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

17. 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 1010 0

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

19. 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 tokenaddress 1-bit delay

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

21. tokenNew/old Full, old Full, new Empty, * Master node Empty, *

22. tokenNew/old Full, old Full, new Sender fails Full, new

23. tokenNew/old Full, new Empty, old Full, old Corruption: empty  full Full, new

24. tokenNew/old Full, old empty, old Empty, old empty, old Corruption: Full  empty

25. tokenNew/old Full, old Full, new Empty, new empty, oldFull, old