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L8: Physical Media Properties LE/EECS 3213 Fall 2014 L8: Physical Media Properties Sebastian Magierowski York University EECS 3213, F14 L8: Physical Media 1 Outline • Key characteristics of physical media – What signals in media are made out of – Delay through media – Attenuation through media – Frequency response of media • Twisted Pair • Coax • Optical • Wireless EECS 3213, F14 L8: Physical Media 2 (‘-. & 0 _1 ‘j ‘3 3 \r (-i) IL I C I’ t —..-- 1 $ Js L8: Physical Media 8.1 Signal Particles 8.1 SignalParticles Electrons throughmetal Photons throughglass and air • • EECS 3213, F14 Communications Systems & EM Spectrum • Frequency of communications signals Optical Analog DSL WiFi Cell fiber telephone phone Frequency (Hz) 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024 X-rays Broadcast radio Powerand telephone Microwave radio Visible light Visible Gammarays Infraredlight Ultraviolet light 106 104 102 10 10-2 10-4 10-6 10-8 10-10 10-12 10-14 Wavelength (meters) EECS 3213, F14 L8: Physical Media 4 8.2 Delay Communication channel d meters t = 0 t = d/c • Propagation speed of signal – c = 3 x 108 meters/second in vacuum – v = c/√ε speed of light in medium • ε>1 is the dielectric constant of the medium • v = 2.3 x 108 m/sec in copper wire • v = 2.0 x 108 m/sec in optical fiber EECS 3213, F14 L8: Physical Media 5 j c • II 1’ i 0 8.2 Attenuation (1 • Usually the signal power that comes out your channel is less than the signal power that comes in your channel 2 – Attenuation = |Ac| = Pin/Pout • Can also think of it in terms of the channel’s frequency‘‘ response (aka transfer function) 2 – |Hc| = Pout/Pin .4 1 ‘ .4 I - 4 — 1 V EECS 3213, F14 L8: Physical Media 1 6 4 Summary: Attenuation in Wired and Wireless • Attenuation varies with media – Dependence on distance of central importance • Wired media attn. has exponential function of distance – Received power at d meters proportional to 10-kd – Attenuation in dB is k • d, where k is dB/meter • Wireless media attn. has power function of distance – Received power at d meters proportional to d-n – Attenuation in dB is n log d, where n is path loss exponent • n=2 in free space – Signal level maintained for much longer distances – Space communications possible EECS 3213, F14 L8: Physical Media 7 8 c • ‘ c • - ‘ V 1- V II 1 II .4 .4 .4 .4 ‘‘ (1 ‘‘ 1 1 4 (1 1 — 1 1’ L8: Physical Media 4 1’ — j j i I 0 4 i I 0 Wired Channel Transfer Characteristics WiredChannel Transfer Characteristics 4 Exponential characteristics • EECS 3213, F14 9 c • ‘ - V 1 II .4 .4 ‘‘ (1 1 1 4 — 1’ L8: Physical Media j relationships c i I 0 4 Channel Transfer Function and Attenuation ChannelTransferFunction and Attenuation and A c H • EECS 3213, F14 1.1. EECS 3213, F14 • broader and broader surface b As your signal leavestheantenna it spreads outover a b II WirelessChannel TransferCharacteristics E E 1 I’ Ii 1 I’ Ii L8: PhysicalMedia L8: I’ It fl p . I’ 9 J4)It Ii II S — U) fl c3 p 0 . J4) (%. 9 c—i Ii II S — U) c3 0 (%. c—i 10 10 EECS 3213, F14 • • wireless Compare theattenuation asafunctiondistance of Compare basic telephone line (k = 0.005 dB/m) to 3-GHz Compare basic telephone line0.0053-GHz (k = dB/m)to Comparison: Wired & Wireless Attenuation Comparison:WiredWireless & Attenuation 1. b I L8: PhysicalMedia L8: E 1 I’ Ii I’ It fl p . 11 11 9 J4) Ii II S — U) c3 0 (%. c—i EECS 3213, F14 • is notflat withfrequency Typicallytheattenuation (and channel transferfunction) C’. C’ C’ I. Li II 8.4 Frequency Response 8.4Frequency Response iO (A C-’ 2 1) L8: PhysicalMedia L8: 3’ --s, 5- cs -b tn 1) m 12 12 1 Sn i-i ‘p C-, Il 4- (1 ‘I 11 U I J —— c) 13 13 v.1) ci ‘p 1 Sn i-i ‘p C-, Il 4- (1 ‘I U 11 ‘%Z -d I J —— c) v.1)L8: Physical Media ci ‘p 8.5 Twisted Pair 8.5 TwistedPair eq I ‘%Z -d eq Common-modeinterference Differential signals Wires wound around eachunshielded other(UTP: I twistedpair) – – • EECS 3213, F14 AWG24 (Telephone/Ethernet) Freq. Response 2 • |Hc| (dB) – 0.511 mm diameter 0 0 -t 7’- 1 I-f.’ N -4 1p EECS 3213, F14 17 L8: Physical Media 14 sc IA N 0 (44 Twisted Pair • Two insulated copper wires arranged in a spiral pattern to 30 26 gauge minimize interference 24 gauge • Various thicknesses, e.g. 24 0.016 inch (24 gauge) 22 gauge • Low cost 18 • Telephone subscriber loop 19 gauge from customer to CO 12 • Intra-building telephone from wiring closet to desktop Attenuation(dB/mi) 6 • In old installations, loading coils added to improve quality f (kHz) in 3 kHz band, but more 1 attenuation at higher 10 100 1000 frequencies Lower attenuation Higher attenuation rate analog rate for DSL telephone EECS 3213, F14 L8: Physical Media 15 Twisted Pair Bit Rates • Twisted pairs provide high • Much higher rates possible bit rates at short distances at shorter distances • Asymmetric Digital – Strategy is to bring fiber close to Subscriber Loop (ADSL) home & then twisted pair – Higher-speed access + video 148 – High-speedTHE Internet PHYSICAL Access LAYER CHAP. 2 – Lower 3 kHz for voice potential bandwidth– Upper as a band function for of distancedata is given in Fig. 2-33.Standard This figure as- R (Mbps) Distance sumes that all the other factors are optimal (new wires, modest bundles, etc.). • 64 kbps inbound T-1 1.544 18,000 feet, 5.5 km 50 • 640 kbps outbound DS2 6.312 12,000 feet, 3.7 km 40 1/4 STS-1 12.960 4500 feet, 1.4 km 30 1/2 STS-1 25.920 3000 feet, 0.9 km Mbps 20 STS-1 51.840 1000 feet, 300 m 10 Table 3.5 Data rates of 24-gauge twisted pair 0 [Tanenbaum, 2011] 0 1000 2000 3000 4000 5000 6000 EECS 3213, F14 Meters L8: Physical Media 16 Figure 2-33. Bandwidth versus distance over Category 3 UTP for DSL. The implication of this figure creates a problem for the telephone company. When it picks a speed to offer, it is simultaneously picking a radius from its end offices beyond which the service cannot be offered. This means that when distant customers try to sign up for the service, they may be told ‘‘Thanks a lot for your interest, but you live 100 meters too far from the nearest end office to get this ser- vice. Could you please move?’’ The lower the chosen speed is, the larger the radius and the more customers are covered. But the lower the speed, the less attractive the service is and the fewer the people who will be willing to pay for it. This is where business meets technology. The xDSL services have all been designed with certain goals in mind. First, the services must work over the existing Category 3 twisted pair local loops. Sec- ond, they must not affect customers’ existing telephones and fax machines. Third, they must be much faster than 56 kbps. Fourth, they should be always on, with just a monthly charge and no per-minute charge. To meet the technical goals, the available 1.1 MHz spectrum on the local loop is divided into 256 independent channels of 4312.5 Hz each. This arrangement is shown in Fig. 2-34. The OFDM scheme, which we saw in the previous section, is used to send data over these channels, though it is often called DMT (Discrete MultiTone) in the context of ADSL. Channel 0 is used for POTS (Plain Old Telephone Service). Channels 1–5 are not used, to keep the voice and data sig- nals from interfering with each other. Of the remaining 250 channels, one is used for upstream control and one is used for downstream control. The rest are avail- able for user data. In principle, each of the remaining channels can be used for a full-duplex data stream, but harmonics, crosstalk, and other effects keep practical systems well 8.6 ADSL Signals • Telephone wire has ~1-MHz reasonable bandwidth – 3-kHz voice bandwidth created by load coils • ADSL dividesSEC. into 2.6 channels THE PUBLIC SWITCHED TELEPHONE NETWORK 149 – 256, 4.3125-kHz channels 256 4-kHz Channels – OFDM (4G) Power 025 1100 kHz Voice Upstream Downstream [Tanenbaum, 2011] – Typically 32 for upstreamFigure 2-34.andOperation 218 for of ADSL downstream using discrete multitone modulation. • ADSL2: 1 Mbps upstream and 12 Mbps downstream • 4000 symbols/sbelow theper theoretical channel limit. It is up to the provider to determine how many chan- nels are used for upstream and how many for downstream. A 50/50 mix of • 1-15 bits perupstream symbol and depending downstream on is technicallySNR possible, but most providers allocate something like 80–90% of the bandwidth to the downstream channel since most users download more data than they upload. This choice gives rise to the ‘‘A’’ in ADSL. A common split is 32 channels for upstream and the rest downstream. It EECS 3213, F14 is also possible toL8: have Physical a few ofMedia the highest upstream channels be bidirectional17 for increased bandwidth, although making this optimization requires adding a special circuit to cancel echoes. The international ADSL standard, known as G.dmt, was approved in 1999. It allows speeds of as much as 8 Mbps downstream and 1 Mbps upstream.
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