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Chapter 3 MAC (Media Address Control) Layer

Chapter 3 MAC (Media Address Control) Layer. Chapter 3 Outline. 3.1. 802.11 碰撞議題相關研究 3.2. 802.11 MAC 機制 3.3. 802.11 節能、省電議題相關研究 3.4. 802.15.4 MAC 3.5. MAC protocols for WSN. Chapter 3 Outline. 3.1. 802.11 碰撞議題相關研究 3.2. 802.11 MAC 機制 3.3. 802.11 節能、省電議題相關研究

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Chapter 3 MAC (Media Address Control) Layer

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  1. Chapter 3 MAC (Media Address Control) Layer

  2. Chapter 3 Outline • 3.1. 802.11 碰撞議題相關研究 • 3.2. 802.11MAC機制 • 3.3. 802.11 節能、省電議題相關研究 • 3.4. 802.15.4 MAC • 3.5. MAC protocols for WSN

  3. Chapter 3 Outline • 3.1. 802.11 碰撞議題相關研究 • 3.2. 802.11MAC機制 • 3.3. 802.11 節能、省電議題相關研究 • 3.4. 802.15.4 MAC • 3.5. MAC protocols for WSN

  4. Collision avoidance Reservation based TDMA、FDMA、CDMA (Slotted)ALOHA、CSMA、MACA Contention based DAMA Hybrid

  5. Reservationbased • TDMA → 一個點可以用到的較多頻寬，輪到時間較短。 F(頻帶) 1234…n 1 T(時間)

  6. Reservationbased Guard Band • FDMA → 一個點可以一直傳送，但頻寬較少。 F(頻帶) T(時間)

  7. Reservationbased • CDMA CDMA can transmission in the same space and time Code Time Frequency FDMA、TDMAcan use resource

  8. Contention based • Pure ALOHA 當想要傳送Data時就直接往外傳送。 特點：traffic load low → 成功率高，反之碰撞率高 • Slotted ALOHA 加入slotted概念，在每個slot的開始點才可以傳送。 特點：改善了隨時隨地都有可能有結點來撞封包的缺點。 S (Throughput per Packet Time) 0.4 Slotted ALOHA 0.3 0.2 Pure ALOHA 0.1 0 0.5 1.0 1.5 2.0 3.0 G (Attempts per Packet Time)

  9. Contention based • 1-persistent CSMA • When medium is • Idle → Transmit • Busy → Continue listening(Carrier Sense) • Non-persistent CSMA • When medium is • Idle → transmit • Busy →Wait an amount of time drawn from a probability distribution and repeat to listen

  10. Contention based • p-persistent CSMA • When medium is • Idle → transmit probability： • transmit probability : p • defer probability : 1−p • Busy → listen until medium is idle Note： For 1-persistent CSMA Transmit probability 1) transmit probability : 1 2)defer probability : 0

  11. Contention based • MACA (Multiple Access with Collision Avoidance) • NAV (Network Allocation Vector) RTS CTS GET RTS- Can transmit but can’t receive Disadvantage ： GET CTS- Can receive but can’t transmit Can’t check frame GET CTS and RTS- Can’t transmit and receive transmission success or not Sender Receiver Sender Receiver

  12. Hybrid • DAMA (Demand Assigned Multiple Access) Two phases 1)Contention-based: use slotted ALOHA 2)Reservation-based: use reservation list Disadvantage : Maintain reservation list Slotted ALOHA Slotted ALOHA Slotted ALOHA reserved reserved time

  13. Chapter 3 Outline • 3.1. 802.11 碰撞議題相關研究 • 3.2. 802.11MAC機制 • 3.3. 802.11 節能、省電議題相關研究 • 3.4. 802.15.4 MAC • 3.5. MAC protocols for WSN

