Lecture 8: Computer Networks - CSMA/CD, Token Ring, FDDI, and ATM, Slides of Computer Networks

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COMPUTER

NETWORKS

Lecture-

Review Lecture 7

 Shared Communication Channel

 Locality of Reference Principle

 LAN Topologies

• Star
• Ring
• Bus

 Ethernet

Collision Detection - CD

 Signals from two computers will interfere with each other

 Overlapping frames is called a Collision

• No harm to hardware
• Data from both frames is Garbled

Ethernet CD

 Ethernet interfaces include hardware to detect transmission

• Monitor outgoing signal
• Garbled signal is interpreted as a collision

 After collision is detected, computer stops transmitting

 So, Ethernet uses CSMA/CD to coordinate transmissions

Recovery from Collision

 After waiting, computers use carrier sense to avoid subsequent collision

• Computer with shorter delay will go first
• Other computers may transmit later

Exponential Backoff

 Even with random delays, collisions may occur

 Especially likely with busy segments

 Computers double delay with each subsequent collision

 Reduces likelihood of sequence of collisions

Limited Connectivity with

Wireless

 In contrast with wired LAN, not all participants may be able to reach each other

• Low signal strength
• Propagation blocked by walls, etc.

 Can't depend on CD; not all participants may hear

Limited Connectivity with

Wireless

Collisions

 Receiver may receive simultaneous requests

• Results in collision at receiver
• Both requests are lost
• Neither transmitter receives reservation; both use backoff and retry

 Receiver may receive closely spaced requests

• Selects one
• Selected transmitter sends message
• Transmitter not selected uses backoff and retries

Local Talk

 LAN technology invented by Apple that uses bus topology

 Interface included with all Macintosh computers

 Relatively low speed - 230.4Kbps

 Low cost (``free'' with a Macintosh); easy to install and connect

 Uses CSMA/CA

Token Ring

 Hardware must be designed to pass token even if attached computer is powered down

Using the token

 When a computer wants to transmit, it waits for the token

 After transmission, computer transmits token on ring

 Next computer ready to transmit receives token and then transmits

Token and Synchronization

 Token gives computer permission to send one frame

• If all ready to transmit, enforces ``Round- Robin'' access
• If none ready to transmit, token circulates around ring

IBM Token Ring

 Very widely used

 Originally 4mbps, now 16Mbps

 Uses special connector cable between computer and ring interface