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CIS 725

CIS 725. Media Access Layer. Medium Access Control Sublayer. MAC sublayer resides between physical and data link layer Broadcast/multiacess channels N independent stations - each station generates traffic independently - if two transmit at the same time, both frames

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CIS 725

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  1. CIS 725 Media Access Layer

  2. Medium Access Control Sublayer • MAC sublayer resides between physical and data link layer • Broadcast/multiacess channels • N independent stations - each station generates traffic independently - if two transmit at the same time, both frames are garbled

  3. Medium Access Control Sublayer • MAC sublayer resides between physical and data link layer • Broadcast/multiacess channels • N independent stations - each station generates traffic independently - if two transmit at the same time, both frames are garbled

  4. Static Allocation • Frequency division multiplexing • Time division multiplexing

  5. Pure ALOHA • A station transmits whenever it wants • Sender detects collision and retransmits after random time

  6. Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

  7. Pure ALOHA Vulnerable period for the shaded frame.

  8. Slotted Aloha • Time is divided into slots • Each station waits until beginning of next slot before transmitting

  9. Pure ALOHA (3) Throughput versus offered traffic for ALOHA systems.

  10. Carrier Sensing • Ability to detect if channel is busy • CSMA Protocols (Carrier Sense Multi Access) • Messages must be long enough to detect collision

  11. 1-persistent CSMA • Listen to the channel • If busy then wait until channel is idle • When idle, transmit frame • If collision then start again after random time

  12. Non-persistent CSMA • Sense the channel • If idle then transmit else start over again after random time If collision then start again after random time

  13. p-persistent CSMA • Listen to the channel • If idle then transmit with probability p else wait for random amount of time

  14. Persistent and Nonpersistent CSMA

  15. Collision-free protocols • Stations are numbered 0..N-1 - 1-bit contention slots are used to determine who wants to transmit

  16. CAN (Controller area network) protocol • Priority-based arbitration mechanism • Message id = priority • For each message, the id is first transmitted Message with the lowest id wins M1 = 0 1 0 0 M2 = 1 1 0 1 M3 = 0 0 1 0 M1 = 0 M2 = 1 M3 = 0 1 0 0

  17. Token Ring • Stations are arranged in a ring • A token circulates in the ring

  18. token address 1-bit delay • To send data, acquire the token; place data on the ring; when data comes back, insert token back • Listen mode: copy input bit to output

  19. Node failures - sender fails Corruption - Full  empty 1  0 - empty  Full 0  1 * cannot include parity/checksum

  20. token New/old Full, new Empty, * Full, new Full, old Empty, * Master node

  21. token New/old Full, new Full, new Full, old Full, new Full, new Sender fails

  22. token New/old Full, new Full, new Full, new Empty, old Full, new Full, old Corruption: empty  full

  23. token New/old empty, old empty, old Full, old Empty, old empty, old Corruption: Full  empty

  24. token New/old Full, new Empty, new Full, new Full, old Full, old empty, old Full, old

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