Transmission Media Chapter 3

Knowledge Checkpoints  Media and their characteristics  Categories of twisted pairs  Applications for wire, cable, fiber  Emerging applications for radiated media

Important Terms  Fiber  Frequency spectrum  Amplifiers  OTDR  Twisted pair  Coax  Cat  UTP, STP  DSLAM  Dark fiber  BRI, PRI  DSL  HFC  Oxygen layer  Line of sight  Footprint  EMI, RFI  Symmetrical & asymmetrical  Propagation delay

Physical Layer: Architectures, Devices and Circuits

Analog Transmission  In analog transmission, the state of line can vary continuously and smoothly among an infinite number of states States can be signal strengths, voltages, or other measurable conditions Time Strength

Digital Transmission  In digital transmission, time is divided into periods of fixed length called clock cycles For modems, a few thousand clock cycles per second For LANs, millions of clock cycles per second Clock Cycle Time Strength

Bits and Baud  Baud Rate = Number of clock cycles/sec 4 baud (not 4 bauds/second)  Bit Rate = Number of bits/second 8 bits/second Second Possible Change Not Made

Equations  For Each Clock Cycle 2 Bits per clock cycle = Number of possible states Overall Bit rate = Baud Rate * Bits per clock cycle  Example Baud rate of 10,000 with four possible states Bits per clock cycle = 2 (2 2 =4) Bit rate = 10,000 * 2 (Eq. 2) Bit rate = 20,000 bps

Transmission Speeds  Bit: A single 1 or 0  Bits per second (bps) Factors of 1,000 (not 1,024 as in memory) kilobits per second (kbps)--Note lower case k megabits per second (Mbps) gigabits per second (Gbps) terabits per second (Tbps)  Occasionally given in bytes per second (Bps) Bits per second / 8 Uncommon

Communication Media  The media is the matter or substance that carries voice, video or data transmission There are two basic types of media:  Guided media - those in which the message flows through a physical media  Radiated media (unguided) - Those in which the message is broadcast through space

Guided Media  Twisted Pair Wire - insulated pairs of wire, twisted to minimize electromagnetic interference between wires  Coaxial Cable - wire with a copper core and an outer cylindrical shell for insulation  Fiber Optic Cable - high speed streams of light pulses from lasers or LEDs carry information inside hair-thin strands of glass or plastic called optic fibers

Important Facts about Twisted Pair  Lowest frequency spectrum –a max of 1MHz  Susceptible to noise and high error rate  Distance between repeaters (cost issue)  Security is poor  Low acquisition cost  Foundation of PSTN  Standards govern categories (maximum data rate)  Applications include: telephone, PBX, desktop, LANs, local loop

Twisted Pair Media  Twisted-pair wires are classified by: v American wire gauge (AWG) rating v Shielding, either unshielded twisted-pair (UTP) or shielded twisted-pair (STP) v Categories that define the wire’s rated acceptable speed and error characteristics  AWG Rating The AWG rating is a measure of the thickness of the copper conductor in the cable The higher the AWG rating, the smaller the diameter of the wire Twisted-pair wiring for LANs have an AWG rating of 22-26

Wire Propagation Effects: Cross-Talk Interference  Cross-Talk Interference Multiple wires in a bundle and each radiates some of its signal Causes “cross-talk” interference in nearby wires

Wire Propagation Effects:Cross Talk  Wire is Twisted Several twists per inch Interference adds to signal over half twist, subtracts over other half Roughly cancels out Simple but effective Single Twist Interference -+ Signal

Wire Propagation Effects:Cross Talk  Terminal Cross-Talk Interference Wire must be untwisted at ends to fit into connectors Cross-talk interference is high at termination Problems severe if untwist more than about 1.25 cm (1/2 inch) Usually the biggest propagation effect Terminal Cross Talk

Characteristics of Common LAN Media Unshielded twisted-pair Shielded twisted-pair Coaxial Cable Fiber optic cable Broadcast radio Spread spectrum radio Microwave radio Infrared light Common Speeds (Mbps) Medium Type 1, 4, 10, 16, 100, , 16, 50 10, 16, 50, 100, 1000, , 10, , 10, 16 Less capable than other conducted media Better than unshielded; less capable than fiber optic or coaxial cables Good; less capable than fiber optic cable Excellent Subject to interference Good Subject to interference Objects can block transmission Error Characteristics

