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Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Updated by Alan Holbrook July 2008 Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book 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 sent individually – Concerned with transmission media, plugs, signaling methods, propagation effects – Chapter 3: Signaling, UTP, optical fiber, and topologies – Wireless transmission is covered in Chapter 5 2 Figure 3-1: Signal and Propagation Received Signal (Attenuated & Distorted) Transmitted Signal Propagation Transmission Medium Sender Receiver 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 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 5 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… 6 Binary-Encoded Data • Some data are inherently binary – 48-bit Ethernet addresses – 32-bit IP addresses – Need no further encoding 7 Figure 3-2: Arithmetic with Binary Numbers Binary Arithmetic for Whole Numbers (Integers) (Counting Begins with 0, not 1) Integer 0 1 2 3 4 5 6 7 8 Binary 0 1 10 11 100 101 110 111 1000 “There are 10 kinds of people— those who understand binary and those who don’t” 8 Figure 3-3: Binary Encoding for Alternatives Encoding Alternatives (Product number, region, gender, etc.) (N bits can represent 2N Alternatives) Number of Bits In Field (N) 1 2 3 4 8 16 … Number of Alternatives That Can be Encoded with N bits 2 (21) 4 (22) 8 (23) 16 (24) 256 (28) 65,536 (216) … Each added bit doubles the number of alternatives that can be represented 9 3-2: Arithmetic with Binary Numbers Binary Arithmetic for Binary Numbers Basic Rules 0 +0 =0 0 +1 =1 1 +0 =1 1 +1 =10 1 +1 +1 =11 3-10 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) 3-11 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 @ 1000000 Special characters Space 0100000 Printing control Carriage Return 0001101 Printing control Line feed 0001010 3-12 3-4: ASCII • Each ASCII Character is Sent in a Byte – 8th Bit in Data Bytes Normally Is Not Used Data Byte 1 0 1 0 0 ASCII Code for Character 1 1 1 Unused. Value does not matter 3-13 3-4: ASCII • To send “Hello world!” (without the quotes), how many bytes will you have to transmit? 3-14 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 3-15 The Physical Layer • The physical layer includes network hardware and circuits. • Network circuits include physical media (e.g., cables) and special purposes devices (e.g., routers and hubs). Networks are made of both physical and logical circuits. – Physical circuits connect devices & include actual wires. – Logical circuits refer to the transmission characteristics of the circuit, such as a T-1 connection. • Sometimes the physical and logical circuits are the same, but they can be different. For example, in multiplexing, one wire carries several logical circuits. Analog and Digital Data • Another fundamental physical layer distinction is between digital and analog forms of data. • Sounds waves, which vary continuously over time are analog data. • Computers produce digital data that is in binary form, that is, it is represented as a series of ones and zeros. Advantages of Digital Transmission • Digital transmission: – produces fewer errors than analog transmission. Because the transmitted data is binary (1s and 0s), it is easier to detect and correct errors. – permits higher transmission rates. Optical fiber, for example, is designed for digital transmission. – is more efficient. It’s possible to send more data through a given circuit using digital rather than analog transmission. – is more secure since it is easier to encrypt. • Integrating voice, video and data on the same circuit is also far simpler with digital transmission since signals made up of digital data are easier to combine. 