Media, LAN Technologies, and Network Topologies

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Media, LAN Technologies, and Network Topologies

• Lecture 5

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Introduction to Media

• Network traffic must flow through some form of
  media, whether it is a cable, or is wireless.
• The most common forms of network media are
  twisted-pair, coaxial, and fiber-optic cable.

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Twisted-Pair Cable

• T-P cable is the most common of all of the media
  types in the average local area network (LAN)
  environment.
• Different categories of T-P cable exist. The
  different categories of cable specify the maximum
  data bandwidth that the cable can withstand.
• T-P comes in two forms, Unshielded (UTP) or
  Shielded (Plenum/STP).

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Twisted-Pair Categories
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Twisted-Pair Categories (cont.)
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Twisted-Pair Comparison

• Advantages
• Cheap
• Easy to implement
• Easy to manage
• LOTS of different applications
• Easy to terminate

• Disadvantages
• Susceptible to EMF,RF interference
• Limited distance 100 meters

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Twisted-Pair (cont.)

• Twisted-pair cable (CAT5 and up) consists of 4
  separate pairs of wires, all wound separately.
• UTP is shown on the right.

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Coaxial Cable

• Coaxial cable (coax) is almost the same thing
  that carries your cable TV signal. Data coax is
  just held to a higher quality.
• Historical Tidbit Coax cable, although not
  commonly seen nowadays, was how Ethernet was
  developed!

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Coax (cont.)

• The physical medium itself consists of an inner
  wire, surrounded by an insulator, which is also
  surrounded by a shield.

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Coax Applications

• Local Area Networks (LANs)
• Thinnet (10base2) 200 meters
• Thicknet (10base5) 500 meters
• Baseband transmissions only

• Wide Area Networks (WANs)
• T3/DS3/E3
• Broadband transmissions

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Baseband v. Broadband

• Baseband is where the medium only carries one
  signal on the line.
• Broadband carries multiple signals on a single
  line.

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Coax Comparison

• Advantages
• Highly shielded from EMF,RF interference
• Signals propagate much farther than TP cable.
• Conforms to standards.
• More channels than TP cable.

• Disadvantages
• One cable for all computers.
• To add additional computers, network must be
  taken down.
• MUST properly terminate.
• Expensive.
• Low channel count compared to fiber.

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Fiber Optic Cable

• Fiber optic cable is where the future of LAN
  wiring exists.
• It is wicked fast.
• It is WICKED fast!

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Fiber Optic Cable (cont.)

• Fiber comes in two different types
• Multimode a channelized fiber-optic circuit.
  Multiple carrier frequencies.
• Singlemode a clear channel circuit. One
  carrier frequency.

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Fiber Comparison

• Advantages
• Wicked fast!
• Handles lots of simultaneous B channels.
• Very reliable.

• Disadvantages
• Cost to implement.
• Splicing kit.
• Cable costs.
• Redundancy (FDDI)?
• When disaster strikes, its a major ordeal.
• Point-to-point only

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Fiber Applications

• High-bandwidth voice transmission.
• Backbone applications.
• Very fast data transfer between network devices.

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Other Media

• Cellular
• Infrared
• Satellite
• Geosynchronous
• Low Earth Orbit
• Packet Radio
• Microwave

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Applying It All To LANs

• Point to point (between all computers) is
  unmanageable. This only becomes apparent when
  looking at multiple machines.

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If We Cant Use Point-To-Point?

• That is where the concept of a shared medium was
  derived.
• If a shared medium was in use, then you would
  only have to connect all of the computers
  together, not each individual computer!

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Locality of Reference

• A computer is more likely to communicate with
  computers that it is physically near than those
  that it is far away from.
• If a pair of computers communicates once, the
  pair is likely to communicate again in the near
  future, and then periodically.

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LAN Topologies

• There are three main types of LAN systems
• Star Topology
• Ring Topology
• Bus Topology
• Some topologies are actually hybrids of the above!

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Star Topology

• All computers connect to a centralized point.
• The central point is called the hub.

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Ring Topology

• One computer is connected to the two computers
  adjacent to it.
• In the traditional case, if a system is
  disconnected from the ring, the network fails.

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Bus Topology

• A bus topology consists of a single cable to
  which each network device attaches.
• Bus topology has the same problems as a ring
  topology.

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Ethernet

• This is the original Ethernet design by Bob
  Metcalfe in 1976.

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Ethernet (cont.)

• The Ethernet specification details how devices
  are supposed to interact on the segment,
  distances between devices, and a whole other
  multitude of other things.
• Carrier Sense On Multi-Access Networks (CSMA)
  with Collision Detection (CD) was the most
  important part of the specification.

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CSMA on Ethernet

• CSMA indicates that computers wait until the
  ether is free. In this case, no electrical signal
  denotes when the ether is free.

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CSMA on Ethernet (cont.)

• The signal is the carrier of the ether.
• The act of waiting for the opportunity to send on
  the ether is carrier sense.

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Collision Detection

• Since networks are not instantaneous, it is
  possible for two stations to check the ether,
  deem it time to send, and both send at the same
  time.
• This is called a collision.
• When a collision occurs, the sender immediately
  stops transmission, and waits some random length
  of time, and then begins transmission again.

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Collision Detection (cont.)

• A busy segment will always have collisions.
• Collisions do not damage the equipment, but it
  forces the two machines that caused the collision
  to wait, and therefore it slows down the network.
• Fewer machines on a segment, smaller segments,
  can improve the collision count.

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LocalTalk

• LocalTalk is another form of a bus network.
• LocalTalk differs from Ethernet in a few ways
• When a machine senses the segment is free, it
  holds it until the frame transmission is over.
  All other machines know to wait for transmission
  to complete.
• Bandwidth is quite small at 230Kbps. Thats 2.3
  of the earliest Ethernet specification!

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IBM Token Ring

• Access mechanism is called token passing.
• Once the sent information makes a complete turn
  around the ring, the sender passes the token to
  the next machine.
• This is a strict-alternation scheme where all
  machines have the same chance to transmit
  information.

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Fiber Distributed Data Interconnect (FDDI)