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TRANSMISSION MEDIA
 

TRANSMISSION MEDIA

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    TRANSMISSION MEDIA TRANSMISSION MEDIA Document Transcript

    • TRANSMISSION MEDIA Transmission medium is physical path between sender and receiver, has finite BW, and may be: • guided (twisted-pair, coaxial cable, fiber) • unguided directional (microwave, satellite) or omnidirectional (broadcast radio, cellular, pagers) With guided, medium acts as conductor and guides signal With unguided (wireless), antenna transmits signal through atmosphere or space, but signal is not guided Signal transmission rate and channel capacity are measured in Hz for analog and in bps for digital 6/9/2010 15
    • Twisted Wire Pair Cables 2 to 1,000s of twisted pairs of insulated (e.g., with plastic) copper wires are bundled together in cable with plastic, vinyl, or Teflon protective jacket 19 to 26 AWG (0.4 to 0.9 mm diam) is common One pair often used to carry voice-grade signal but can carry 1 Gbps over short distances UTP is easy to install (can be hung or buried) Lower cost (per foot), distance, BW, and data rate versus other guided media Need amplifiers/repeaters approx. every few km Shielding and twisting, with different twist lengths in adjacent pairs, ↓ crosstalk Usually installed (with spares) in buildings during construction and used to connect phones to PBX or Centrex facility, for local loops, and in LANs RJ-11 (2-pair) modular jack is common for home use and RJ-45 (4-pair) is common for business When UTP enters building, it is connected to terminating punchdown block which is “demarcation point” for maintenance responsibilities 6/9/2010 16
    • EIA/TIA 568-A (Commercial Building Telecommunications Cabling Standard) has specs for categories 3, 4, 5, and 5E 100 Ω UTP cables, including topology and connectors It specifies max bps and transmission characteristics (attenuation, crosstalk) over freq ranges of operation Cat 3 supports up to 10 Mbps; Cat 4 supports up to 16 Mbps and is rarely used; Cat 5 supports up to 100 Mbps, e.g., for fast Ethernet; 4-pair enhanced Cat 5E may be used for GigE Twist lengths may be selected by manufacturer to comply with specs, e.g., 3 to 4 twists per foot for Cat 3, and 3 to 4 twists per inch for Cat 5 EIA 568-A also recognizes 150 Ω STP Can use STP (with wire pairs individually/collectively wrapped in metal foil/braided mesh) to protect from interference, or Plenum wire to ↓ toxic fumes in fire, but size and cost ↑ and cable is more difficult to work with EIA/TIA 568-B has Cat 6 UTP specs with 200 MHz BW and ↓ NEXT and FEXT; work is in progress on Cat 7 6/9/2010 17
    • Attenuation (dB per 100 m at 20°C) Frequency Category 3 Category 5 (MHz) UTP UTP STP 1 2.6 2.0 1.1 4 5.6 4.1 2.2 16 13.1 8.2 4.4 25 - 10.4 6.2 100 - 22.0 12.3 300 - - 21.4 Near-End Crosstalk (dB)* Frequency Category 3 Category 5 (MHz) UTP UTP STP 1 41 62 58 4 32 53 58 16 23 44 50.4 25 - 41 47.5 100 - 32 38.5 300 - - 31.3 * NEXT is coupling of signal from 1 conductor pair to another, i.e., when transmit signal entering link couples back to receive conductor pair at same end of link Point-to-Point Transmission Characteristics of Guided Media: Transmission Frequency Typical Repeater Medium Range Attenuation Spacing Twisted Pair 0 to 1 MHz 3 dB/km @ 1 kHz 2 km (multipair cables) Coaxial Cable 0 to 500 MHz 7 dB/km @ 10 MHz 1 to 9 km Optical Fiber 180 to 370 THz 0.2 to 0.5 dB/km 40 km 6/9/2010 18