  14. MAC • Medium Access Control(MAC) • 無線網路中主要的功能為 • 碰撞控制 • 存取控制 • 排程機制 • 醒睡省電機制 (Wireless STD)

  15. 802.11訊框結構(Frame Structure) 2 byte 2 byte 6+6+6 byte 2 byte 6 byte 0 ~ 2312 byte 4 byte Frame control Duration Address 1 ~ 3 Seq. Address 4 Data Checksum Version Type Subtype To DS From DS MF Retry Pwr. W O 1 bit 2 bit 2 bit 4 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit

  16. 802.11訊框結構(Frame Structure) Version Type Subtype To DS From DS MF Retry Pwr. W O Different type for each frame type (EX-in type control has subtype -CTS/RTS) Frame type (Data、Control、Management)

  17. 802.11訊框結構(Frame Structure) Version Type Subtype To DS From DS MF Retry Pwr. W O ESS BSS2 BSS1 STA STA STA STA STA IBSS STA AP1 AP2 Distribution System To DS =0 From DS =0 To DS =0 From DS =1 To DS =1 From DS =0 Portal To DS =1 From DS =1 802.X (EX:802.3、802.16)

  18. 802.11訊框結構(Frame Structure) Version Type Subtype To DS From DS MF Retry Pwr. W O More fragment? Retransmit ? Sleep ?

  19. 802.11訊框結構(Frame Structure) 2 byte 2 byte 6+6+6 byte 2 byte 6 byte 0 ~ 2312 byte 4 byte Frame control Duration Address 1 ~ 3 Seq. Address 4 Data Checksum Duration of frame Four address： (by To DS/ From DS) Source Address(SA) Destination Address(DA) Transmitter Address(TA)–(now address) Receiver Address(RA)–(next address)

  20. 競爭式服務 (非同步傳輸) 免競爭式服務 (具時限傳輸) Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF) MAC Architecture

  21. MAC Architecture • Distributed Coordination Function (DCF) • The fundamental access method for the 802.11 MAC, known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). • Shall be implemented in ALL stations and APs. • Used within both ad hoc and infrastructure configurations. • Point Coordination Function (PCF) • An alternative access method • Shall be implemented on top of the DCF • A point coordinator (polling master) is used to determine which station currently has the right to transmit. • Shall be built up from the DCF through the use of an access priority mechanism.

  22. 802.11傳遞模式 Super frame Super frame PCF period DCF period AP time Beacon CF_END Beacon STA1 NAV STA2 PCF週期中沒拿到資料傳送權的STA ，會進入NAV休息狀態 PCF period ,根據排程好的傳送者進行傳送 DCF period ,節點與節點間傳送是互相競爭傳送權的

  23. 802.11傳遞模式 - PCF週期 • DL-下傳封包 • ACK-回應封包 • Polling-詢問是否有資料上傳 • UL-上傳封包 • 沒傳完的資料怎辦？ • 去DCF競爭 or 等待下一個PCF(DCF沒競爭到) PCF Beacon ACK DL Polling ACK DL Polling ACK Polling AP time STA1 ACK UL UL STA2 UL ACK

  24. 802.11傳遞模式 - PCF週期 Defer beacon The beginning of DCF CF_END Beacon AP STA1 Data time STA2 Data Data DIFS (DCF Interframe Space ), 一段固定的等待時間 Random backoff ,亂數等待時間 PIFS (PCF Interframe Space ), 一段固定的等待時間 , (DIFS>PIFS) 24

  25. Piggyback機制 • Problem in Original PCF ? • 封包來回傳遞太多次，浪費資源。 • One frame in multi-message • Piggyback Beacon ACK+ DL2+ Polling2 ACK+ DL3+ Polling3 DL1+ Polling1 DL1+ Polling1 CF_END AP STA1 ACK+ UL1 ACK+ UL1 STA2 time ACK+ UL2 STA3沒回ACK (超過PIFS認定他不在) PIFS (PCF Interframe Space )

  26. DCF Operation • MAC begins frame transmission • If both PHY and virtual carrier sense mechanisms indicate the medium is idle for an interval of DIFS (or EIFS if previously received frame contained errors). • If medium is busy during the DIFS interval, • Backoff interval is selected and increment retry counter • For each slot time, if medium is detected to be idle, decrement backoff interval; MAC begins to transmit if backoff interval is expired. • If the transmission is not successful (I.e. collision), CW is doubled and new backoff interval is selected and countdown is begun, again. When to stop?

