Small Local Area Networks: Single Collision-Domain Networks

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Small Local Area NetworksSingle
Collision-Domain Networks
Kevin BoldingElectrical EngineeringSeattle
Pacific University
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Local Area Networks

• General Definition(s) of a LAN
• Small (lt100 stations, single building)
• Trusted users
• Single ownership
• Fast

• Single Collision Domain networks
• All devices connected to the network share the
  network at the same time
• Only one device may send data on the network at a
  time
• Data is sent to all receivers no routing occurs

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Network Topologies

• Network topology
• Describes the physical layout of a network
• How are the computers and devices connected?

• Basic shape of design
• Bus (line)
• Ring
• Star

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Bus Topology
37
Ö
12
13
09
X
X
To 37 Dataxxxx

• One cable connects all components
• Cable may be straight or snake through the
  building
• Individual devices are connected on branches of
  the backbone

• Each message is sent to all computers

• Only the addressed computer responds to the
  message

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10Base-2 Bus LANs

• Most Bus LANs are connected using 10Base-2
  Coaxial Cable
• 10Mbps, baseband transmission, 200m max
• 50W, RG-58 A/U

• BNC-type connections
• Use a T to make a spur to connect a workstation

• Terminators
• All cable ends must be terminated (50W) to
  prevent reflections

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Bus Issues

• Theres only one cable system
• Any breaks will isolate part of the network

• Its even worse!
• Unplugging a single workstation creates an
  un-terminated end in the network
• Communication in the whole network is stopped
• Like trying to talk in an echo chamber

• The Bus is a passive system
• Signals are simply sent through the wires
• No active components refresh the signals
• Repeaters can be used if necessary

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Bus Pros and Cons
Disadvantages
Advantages

• Many single-point failure modes
• Broken backbone
• Any unplugged computer that isnt terminated

• Simple
• Plug and play

• Cheap
• Mostly all cables
• Cables are relatively inexpensive

• Difficult to trace errors
• All one big wire
• Problem could be anywhere

• Linear arrangement
• Just run your backbone in a path that goes near
  the workstations

• Limits on cable length seriously constrain size
• Repeaters needed

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Star (Hub) topologies
Standard Ports

• A Hub is a multi-port repeater
• Reads input from any one of its ports
• Repeats it to all other ports

Crossover Port

• Systems built around a hub usually have a star
  topology
• Usually use 10Base-T or 100Base-T
• 10/100Mbps, Baseband, UTP
• RJ-45 Connectors

• Logically, the same as a bus
• Medium is still shared by all stations at all
  times

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Star (Hub) Pros and Cons
Disadvantages
Advantages

• Termination is no longer an issue
• An un-terminated line only effects communication
  to one device
• Cant bring the network down by unplugging your
  network connection

• Single-point failure
• If the hub goes down, all is lost

• More cable needed
• All cables must reach from workstations to the
  hub

• Expansion is easier
• Active hubs reduce loading limitations

• Easier troubleshooting
• Isolated connections

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Star-Star Multi-Hub LANs
Star-Star

• Connect multiple stars (hubs) together in a bus
  or star
• Isolates individual groups from each other
• Hierarchical

• Logically, still one single shared medium

A Star-Star hub network is logically the same as
a bus network
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Connecting Hubs

• Connection from hub-to-hub requires a crossover
  port or crossover cable

Crossover Port
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Ethernet Physical Media and Signals

• Most widely-accepted Ethernet standards use the
  same medium and connectors

• 4-pair UTP Cat 5e is the most common today

• RJ-45 connectors

• Cable length of up to 100m

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Ethernet 10BaseT

• 10BaseT

• Uses Manchester coding at 10Msps to send 10Mbps

• Uses only 2 of the 4 pairs one for each
  direction

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Ethernet 100BaseTX

• 100BaseTX

• For error correction, uses a 4B/5B code sends 5
  bits to represent 4 actual bits (80 efficient)

• Uses MLT-3 3-level coding (similar to AMI) at
  125M symbols/s
• 125M symbols/s x 4/5 bits/symbol 100Mbps

• Uses only 2 of the 4 pairs one for each
  direction

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Ethernet 1000BaseT

• 1000BaseT

• Each UTP pair uses a 5-level coding (2-bits per
  symbol) at 125M symbols/s to send 250Mbps
• Warning this reduces the tolerable SNR by 6dB,
  which increases the error rate

• Uses all four pairs at the same time 4 x
  250Mbps 1Gbps in one direction

• Uses simultaneous bi-directional signaling (echo
  cancellation) to send signals at the same time in
  both directions on the wire 1Gbps in both
  directions

• Uses a complex coding scheme (Trellis coding) to
  decrease the error rate. This is equivalent to
  adding back 6dB to the SNR

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Ethernet 10GBaseT

• 10GBaseT
• 10Gbps over twisted pair
• 45m over Cat-5e UTP
• 55m over Cat-6 UTP
• 100m over Cat-6a UTP

• Fiber Several standards using single- or
  multi-mode fiber