18 Figure 3-6: Data Encoding and Signaling Data “Now is the …” Male or Female Graphics Human Voice 1. First, data must be converted to binary, as we have just seen Binary Encoding BinaryEncoded Data 1101010 Signaling 2. Second, bits must be covered Into signals (voltage changes, etc.). Voltage change, etc. 19 Serial transmission Digital Transmission • Digital signals are sent as a series of “square waves” of either positive or negative voltage. Voltages vary between +3/-3 and +24/-24 depending on the circuit. • Each digital transmission standard defines what voltage levels correspond to a bit value of 0 or 1. • Unipolar signal voltages either vary between 0 and a positive value or between 0 and some negative value. Digital Transmission (cont.) • With bipolar signals, signals are sent using both positive and negative voltages. • A second digital transmission factor, called return to zero (RZ) means the signal returns to the 0 voltage level after sending a bit. In non return to zero (NRZ), the signals maintains its voltage at the end of a bit. • Ethernet uses Manchester encoding in which the bit value is defined by a mid-bit transition. A high to low voltage transition is a binary 0 and a low-high mid-bit transition defines a binary 1. Digital transmission types Figure 3-7: On/Off Binary Signaling Clock Cycle Light Source Off= 0 On= 1 On= 1 Off= 0 On= 1 Off= 0 On= 1 Optical Fiber During each clock cycle, light is turned on for a one or off for a zero. 24 Figure 3-8: Binary Signaling in 232 Serial Ports In a clock cycle, 15 Volts Clock Cycle -2 to -15 volts is a zero 0 3 Volts 3 to 15 volts represents a one 0 0 0 Volts -3 Volts 1 -15 Volts 1 This type of signaling is used in 232 serial ports. 25 Figure 3-9: Relative Immunity to Errors in Binary Signaling 15 Volts 0 Transmitted Signal (12 Volts) Received Signal (6 volts) 3 Volts 0 Volts -3 Volts 1 Despite a 50% drop in voltage, the receiver will still know that the signal is a zero -15 Volts 26 Binary and Binary Signaling • In binary signaling, there are two states – This can represent a single bit per clock cycle. • In digital signaling, there are a few bits per clock cycle—2, 4, 8, 16, 32, … • With more states, several bits to be sent per clock cycle • Note that all binary transmission (2 states) is digital (few states) • But not all digital transmission is binary 11 11 10 01 00 10 01 Clock Cycle 01 00 27 Figure 3-10: 4-State Digital Signaling Box Clock Cycle 11 11 10 01 00 Client PC 10 01 01 00 Server Digital signaling has a FEW possible states per clock cycle (4 in this slide) This allows it to send multiple bits per clock cycle This increases the bit transmission rate per clock cycle It reduces error resistance because differences between states are smaller 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 3-29 Transmission Media Communications Media • Medium: the physical matter that carries the transmission. Two basic categories of media: • With Guided media the transmission flows along a physical guide. The three main types of guided media: twisted pair wiring, coaxial cable and optical fiber cable. • With Wireless media there is no wave guide and the transmission just flows through the air (or space). The main forms of wireless communications are radio, infrared, microwave, and satellite communications. Guided Media: Twisted Pair Wires • Twisted pair wire cables are commonly used for telephones and local area networks. • Twisting two wires together reduces electromagnetic interference. • TP cables have a number of pairs of wires. – Telephone lines have two pairs (4 wires, usually only one pair is used by the telephone) – LAN cables have 4 pairs (8 wires) • Shielded twisted pair also exists, but is more expensive. • TP cables are also used in telephone trunk lines and can have up to several thousand pairs. UTP Propagation Unshielded Twisted Pair wiring Figure 3-12: 4-Pair UTP Cord with RJ45 Connector 3. RJ-45 Connector 1. UTP Cord Industry Standard Pen 2. 