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Local Area Networks (LANs)
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Introduction
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  • Definition:
    • A shared transmission medium owned by a single individual/organization and hardware/software for interfacing devices to the medium
  • Issues to Consider:
    • Topolgies
    • Access Methods
Bus Topology
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  • Components:
    • Single cable (called a trunk/backbone)
    • Terminators at each end of the cable
    • Each device connects to the trunk with a tap
  • Properties:
    • Signals travel in both direction from the insertion point
    • The signal is stopped by the terminators
    • Devices are passive (i.e., do not re-transmit signals)
Ring Topology
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  • Components:
    • One cable
  • Properties:
    • Signals are sent in one direction
    • Devices are active (i.e., re-transmit signals)
    • If one device fails, the network fails (unless a shunt is used)
    • Modifications disrupt service
Star Topology
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  • Components:
    • One cable for each device
    • A central hub
  • Properties:
    • Devices must communicate through the hub
    • Easy to modify
LAN Access Methods
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  • The Issue:
    • Only one device can access the LAN (e.g., trunk, hub) at a time
  • Gaining Intuition:
    • What access methods are used in a classroom?
  • Approaches in LANs:
    • Token Passing
    • Contention Detection/Resolution (e.g., Carrier Sense Multiple Access/Collision Detection)
Token Passing
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  • A special packet (called a token) is used to control access to the network
  • When a device wants to transmit data it "takes" the token (which usually means that it doesn't re-transmit it) and transmits the data packet
  • The data packet is forwarded until it reaches the intended recipient who reads it, marks it as read and re-transmits it (so that the sender can know it was received and generate a new token)
Contention Resolution (e.g., ALOHA, CSMA/CD)
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  • Any device can send information if the network is clear
  • This means that collisions are possible; when a collision occurs, the devices wait a random amount of time and try again
Examples
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  • Token Ring:
    • Uses token passing on a ring network
    • It is deterministic (i.e., the maximum delay is known) if there is a maximum token hold time
  • Ethernet (Metcalfe and Boogs, 1976):
    • Used contention resolution on a bus network (to connect PCs, printers and shared disks)
  • Wi-Fi (IEEE802.11) Infrastructure:
    • Uses a star network
There's Always More to Learn
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