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通訊所 一年級 693430028 鄭筱親

A Survey of Quality of Service in IEEE 802.11 Networks IEEE 802.11E: QoS Provisioning at the MAC Layer. 通訊所 一年級 693430028 鄭筱親. Abstract. IEEE 802.11 experiences serious challenges in meeting the demands of multimedia services and applications.

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通訊所 一年級 693430028 鄭筱親

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  1. A Survey of Quality of Service in IEEE 802.11 NetworksIEEE 802.11E: QoS Provisioning at the MAC Layer 通訊所 一年級 693430028 鄭筱親

  2. Abstract • IEEE 802.11 experiences serious challenges in meeting the demands of multimedia services and applications. • IEEE 802.11e standard support quality of service at MAC layer. • The viewpoint • 802.11 QoS schemes • 802.11e

  3. Outline • Introduction • An Overview of IEEE 802.11 • 802.11MAC • QoS Mechanisms • IEEE 802.11E • Conclusion and Future Work

  4. Introduction(1/2) • WLANs are becoming ubiquitous and increasingly relied on 802.11。 • Wireless users can access real-time and Internet services virtually anytime, anywhere. • In wireless home and office networks, QoS and multimedia support are critical. • QoS and multimedia support are essential ingredients to offer VOD、audio on demand and high-speed Internet access.

  5. Introduction(2/2) • The lack of a built-in mechanism for support of real time services makes it difficult to provide QoS guaranteed for throughput-sensitive and delay-sensitive multimedia applications. • IEEE 802.11e is being proposed as the upcoming standard for the enhancement of the vice differentiation.

  6. An Overview of IEEE 802.11

  7. 802.11MAC (1/4) 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)

  8. 802.11MAC (2/4) • Distributed Coordination Function (DCF) • Defines a basic access mechanism and optional RTS/CTS mechanism. • 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

  9. 超級訊框 免競爭訊框 需競爭訊框 802.11MAC (3/4) • Different accesses to medium can be defined through the use of different values of IFS (inter-frame space). • PCF IFS (PIFS) < DCF IFS (DIFS) • PCF traffic should have higher priority to access the medium, to provide a contention-free access. • This PIFS allows the PC (point coordinator) to seize control of the medium away from the other stations. • Coexistence of DCF and PCF • DCF and PCF can coexist through superframe. • superframe: a contention-free period followed by a contention period.

  10. 802.11MAC (4/4) Figure：Coexistence of DCF and PCF

  11. Distributed Coordination Function (1/3) • Allows sharing of medium between PHYs through • CSMA/CA • random backoff following a busy medium. • All packets should be acknowledged (through ACK frame) immediately and positively. • Retransmission should be scheduled immediately if no ACK is received.

  12. Distributed Coordination Function (2/3) • Carrier Sense shall be performed through 2 ways: • physical carrier sensing: provided by the PHY • virtual carrier sensing: provided by MAC • by sending medium reservation through RTS and CTS frames • duration field in these frames • The use of RTS/CTS is under control of RTS_Threshold. • An NAV (Net Allocation Vector) is calculated to estimate the amount of medium busy time in the future. • Requirements on STAs: • can receive any frame transmitted on a given set of rates • can transmit in at least one of these rates • This assures that the Virtual Carrier Sense mechanism work on multiple-rate environments

  13. Distributed Coordination Function (3/3) • MAC-Level ACKs • Frames that should be ACKed: • Data • Poll • Request • Response • An ACK shall be returned immediately following a successfully received frame. • After receiving a frame, an ACK shall be sent after SIFS (Short IFS). • SIFS < PIFS < DIFS • So ACK has the highest priority

  14. DCF: the Random Backoff Time (1/2) • Before transmitting asynchronous MPDUs, a STA shall use the CS function to determine the medium state. • If idle, the STA • defer a DIFS gap • transmit MPDU • If busy, the STA • defer a DIFS gap • then generate a random backoff period (within the contention window CW) for an additional deferral time to resolve contention.

  15. 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 DCF: the Random Backoff Time (2/2) Backoff time = CW* Random() * Slot time where 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

  16. Duration Reservation Strategy (1/2) • Each Fragment and ACK acts as a “virtual” RTS and CTS for the next fragment. • The duration field in the data and ACK specifies the total duration of the next fragment and ACK. • The last fragment and ACK will have the duration set to zero.

  17. Duration Reservation Strategy (2/2) • Goal of fragmentation: • shorter frames are less suspectable to transmission errors, especially under bad channel conditions

  18. Point Coordination Function (1/6) • The PCF provides contention-free services. • One STA will serve as the Point Coordinator (PC), which is responsible of generating the Superframe (SF). • The SF starts with a beacon and consists of a Contention Free period and a Contention Period. • The length of a SF is a manageable parameter and that of the CF period may be variable on a per SF basis. • There is one PC per BSS. • This is an option; it is not necessary that all stations are capable of transmitting PCF data frames

  19. Point Coordination Function (2/6) • The PC first waits for a PIFS period. • PC sends a data frame (CF-Down) with the CF-Poll Subtype bit = 1, to the next station on the polling list. • When a STA is polled, if there is a data frame (CF-Up) in its queue, the frame is sent after SIFS with CF-Poll bit = 1. • Then after another SIFS, the CF polls the next STA. • This results in a burst of CF traffic. • To end the CF period, a CF-End frame is sent.

  20. Point Coordination Function (3/6) • If a polled STA has nothing to send, after PIFS the PC will poll the next STA. • NAV setup: • Each STA should preset it’s NAV to the maximum CF-Period Length at the beginning of every SF. • On receiving the PC’s CF-End frame, the NAV can be reset (thus may terminate the CF period earlier).