8 Wires Organized as 4 Twisted Pairs UTP Cord 34 RJ-45 Jacks and Connectors RJ-45 Jack RJ-45 Jack RJ-45 Jack RJ-45 Connectors 35 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued • UTP Characteristics – Inexpensive and to purchase and install – Dominates media for access links between computers and the nearest switch 36 Coax Cable Guided Media: Coaxial Cable • Formerly common on LANs, but now disappearing (but still used on other comm. equipment, e.g., CATV). • More expensive than twisted pair, but coax is shielded, so it’s less prone to interference than twisted pair. • Coaxial Cable Structure – Inner conductor – Insulator – Wire mesh ground – Outer protective jacket or shell 38 Figure 3-5 Coaxial Cable 39 Optical Fiber Transmission Light through Glass Better than UTP: More Easily Spans Longer Distances at High Speeds Figure 3-19: UTP in Access Lines and Optical Fiber in Trunk Lines 1. Workgroup Switches Link Computers to the Network Workgroup Switch UTP Access Line 2. UTP dominates access lines between stations and their workgroup switches UTP Access Line UTP Access Line 41 Figure 3-19: UTP in Access Lines and Optical Fiber in Trunk Lines, Continued 1. Core switches connect other switches Fiber Trunk Fiber Trunk Fiber Trunk Core Switch Core Switch Core Fiber Trunk Core Switch Fiber Trunk 2. Fiber dominates trunk lines between switches 42 Figure 3-20: Optical Fiber Transceiver and Strand Strand 3. Cladding 125 micron diameter Transceiver 1. (Transmitter/Receiver) Light Source 5. 850 nm, Perfect internal reflection at 1,310 nm, core/cladding boundary; and 1,550 nm No signal loss, so low attenuation 2. Core 8.3, 50 or 62.5 micron diameter 4. Light Ray 43 Figure 3-22: Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors Cord Two Strands A fiber cord has two-fiber strands for full-duplex (twoway) transmission SC Connectors ST Connectors 44 Figure 3-22: Pen and Full-Duplex Optical Fiber Cord with SC and ST Connectors SC Connectors (Push in and Snap) ST Connectors (Bayonet: Push in and Twist) 45 Figure 3-23: Frequency and Wavelengths 2. Wavelength Distance between comparable points in successive cycles (Measured in nanometers for light) Wave 1. Amplitude Power, Voltage, etc. Amplitude 1 Second 3. Frequency is the number of cycles per second. 1 Hz = 1 cycle per second In this case, there are two cycles in 1 second, so frequency is two hertz (2 Hz). 46 Light Wavelengths • Light signals are measured by wavelength • Light wavelengths measured in nanometers (nm) • There are three fiber wavelength “windows” with good propagation characteristics – 850 nm – 1310 nm – 1550 nm • Shorter wavelength allows cheaper transceivers • Longer-wavelength light travels farther 47 Figure 3-24: Carrier Fiber and LAN Fiber • LAN Fiber – Uses multimode fiber, which has a “thick” core diameter of 50 or 62.5 microns • Less expensive than single-mode fiber (later) • 62.5 micron fiber is more common in the US but does not carry signals as far as 50 micron fiber – Also uses inexpensive 850 nm transceivers – Multimode fiber with 850 nm signaling cannot span the kilometer distances needed by carriers, but can span the 200-300 meters needed in LAN fiber cords 48 Figure 3-24: Multimode and Single-Mode Optical Fiber Mode 2 Light Source (Usually Laser) Core Multimode Fiber Mode 1 Arrives Later In thicker fiber, light only travels in one of several allowed modes. Different modes travel different distances and arrive at different times (See that Mode 1 light takes longer to arrive than Mode 2 light.) If distance is too long, modes from successive light pulses will overlap. This is modal distortion. If it is too large, signals will be unreadable. Modal distortion is the main limitation on distance in multimode fiber. 49 Figure 3-24: Carrier Fiber and LAN Fiber • LAN Fiber – All multimode fiber today is graded-index multimode fiber • The index of refraction decreases from the center of the core to the core’s outer edge. Lower Higher Incidence of Refraction 50 Figure 3-24: Carrier Fiber and LAN Fiber • LAN Fiber – Graded-index multimode fiber • Light speed increases when the index decreases • The central mode (Mode 2) is slowed • High-angle modes (Mode 1) are speeded up • Modal dispersion between the modes is reduced Mode 2 (Slowed) Mode 1 (Speeded Up Near Edge of Core) Lower Modal Dispersion 51 Figure 3-24: Carrier Fiber and LAN Fiber • LAN Fiber – UTP quality is measured by category number. – Multimode Fiber Quality • Measured as modal bandwidth (MHz.km or