CHAPTER TRANSMISSION 4.1 MEDIA Guided Transmission Media Twisted Pair Coaxial Cable Optical Fiber 42 Wireless Transmission Antennas Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared 43 Wireless Propagation Ground Wave Propagation Sky Wave Propagation Line-of-Sight Propagation 44 Line-of-Sight Transmission Free Space Loss Atmospheric Absorption Multipath Refraction Recommended Reading and Web Sites Key Terms, Review Questions, and Problems Key Terms Review Questions Problems CHAPTER Po TRANG SS1SjCS VINH N KEY POINTS tion can be classi« The transmission media that are used to convey informa l path along fied as guided or unguided Guided media provide a physica coaxial cable, which the signals are propagated; these include twisted pair, transmitting and optical fiber Unguided media employ an antenna for through air, vacuum, or water ications of * Traditionally, twisted pair has been the workhorse for commun with coaxd all sorts Higher data rates over longer distances can be achieve local area eed ial cable, and so coaxial cable has often been used for high-sp applications However, network and for high-capacity long-distance trunk medium more attracthe tremendous capacity of optical fiber has made that much of the tive than coaxial cable, and thus optical fiber has taken over applications market for high-speed LANs and for Jong-distance information com* Unguided transmission techniques commonly used for and satellite ave, microw rial terrest munications include broadcast radio, tions applica LAN some Infrared transmission is used in m is the physical path between Ina data transmission system, the transmission mediu for guided media, electromagtransmitter and receiver Recall from Chapter that as copper twisted pair, copper netic waves are guided along a solid medium, such s transmission occurs coaxial cable, and optical fiber For unguided media, wireles through the atmosphere, outer space, or water are determined both by The characteristics and quality of a data transmission of the signal In the case of the characteristics of the medium and the characteristics ining the limitations of guided media, the medium itself is more important in determ transmission by the transmitting For unguided media, the bandwidth of the signal produced ssion charactertransmi ining antenna is more important than the medium in determ a is directionality In generistics One key property of signals transmitted by antenn the signal propagates in al, signals at lower frequencies are omnidirectional; that is, e to focus the sigpossibl is it cies, all directions from the antenna At higher frequen nal into a directional beam , key concerns are data In considering the design of data transmission systems better.A number of dethe e distanc and rate rate and distance: the greater the data signal determine the data sign factors relating to the transmission medium and the rate and distance: * Bandwidth: bandwidth All other factors remaining constant, the greater the of a signal, the higher the data rate that can be achieved tion, limit the dis© Transmission impairments: Impairments, such as attenua impairment than tance For guided media, twisted pair generally suffers more fiber optical than more suffers coaxial cable, which in turn overlapping frequency » Interference: Interference from competing signals in particular concern for of is rence bands can distort or wipe out a signal Interfe guided media, unguided media but is also a problem with guided media For pe 94 4.41 / GUIDED TRANSMISSION Frequency (Hertz) VF 10 ELF| T ] oto” 10 LF [ MF | HE 1056 ] VLF] | Radio Power and telephone † Electronic tubes Infrared ` tol lợ | Lasers Micraware antennas Visible light Guided missiles Rangefinders Magn Integrated circuits Voice microphones | ` | Radar Radios and televisions Rotating generators Musical instruments Microwave L 95 102 [ 102 102 109 SHE[ EHF[ 109 UHF] 109 ] VHF] MEDIA Twisted pair Coaxial cable ~ FM radio) AM radio Wavelength in space L 49° { 105 L {meters) ELF = Extremely low frequency VF = Voice frequency VLE = Very low trequency LF ii = Low frequency Figure 4.1 L 100 “pe | 102 and TV | L 10 ME = Medium trequency HF = High frequency VHF = Very high frequeacy k ¡09 Terrestrial and satellite transmission L 10) [ 107 M2 : tL 102 | 109 i 10Ẽ 2) 107% UHF = Ultrahigh frequency SHF = Superhigh frequency EH! = Extremely high frequency Electromagnetic Spectrum for Telecommunications interference can be caused