  21. 超級訊框 免競爭週期 競爭週期 PIFS PIFS SIFS SIFS SIFS 媒介忙碌中 CF-D1 CF-D2 CF-D3 CF-D4 CF-End CF-U1 CF-U2 CF-U4 重設NAV CF-邊界 SIFS SIFS SIFS NAV Dx = Down Traffic Ux = Up Traffic Point Coordination Function (4/6)

  22. Point Coordination Function (5/6) • When the PC is neither a transmitter nor a recipient: • When the polled STA hears the CF-Down: • It may send a Data frame to any STA in the BSS after an SIFS period. • The recipient (.neq. PC) of the Data frame returns an ACK after SIFS. • Then PC transmits the next CF-Down after an SIFS period after the ACK frame. • If no ACK is heard, the next poll will start after a PIFS period

  23. 超級訊框 免競爭週期 競爭週期 PIFS SIFS SIFS 媒介忙碌中 CF-D1 CF-D2 CF-End S-To-S ACK CF-U2 重設NAV CF-邊界 SIFS SIFS SIFS NAV Dx = Down Traffic Ux = Up Traffic Point Coordination Function (6/6)

  24. QoS Mechanisms • QoS mechanisms for 802.11 can be classified into three categories: • Service differentiation • Admission control and bandwidth reservation • Link adaptation

  25. BETTER THAN BEST EFFORT SCHEMES:SERVICE DIFFERENTIATION (1/3) • Enhanced DCF (EDCF) • prioritizes traffic categories by different contention parameters, including • arbitrary interframe space (AIFS), • maximum and minimum backoff window size • (CWmax/min), and a multiplication factor for expanding the backoff window. • Persistent Factor DCF (P-DCF) • each traffic class is associated with a persistent factor P • a uniformly distributed random number r is generated in every slot time • Each flow stops the backoff and starts transmission only if (r > P)

  26. BETTER THAN BEST EFFORT SCHEMES:SERVICE DIFFERENTIATION (2/3) • Distributed Weighted Fair Queue (DWFQ) • the backoff window size CW of any traffic flow is adjusted based on the difference between the actual and expected throughputs. • a ratio (Li′ = Ri/Wi) is calculated, where Ri is the actual throughput and Wi the corresponding weight of the ith station. • Distributed Fair Scheduling (DFS) • differentiate thebackoff interval (BI) based on the packet length and traffic class • For the ith flow, BIi = ρi × scaling × factor × Li/ϕi, • Distributed Deficit Round Robin (DDRR) • the ith throughput class at the jth station is assigned with a service quantum rate (Qi,j) equal to the throughput it requires

  27. BETTER THAN BEST EFFORT SCHEMES:SERVICE DIFFERENTIATION (3/3)

  28. QOS MECHANISMS FOR ADMISSION CONTROL AND BANDWIDTH RESERVATION (1/2) • Measurement-based approaches • Calculation-based approaches • Scheduling and reservation-based approaches

  29. QOS MECHANISMS FOR ADMISSION CONTROL AND BANDWIDTH RESERVATION (2/2)

  30. QOS MECHANISM FOR LINK ADAPTATION (1/2) • Received signal strength (RSS) • PER-prediction • MPDU-based link adaptation • Link adaptation with success/fail (S/F) thresholds • Code Adapts To Enhance Reliability (CATER)

  31. QOS MECHANISM FOR LINK ADAPTATION (2/2)

  32. IEEE 802.11E • Main new features of 802.11e: • The Enhanced DCF • THE CONTROLLED HCF

  33. The Enhanced DCF (1/2)

  34. The Enhanced DCF (2/2)

  35. DISTRIBUTED ADMISSION CONTROL FOR EDCF • TXOPBudget[i] =Max(ATL[i] – TxTime[i]*SurplusFactor[i],0) • If TXOPBudget[i] = 0 –TxMemory[i] shall be set to zero all other QSTAs TxMemory[i] remains unchanged • If the TXOPBudget[i] >0 –TxMemory[i] = f*TxMemory[i] + (1 – f)* (TxCounter[i]*SurplusFactor[i] + TXOPBudget[i]) –TxCounter[i] = 0 –TxLimit[i] = TxMemory[i] + TxRemainder[i]

  36. THE CONTROLLED HCF • Controlled channel access function • allows reservation of transmission opportunities (TXOPs) with a hybrid coordinator (HC) • a type of PC handling rules defined by the HCF

  37. ADMISSION CONTROL ANDSCHEDULING FOR THE CONTROLLED HCF • The behavior of the scheduler is as follows: • The scheduler shall be implemented • if a traffic stream is admitted by the HC, the scheduler shall send polls anywhere between the minimum service interval and the maximum service interval within the specification interval.

  38. PRELIMINARY RESULTS

  39. Conclusion and Future Work • Traffic Differentiation • 將資料分類並規範其優先順序 • HCF結合DCF和PCF • Block Acknowledgement • 無線環境下，資料發送出去，需接收端發出回應 • 資料傳送達一定量之後，接收端再統一送出一個Summary的確認訊息 • Direct Link Protocol (DLP) • 當兩個Station要直接互通時，必需確定雙方均有此項功能，即可直接互通 • 但同時擁有Infrastructure的模式。

  40. 看法及感想 • QoS是網路的一種功能，能夠對特定的網路流量，提供較佳的服務 • 由於一個傳輸頻率在一個時間與一個特定地點內僅能被一個站點所使用，因此須透過CSMA程序將資源儘可能公平地分配給所有站點使用 • 每個站點可透過網路接收到同等的資料量，但資料量等級不僅沒有可靠的保證，而且傳輸流量在短期也不平均 • QoS希望能達到增加Throughput、降低latency的目的

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