Practical Issues in Propagation Effects  Distance limits in standards prevent serious propagation effects For instance, usually 100 meters maximum for ordinary copper wire Stay within limits, usually no serious problems  Problems usually occur at connectors Crossed wires Poor connections Cross-talk interference

Wire Media: UTP  Unshielded Twisted Pair (UTP) Ordinary copper wire Twisted several times per inch to reduce interference Pair of wires needed for a complete electrical signal Unshielded: nothing but plastic coating v No protection from interference

Wire Media: UTP  Unshielded Twisted Pair (UTP) Business telephone wiring traditionally comes in 4-pair UTP wire bundles Used in LAN wiring to use existing building wiring technology

Wire Propagation: RJ-45  RJ-45 connector terminates a UTP bundle Slightly wider than RJ-11 residential connector Width needed for 8 wires

Wire Media: UTP to the Desktop  UTP Dominant for line from desktop to first hub or switch Inexpensive to buy and install Rugged: can take punishment of office work Easily 100 Mbps, 1 Gbps with careful insulation UTP First Hub or Switch

ISDN  Two B Channels are 64 kbps Original idea: one for voice, one for data 1101 B Channel B Channel

ISDN  Multiplexes (mixes) three channels on one UTP wire pair to the desktop  BRI & PRI 1101 B Channel B Channel 2B+D Multiplexed Onto One Set of Wires ISDN Modem D Channel: control signals

ISDN Costs  Line is expensive $60-$80/mo plus installation fee ISP charge is separate; may charge more for ISDN access  Dial Up: Not always connected Do not have to pay for full-time use  Need “ISDN modem” (expensive)

ISDN Modem ISDN “Modem” is a Misnomer Modem is for digital device on an analog line ISDN line is digital  ISDN modem really contains Codec to link analog telephone to digital ISDN line Data service unit (DSU) to translate between PC digital format and ISDN digital format (voltage levels, timing, etc.,)

Twisted-Pair Wire Category Summary Maximum Data Rate Category 1 Mbps 4 Mbps 10 Mbps 16 Mbps 100, 155, and 1,000 Mbps Telephones Token Ring LANs Ethernet LANs Token ring LANs Ethernet, fast ethernet, and gigabit ethernet LANs, CDDI LANs and asynchronous transfer mode (ATM) Typical Use Cost (Relative to Category 1)

DSLs Digital Subscriber Lines

Digital Subscriber Lines (DSLs)  Offered by Telephone Companies Lines to customer premises are subscriber lines, which connect subscribers to the telephone system--These are digital “Digital subscriber line” Telephone Network Telephone Network ISP DSL Modem DSL Modem DSL

Digital Subscriber Lines (DSLs)  Several types of digital lines for subscribers: for homes, others for businesses For residential customers, usually multiplexes regular phone, high-speed data v Use existing phone line coming into house v Use Internet without tying up phone Telephone Network Telephone Network ISP DSL Modem DSL Modem Existing Phone Line

Digital Subscriber Lines Most common for home is Asymmetric DSL (ADSL) Upstream and downstream speeds are different (asymmetrical) Upstream at 64 kbps or more Downstream at 256 kbps to a few Mbps Asymmetric speed is good for web surfing About $50 per month incl. ISP; more for faster service Telephone Network Telephone Network ISP DSL Modem DSL Modem ADSL 64 or more kbps 256 kbps-a few Mbps

Digital Subscriber Lines  G.Lite (G.992.2) Standard (ADSL1) New ADSL standard from ITU-T Up to 1.5 Mbps downstream speed No carrier installation is necessary Not being widely adopted by ADSL vendors yet Telephone Network Telephone Network ISP DSL Modem DSL Modem ADSL 64 or more kbps Up to 1.5 Mbps

Digital Subscriber Lines  Digital Subscriber Line Access Multiplexer Telco must install a DSLAM at end switching office (CO) Telephone Network Telephone Network ISP DSLAM DSL

Digital Subscriber Lines  Splitting Voice and Data Voice and data are split at home by the DSL modem Voice and data are also split at the telephone company’s first switching office DSL Modem DSL Splitter DSL DSLAM Voice Network Computer Phone