MHz-km) • More modal bandwidth is better • Increases the speed–distance product – With greater mobile bandwidth, can go faster, farther, or some combination of the two 52 Figure 3-24: Carrier Fiber and LAN Fiber • LAN Fiber – Example: 1000BASE-SX Ethernet • Uses inexpensive 850 nm light • With 62.5 micron fiber and 160 MHz-km modal bandwidth, maximum distance is 220 m • With 62.5 micron fiber and 200 MHz-km bandwidth, maximum distance is 275 m • Some vendors with higher-than-standard modal bandwidth can carry traffic farther 53 Figure 3-24: Carrier Fiber and LAN Fiber • LANs and WAN carriers use different types of fiber • Carrier Fiber – Carrier fiber must span long distances – This requires expensive long-wavelength laser light sources (1,310 and 1,550 nm) – It also requires expensive “single-mode” fiber with a very narrow core (8.3 microns) 54 Figure 3-24: Multimode and Single-Mode Optical Fiber , Continued Single Mode Light Source Cladding Core Single-Mode Fiber Light enters only at certain angles called modes Single-mode fiber cores are so thin that only one mode can propagate—the one traveling straight through No modal dispersion (discussed earlier), so can span long distances without this distortion Expensive but necessary in WANs 55 Figure 3-24: Carrier Fiber and LAN Fiber • Carrier Fiber – Main propagation effect for single-mode fiber is attenuation, which is very low • For 850 nm light, attenuation is around 2.5 dB/km • At 1,310 nm, attenuation is lower—about 0.8 dB/km • At 1,550 nm, attenuation falls even lower—about 0.2 dB/km – Longer wavelengths carry farther but cost more – Carrier fiber uses wavelengths of 1,310 or 1,550 nm 56 Figure 3-24: Carrier Fiber and LAN Fiber • Noise and Electromagnetic Interference (EMI) Are Not Problems for Either LAN or Carrier Fiber – Noise from moving electrons cannot interfere with light signals – EMI would have to be light signals • Wrapping the cladding in an opaque covering prevents light from coming in 57 Figure 3-24: Carrier Fiber and LAN Fiber Cost Fiber Type Corporate LAN Multimode Fiber Only 200-300 meters Much Lower ($) Multimode ($) Wavelength Usually 850 nm ($) Needed Distance Carrier (WAN) Single-Mode Fiber Many kilometers Very high ($$$$) Single-mode ($$$$) Typical Core Usually 1,310 or 1,550 nm ($$$$) 50/62.5 microns ($) 8.3 microns ($$$) Propagation Limit Modal Distortion Is Modal Bandwidth Yes Important? Attenuation No. Only attenuation matters 58 Wireless Media 59 Wireless Media • Wireless media signals are becoming popular for LAN use. The main forms are: – Radio: wireless transmission of electrical waves. Includes AM and FM radio bands. Microwave is also a form of radio transmission. – Infrared: “invisible” light waves whose frequency is below that of red light. Requires line of sight and are generally subject to interference from heavy rain. Used in remote control units (e.g., TV). – Microwave: high frequency form of radio with extremely short wavelength (1 cm to 1 m). Often used for long distance, terrestrial transmissions and cellular telephones. Requires line-of-sight. Satellite Communications Satellite & Microwave Communications • Common Applications - Radio Relay - data, voice, video • Microwave – Characteristics – +2 GHz – Directional – Line of Sight –… Satellite Communications • RF Bands Band Name HF-band VHF-band P-band UHF-band L-band FCC's digital radio S-band C-band X-band Ku-band (Europe) Ku-band (America) Ka-band Frequency Range 1.8-30 MHz 50-146 MHz 0.230-1.000 GHz 0.430-1.300 GHz 1.530-2.700 GHz 2.310-2.360 GHz 2.700-3.500 GHz Downlink: 3.700-4.200 GHz Uplink: 5.925-6.425 GHz Downlink: 7.250-7.745 GHz Uplink: 7.900-8.395 GHz Downlink: FSS: 10.700-11.700 GHz DBS: 11.700-12.50 0 GHz Telecom: 12.500-12.750 GHz Uplink: FSS and Telecom: 14.000-14.800 GHz; DBS: 17.300-18.100 GHz Downlink: FSS: 11.700-12.200 GHz DBS: 12.200-12.700 GHz Uplink: FSS: 14.000-14.500 GHz DBS: 17.300-17.800 GHz Roughly 18-31 GHz Satellite Communications • Orbits - GEO – 35,786 kilometers 22,241 statute miles – 6,900 mph – 7,000 circular footprint – Spacing >= 2o