1 Chapter 8 - Physical Transmission Media Class #8Chapter 8 - Physical Transmission Media
2 Physical Media Objectives In chapter 8, you will learn to:Identify the characteristics of wireline transmissionDescribe the properties and uses of coaxial cableDescribe the properties and uses of different types of twisted-pair wireIdentify the characteristics of lightwave transmissionDescribe the properties and uses of fiber optic cableIdentify factors to consider when selecting a telecommunications mediumExplain and apply cabling standardsDescribe best practices for installing wire and fiber optic cablingIdentify techniques for testing the continuity and performance of physical transmission media
3 Characteristics of Wireline Transmission Impedance: expressed in Ohms, is the combined effect of a circuit’s inductance and capacitance.In the case of a DC circuit, impedance equals the circuit’s resistance.Abrupt changes in impedance is a circuit results in part of the signal being reflected back toward the source, and thus signal loss of amplitudePropagation Delay and Latency: the difference in time between a data packet’s transmission and its reception over a specific route.Latency is typically the speed of light multiplied by the distance between source and destination, times 2 (for the round trip), plus some misc. processing time.
4 Characteristics of Wireline Transmission, cont. Distortion: the unintended and undesirable modification of at least one signal component, which makes the signal different from how it was originally transmitted.This can happen because of changes in a wave’s phase, amplitude, and/or frequencyAttenuation causes the loss of amplitude over distance. Attenuation distortion occurs because attenuation is more severe at higher frequencies.Phase distortion occurs because propagation of middle frequencies is faster than frequencies at the edges of a transmission envelope, and is corrected by the use of an equalizer.Noise: any unwanted interference (electromagnetic or radio frequency) from external sources.Forms of noise include crosstalk, intermodulation, impulse, and thermal.
5 CrosstalkCrosstalk is the unwanted mutual inductance, or unwanted transformer effect, between 2 adjacent data-carrying wires.
6 IntermodulationIntermodulation occurs when two nearby channels on the same media interfere with each other.This is usually because the guard band is non-existent or not wide enough.
7 Impulse NoiseImpulses are unwanted spikes on an analog or digital signal.
8 Thermal NoiseThermal noise (or white noise) occurs when heat or change in temperature affects the movement of electrons in a media.
9 Ways to overcome interference Increase your signal-to-noise ratio (make sure your signal is louder than your noise)Filter out the noiseA bandpass filter, applied to a circuit, blocks out selected frequenciesUse amplifiers/repeaters to overcome attenuationTake steps to eliminate crosstalk and other external interference by choosing certain features in your transmission media.
10 Copper Transmission Media Coaxial Cable (coax): A common type of cabling used in TV and data transmissionsDesigned with shielding in mindCan carry signals farther than other types of cableFairly expensiveSomewhat (but not totally) flexibleMust be impedance-matched for the type of signal it is carrying
11 Copper Transmission Media, cont. Non-twisted Wire: 2 colored pairs of 28 gauge wire.Also known as CAT1, quad-pair, or Silver SatinInexpensiveDesigned for short-haul (50 feet) PSTN inside the homeHighly susceptible to attenuation, interference, and crosstalk.
13 Copper Transmission Media, cont. Shielded Twisted Pair (STP): 2 colored pairs of 28 gauge wire.Shielded against outside interferenceTwists in pairs work against crosstalkNot very common due to cost
14 Copper Transmission Media, cont. Unshielded Twisted Pair (UTP): colored pairs of gauge wire.Fairly inexpensive, and very commonNo shielding to protect against outside interference.Somewhat susceptible to attenuationTwists in pairs overcome crosstalk
15 Copper Transmission Media, cont. CAT3 UTP: 4 colored pairs of 24 gauge wire.Designed for telephony and data feed up to 10Mbps at 16MHz.
