LAN Wiring, Physical Topology, and Interface Hardware

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LAN Wiring, Physical Topology, and Interface
Hardware
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The Need for Speed

• Computers and LAN operate at different speeds
• Computers get faster
• Shouldnt operate at speed of slowest
• Doesnt make sense to involve expensive CPU in
  easy, repetitive, tasks

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Speed Differences

• NICs can work more quickly than CPU
• CPU not involved in individual frames
• Network Interface Card does this work
• Plugs into backplane or special socket

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Back of Computer
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Hardware Map of a Standard Computer
I/O B U S
Central Processing Unit
NETWORK
Bridge
System Bus
Network Interface HW (NIC)
Main Memory DRAM
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Inside the CPU
Performs calculations and decisions
Arithmetic / Logic Unit
Coordinates processing steps
Control Unit
Small storage areas (program counter, IR, the
stack, etc.)
Registers
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DMA

• Direct Memory Access
• Allows NIC to move data directly into memory
• Completely removes CPU from transfer
• NIC looks like all other I/O devices
• Very general interface

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DMA Continued

• To send, CPU puts information into memory and
  notifies NIC
• CPU continues with other work
• To receive, CPU allocates buffer for NIC to write
  to
• NIC grabs frames destined for this computer,
  checks, copies, and interrupts CPU

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10base5

• Original wiring large coax cable typically in
  straight line away from computers (ceiling, etc.)
• Called Thicknet analog transmission on the wire
• Thicknet NICs did not have analog signaling
  capability
• Instead connected to transceiver that was
  typically remote from computer (and rest if NIC)
• Connected via Attachment Unit Interface (AUI)

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10base5 Continued

• Each end of cable has terminator
• Resistor that connects signal and shield
• Reduces reflections

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Multiplexing

• Reduces the number of AUI connections from the
  cable
• 10base5 has minimum spacing
• Multiplexor allows multiple machines to be
  connected to one AUI connection share one
  transceiver
• MUX replicates all signals exactly

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10base2

• Thinnet thinner more flexible coax cable
• Costs less to install and operate than 10base5
• NICs have built-in transceiver so external one
  not needed
• No AUI cable to remote medium medium goes right
  up to computers and connects to each via BNC
• Like 10Base5,
• requires termination, uses coax shielding, same
  physical bus architecture.
• Same resistance and capacitance, signals
  propagate same way.

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Thinnet
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Twisted Pair

• AKA 10baseT
• Connected at computer end (to NIC) with RJ-45
  (larger, but similar to telephone connectors)
• Connects all computers to a hub via twisted-pair
  wiring to an Ethernet hub
• While there really is no shared physical medium,
  the hardware in the hub simulates a common
  physical cable operates like conventional
  Ethernet
• Hubs are similar to multiplexing but endpoints
  operate as normal Ethernet (CSMA/CD, Ethernet
  Framing, etc.)
• CPU and software doesnt know difference between
  connection techniques

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Pluses and Minuses

• Cost the major factor but 10BaseT has proved
  generally cheaper
• Cost varies with number of computers, distance
  between, type of existing wiring, reliability,
  etc.
• Can have more than one wiring scheme

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The Topology Paradox

• Is 10baseT Ethernet a bus or a star?
• Can be both
• Hub operates as a bus
• Switch makes a star

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NIC

• A NIC can have more than one type of connector
• NICs commonly made to support all three types
• Only one is active at a time
• Software chooses which is active

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