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Mesh networks
LAN technologies and network

topology

• Early local networks used dedicated links between each pair of computers
LANs and shared media Locality of reference
• Some useful properties

– hardware and frame details can be tailored for each link

Star, bus and ring topologies Medium access control protocols

– easy to enforce security and privacy

Disadvantages of meshes
Links between rooms/buildings

• Poor scalability
• Many links would follow the same physical path

Locality of reference
Shared Communication Channels

• LANs now connect more computers than any other form of network
• Shared LANs invented in the 1960s • Rely on computers sharing a single medium • Computers coordinate their access • Low cost
• The reason LANs are so popular is due to the principle of locality of reference

– physical locality of reference - computers more

• But not suitable for wide area -

likely to communicate with those nearby

communication delays inhibit coordination

– temporal locality of reference - computer is more likely to communicate with the same computers repeatedly

1

LAN topologies

• LANs may be categorised according to topology

ring bus

star

  • Pros and cons
  • Example bus network: Ethernet

  • • Star is more robust but hub may be a
  • • Single coaxial cable - the ether - to which

  • bottleneck
  • computers connect

• Ring enables easy coordination but is sensitive to a cable being cut
• IEEE standard specifies details

– data rates – maximum length and minimum separation – frame formats

• Bus requires less wiring but is also sensitive to a cable being cut

– electrical and physical details

  • Conceptual flow of Ethernet data
  • Ethernet coordination

• The computers can detect when a signal is

on the Ether - carrier sense

• Can only transmit when the Ether is free -

carrier sense with multiple access (CSMA)

• Prevents a computer interrupting an ongoing transmission
• Transmitter has exclusive use of the medium

2

Collision detection
Collision recovery

• Collisions can occur if computers decide to transmit at the same time
• Computers must wait after collision before retransmission
• Each computer also senses for garbled

transmission - a collision
• Choose random delay up to specified max • Double the delay for each subsequent collision - binary exponential backoff
• Ethernet mechanism is called - carrier

sense multiple access with collision detect - CSMA/CD

• Difference between bandwidth and throughput

• Example of a medium access control (MAC)

protocol

What to do if the medium is busy?
Throughput vs. offered traffic

• Non persistent CSMA (deferential)

– if medium is idle, transmit

• More offered traffic results in more collisions, more backing-off and eventually congestion and reduced throughput

– if busy, wait a random time then try again

• 1-persistent CSMA (selfish)

– if idle, transmit – if busy, listen until idle then transmit

• p-persistent (compromise)

– if idle transmit with probability p and delay one time unit with probability (1-p)

Wireless LANs and CSMA/CA
Example bus network -

LocalTalk

• Collision detection does not work because a transmission from one computer may only be received by its immediate neighbours

• LAN technology for Apple computers

• Solution is collision avoidance

• MAC protocol is CSMA/CA (collision

avoidance)

– sender sends small request message to receiver

– receiver responds with a ‘clear to send’ message that

received by all adjacent computers

• Each computer first sends a small message to reserve the bus

3

• Special message called the token grants

Example ring network -
IBM Token Ring

permission (needs bit stuffing)
• Computer grabs token, removes it, sends one frame, checks for errors then replaces it
• MAC protocol based on token passing

• Computer must wait for permission before transmitting
• Computer controls the ring until finished • Data flows right round the ring

– receiver makes a copy – transmitter checks for errors and then removes

Example star network - ATM
Example ring network - FDDI

• Overcomes token ring susceptibility to failure through two counter-rotating cables
• Asynchronous

Transfer Mode
• Uses pairs of optical fibres to connect computers to a central hub

Summary

• Locality of reference • Shared medium • Star, bus and ring topologies • Medium access control protocols

4

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