Cabling in a Network Environment

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Cabling in a Network Environment
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Ethernet Cabling Schemes
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Coaxial Cable

• Coaxial cabling has a single copper conductor at
  its center. A plastic layer provides insulation
  between the center conductor and a braided metal
  shield. This helps in blocking outside
  interference from fluorescent lights, motors and
  other computers.
• The braided shield prevents crosstalk which is
  unwanted interference from another adjacent
  communications channel . (The signal from the
  adjacent channel is inserted into the original
  communications channel.)
• It is more resistant to interference and
  attenuation, (which is the reduction of signal
  power that travels over a wire), than twisted
  pair cabling.
• Can transmit data, voice, and video.
• The 2 types of coaxial cabling are thin coaxial
  and thick coaxial.

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Thin Coaxial Cable

• Also referred to as thinnet RG58 cable
  10Base2.
• BNC connectors are commonly used with coaxial
  cables. (Twist, barrel and T connectors).
• Each cable must have a terminator whose impedance
  matches the cable type.

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Thick Coaxial Cable

• Also referred to as thicknet RG-8 and RG-11
  10Base5.
• Thick coaxial cable has an extra protective
  plastic cover that helps keep moisture away from
  the center conductor. This makes it an ideal
  choice when running longer lengths in a linear
  bus network.
• 0.5 inches thick
• Vampire taps are used to attach a transceiver to
  a thicknet trunk. Instead of cutting the cable
  and attaching connectors to both ends of the
  severed coaxial cable, a vampire tap pierces
  through the insulating layer of the cable and
  makes direct contact with the cables conductor.
  
• Less flexible, more bulky, more expensive, and
  harder to install than thinnet.

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Unshielded Twisted Pair (UTP)

• 8 copper wires grouped into pairs.
• Known as 4-pair wiring.

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UTP

• The quality of UTP may vary.
• The cable has four pairs of wires inside the
  jacket. Each pair is twisted with a different
  number of twists per inch to help eliminate
  interference from adjacent pairs and other
  electrical devices. The tighter the twisting, the
  higher the supported transmission rate and the
  greater the cost per foot.
• Categories of UTP

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UTP

• Twisting of copper wires cancels out crosstalk
  from adjacent pairs and external sources.
• The standard connector for unshielded twisted
  pair cabling is an RJ-45 connector. This is a
  plastic connector that looks like a large
  telephone-style connector (8 wires vs.
  telephones 4 wires RJ11/12).
• The minimum cable length between workstations is
  8 feet.
• 1,023 stations are allowed on a segment without
  bridging.
• Easily tapped because there is no shielding.
• With 100 metres distance, attenuation is the
  biggest problem.

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Shielded Twisted Pair (STP)

• A disadvantage of UTP is that it may be
  susceptible to radio and electrical frequency
  interference. Shielded twisted pair (STP) is
  suitable for environments with electrical
  interference however, the extra shielding can
  make the cables quite bulky.
• STP is often used in AppleTalk and Token Ring
  topology.
• Same 100 meter limit as UTP.
• Harder to tap.
• STP uses a woven copper braid jacket and a higher
  quality protective jacket. Also uses foil wrap
  between and around the wire pairs.

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Fiber Optic Cable

• Facts about fiber optic cables
• Outer insulating jacket is made of Teflon or PVC.
  
• Kevlar fiber helps to strengthen the cable and
  prevent breakage.
• A plastic coating is used to cushion the fiber
  center.
• Center (core) is made of glass or plastic fibers.

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Fiber Optic Cable

• Fiber optic cabling consists of a center glass
  core surrounded by several layers of protective
  materials.
• It transmits light rather than electronic signals
  eliminating the problem of electrical
  interference. This makes it ideal for certain
  environments that contain a large amount of
  electrical interference.
• Fibres are unidirectional (light only travels in
  one direction), so two fibres are used, one for
  sending and one for receiving.
• Good for high speed, and long distance data
  transmission.
• Communication possibilities are increased to
  include services such as video conferencing and
  interactive services.
• Fiber optic cabling is more difficult to install
  and modify.
• 10BaseF refers to the specifications for fiber
  optic cable carrying Ethernet signals. (2000
  meters)

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Fiber Optic Connector

• The most common connector used with fiber optic
  cable is an ST connector.
• It is barrel shaped, similar to a BNC connector.
• A newer connector, the SC, is becoming more
  popular. It has a squared face and is easier to
  connect in a confined space.

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Plenum Grade Cable

• Plenum cable is cable which is rated to be run
  through plenum spaces. Plenum is hidden space
  within a building which is used for HVAC
  (Heating, Ventilation, and Air Conditioning) and
  to run cables.
• Plenum cable must meet two requirements to meet
  fire codes
• Plenum cable must not burn easily
• Plenum cable must not emit toxic fumes when
  exposed to extreme heat
• Because of these requirements, plenum cable is
  usually more expensive than non-plenum cable.
• Plenum cable is also more stiff and difficult to
  work with than regular network cable.
• Most plenum cable is covered with Teflon or PVC.
• Characteristics
• Biodegradable
• Non-flammable
• Smokeless
• Plenum is a fire code rating! - There is no
  difference between the data transmission
  performance of plenum and non-plenum cable.