16 Copper Transmission Media, cont. CAT5 and CAT5e UTP: 4 colored pairs of 24 gauge wire.Designed for telephony and data feed up to 100Mbps at 100MHz.Most communications protocols using this media are spec’d out to 100 meters of cable run.Higher resistance to crosstalk and interference due to advanced pair-twisting techniques.Due to minimal price differences between CAT1 and CAT5, most modern installations will use the most current media form.
17 Fiber Optic CableIn contrast to copper, which carries signals as voltage, fiber optic cable carries discreet pulses of light from a laser or light-emitting diode.Compared to copper media, fiber can reliably transmit more data at faster rates – up to 1 billion light pulses per second – at long cable runs with little risk of interference.The only real limit to using extremely long runs is attenuation through optical loss.Fiber consists of glass or plastic filaments at the core, surrounded by a kevlar and glass reflective cladding and a flexible outer jacket.Fiber optic cable is fairly expensive, but highly reliable and tolerant of abuse. Very secure (no transformer effect) and impervious to most noise.
18 Fiber Optic Cable, cont. Single Mode Fiber: a narrow core (10 microns) Highly expensive, and uses a laserHas a graded index of refraction, causing all light in the core to move in one direction.Less light dispersion along the cable runHigh frequencies and long runs (25km)Multi-mode Fiber: a larger core (100 microns)Less expensive, and uses an LEDLight bounces off of all internal surfacesShorted distances due to phase distortion and attenuation (1km)
19 Fiber Optic Cable, cont. Popular uses includes connecting: Regional and local cable TV facilitiesInternet NAPs with other large telecommunications exchange pointCentral offices with other central officesMain feeders with central officesA telecommunication’s network with private LANsA telecommunication’s network with private switching systems, such as PBXBackbone connectionsHigh security needs
20 Selecting Appropriate Media When selecting telecommunications media consider:Existing infrastructure –What is easiest to run in existing buildings?Where do you have to run the cable?Throughput potential –How much throughput do you need versus the installation cost?Cost of installation –Outside contractors vs. do-it-yourself?Special techniques for certain media.Most expensive part of the installationNoise immunity –Certain media more immune than others in various situationsSecurity –How easy is it to tap the media?What protocols are available for use on the media?Size and scalabilityCable run lengthsEase of splitting mediaEase of patching into existing media.
23 Class Exercise #1What media would you choose in the following situations:Small classroomHouse remodelBackbone through elevator shaft, with lots of sharp angles to get aroundNoisy defense contractorSchool with CATV already installedNew installation in new office building
24 Structured CablingStructured Cabling is based off of the TIA/EIA 568 Commercial Building Wiring Standard for enterprise cabling systems. It suggests how voice and data can best be installed to maximize performance and minimize upkeep.Entrance: separates LAN from WAN, and LEC’s POP from interoffice trunks.Backbone: interconnection between equipment rooms and closets, floors, buildings, and the entrance facility.Plenum: an NEC standard cable conduit, often rated for flame/fume resistance.
26 Structured Cabling, cont. Equipment Rooms: locations of significant hardware, such as servers, PBXs, etc.Telecommunications Closets: rooms that contain connectivity (cross-connects, patch panels, etc) for workstation equipment like phones, faxes, PCs, printers, etc., for limited work areas.Note: The equipment rooms and telecom closets are often the same area at smaller companies.
27 Structured Cabling, cont. In the telecom closet, cable punch-downs and cross-connects are done at blocks designed by AT&T. Phone line punch-downs are done on an AT&T “66” block, and network is done at the “110” block.
28 Structured Cabling, cont. Patch Panels are used to interconnect the blocks (which connect the workstation equipment) to connectivity devices such as hubs or switches.
29 Structured Cabling, cont. Horizontal Wiring connects the workstations to the blocks and patch panels
30 Structured Cabling, cont. The Work Area is terminated in a mini patch-panel. The standard calls for at least 1 voice and 1 data jack; realistically each workstation will be equipped with the number and kind of jacks most suited to the enterprise.