by emanations from nearby cables For example, twisted pairs are often bundled together and conduits often carry multiple cables Interference can also be experienced from unguided transmissions Proper shielding of a guided medium can minimize this problem ¢ Number of receivers: A guided medium can be used to construct a point-to- point link or a shared link with multiple attachments In the latter case, each attachment introduces some attenuation and distortion on the line, limiting distance and/or data rate Figure 4.1 depicts the electromagnetic spectrum and indicates the frequencies at which various guided media and unguided transmission techniques operate In this chapter we examine these guided and unguided alternatives In all cases, we describe the systems physically, briefly discuss applications, and summarize key trans- mission characteristics ti torn 286v ta GUIDED TRANSMISSION MEDIA For guided transmission media, the transmission capacity, in terms of either data rate or bandwidth, depends critically on the distance and on whether the medium is point-to-point or multipoint Table 4.1 indicates the characteristics typical for the common guided media for long-distance point-to-point applications: we defer a dis- cussion of the use of these media for multipoint LANs to Part Four +4 /TRANSMISSION CHAPTER Table 4.1 MEDIA Point-to-Point Transmission Characteristics of Guided Media [GLOV9%8] ¿.82 dB/km @ 1kHz Twisted pair (with loading) 0103.5 kHz Twisted pairs (multi-pair cables) Oto MHz Coaxial cable 0to500 MHZ- - |: - 18B/km'@ 10 MHz Optical fiber 180 to 370THz : Ƒ- “Õ.2to 0.5 đB/km : dB/km @ 1kHz ` Repeater Spacing Typical Delay Typical Attenuation Erequency Range - 2km 50 ps/km 2km ws/km Tuân Aus/km `#ws/km - to9 km: 40km THz = TeraHerz = L0 Hz The three guided media commonly used for data transmission are twisted pair, coaxial cable, and optical fiber (Figure 4.2) We examine each of these in turn Twisted Pair The least expensive and most widely used guided transmission medium is twisted pair Physical Description A twisted pair consists of two insulated copper wires arranged in a regular spi- of ral pattern A wire pair acts as a single communication link Typically, a number protectough a in them wrapping by cable a into together bundled these pairs are twisttive sheath Over longer distances, cables may contain hundreds of pairs The in a cable ing tends to decrease the crosstalk interference between adjacent pairs to relengths Neighboring pairs in a bundle typically have somewhat different twist typically length duce the crosstalk interference On long-distance links, the twist 0.9 mm varies from to 15 cm The wires in a pair have thicknesses of from 0.4 to Applications By far the most common transmission medium for both analog and digital sig- network nals is twisted pair It is the most commonly used medium in the telephone and is the workhorse for communications within buildings In the telephone system, individual residential telephone sets are connected to the local telephone exchange, or “end office,” by twisted-pair wire These are referred to as subscriber loops Within an office building, each telephone is also connected to a twisted pair, which goes to the in-house private branch exchange (PBX) system or to a Centrex facility at the end office These twisted-pair installations were designed to support voice traffic using analog signaling However, by means of a modem, these facilities can handle digital data traffic at modest data rates Twisted pair is also the most common medium used for digital signaling For connections to a digital data switch or digital PBX within a building, a data rate of 64 kbps is common Twisted pair is also commonly used within a building for local area networks supporting personal computers Data rates for such products are typically in the neighborhood of 10 Mbps However, twisted-pair networks with data rates of to Gbps have been developed, although these are quite limited in terms of fe 96 4.1 / GUIDED TRANSMISSION MEDIA — 97 Twist length Separately insulated —Oiten bundled" into cables —Usually installed in building during construction (a) Twisted pair Outer conductor Outer sheath Insulation Inner conductor —Outer conductor is braided shield ~—Inner conductor is solid metal — Separated by insulating material —Covered by padding a £ Ệ H g (b) Coaxial cable Jacket Core ⁄ Cladding C) — Glass or plastic core —Laser or light emitting diode —-Specially designed jacket —Small size and weight Light at less than critical angle is absorbed in jacket Angie of Angle of incidence reflection (c) Optical fiber Figure 4.2 i Guided Transmission Media the number of devices and geographic scope of plications, twisted pair can be used at data rates Twisted pair is much less expensive than transmission media (coaxial cable, optical fiber) the of the and network For long-distance apMbps or more other commonly used guided is easier to work with “ Ee Transmission Characteristics ‘Twisted pair may be used to transmit both analog and digital transmission For analog signals, amplifiers are required about every to km For digital transmission (using either analog or digital signals) repeaters are required every or3 km Compared to other commonly used guided transmission media (coaxial cable, optical fiber), twisted pair is limited in distance, bandwidth, and data rate As / TRANSMISSION CHAPTER MEDIA 3ã s tử a 10" Frequency (Hz) (a) Twisted pair (based on (REEV9S) 10 10° La t5 ứ I0 #00 Q01 1000 1100 1300 1300 (400 (500 1690 1700 Wavelength in vacuum (nen) (c) Optical fiber (based on [FREE02]} Attenuation (dB/km) Atienuation (dBékin} Attenuation (dB/km) 26-AWG (0.4 mm} 24-AWG (05 mm) 22-AWG(06 (AWG (0.9 mm) Attenuation (dB/km) 10° kHz 108 Ì MHz Frequency (Hz) (by Couxial cable (bused on [BELL90)) Figure 4.3 10° 1GHz Frequency (H2) 10? THz 10'S {d) Composite graph Attenuation of Typical Guided Media a very strong function of freFigure 4.3a shows, the attenuation for twisted pair is pair The medium is quite susquency Other impairments are also severe for twisted coupling with electromagnetic ceptible to interference and noise because of its easy will pick up 60-Hz energy fields, For example, a wire run parallel to an ac power line Several measures are taken to Impuise noise also easily intrudes into twisted pair braid or sheathing reduces inreduce impairments Shielding the wire with metallic the use equency interference, and terference The twisting of the wire reduces low-fr crosstalk s of different twist lengths in adjacent pairs reduce up to about MHz is posFor point-to-point analog signaling, a bandwidth of For jong-distance digital ls channe sible This accommodates a number of voice for very short Mbps are possible; point-to-point signaling, data rates of up to a few ed in commercially available distances, data rates of up to Gbps have been achiev products Unshielded and Shielded Twisted Pair Twisted pair comes in two varieties: unshielded and shielded Unshielded gs, by universal practwisted pair (UTP) is ordinary telephone wire Office buildin sim- more than is needed for tice, are prewired with excess unshielded twisted pair, all the transmission media ple telephone support This is the least expensive of to work with and easy to install commonly used for local area networks and is easy interference, inUnshielded twisted pair is subject to external electromagnetic noise generated in the cluding interference from nearby twisted pair and from Vet 98 4.1 / GUIDED TRANSMISSION MEDIA 99 environment A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference This shield- ed twisted pair (STP) provides better performance at higher data rates However, it is more expensive and more difficult to work with than unshielded twisted pair Category and Category UTP Most office buildings are prewired with a type of 100-ohm twisted pair cable commonly referred to as voice grade Because voice-grade twisted pair is already in- stalled, it is an attractive alternative for use as a LAN medium Unfortunately, the data rates and distances achievable with voice-grade twisted pair are limited In 1991, the Electronic Industries Association published standard EIA-568, Commercial Building Telecommunications Cabling Standard, which specifies the use of voice-grade unshielded twisted pair as well as shielded twisted pair for in-building data applications At that time, the specification was felt to be adequate for the range of frequencies and data rates found in office environments Up to that time, the principal interest for LAN designs was in the range of data rates from Mbps to 16 Mbps Subsequently, as users migrated to higher-performance workstations and applications, there was increasing interest in providing LANs that could operate up to 100 Mbps over inexpensive cable In response to this need, EIA-568-A was issued in 1995 The new standard reflects advances in