9 multiple access


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9 multiple access

  1. 1. Multiple Access Komunikasi Data Adapted from lecture slides by Behrouz A. Forouzan © The McGraw-Hill Companies, Inc. All rights reserved Anhar, ST., MT anhar19@gmail.com http://anhar.staff.unri.ac.id Jurusan Teknik Elektro Univ. Riau
  2. 2. Outline     Multiple access mechanisms Random access Controlled access Channelization 2
  3. 3. Data Link Layer
  4. 4. Sublayers of Data Link Layer 4
  5. 5. Multiple Access Links and Protocols Three types of “links”:  Point-to-point (single wire, e.g. PPP, SLIP)  Broadcast (shared wire or medium; e.g, Ethernet, WiFi/WaveLAN, etc.)  Switched (e.g., switched Ethernet, ATM etc)
  6. 6. Multiple access problem     Example: Classroom– many people gather together in a large room Broadcast medium – air Human protocols:       “Give everyone a chance to speak” “Don’t speak until you are spoken to” “Don’t monopolize the conversation” “Raise your hand if you have a question” “Don’t interrupt when someone is speaking” “Don’t fall asleep when someone else is talking”
  7. 7. Multiple access protocols     In LANs, WiFi, satellite networks If more than 2 users send @ the same time collision All collided packets are lost -> waste of bandwidth Ideally, the MAC protocol for a broadcast channel with the bit-rate R bps should satisfy:     if only 1 node is sending than the throughput is R when M nodes have data to send than the throughput is R/M decentralized protocol – no master simple & inexpensive to implement
  8. 8. MAC Protocols: Taxonomy Three broad classes:  Channel Partitioning    Random Access    divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use allow collisions “recover” from collisions “Taking turns”  tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized
  9. 9. Multiple Access Mechanisms 9
  10. 10. Random Access
  11. 11. Random Access   Also called contention-based access No station is assigned to control another 11
  12. 12. Random Access Protocols  In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send.
  13. 13. Random Access   1. 2. 3. 4. If more than one station wants to send, there is an access conflict -- Collision— To avoid access conflict each station has to follow procedure that will answers the following questions: When can the station access the medium? What can the station do if the medium is busy? How can the station determine the success or failure of the transmission What can the station do if there is an access conflict?
  14. 14. ALOHA Network  Developed by Norm Abramson at the Univ. of Hawaii    the guy had interest in surfing and packet switching mountainous islands → land-based network difficult to install fully decentralized protocol ACK ACK ACK ACK
  15. 15. Frames in Pure ALOHA 15
  16. 16. ALOHA Protocol 16
  17. 17. 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}  : 17
  18. 18. ALOHA: Vulnerable Time 18
  19. 19. 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 19
  20. 20. 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 20
  21. 21. Slotted ALOHA 21
  22. 22. Slotted ALOHA: Vulnerable Time 22
  23. 23. 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 23
  24. 24. Example  A Slotted ALOHA network transmits 200bit 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 24
  25. 25. Multiple Access Protocols ALOHA
  26. 26. CSMA  C arrier S ense M ultiple A ccess   "Listen before talk" Reduce the possibility of collision  But cannot completely eliminate it 26
  27. 27. Collision in CSMA 27
  28. 28. CSMA: Vulnerable Time 28
  29. 29. Persistence Methods  What a station does when channel is idle or busy 29
  30. 30. Persistence Methods 30
  31. 31. CSMA/CD   C arrier S ense M ultiple A ccess with C ollision D etection Station monitors channel when sending a frame 31
  32. 32. Carrier Sensing Multiple Access with Collision detection (CSMA-CD)  Jika station dp mengetahui apakah collision terjadi, maka bandwith yang terbuang dpt dikurangi dengan menghentikan transmisi
  33. 33. Carrier Sensing Multiple Access with Collision detection (CSMA-CD)
  34. 34. Carrier Sensing Multiple Access with Collision detection (CSMA-CD)    Station yang mempunyai paket mendeteksi kanal dan transmit jika kanal idle Jika kanal sibuk, gunakan strategi dari CSMA (persist, backoff segera atau persist dengan prob. p) Jika collision terdeteksi saat transmisi, sinyal short jamming ditransmisikan untuk meyakinkan semua station mengetahui terjadi collision sebelum menghentikan transmisi, selanjutnya algoritma backoff digunakan untuk rescheduling waktu resensing
  35. 35. Carrier Sensing Multiple Access with Collision detection (CSMA-CD)  Kanal mempunyai 3 kondisi (state):      sibuk mentransmisikan frame idle perioda contention (dimana station berusaha menduduki kanal) Throughput 1-Persistent CSMA-CD dapat dianalisa dg asumsi waktu dibagi dalam minislot sebesar 2tprop det (untuk menjamin station selalu dapat mendeteksi collision) Setiap kanal menjadi idle, station memperebutkan (contend) kanal dengan transmit dan mendengar untuk mengetahui apakah sukses menduduki kanal
  36. 36. 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 36
  37. 37. 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? 37
  38. 38. CSMA/CD: Flow Diagram 38
  39. 39. Multiple Access Protocols ALOHA
  40. 40. CSMA/CA   C arrier S ense M ultiple A ccess with C ollision A voidance Used in a network where collision cannot be detected  E.g., wireless LAN IFS – Interframe Space 40
  41. 41. CSMA/ CA In wireless networks collision is avoided.  Collisions are avoided through following strategies: 1. Interframe Space 2. Contention Window 3. Acknowledgment 
  42. 42. CSMA/CA: Flow Diagram contention window size is 2K-1 After each slot: - If idle, continue counting - If busy, stop counting 42
  43. 43. Controlled Access
  44. 44. Control Access   A station must be authorized by someone (e.g., other stations) before transmitting Three common methods:    Reservation Polling Token passing 44
  45. 45. Reservation   A station must make a reservation before sending data Time is divided into intervals    A reservation frame proceeds each time interval Number of stations and number of time slots in the reservation frame are equal Each time slot belongs to a particular station
  46. 46. Polling  Devises are categorized into:       All data exchange must go through the primary station Primary station controls the link and initiates the session Secondary station obey the instructions of PS. PS polls stations   Primary station (PS) Secondary station (SS) Asking SS if they have something to send PS select a SS  Telling it to get ready to receive data
  47. 47. Poll procedure
  48. 48. Select procedure
  49. 49. Token passing       the stations in a network are organized in a logical ring for each station, there is a predecessor and a successor for a station to access the channel, it must posses a token (special packet) that gives the station the right to access the channel and send its data once the station has finished its task, the token will then be passed to the successor (next station) the station cannot send data until it receives the token again in the next round token management is necessary     Every station is limited in the time of token possession Token must be monitored to ensure no lose or destroyed Assign priorities to the stations and to the types of data transmitted To make low-priority stations release the token to high priority stations
  50. 50. Token Passing procedure
  51. 51. Token passing  Logical Ring  in a token passing network, stations do not have to be physically connected in a ring; the ring can be a logical one.
  52. 52. Channelization
  53. 53. Channelization   Similar to multiplexing Three schemes    Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA) 53