degrees, or >= 9o (broadcast) – 1 revolution per day – Prop delay = 22,300 miles/186,000 miles/sec = 0.1198 sec 0.1199 sec x 2 = 0.2398 seconds (one way delay) – Freq. • C Band 4GHz-6-GHz - Interference from Microwave • Ku 11GHz - 12 GHz - atmospheric attenuation • Ka 14GHz - atmospheric attenuation – Higher Frequencies used in uplink, lower loss for lower bands used in downlink Satellite Communications • Orbits - MEO • ~6,000 miles • 5,000-6,000 mile circular footprint • 5 revolutions per day • Prop delay = 6,000 miles/186,000 miles/sec = 0.0322 sec 0.0322 sec x 2 = 0.0644seconds (one way delay) • Freq. – Ranges from 300 MHz - 2200 MHz – C, S, K Band Satellite Communications • Orbits - LEO• >1,000 miles • 1,000 - 3,500 mile circular footprint • ~12 revolutions per day • Prop delay = 1,000 miles/186,000 miles/sec = 0.0054 sec 0.0054 sec x 2 = 0. 0108 seconds (one way delay) • Freq. – Ranges from 300 MHz - 2200 MHz – C, S, K Band Satellite Communications • Orbit Tracks • http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html Satellite Communications • Power and Footprint – Low Power - 10’s to 100 watts – Free Space Loss ~200 dB for GEOs – Very low power at receiver – Restricting radiated energy to specific area allow power to be concentrated – Antenna design - Gain • Effective/Equivalent Isotropic Radiated Power : It is the ouptut power at the transmitter terminal, minus feeder and mismatch losses, plus average antenna gain relative to an isotropic radiator in the horizontal direction in dBW – Spot Beam Satellite Communications • http://www.intelsat.com/satellites/covmaps/[email protected]# • http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html Satellite Communications • Transponder – Satellites have some number of transponders – receives a signal, amplifies it, and retransmits it (typically at 8.5 to 60 watts – new direct broadcast satellites use up to 120 watts so that very small receiving antennas can be used) – Transponders typically have a bandwidth of 36 to 72 MHz each (though newer satellites have up to 108-MHz transponder bandwidths). • http://www.oreilly.com/reference/dictionary/terms/S/Satellite.htm Satellite Communications • Transponder – NTSC standard analog television video (with audio) signal requires 24 to 36 MHz of transponder bandwidth – Each transponder typically carries one, two, or three television signals (two for a 54-MHz transponder, three for a 72-MHz transponder). – Video signal digitization and compression schemes allow up to eight television signals to share the bandwidth required by a single uncompressed video signal. Satellite Communications • VSAT Satellite Communications • http://www.gilat.com/Technology_SatelliteBas ics.asp • http://www.tbssatellite.com/tse/online/mis_telecom_geo.htm l • http://www.ssloral.com/products/satint.html Media Selection depends on many factors including: • Type of network • Cost • Transmission distance • Security • Error rates • Transmission speeds IMPAIRMENTS TO TRANSMISSION Transmission Media Data Rate • Data rate is effected by – Bandwidth – Transmission impairments – Interference – Number of nodes (guided medium only) Transmission Impairment/Limitations • Guided medium impairments – Noise • • • • Thermal (white) noise Intermodulation noise Crosstalk Impulse noise – Attenuation - signal amplitude reduced • rcvr signal strength, signal/noise,greater at higher freq.... – Delay distortion - different freq..... propagate at different rate. Highest near center freq.