    • Coaxial Cable Coaxial cable has 4 concentric sections: 1. Inner copper or aluminum conductor 2. Plastic, foam, or air insulation 3. Cylindrical aluminum or copper shield (e.g., braided wire mesh) acting as outer conductor 4. Outer jacket that physically protects cable Coax supports higher BW and data rates, longer distances, and more sharing devices than UTP, and is less susceptible to crosstalk Relatively easy to install and tap and may be buried or in building walls Used for TV distribution, long distance phone networks, LANs, and terminal to mainframe connections Baseband coaxial cable transmits 1 digital signal Broadband coaxial cable can carry multiple analog signals (voice, data, video) via FDM; interference immunity, $, and complexity ↑ and need RF modems Thick coax (RG-8/frozen yellow garden hose) uses type N connectors and vampire taps Thin coax (RG-58/ThinNet/CheaperNet) uses T connector with stem screwed into BNC plug on NIC; easier to bend but distance and no. devices ↓ Balun can interconnect UTP and coaxial cable 6/9/2010 19
    • Fiber Optics Fiber optic cable contains cladded fiber core(s) in protective jacket: 1. Core is thin strand of glass or plastic 2. Cladding surrounds each fiber core and is glass or plastic coating with different optical properties to core; cladded fibers are often covered by buffer layer 3. Jacket surrounds 1 or bundle of fibers and protects against moisture, heat, bending, and crushing Fiber → high BW and data rates; low $ (per bps capacity), noise, distortion, attenuation, and BER; small size; light weight; no radiation; secure; less fragile, brittle, and corrosive than copper; and immune to interference from motors, lightning, and crosstalk Glass fibers offer ↑ data rates and distances; plastic is cheaper Fiber is difficult to tap ⇒ primarily point-to-point Used for long distance commns, MANs, LANs, local loops, and video TAT-8, with 2 fiber pairs for voice/data and 1 pair for backup, can carry 40,000 simultaneous calls WDM can use about 100 colors to send Tbps 6/9/2010 20
    • If voltage is applied, LED or laser emits light beam which propagates through fiber via total internal reflection; photoreceptor converts light to electrical signal LED transmits bit by presence and absence of light pulse and is cheaper than laser Laser transmits bits as high or low amplitude lightwaves and has less dispersion, but installation requires ↑ skill A single mode fiber’s radius (e.g., 8.3/125) is so small that only axial ray enters core (requires laser) Single path ↓ distortion and ↑data rate and distance With multimode fiber, shallow angle light rays enter core, reflect off cladding, and propagate along fiber Multiple propagation paths with different reflections and delays ↑ attenuation and dispersion (i.e., signal spreads out and bits overlap) ⇒ data rate and distance ↓ MMF (50/125, 62.5/125) is less expensive than SMF; graded-index MMF ↓ dispersion vs. step-index MMF ↑ H/W cost versus other guided media and, before transmission on next link, signal may need to be converted to electrical form and amplified Can buy fiber with connectors at ends 6/9/2010 21
    • WIRELESS COMMUNICATIONS EM waves (radio, light) carry signals through air/space; offer mobility, commn in hostile environment, quick deployment, low installation $, and/or broadcasting FCC regulates freq use and auctions BW At high freqs, can focus signal into narrow directional beam ⇒ must align antennas Low freq signals are usually omnidirectional ⇒ precise antenna alignment not needed, but data rates ↓ Terrestrial Microwave Fixed antenna (e.g., parabolic dish or horn) transmits narrow LOS beam (2 to 40 GHz) to receiving antenna Need amplifiers/repeaters at 30 mile intervals on top of mountains, tall buildings, or towers Above 10 GHz, precipitation or humidity ↑ attenuation Need FCC license for frequency use; in cities, BW may not be available or congestion can cause crosstalk Sent through free space ⇒ security risk Applications include intra- and inter-state phone, inter- LAN data, and bypass 6/9/2010 22