  27. Example of Backoff Intervals (2) (3) Backoff=2 DIFS DIFS Backoff=9 DIFS Backoff=4 DIFS (5) busy Station 1 Backoff=5 Packet arrival at MAC busy Station 2 (1) busy Station 3 Backoff=7 Backoff=2 (4) busy Station 4 • After packet arrival at MAC, station 3 senses medium free for DIFS, so it starts transmission immediately (without backoff interval). • For station 1,2, and 4, their DIFS intervals are interrupted by station 3. Thus, backoff intervals for station 1,2, and 4, are generated randomly (i.e. 9,5, and 7, respectively). • After transmission of station 2, the remaining backoff interval of station 1 is (9-5)=4. • After transmission of station 2, the remaining backoff interval of station 4 is (7-5)=2. • After transmission of station 4, the remaining backoff interval of station 1 is (4-2)=2.

  28. Random backoff 機制 • Backoff Counter ： • when network busy → B.C. freeze • network idle → B.C. decrease BC=3 BC=5 STA1 STA2 BC=3 STA3 BC=5 BC=2 STA4 DIFS

  29. DCF: the Random BackoffTime CWmax 255 255 127 8 63 31 15 CWmin 7 第三次重送 初始值 第二次重送 第一次重送 • Backoff time = CW* Random() * Slot time • CW = starts at CWmin and doubles after each failure until reaching CWmax and remains there in all remaining retries • e.g., CWmin = 7, CWmax = 255 • Random() = (0,1) • Slot Time = Transmitter turn-on delay + medium propagation delay + medium busy detect response time

  30. Priority Scheme • Goal：Let each frame has different priority • SIFS → PIFS → DIFS → EIFS • 802.11 DSSS– SIFS(10μs)，PIFS(30μs)，DIFS(50μs)，EIFS(>50μs) DIFS PIFS SIFS time 1st Priority 2nd Priority 3rd Priority

  31. CSMA/CA with RTS/CTS • Hidden terminal problem → Collision • Exposed terminal problem → Waste bandwidth C A B D C can send data. But carrier the network is busy A B C D

  32. CSMA/CA with RTS/CTS • Solve hidden terminal problem • Highoverhead NAV(RTS) [LOCK] Sender Neighbor Data RTS Sender Sender Receiver Receiver ACK CTS NAV(CTS) [LOCK] Receiver Neighbor time

  33. Chapter 3 Outline • 3.1. 802.11MAC機制 • 3.2. 802.11 碰撞議題相關研究 • 3.3. 802.11 節能、省電議題相關研究 • 3.4. 802.15.4 MAC • 3.5. MAC protocols for WSN

  34. 802.11內建省電模式 • In 802.11 Power Saving mode • 802.11Infrastructure mode的省電模式 • Have AP • Ad-hocmode的省802.11電模式 • Only node

  35. 802.11Infrastructure mode的省電模式 • TIM(Traffic Indication Map) • TIM record data：Association ID、Buffered(0/1) • Mechanism • Listen Beacon • 1.TIM (if Buffered is 0) • Go to SLEEP STATE • 2. If Buffer is 1： • a.in PCF • waiting AP transmit data • b. in DCF • 1. STA send PS-Poll to AP • 2. AP receive PS-Poll and transmit buffered data 0：no data 1：have data

  36. 802.11Ad-hocmode的省電模式 Beacon interval Beacon interval Data STA1 STA2 Beacon ACK STA3 time Beacon Sleep Active TBIT window ATIM DATA /ACK ATIMwindow ATIM_ACK