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Crosstalk (NEXT and FEXT)

• Crosstalk is signal interference between pairs
  that may be caused by a pair picking up unwanted
  signals from either adjacent pairs of conductors
  or nearby cables.
• For example, this interference can result from
  the magnetic field that surrounds any
  current-carrying conductor.
• Two types of crosstalk are
• Near End Crosstalk (NEXT) and
• Far End Crosstalk (FEXT)
• Near End Crosstalk (NEXT) is measured by applying
  and measuring the signal transfer level at the
  same end. Leakage is measured at transmission
  end.
• Far End Crosstalk (FEXT) is measured by applying
  the disturbing signal on one pair at the near end
  and measuring the signal transfer level on the
  disturbed pair at the far end. Occurs at the far
  end of a cable.

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NEXT and FEXT

• NEXT results when some of the energy that is
  transmitted in the desired direction seeps into
  one or more adjacent communication lines from the
  originating source.

• FEXT occurs when some of the digital signal
  energy leaks from one twisted pair and is coupled
  back to a communications line that is
  transferring a signal in the opposite direction.

Generally, NEXT is more serious than FEXT as the
signal interference levels from NEXT are higher.
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More terms used in testing
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More terms used in testing

• NEXT (Near End Cross(X) Talk) A measurement of
  crosstalk between two wire pairs. It is the
  difference in signal strength between the
  interfering pair and the interfered pair. The
  higher the number, the less crosstalk.
• PSNEXT (Power Sum NEXT) In a four-pair cable, it
  is the algebraic sum of near end crosstalk (NEXT)
  of three pairs as they affect one pair.

• FEXT (Far End Cross(X) Talk) A measurement of
  crosstalk between two wire pairs taken at the far
  end of the line.
• ELFEXT (Equal Level FEXT) A calculation that
  normalizes the results of a FEXT measurement,
  because it takes attenuation into account. It is
  derived by subtracting the attenuation of the
  interfering pair from the far end crosstalk
  (FEXT) that it has caused in the interfered pair.

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More terms used in testing

• PSELFEXT Power Sum Equal Level Far End
  Crosstalk
• The crosstalk noise caused by far end
  transmitters on neighboring cabling pairs
  interfering with the signal measured at the near
  end of the same channel.
• Delay - the time that it takes for a signal to
  travel from one end of a channel to the other end
  of the channel. Expressed in nanoseconds and
  lower values are better.
• Delay Skew - Delay skew is the difference between
  the propagation delay between any two pairs
  within the same cable sheath.

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Testing Cables

• Length
• Length of the cable
• Attenuation
• Decrease in magnitude of power of a signal in
  transmission between points.
• Wire Map
• Used to identify installation wiring errors (Ex.
  Shorted, Crossed, Split).
• Return Loss
• The ratio in dB (Decibel unit to measure
  strength of a signal) of maximum power sent down
  a transmission line to the power returned toward
  the source.
• DC Loop Resistance
• The total resistance through 2 conductors looped
  at one end of the link. This is usually a
  function of the conductor diameter and varies
  only with distance. This measurement is sometimes
  done to ensure that there are no gross
  misconnections which can add significant
  resistance to the link. Note that the wire map
  test automatically isolates breaks but not high
  resistance connections. From a signal
  perspective, attenuation may be a more useful
  measurement

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Testing Cables

• Attenuation to Crosstalk Ratio is the most
  important result when testing a link because it
  represents the overall performance of the cable.
• What causes attenuation?
• Where does crosstalk come from?

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Testing Cables - ACR

• ACR Attenuation to Crosstalk Ratio
• Attenuation is the reduction in signal strength
  over the length of the cable and frequency range.
• Crosstalk is the external noise that is
  introduced into the cable.

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Structured Cable Installation

• Quantity of cable required will vary with the
  project you choose.
• Your plan should include the following
• Building and networking materials
• Suppliers
• Tools
• Date and length of time tools are required.

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Some Guidelines to Install Cables

• When running cable, it is best to run a few
  simple rules
• Always use more cable than you need. Leave plenty
  of slack.
• Test every part of the network as you install it.
  Even if it is brand new, there may be problem
  which may be difficult to isolate later.

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Installing Cable

• Stay at least 3 feet away from fluorescent light
  boxes and other sources of electrical
  interference.
• If it is necessary to run cable across the floor,
  cover the cable with cable protectors.
• Label both ends of each cable.
• Use cable ties (not tape) to keep cables in the
  same location together.

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To Conclude ..

• It is important to remember what you have learnt
  with regards to different types of cables and how
  they are affected by crosstalk, different types
  of crosstalk, attenuation, and more, since these
  affect us in real life.