Important Facts about Coax  Greater frequency spectrum  Capable of multiple channels  Better performance (lower error rate & better bandwidth)  Amplifiers every 1.5 miles  Requires a bus topology  Noise at connection points & susceptible to lightening  Applications include: interoffice trunks, LANs, international lines, cable TV, local loop  Bi-directional upgrade required  High installation cost

Wire Media: Coax  Coaxial Cable Used in cable TV, VCRs Central wire, external concentric cylinder Outer conductor wrapped in PVC Screw-On Connector Inner Wire Outer Conductor Wrapped in PVC

Wire Media: Coaxial Cable  Coaxial Cable Installed widely today in old 10 Mbps Ethernet LANs Not being used in new installations v Optical fiber more cost-effective for long links v UTP more cost-effective for desktop links

A Single Conductor Coaxial Cable Outer Insulation Mesh Shielding InsulationConductor

Guided Media

Cable Modem Service Broadband Internet Access

Cable Modem Service Service of Cable Television Companies Deliver 10 Mbps downstream to the home Capacity is shared by multiple subscribers, so real speed is more limited Limited to about 64 kbps to 256 kbps upstream Does not tie up telephone line--Always available Cable TV Network Cable TV Network Also ISP Functions Also ISP Functions Cable Modem Cable Modem

Cable Modem Sharing in Perspective  Sharing is Not as Extreme as it May First Appear Shared within Blocks of 500 Houses Only Some Households in Block will Subscribe Only Some Subscribers will be Online at Any Moment Only Some of Online Subscribers will Send and Receive at Any Moment Only these will Share the Capacity  So Sharing Does Not Reduce Speed Unless the Adoption Rate is Large

Cable Modem Connection

Cable Modem Service  Cost is about $50 per Month Includes ISP service! Installation usually costs $100 to $150 and includes a cable modem and a network interface card Best alternative today beyond V.90 modem Cost-competitive with adding a second phone line to handle your modem communication

Views of a Fiber Optic Cable Plastic Covering Glass Cladding Glass Conductor

Important Facts about Fiber  Greater distance between repeaters (500 miles)  Single and multi-mode fibers  Applications include: backbone for public and private networks, the Internet, LANs  High bandwidth  Elastic traffic carrying capacity  Low error rate  Secure transmission  High installation cost

Guided Media  The earliest fiber optic systems were multimode, (light could reflect inside the cable at many different angles)  Single mode fiber optic cables transmit a single direct beam of light through a cable that ensures the light only reflects in one pattern  Fiber is more secure than coax or wire

Guided Media

Optical Fiber  Optical Fiber Glass core, surrounding glass cladding Light source turned on/off (pulses) for 1/0 Total internal reflection at boundary Almost no attenuation Light Source Cladding Core Reflection

Optical Fiber  Multimode Fiber Wide core makes easy to splice (50 or 62 microns) Many angles for rays (modes) Short propagation distance (usually 200 m to 500 m) Light Source

Wire Media: Optical Fiber  Single Mode Fiber Narrow core difficult to splice (5 or 8 microns) Only one angle for rays (one mode), so (almost) no distortion Longer propagation distance (usually up to 2 km for LAN fiber, longer for long-distance fiber) Narrow core makes fiber fragile and difficult to splice

Wire Media: Optical Fiber  Optical Fiber High speeds over long distances v 200 m to 2 km Costs more than UTP, but worth it on long runs Good for all links between hubs and switches within and between buildings in a site network Optical Fiber

Wire Media: UTP and Optical Fiber  The emerging pattern: UTP from first hub or switch to desk, fiber everywhere else

Optical Fiber  Limited by Distortion Light entering at different angles travels different distances (different number of reflections) Called different modes Light from successive bits becomes mixed over long distances Light Source

Radiated Media  Radio (wireless) data transmission uses the same basic principles as standard radio transmission  Infrared Transmission uses low frequency light waves to carry data through the air on a direct line-of-sight path between two points

Radiated Media  A microwave is an extremely high frequency radio communication beam that is transmitted over a direct line-of-sight path between two points  Transmission via GEO satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space