cable and connector design and test methods It covers 150-ohm shielded twisted pair and 100-ohm unshielded twisted pair EIA-568-A recognizes three categories of UTP cabling: * Category 3: UTP cables and associated connecting hardware whose transmis- sion characteristics are specified up to 16 MHz * Category 4: UTP cables and associated connecting hardware whose transmis- sion characteristics are specified up to 20 MHz * Category 5: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 100 MHz Of these, it is Category and Category cable that have received the most at- tention for LAN applications Category corresponds to the voice-grade cable found in abundance in most office buildings Over limited distances, and with proper design, data rates of up to 16 Mbps should be achievable with Category Category Š is a data-grade cable that is becoming increasingly common for preinstallation in new office buildings Over limited distances, and with proper design, data rates of up to 100 Mbps should be achievable with Category A key difference between Category and Category cable is the number of twists in the cable per unit distance Category is much more tightly twisted, with a typical twist length of 0.6 to 0.85 cm, compared to 7.5 to 10 cm for Category The tighter twisting of Category is more expensive but provides much mance than Category better perfor- Table 4.2 summarizes the performance of Category and UTP, as well as the The first parameter used for comparison, attenuation, in ELA-568-A specified STP strength of a signal falls off with distance over any The straightforward fairly is transmission medium For guided media attenuation is gencrally exponential and therefore is typically expressed as a constant number of decibels per unit distance 100) CHAPTER Table 4.2 TRANSNHSSION MDLA Comparison of Shielded and Unshielded Twisted Pair Attenuation (dB per 100 m) Frequency | Category3 | Category (MHz) UTP UTP 2.6 20 16 5.6 cIÁU 41 82 25 _— 300 Sooke 100 — ` - 104 220 oe - Near-end Crosstalk (dB) 150-ohm - | Category | Category STP 1q c7 22 A4 150-ohm UTP UTP “al 62 58 |: in - 58 50.4 oo 47g oe 313 S623 12377 wars STP 32.) | ` 385 Near-end crosstalk as it applies to twisted pair wiring systems is the coupling of the signal from one pair of conductors to another pair These conductors may be the metal pins in a connector or wire pairs in a cable The near end refers to coupling that takes place when the transmit signal entering the link couples back to the receiving conductor pair at that same end of the link (i.e., the near transmitted signal is picked up by the near receive pair) Since the publication of E[A-568-A, there has been ongoing work on the development of standards for premises cabling, driven by two issues First, the Gigabit Ethernet specification requires the definition of parameters that are not specified completely in any published cabling standard Second, there is a desire to specify cabling performance to higher levels, namely Enhanced Category (Cat 5E), Category 6, and Category Tables 4.3 and 4.4 summarize these new cabling schemes and compare them to the existing standards Coaxial Cable Physical Description Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor (Figure 4.2b) The inner conductor is heid in place by either regularly spaced insulating Table 4.3 UTP FTP SSTP Twisted Pair Categories and Classes = Unshielded twisted pair = Foil twisted pair = Shielded screen twisted pair Table 4.4 Name High-Performance LAN Copper Cabling Alternatives [JOHN98] : Construction Cable consists of pairs of 24 AWG {0.50 mm) copper with thermopiastic polyolefin or fluorinated ethylene L CategorySUTP propylene (FEP) jacket Outside sheath consists of polyviny|chiorides: (PV©) a fire-retardantpolyolefin or fludropolymers.” Cable consists of pairs of 24 AWG (0.50 mm) copper with: thermoplastic polyolefin or fluorinated ethylene propylene (FEP) jacket Outsi sheath consists of polyvinylchlorides” (PVC); fire-retardant polyolefin or Expected Performance | Mixed and matched cables anđ:co necting hardware from various man facturers that have a reasonable ‘chance of meeting TIA,Cai pel and ISO Class D requi: : No manufacturer’s warrant invoived : ‘opolymers: Higher care taken in ROA FH; and manufacturing.