32 Installing UTPFor a 10BaseT circuit, you would use pairs 2 and 3. You could also “split out” pairs 1 and 4 if you needed a second circuit and couldn’t lay new media.For a 100BaseT circuit, you would use all 4 pairs.
33 Crossover CableA crossover cable is used to connect 2 similar devices (aka NIC-NIC, hub-hub, switch-switch, router-router, etc.) without going through an intermediate device.Essentially, a crossover cable reverses the matched pairs (you would switch pairs 2/3 and 1/4 on one end of the cable to 3/2 and 4/1).
34 Installation Tips for CAT5 UTP Do not untwist twisted-pair cables more than one-half inch before inserting them into the punch-down block or connector.Pay attention to the bend radius limitations for the type of cable you are installing.Test each segment of cabling as you install it with a cable tester.Use only cable ties to cinch groups of cables together
35 Installation Tips for CAT5 UTP, cont. When pulling cable, do not exert more than 25 pounds of pressure on the cable.Avoid laying cable across the floor where it might sustain damage from rolling chairs or foot traffic.Install cable at least three feet away from fluorescent lights or other sources of EMI.Always leave slack in cable runs.
36 Maintaining FiberSplice - the physical joining of two facing and aligned pieces of wire or fiber.Mechanical splicing - the two ends of a fiber optic cable are fixed in position within a tube so that they form one continuous communications channel.Fusion splicing - a connection between fibers is accomplished through the application of heat and the resulting melting and fusion of two fiber strands.
38 Cable Installation Tips for Fiber Optic Cable When pulling fiber optic cable, do not exert pressure on the cable.Fiber optic cable should be installed within a conduit whenever you are concerned about the potential for environmental damage.Do not exceed the minimum bend radius.
39 Troubleshooting Connectivity Problems Identify the symptomsIdentify the scope of the problemEstablish what has changed on the networkDetermine the most probable cause of the problemImplement a solutionTest the solutionRecognize the potential effects of the solutionDocument the solution
40 Troubleshooting Tools Tone Generator - a small electronic device that issues a signal on a wire pair.Tone Locator - a type of amplifier that can detect the inductive energy emitted by the tone (current) on a wire.Used together, these tools can:Locate individual pairs in bundles/blocksTrace circuit breaks to the area of breakage
41 Troubleshooting Tools, cont. Continuity Tester: a simple device that looks for line breaks in a cable in situations where either a quick test of a cable is needed, or where a tone generator cannot be used.
42 Troubleshooting Tools, cont. A Time/Domain Reflectometer (TDR) is an advanced version of a continuity tester. It can tell exactly where along a cable run that a pair fails, so that a patch can be installed.
43 Troubleshooting Tools, cont. A Performance Tester provides the following functions:Measures the length of each wire pairEnsures that the cable does not exceed recommended maximum lengthsMeasures the distance from the tester to a cable faultMeasures attenuation along a cableMeasures cross-talk between wires
44 Troubleshooting Tools, cont. The Telephone Test Set is essentially a rugged and sophisticated phone. Its basic purpose is to detect dial-tone at various points. Some versions can carry out simple cable tests and generate dial tone.
45 Summary of Physical Media Characteristics that affect wireline transmission include impedance changes, latency, delay distortion, and noise.Traditional four-pair, non-twisted copper telephone wiring is known as Level 1 cable or quad wire.Category (CAT3) UTP cable is the minimum grade of unshielded twisted-pair cabling for use in data systems. CAT5 is the minimum recommended grade, however.To identify the source of cabling infrastructure problems, follow a logical troubleshooting methodology and have the appropriate testing tools handy.
47 End of Class QuizWhat happens to a signal as a result of impedance mismatch?What type of device is used to compensate for delay/phase distortion?A lightning strike near a drop cable causes what kind of noise?What is caused by the induction of one wire’s current on an adjacent cable?What is the most common type of cable in a modern LAN?
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