MultipleAccess.ppt
- 1. Multiple Access 01204325: Data Communication and Computer Networks Asst. Prof. Chaiporn Jaikaeo, Ph.D. chaiporn.j@ku.ac.th http://www.cpe.ku.ac.th/~cpj Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from lecture slides by Behrouz A. Forouzan © The McGraw-Hill Companies, Inc. All rights reserved
- 2. 2 Outline Multiple access mechanisms Random access Controlled access Channelization
- 3. 3 Sublayers of Data Link Layer
- 6. 6 Random Access Also called contention-based access No station is assigned to control another
- 8. 8 Frames in Pure ALOHA
- 10. 10 Example Calculate possible values of TB when stations on an ALOHA network are a maximum of 600 km apart Tp = (600 × 103) / (3 × 108) = 2 ms When K=1, TB {0ms,2ms} When K=2, TB {0ms,2ms,4ms,6ms} :
- 12. 12 ALOHA: Throughput Assume number of stations trying to transmit follow Poisson Distribution The throughput for pure ALOHA is S = G × e−2G where G is the average number of frames requested per frame-time The maximum throughput Smax = 0.184 when G= 1/2
- 13. 13 Example A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces 1000 frames per second 500 frames per second 250 frames per second
- 14. 14 Slotted ALOHA
- 15. 15 Slotted ALOHA: Vulnerable Time
- 16. 16 Slotted ALOHA: Throughput The throughput for Slotted ALOHA is S = G × e−G where G is the average number of frames requested per frame-time The maximum throughput Smax = 0.368 when G= 1
- 17. 17 Example A Slotted ALOHA network transmits 200- bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces 1000 frames per second 500 frames per second 250 frames per second
- 18. 18 CSMA Carrier Sense Multiple Access "Listen before talk" Reduce the possibility of collision But cannot completely eliminate it
- 21. 21 Persistence Methods What a station does when channel is idle or busy
- 23. 23 CSMA/CD Carrier Sense Multiple Access with Collision Detection Station monitors channel while sending a frame
- 24. 24 Energy Levels
- 25. 25 CSMA/CD: Minimum Frame Size Each frame must be large enough for a sender to detect a collision Worst case scenario: "A" is transmitting "D" starts transmitting just before A's signal arrives A B C D Long enough to hear colliding signal from D
- 26. 26 Example A CSMA/CD network has a bandwidth of 10 Mbps. If the maximum propagation time is 25.6 μs, what is the minimum size of the frame?
- 28. 28 CSMA/CA Carrier Sense Multiple Access with Collision Avoidance Used in a network where collision cannot be detected E.g., wireless LAN IFS – Interframe Space
- 29. 29 CSMA/CA: Flow Diagram contention window size is 2K-1 After each slot: - If idle, continue counting - If busy, stop counting
- 31. 31 Control Access A station must be authorized by someone (e.g., other stations) before transmitting Three common methods: Reservation Polling Token passing
- 34. 34 Token Passing
- 35. Channelization
- 36. 36 Channelization Similar to multiplexing Three schemes Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA)
- 37. 37 FDMA
- 38. 38 TDMA
- 39. 39 CDMA One channel carries all transmissions at the same time Each channel is separated by code
- 40. 40 CDMA: Chip Sequences Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors Inner product of any pair must be zero With N stations, sequences must have the following properties: They are of length N Their self inner product is always N
- 43. 43 CDMA Encoding
- 44. 44 Signal Created by CDMA
- 45. 45 CDMA Decoding
- 46. 46 Sequence Generation Common method: Walsh Table Number of sequences is always a power of two
- 47. 47 Example: Walsh Table Find chip sequences for eight stations
- 48. 48 Example: Walsh Table There are 80 stations in a CDMA network. What is the length of the sequences generated by Walsh Table?