Class #8 Chapter 8 - Physical Transmission Media
Physical Media Objectives In chapter 8, you will learn to: Identify the characteristics of wireline transmission Describe the properties and uses of coaxial cable Describe the properties and uses of different types of twisted- pair wire Identify the characteristics of lightwave transmission Describe the properties and uses of fiber optic cable Identify factors to consider when selecting a telecommunications medium Explain and apply cabling standards Describe best practices for installing wire and fiber optic cabling Identify techniques for testing the continuity and performance of physical transmission media
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 amplitude Propagation 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.
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 frequency Attenuation 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.
Crosstalk Crosstalk is the unwanted mutual inductance, or unwanted transformer effect, between 2 adjacent data-carrying wires.
Intermodulation Intermodulation 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.
Impulse Noise Impulses are unwanted spikes on an analog or digital signal.
Thermal Noise Thermal noise (or white noise) occurs when heat or change in temperature affects the movement of electrons in a media.
Ways to overcome interference 1. Increase your signal-to-noise ratio (make sure your signal is louder than your noise) 2. Filter out the noise A bandpass filter, applied to a circuit, blocks out selected frequencies 3. Use amplifiers/repeaters to overcome attenuation 4. Take steps to eliminate crosstalk and other external interference by choosing certain features in your transmission media.
Copper Transmission Media Coaxial Cable (coax): A common type of cabling used in TV and data transmissions Designed with shielding in mind Can carry signals farther than other types of cable Fairly expensive Somewhat (but not totally) flexible Must be impedance- matched for the type of signal it is carrying
Copper Transmission Media, cont. Non-twisted Wire: 2 colored pairs of 28 gauge wire. Also known as CAT1, quad-pair, or Silver Satin Inexpensive Designed for short-haul (50 feet) PSTN inside the home Highly susceptible to attenuation, interference, and crosstalk.
Copper Transmission Media, cont.
Shielded Twisted Pair (STP): 2 colored pairs of 28 gauge wire. Shielded against outside interference Twists in pairs work against crosstalk Not very common due to cost
Copper Transmission Media, cont. Unshielded Twisted Pair (UTP): colored pairs of gauge wire. Fairly inexpensive, and very common No shielding to protect against outside interference. Somewhat susceptible to attenuation Twists in pairs overcome crosstalk
Copper Transmission Media, cont. CAT3 UTP: 4 colored pairs of 24 gauge wire. Designed for telephony and data feed up to 10Mbps at 16MHz.
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.
Fiber Optic Cable In 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.
Fiber Optic Cable, cont. Single Mode Fiber: a narrow core (10 microns) Highly expensive, and uses a laser Has a graded index of refraction, causing all light in the core to move in one direction. Less light dispersion along the cable run High frequencies and long runs (25km) Multi-mode Fiber: a larger core (100 microns) Less expensive, and uses an LED Light bounces off of all internal surfaces Shorted distances due to phase distortion and attenuation (1km)
Fiber Optic Cable, cont. Popular uses includes connecting: Regional and local cable TV facilities Internet NAPs with other large telecommunications exchange point Central offices with other central offices Main feeders with central offices A telecommunication’s network with private LANs A telecommunication’s network with private switching systems, such as PBX Backbone connections High security needs
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 installation Noise immunity – Certain media more immune than others in various situations Security – How easy is it to tap the media? What protocols are available for use on the media? Size and scalability Cable run lengths Ease of splitting media Ease of patching into existing media.
Selecting Appropriate Media, cont.
Class Exercise #1 What media would you choose in the following situations: 1. Small classroom 2. House remodel 3. Backbone through elevator shaft, with lots of sharp angles to get around 4. Noisy defense contractor 5. School with CATV already installed 6. New installation in new office building
Structured Cabling Structured 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.
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.
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.
Structured Cabling, cont. Patch Panels are used to interconnect the blocks (which connect the workstation equipment) to connectivity devices such as hubs or switches.
Structured Cabling, cont. Horizontal Wiring connects the workstations to the blocks and patch panels
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.
Structured Cabling, cont.
Installing UTP For 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.
Crossover Cable A 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).
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
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.
Maintaining Fiber Splice - 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.
Maintaining Fiber, cont.
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.
Troubleshooting Connectivity Problems Identify the symptoms Identify the scope of the problem Establish what has changed on the network Determine the most probable cause of the problem Implement a solution Test the solution Recognize the potential effects of the solution Document the solution
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/blocks Trace circuit breaks to the area of breakage
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.
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.
Troubleshooting Tools, cont. A Performance Tester provides the following functions: Measures the length of each wire pair Ensures that the cable does not exceed recommended maximum lengths Measures the distance from the tester to a cable fault Measures attenuation along a cable Measures cross-talk between wires
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.
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.
End of Class Quiz 1. What happens to a signal as a result of impedance mismatch? 2. What type of device is used to compensate for delay/phase distortion? 3. A lightning strike near a drop cable causes what kind of noise? 4. What is caused by the induction of one wire’s current on an adjacent cable? 5. What is the most common type of cable in a modern LAN?