… – Phase Jitter, Echo, Dropouts Transmission Impairment/Limitations • Unguided medium impairments – Free-space loss - signal disperses – Atmospheric absorption - water vapor, oxygen – Multipath - reflections – Refraction - signals bend through atmosphere – Thermal Noise - thermal activity Figure 3-13: Attenuation and Noise Power 1. Signal Signals in UTP attenuate with propagation distance. If attenuation is too great, the signal will not be readable by the receiver. Distance 79 Figure 3-14: Decibels • Attenuation is Sometimes Expressed in Decibels (dB) • The equation for 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 80 Figure 3-14: Decibels, Continued • Example – Over a transmission link, power drops to 37% of its original value – P2/P1 = 37/100 = .37 (37%/100%) – LOG10(0.37) = -0.4318 – 10*LOG10(0.37) = -4.3 dB (negative, reflecting power reduction through attenuation) – In calculations, the Excel LOG10 function can be used 81 Figure 3-14: Decibels, Continued • There are two useful approximations • 3 dB loss is a reduction to very nearly 1/2 the original power – 6 dB loss is a decrease to 1/4 the original power – 9 dB loss is a decrease to 1/8 the original power –… • 10 dB loss is a reduction to very nearly 1/10 the original power – 20 dB loss is a decrease to 1/100 the original power –… 82 Figure 3-13: Attenuation and Noise, Continued Power Signal Signalto-Noise Ratio (SNR) Noise Spike Error Noise Floor Noise Distance Noise is random unwanted energy within the wire Its average is called the noise floor Random noise spikes cause errors -A high signal-to-noise ratio reduces noise error problems As a signal attenuates with distance, damaging noise spikes become more common 83 Limiting UTP Cord Length • Limit UTP cord length to 100 meters – Limits attenuation to being a negligible problem – Limits noise problems being a negligible problem – Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length 84 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued • Electromagnetic Interference (EMI) (Fig. 3-15) – 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 85 Effect of Noise Amplifier - Repeater Figure 3-16: Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Crosstalk Interference Terminal Crosstalk Interference Terminal crosstalk interference Normally is the biggest EMI problem for UTP 88 Figure 3-16: Crosstalk Interference and Terminal Crosstalk Interference, Continued • 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 89 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued • Electromagnetic Interference (EMI) (Fig. 3-15) – NEXT – 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 1.25 cm or 0.5 inches to a negligible level. 90 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 91 Figure 3-17: Serial Versus Parallel Transmission One Clock Cycle 1. Serial 1 bit Transmission (1 bit per clock cycle) 2. Parallel Transmission (1 bit per clock cycle per wire pair) 4 bits per clock cycle on 4 pairs 1 bit 1 bit 1 bit 1 bit Parallel transmission increases speed. But it is only workable over short distances. Parallel is not 4. It is more than one. 92 Figure 3-18: Wire Quality Standards • Wiring Quality Standards – Rated by Category (Cat) Numbers • Category Standards are Set by ANSI/TIA/EIA and ISO/IEC – In the United States, the TIA/EIA/ANSI-568 governs UTP and optical fiber standards – In Europe and many other parts of the world, the standard is ISO/IEC 11801 – The two sets of standards are close but not identical 93 Figure 3-18: Wire Quality Standards • UTP Categories 3 and 4 – Early data wiring, which could only handle Ethernet speeds up to 10 Mbps • UTP Categories 5 and 5e – Most wiring installed today is Category 5e (enhanced) – Cat 5e and Cat 5 can handle Ethernet up to 1 Gbps – Most wiring sold today is Cat 5e 94 Figure 3-18: Wire Quality Standards • UTP Category 6 Errors – Relatively new – No better than Cat 5 or Cat 5e at 1 Gbps – Developed for higher Ethernet speeds of 10 Gbps • But can only span 55 meters at that speed • Book says cannot be used. This is an error. • Category 6A (Augmented) – Able to carry Ethernet signals at 10 Gbps up 100 meters – The book said 55 meters, but this is an error 95 Figure 3-18: Wire Quality Standards • Category 7 STP – Shielded twisted pair (STP) rather than unshielded twisted pair (UTP) • Metal foil shield around each pair to reduce crosstalk interference • Metal mesh around all four pairs to reduce crosstalk from other cords – STP is expensive and awkward to lay – Can 10 Gbps Ethernet to 100 meters 96 Figure 3-19: Wire Quality Standards Category Technology Maximum Speed 1 2 3 4 5 5e 6 6 6A 7 UTP UTP UTP UTP UTP UTP UTP UTP UTP STP1 Never