    • Satellite Microwave Orbiting satellite is microwave relay station linking ≥2 earth/ground stations Operates on various frequency bands called transponders or transponder channels; satellite receives on uplink band, repeats signal, and transmits on downlink band ITU regulates orbits, spectrum use, and “footprints”; spot and steered beam antennas can control footprint High capacity, quick installation (install earth station or terrestrial link to earth station), broadcast in nature, and wide coverage area Attenuation, propagation delay, and security problems; solar flares and sunspots can cause EMI Above 10 GHz, attenuation ↑ but smaller (few ft.) and cheaper receivers (VSATs) may be used and BW ↑ Applications include TV, intl phone, mobile commns (trucks, ships, planes), private business nets, bypass, LAN interconnect, diversity, remote areas, GPS, tracking, data, paging, and fax services Traditional satellites broadcast TV signals to ground stations for distribution to viewers; DBS transmits programs directly to viewers 6/9/2010 23
    • GEOSs are in circular orbit 22,300 miles above equator, have 24 hour rotation period, and appear stationary relative to earth → simple tracking once dish aimed Spaced 4° apart in 6/4 GHz C-band and 3° apart in 14/12 GHz Ku-band; 3 cover most of Earth’s surface Propagation delays of approx. 0.25 sec between earth stations → problems with HDX stop-and-wait protocols MEOSs orbit at 3,000 to 9,000 miles (between Van Allen radiation belts) ⇒ time overhead ↓ and need HOs LEOSs (e.g., Iridium, Teledesic, Globalstar) orbit at 100-1,200 miles LEOSs (e.g., Iridium, Teledesic, Globalstar) use less expensive satellites, orbit at 100-1,200 miles, and support hand held phones (without satellite dishes) Least attenuation, power reqts, delay, coverage area, and life span; "in view" for <2 hours, i.e., "race across sky," like cellular system with cell site moving! Iridium was to use 66 satellite constellation and inter- satellite commns for global phone and paging network Russia uses HEOSs (Molniya series) with elliptical orbit to ↓ "shadowing" problems in northern regions 6/9/2010 24
    • Comparison of Optical Fiber and Satellite Transmission: Characteristic Optical Fiber Satellite Bandwidth Theoretical limit 1THz; Typical transponder BW currently 1-10 GHz of 36-72 MHz Interference Immune to EMI Subject to EMI Immunity Security Difficult to tap without Requires encryption detection Multipoint Primarily point-to-point Easy to implement Capability point-to-multipoint Reconfiguration Difficult Easy Flexibility Connectivity to Local loop required If earth station installed Customer Site on customer premises, local loop not required 6/9/2010 25
    • Mobile/Cellular Telephony AMPS is 1st generation and analog Service region is divided into cells, with 1 fixed antenna per cell, and call is handed off to closest fixed antenna as user travels between cells; network may support roaming outside service area Fixed antennas are connected to MTSO which has connection to local phone system, controls HOs (if power < threshold), and collects billing info Each cell has setup channel to set up and control calls and to carry phone ID info AMPS is allocated 869-894 MHz (824-849 MHz) band for transmission to (from) mobile and each 25 MHz band is split in 2 to ↑ competition Each carrier uses 30 kHz/channel and can support ≤416 simultaneous 2-way calls via FDM To ↑ capacity, can reuse freqs in nonadjacent cells and/ or split cells; AMPS uses 7 cell reuse pattern If cell size ↓, no. cells ↑, power reqts ↓, battery life ↑, unit size and weight ↓, $ ↓, radiation ↓, and HOs ↑ D-AMPS triples no. channels/cell by adding TDM to FDM and has ↑ clarity and security (scanner issue) 6/9/2010 26
    • PCS/PCN is 2nd generation, digital, and incompatible with 1G Uses TDMA (allocates time slots), CDMA (based on SS), or GSM (different form of TDMA); GSM is least popular in U.S. and most popular in Europe GSM phone has SIM smart card to provide ID, authentication, and billing 3rd generation phones can provide call waiting, caller ID, and v-mail and use WAP to communicate with ISP for stock prices, weather, sports scores, etc. CDPD Mobile PC (e.g., in police, fire, or EMS vehicle) can send data on cellular network when not in use by voice Low $ and can use encryption, but data rate ≤19.2 kbps Pagers May use satellite network to locate receiving pager Can receive and display phone no. or text and, if 2-way, return short pre-programmed reply Restaurant use ↑ 6/9/2010 27
