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

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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

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EIA/TIA 568-A (Commercial Building Telecommunications Cabling Standard) has specs for categories 3, 4, 5, and 5E 100 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 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

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Attenuation (dB per 100 m at 20°C)

Frequency(MHz)

Category 3UTP

Category 5UTP STP

1 2.6 2.0 1.14 5.6 4.1 2.216 13.1 8.2 4.425 - 10.4 6.2

100 - 22.0 12.3300 - - 21.4

Near-End Crosstalk (dB)*

Frequency(MHz)

Category 3UTP

Category 5UTP STP

1 41 62 584 32 53 5816 23 44 50.425 - 41 47.5

100 - 32 38.5300 - - 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:

TransmissionMedium

FrequencyRange

TypicalAttenuation

RepeaterSpacing

Twisted Pair (multipair cables)

0 to 1 MHz 3 dB/km @ 1 kHz 2 km

Coaxial Cable 0 to 500 MHz 7 dB/km @ 10 MHz 1 to 9 kmOptical Fiber 180 to 370 THz 0.2 to 0.5 dB/km 40 km

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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 conductor4. 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

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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

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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

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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

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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

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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

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Comparison of Optical Fiber and Satellite Transmission:

Characteristic Optical Fiber Satellite

Bandwidth Theoretical limit 1THz; currently 1-10 GHz

Typical transponder BW of 36-72 MHz

Interference Immunity

Immune to EMI Subject to EMI

Security Difficult to tap without detection

Requires encryption

Multipoint Capability

Primarily point-to-point Easy to implement point-to-multipoint

Reconfiguration Flexibility

Difficult Easy

Connectivity to Customer Site

Local loop required If earth station installed on customer premises, local loop not required

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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)

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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

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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

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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

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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

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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

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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)

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