5 Binary-Encoded Data Computers store and process data in binary representations –Binary means “two” –There are only ones and zeros –Called bits

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-6 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 …

© 2009 Pearson Education, Inc. Publishing as Prentice Hall3-7 Binary-Encoded Data Some data are inherently binary –48-bit Ethernet addresses –32-bit IP addresses –Need no further encoding

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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?

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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 –2 7 (128) characters possible –Sufficient for all keyboard characters (including shifted values)

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII ASCII –Sufficient for all keyboard characters CategoryMeaningASCII Capital lettersA Lower-case lettersa Digits Punctuation Special Space Printing controlCarriage Return Printing controlLine feed

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII Each ASCII Character is Sent in a Byte –8 th Bit in Data Bytes Normally Is Not Used Data Byte ASCII Code for Character Unused. Value does not matter

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : ASCII To send “Hello world!” (without the quotes), how many bytes will you have to transmit?

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Graphics Image and Conversion to Binary 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 2 Example 1: 8 bits per base color gives 256 levels per base color (2 8 ). Three base colors gives or over 16 million colors

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Layering Perspective 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 3-16 New: Not in the Book

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Signaling

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-18 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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)

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Binary Voltage Signaling in 232 Serial Ports The high state (0) is anything from +3 to +15 volts The low state (1) is anything from -3 to -15 volts 1

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Relative Immunity to Errors in Binary Signaling Binary signaling gives some immunity to errors. This is one of its major attractions.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Frequency Modulation frequency shift keying or FSK

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Phase Phase describes the position of the wavelength of the waveform relative to time 0. Phase is measured in degrees, –a phase shift of corresponds to a shift of complete period; –a phase shift of corresponds to a shift of one-half of a period; –a phase shift of 90 0 corresponds to a shift of one-quarter of a period.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall WCB/McGraw-Hill  The McGraw-Hill Companies, Inc., 1998 Phases

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling 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 Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling Concepts –Bits per baud: Number of bits that can be sent per clock cycle 1 if two states 2 if four states … Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling 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 Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling 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 Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Multistate Digital Signaling 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 Box

© 2009 Pearson Education, Inc. Publishing as Prentice Hall UTP Propagation Unshielded Twisted Pair wiring

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Unshielded Twisted Pair (UTP) Wiring UTP Characteristics –Inexpensive and to purchase and install –Dominates media for access links between computers and the nearest switch

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 4-Pair UTP Cord with RJ45 Connector 3. 8-pin RJ-45 Connector 2. 8 Wires organized as 4 twisted pairs Industry standard pen 1. UTP cord

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Unshielded Twisted Pair (UTP) Wiring Connector –RJ-45 connector is the standard connector –Plugs into an RJ-45 jack in a NIC, switch, or wall jack RJ-45 Jack RJ-45 Jack 8-pin RJ-45 connectors

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Attenuation and Noise Power Distance 3. Noise Floor (Average Noise level) 2. Noise 4. Noise Spike 1. Signal 2. Signal- to-Noise Ratio (SNR) 5. Error 1.The signal attenuates (falls in power) as it propagates 2.There is noise (random energy) in the wire that adds to the signal 3.The average noise level is called the noise floor 4.Noise is random. Occasionally, there will be large noise spikes 5.Noise spikes as large as the signal cause errors 6.You want to keep the signal-to-noise ratio high

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-41 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-42 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Electromagnetic Interference (EMI) and Twisting Interference on the Two Halves of a Twist Cancels Out Twisted Wire Electromagnetic Interference (EMI) UTP is twisted dpecifically to reduce EMI

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference Terminal crosstalk interference normally is the biggest EMI problem for UTP

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-45 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-46 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 cm or 0.5 inches

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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 3-47 New Not in Book

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-48 UTP Limitations 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 2

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Serial Versus Parallel Transmission

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-50 Figure 3-19: Wire Quality Standards CategoryTechnologyMaximum SpeedMaximum Ethernet Distance at this Speed 1UTPNever definedNot Applicable 2UTPNever definedNot Applicable 3UTP10 Mbps100 meters 4UTP10 Mbps100 meters 5UTP1 Gbps100 meters 5eUTP1 Gbps100 meters 6UTP1 Gbps100 meters 6UTP10 Gbps55 meters 6AUTP10 Gbps100 meters 7STP 1 10 Gbps+100 meters Category numbers indicate wire quality

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Roles of UTP and Optical Fiber in LANs

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-55 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 SC Connectors ST Connectors Two Strands Cord

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Full-Duplex Optical Fiber Cord with SC and ST Connectors SC Connector (push and click) ST Connector (bayonet connectors: 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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 Wave

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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 Wave

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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 Wave

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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)

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Wavelength, Core Diameters, Modal Bandwidth, and Maximum Propagation Distance for Ethernet 1000BASE-SX WavelengthCore DiameterModal BandwidthMaximum Propagation Distance 850 nm62.5 microns160 MHz.km220 m 850 nm62.5 microns200 MHz.km275 m 850 nm50 microns500 MHz.km550 m With 850 nm light, distance can be increased by using a smaller core diameter or using better-quality fiber with higher modal bandwidth

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : LAN Fiber Versus Carrier WAN Fiber LAN FiberCarrier WAN Fiber Required Distance Span 200 m to 300 m1 to 40 kilometers Transceiver Wavelength 850 nm1,310 nm (and sometimes 1,550 nm) Type of FiberMultimode (thick core)Single mode (thin core) Core Diameter50 microns or 62.5 microns 8.3 microns Primary Distance Limitation Modal dispersionAbsorptive attenuation Quality MetricModal bandwidth (MHz.km) NA LAN distance requirements are so short ( m) that multimode fiber and 850 nm light are sufficient. Multimode fiber quality (modal bandwidth), however, is important.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : LAN Fiber Versus Carrier WAN Fiber LAN FiberCarrier WAN Fiber Required Distance Span 200 m to 300 m1 to 40 kilometers Transceiver Wavelength 850 nm1,310 nm (and sometimes 1,550 nm) Type of FiberMultimode (thick core)Single mode (thin core) Core Diameter50 microns or 62.5 microns 8.3 microns Primary Distance Limitation Modal dispersionAbsorptive attenuation Quality MetricModal 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Radio Propagation

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-69 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Omnidirectional and Dish Antennas

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : 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.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines It is a physical layer concept

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies The simplest topology is the point-to-point topology

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies Ethernet uses a star topology Note that the switch does not have to be in the middle of the star

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies Larger Ethernet LANs use an extended star topology This is better called a hierarchical topology

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies In a mesh topology, there are many connections between switches or routers Consequently, there are many alternative routes between hosts

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies In the ring topology, messages travel around a loop

© 2009 Pearson Education, Inc. Publishing as Prentice Hall : Major Topologies The bus topology uses broadcasting. The message receives each host at almost the same time. All wireless transmission uses a bus topology.

© 2009 Pearson Education, Inc. Publishing as Prentice Hall Topics Covered

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-86 Topics Covered Binary Data Representation –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 86

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-87 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-88 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-89 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

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-90 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