    • Infrared IR (0.3-200 THz) transceivers must be within direct LOS or LOS via reflection, e.g., from ceiling or wall Do not require license, penetrate walls, or bend around objects ⇒ secure and limited distance, e.g., within room or inter-building Bright light, heat, rain, fog, or smoke can interfere Can interconnect desktop, laptop, PDAs (business cards), and peripherals; standards from IrDA Bluetooth Bluetooth piconet/PAN (IEEE 802.15) uses FH-SS at 1 Mbps in 2.45 GHz ISM band Piconet connects 1 master (e.g., PC) with ≤7 slave devices (e.g., PDA, mouse, keyboard, headset, printer, fax, copier); scatternet can interconnect piconets Low power, distance ≤30 meters, and risk of interference from IEEE 802.11 WLANs, cordless phones, baby monitors, and microwave ovens Incompatible with 802.11, 802.11b (WiFi), and HomeRF HomeRF is also in 2.45 GHz band and can send 1.6 Mbps up to 300’ indoors 6/9/2010 28
    • WAP Allows mobile phone, PDA, laptop, or 2-way radio to access Internet and perform subset of usual activities Displays web pages via WML; problems with low data rates and small text-based screens Broadband/Fixed-Point Wireless and WLL LMDS (28-30 GHz) and MMDS (2.5 GHz) are point- multipoint and shared; can bypass LEC and carry 2- way voice, data, or video LMDS range is few miles; good for densely populated areas, susceptible to weather interference, requires many towers, and supports ≤45 Mbps down MMDS range is ≤35 miles; good for rural/suburban areas and supports ≤10 Mbps down UWB uses low power radio pulses across broad frequency range 6/9/2010 29
    • PRIVATE, LEASED, SWITCHED AND BYPASS CIRCUITS Private Circuits Not owned, installed, or maintained by common carrier Usually within building or between buildings on campus; need right of way to cross public roads or property owned by others Circuit is available on full-time basis and inexpensive to operate Leased/Dedicated Circuits Owned and maintained by telco or satellite provider and may be leased by customer for full time exclusive use Alternative to private circuits for FDX transmission, e.g., at DS-0 (56 or 64 kbps), fractional T1, T1/E1, T3/E3, or SONET/SDH rates Provide nonswitched point-to-point connections between 2 sites ⇒ no dial tone Cost includes installation charge plus flat monthly fee (based on data rate and maybe distance) for local and/ or IXC (e.g., AT&T, PTT) channels Enterprise network might be mesh with N*(N-1)/2 leased lines or have 1 leased line per site to PSDN 6/9/2010 30
    • Switched/Dialup Circuits Circuit switched channels temporarily provided by POTS when call placed by dialing phone number Tariff specifies detailed description of service and pricing Price might be (!) based on call duration, distance, time of day, and/or data rate LEC (ILEC, RBOC, CLEC, CAP, or ALT) handles intra- LATA traffic and must hand off inter-LATA traffic to user's choice of IXC In LATA, each competing IXC has POP which routes calls to destination LATA U.S. has approx. 200 LATAs with 1 ILEC per LATA Bypass Used to eliminate cost of local channel or to obtain capability (e.g., fractional T1) not provided by LEC Directly connects user to IXC's POP via microwave, satellite or, depending on local regulations, cable TV facilities 6/9/2010 31
    • PRIVATE/LEASED VERSUS SWITCHED/DIALUP Pro Private/Leased Economical if high usage; break-even point depends on hours used per day No switching ⇒ fast access and less noise Can use conditioning to ↑ line quality ⇒ BER ↓ and data rate ↑ Under control of user Pro Switched/Dialup Economical when usage is low Offer flexibility to dial different computers and services User can redial and obtain alternate path if line failure occurs Telco is responsible for installation, maintenance, and management (also with leased lines) 6/9/2010 32