Centralized MAC and Switching in Communication Networks - Prof. Thomas C. Clancy, Study notes of Organizational Communication

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Centralized MAC and Switching

Multiple Access

• Infrastructure:
  • Control access to uplink
• Distributed:
  • Control access to shared medium
  • Distributed networks are half-duplex
  • No central controller -> users = network

Token Ring

• Token frame (no payload) circulates network until a device needs to transmit
• Transmitting device receives token, attaches payload, transmits
• Packet circulates around the network
• When it returns to the originating device, the payload is removed and the token is forwarded on

Token Ring Performance

• Deterministic
  • Same amount of time to transmit every packet
  • Finite maximum amount of time between transmissions
• Compared to CSMA/CD
  • Better overall performance
  • No chance of collisions
  • But more overhead to maintain tokens
• Switched Ethernet put Token Ring out of

business

FDDI and RPR

• FDDI was fiber version of token ring
• Initial interest, but died out
• RPR is ring-like topology for SONET/SDH
• Used in some MANs

Centralized MAC

• Centralized star topologies
• All devices communicate with a single relay

node rather than with each other

• Wireless: base station
• Ethernet: switch
• Wired networks: each user has own channel
• Wireless: shared channels

Comparison

• FDMA
  • Typically requires fixed-width channels
  • Good for circuit switched networks
  • Too few users, wasted capacity
  • Too many users, some users receive no service
• TDMA
  • Size of time blocks typically fixed
  • Vary who gets assigned to which blocks when
  • Users get 1/n of the available bandwidth
  • If underutilized, others can increase speed
• CDMA
  • Power levels fixed
  • More users means more interference, so more coding necessary, reducing overall rate

Switched Networks

• Virtual Circuits
  • Connection-oriented communications on packet-switched network
  • Network preconfigured with identifiers for source-destination pairs
  • Virtual Circuit IDs (VCIs)
  • Packets transmitted with virtual circuit ID in the header
  • Devices map this VCI to physical interfaces as the packet transits to properly route it
• Good for QoS
  • Can guarantee enough capacity to support all the VCs at particular rate
• Bad for dynamic networks
  • Overhead associated with setup/teardown of VCs

Switched Networks

• Packet Switching
  • Dumb hosts, but smart network
  • Hosts address packets, network figures out how to deliver them
• Bridges/Switches
  • Learn location of MAC addresses through bridging tables - Associate MAC addresses with certain ports - Bridging loops?

Bridging/Switching Loops

Spanning Tree Protocol

• Each bridge has unique ID and priority number
• Basic Idea
  • Elect a root bridge (lowest priority)
  • Each bridge computes least-cost path from itself to the root bridge (provisions for breaking ties)
  • Cost is function of link speeds
• Protocol
  • STP has unique MAC address (broadcast to other switches)
  • Every two seconds STP messages are exchanged
    • Weight to believed root bridge
    • Notification of topology change

STP Example