« propylen (FEP) jacket Outsid sheath consists | of polyvinylchlo (PVC) a fire-retardant polyolefin or fluoropolymers Extremely high care : ( ` bandwidth) iis guaranteed to 20 or beyond Best available UT! MHz: mance specifications for Category £ UTP to 250 MHz areunder | development propylene (FEP) jacket: Pairs'are sure © ‘ounded by.a.common metallic foil shield ‘Outside sheath consists of lyvinylchlorides (PVC), a fire .ô retardantpolyolefin, âr ủuoropolmers | Shiel Jed Foil Twisted d Pair > Category ốc Shielded-Screen Twisted Pair ACR EMI ` | Cable consists of pairs of 24 AWG with thermoplastic “polyolefir in or fluorinated ethylene - propylene (FEP} jacket Pairs are sur-: rounded by a commion metallic foil shield, followed by a braided metallic : Shield: Outside sheath consists of polyvinylchlorides (PVC), a fireretardant polydlefin, or fluoropolymers iz: fers superior EME protection to FTP: ` ý : Also called‘ PiMF (for Pairs in Metal Category cabling provides positive Foil}, SSTP of.4 pairs of 22-23AWG ACR to 600 to 1200 MHz Shielding copper with thermoplastic polyolefin | on the individual! pairs gives it or fluorinated ethylenepropylene phenomenal ACR (FEP) jacket Pairs are individually surrounded by a helical or longitudinal metallic foil shield, followed by a braided metallic shield Outside sheath of polyvinylchlorides (PVC), a fireretardant potyaletin, or fluoropolymers = Attenuation to crosstalk ratio = Electromagnetic interference + CHAPTER / TRANSMISSION MEDIA rings or a solid dielectric material The outer conductor is covered with a jacket or shield A single coaxial cable has a diameter of from | to 2.5 cm Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair Applications Coaxial cable is perhaps the most versatile transmission medium and is enjoyThe most important of these are ing widespread use in a wide variety of applications * * ¢ * Television distribution Long-distance telephone transmission Short-run computer system links Local area networks TV signals to individual Coaxial cable is widely used as a means of distributing homes—cable TV From its modest beginnings as Community Antenna Television (CATV), designed to provide service to remote areas, cable TV reaches almost as many homes and offices as the telephone A cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few tens of kilometers Coaxial cable has traditionally been an important part of the long-distance telephone network Today, it faces increasing competition from optical fiber, terres- trial microwave, and satellite Using frequency division multiplexing (FDM, see Chapter 8), a coaxial cable can carry over 10,000 voice channels simultaneously Coaxial cable is also commonly used for short-range connections between devices Using digital signaling, coaxial cable can be used to provide high-speed VO channels on computer systems Transmission Characteristics Coaxial cable is used to transmit both analog and digital signals As can be seen from Figure 4.3b, coaxial cable has frequency characteristics that are superior to those of twisted pair, and can hence be used effectively at higher frequencies and data rates Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair The principal constraints on perfor- mance are attenuation, thermal noise, and intermodulation noise The latter is present only when several channels (FDM) or frequency bands are in use on the cable For long-distance transmission of analog signals, amplifiers are needed every few kilometers, with closer spacing required if higher frequencies are used The usable spectrum for analog signaling extends to about 500 MHz For digital signaling, repeaters are needed every kilometer or so, with closer spacing needed for higher Optical Fiber Physical Description An optical fiber is a thin (2 to 125 ym), flexible medium capable of guiding an optical ray Various glasses and plastics can be used to make optical fibers The lowest losses have been obtained using fibers of ultrapure fused silica Ultrapure fiber is mm data rates rer9rirotrnnvirnh 102 ... summarize key trans- mission characteristics ti torn 286v ta GUIDED TRANSMISSION MEDIA For guided transmission media, the transmission capacity, in terms of either data rate or bandwidth, depends... bands can distort or wipe out a signal Interfe guided media, unguided media but is also a problem with guided media For pe 94 4.41 / GUIDED TRANSMISSION Frequency (Hertz) VF 10 ELF| T ] oto” 10... common guided media for long-distance point-to-point applications: we defer a dis- cussion of the use of these media for multipoint LANs to Part Four +4 /TRANSMISSION CHAPTER Table 4.1 MEDIA Point-to-Point