Radiated Media

 A disadvantage of satellite transmission is the delay that occurs because the signal has to travel out into space and back to Earth (propagation delay)  A problem associated with some types of satellite transmission is raindrop attenuation (some waves at the high end of the spectrum are so short they can be absorbed by raindrops)

Radiated Media  Ku-band satellites use waves that are so short they can be caught and concentrated in much smaller dish antennas, called very small aperture terminals (VSAT)  The larger Earth dish hubs can cost as much as several hundred thousand dollars

Radio Propagation  Broadcast signal Not confined to a wire Moves through the air Security risk

Radio Waves When Electron Oscillates, Gives Off Radio Waves Single electron gives a very weak signal Many electrons in an antenna are forced to oscillate in unison to give a practical signal

Radio Propagation Problems  Wire Propagation is Predictable Signals go through a fixed path: the wire Propagation problems can be easily anticipated Problems can be addressed easily  Radio Propagation is Difficult Signals begin propagating as a simple sphere But they can be blocked There are shadow zones Shadow Zone

Radio Propagation Problems Radio Propagation is Difficult Signals are reflected May arrive at a destination via multiple paths Signals arriving by different paths can interfere with one another This is called multi-path interference

Radio Propagation: Service Bands  Service Bands Divide spectrum into bands for services A band is a contiguous range of frequencies FM radio, cellular telephone service bands 0 Hz Cellular Telephone FM Radio AM Radio Service Bands

Radio Propagation: Channels and Bandwidth  Service Bands are Further Divided into Channels Like television channels Bandwidth of a channel is highest frequency minus lowest frequency 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

Radio Propagation: Channels and Bandwidth  Example Highest frequency of a radio channel is 43 kHz Lowest frequency of the radio channel is 38 kHz Bandwidth of radio channel is 5 kHz (43-38 kHz) 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

Radio Propagation Problems  Wire Propagation is Predictable Signals go through a fixed path: the wire Propagation problems can be easily anticipated Problems can be addressed easily  Radio Propagation is Difficult Signals begin propagating as a simple sphere But they can be blocked There are shadow zones Shadow Zone

Radio Propagation: Waves  Waves Amplitude (strength) Wavelength (meters) Frequency in hertz (Hz) Cycles per Second One Second 7 Cycles 1 Hz = 1 cycle per second

Radio Propagation: Frequency Spectrum  Frequency Spectrum Frequencies vary (like strings in a harp) Frequencies measured in hertz (Hz) Frequency spectrum: all possible frequencies from 0 Hz to infinity 0 Hz

Frequencies  Metric system kHz (1,000 Hz) kilohertz; note lower-case k MHz (1,000 kHz) megahertz GHz (1,000 MHz) gigahertz THz (1,000 GHz) terahertz

Radio Propagation: Channels and Bandwidth  Shannon’s Equation W is maximum possible (not actual) transmission sped in a channel B is bandwidth of the channel: highest frequency minus lowest frequency S/N is the signal-to-noise ratio W = B Log 2 (1 + S/N)

Radio Transmission: Broadband  Speed and Bandwidth The wider the channel bandwidth (B), the faster the maximum possible transmission speed (W) W = B Log 2 (1+S/N) Maximum Possible Speed Bandwidth

Broadband  Two Uses of the Term “Broadband”  Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large Therefore, maximum possible transmission speed is high  Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all

Media Selection NetworkTransmissionError MediaTypeCostDistanceSecurityRatesSpeed Twisted PairLANLowShortGoodLowLow-high Coaxial CableLANMod.ShortGoodLowLow-high Fiber OpticsanyHighMod.-longV. GoodV.LowHigh-V.High NetworkTransmissionError MediaTypeCostDistance SecurityRates Speed RadioLAN LowShort PoorMod Low InfraredLAN, BN LowShort PoorMod Low MicrowaveWAN ModLong PoorLow-Mod Mod SatelliteWAN ModLong PoorLow-Mod Mod Guided Media Radiated Media

Important Figures to Study  P. 55 Table 3.1  P. 68 Figure 3.3

Chapter Questions  Why is coax more secure than twisted pair?  Why is fiber the best choice for maximum security?  How does physical lay-out of a building influence your selection?  What does this statement mean? The frequency spectrum in which a medium operates directly relates to the bit rate you can obtain with the medium.