  37. References [1] Andrew S. Tanenbaum , “Computer Network 4/e” , PHPTR [2] 曾煜棋, 潘孟鉉, 林致宇 , “無線網域及個人網路-隨意及感測網路之技術與應用”, 知城 [3]N.Abramson, “The ALOHA system – another alternative for computer communications” , in proc. Fall Joint Computer Conference. [4] Jung-Hyon Jun, Young-June Choi, and Saewoong Bahk , “Affinity-Based Power Saving MAC Protocol in Ad Hoc Network” , in proc. IEEE PerCom2005 [5] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, “ MACAW: A media access protocol for wireless LAN's.” in proc. ACM SIGCOMM '94 [6] IEEE Std 802.11-1997 [7] IEEE Std 802.11a-1999 [8] IEEE Std 802.11b-1999

  38. Chapter 3 Outline • 3.1. 802.11MAC機制 • 3.2. 802.11 碰撞議題相關研究 • 3.3. 802.11 節能、省電議題相關研究 • 3.4. 802.15.4 MAC • 3.5. MAC protocols for WSN

  39. IEEE 802.15.4 MAC • Architecture Applications ZigBee Network • Channel acquisition • • Contention Window IEEE 802.15.4 MAC IEEE 802.15.4 PHY

  40. IEEE 802.15.4 MAC • Architecture Applications • Device join and leave • Frame routing • And so on ZigBee Network IEEE 802.15.4 MAC IEEE 802.15.4 PHY

  41. IEEE 802.15.4 MAC Network topology FFD vs. RFD • Full function device (FFD) • Any topology • Network coordinator capable • Talks to any other device • Reduced function device (RFD) • Limited to star topology • Cannot become a network coordinator • Talks only to a FFD • Very simple implementation

  42. IEEE 802.15.4 MAC - Star Topology FFD Communications flow RFD PAN coordinator Master/slave

  43. IEEE 802.15.4 MAC – Tree and Mesh Topologies FFD Communications flow RFD PAN coordinators Cluster tree Point to point

  44. Transfer mode – Superframe Structure CAP CFP GTS Active portion Inactive portion Beacon interval Beacon frame CAP︰ Contention-Access Period CFP︰ Contention-Free Period GTS︰ Guaranteed Time Slot Beacon frame

  45. Transfer mode – GTS Concepts • Beacon interval = aBaseSuperframeDuration × 2SO symbols • aBaseSuperframeDuration為IEEE 802.15.4預設參數。 • Active portion的長度為: aBaseSuperframeDuration × 2BO symbols (BO≦SO≦14) • 當SO =15時，代表不使用superframe的架構。 • A Guaranteed Time Slot (GTS) allows a device to operate on the channel within a portion of the superframe • A GTS shall only be allocated by the PAN coordinator • The PAN coordinator can allocated up to seven GTSs at the same time

  46. Transfer mode – GTS Allocation • If and only if PAN coordinator has enough capacity for the requested GTS • GTSs shall be allocated on a first-come-first-served basis by the PAN coordinator Coordinator MAC ACK Beacon(with GTS descriptor) Device MAC GTS request

  47. Transfer mode – GTS deallocation • PAN coordinator shall update the final CAP slot subfield of the superframe Coordinator MAC ACK Beacon(with GTS descriptor) Device MAC GTS release

  48. Transfer mode – GTS reallocation • The deallocation of a GTS may result in the superframe becoming fragmented. CAP CFP GTS1 GTS2 GTS3 8 10 13

  49. Transfer mode – GTS reallocation CAP CFP GTS1 GTS3 11 13 Maximize CAP

  50. Data Transfer Model - Channel Access • Beacon-enable networks • With beacon frame • Slotted CSMA/CA channel access mechanism • Non Beacon-enable networks • No beacon frame • Unslotted CSMA/CA channel access mechanism

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