defined Never defined 10 Mbps 10 Mbps 1 Gbps 1 Gbps 1 Gbps 10 Gbps 10 Gbps 10 Gbps+ Maximum Ethernet Distance at this Speed Not Applicable Not Applicable 100 meters 100 meters 100 meters 100 meters 100 meters 55 meters 100 meters 100 meters Category numbers indicate wire quality 3-97 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 3-99 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 3-100 3-21: Roles of UTP and Optical Fiber in LANs 3-101 Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors Cord Two Strands A fiber cord has two-fiber strands for full-duplex (twoway) transmission SC Connectors ST Connectors 3-102 3-22: 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 3-103 3-23: Frequency and Wavelength Wave Light travels in waves The amplitude is the intensity of the wave In sound waves, amplitude is loudness Amplitude is a measure of power 3-104 3-23: Frequency and Wavelength Wave 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 3-105 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 3-106 3-23: Frequency and Wavelength Wave 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 3-107 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 3-108 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 3-109 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) 3-110 3-26: Wavelength, Core Diameters, Modal Bandwidth, and Maximum Propagation Distance for Ethernet 1000BASE-SX Wavelength Core Diameter Modal Bandwidth 850 nm 62.5 microns 160 MHz.km Maximum Propagation Distance 220 m 850 nm 62.5 microns 200 MHz.km 275 m 850 nm 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 3-111 3-25: Multimode Fiber and SingleMode 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 3-112 3-24: LAN Fiber Versus Carrier WAN Fiber Required Distance Span Transceiver Wavelength Type of Fiber Core Diameter Primary Distance Limitation Quality Metric LAN Fiber Carrier WAN Fiber 200 m to 300 m 1 to 40 kilometers 850 nm 1,310 nm (and sometimes 1,550 nm) Multimode (thick core) Single mode (thin core) 50 microns or 62.5 8.3 microns microns Modal dispersion Absorptive attenuation 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. 3-113 3-24: LAN Fiber Versus Carrier WAN Fiber Required Distance Span Transceiver Wavelength Type of Fiber Core Diameter Primary Distance Limitation Quality Metric LAN Fiber Carrier WAN Fiber 200 m to 300 m 1 to 40 kilometers 850 nm 1,310 nm (and sometimes 1,550 nm) Multimode (thick core) Single mode (thin core) 50 microns or 62.5 8.3 microns microns Modal dispersion Absorptive attenuation 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. 3-114 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. 3-116 3-27: Omnidirectional and Dish Antennas 3-117 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. 3-118 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 3-119 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. 3-120 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 3-121 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? 3-122 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. 3-123 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. 3-124 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. 3-125 Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines It is a physical layer concept 3-29: Major Topologies The simplest topology is the point-to-point topology 3-127 3-29: Major Topologies Ethernet uses a star topology Note that the switch does not have to be in the middle of the star 3-128 3-29: Major Topologies Larger Ethernet LANs use an extended star topology This is better called a hierarchical topology 3-129 3-29: Major Topologies In a mesh topology, there are many connections between switches or routers Consequently, there are many alternative routes between hosts 3-130 3-29: Major Topologies In the ring topology, messages travel around a loop 3-131 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. 3-132 Topics Covered 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 3-134 134 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 3-135