QoS in 802.11 Reference : IEEE 802.11e: QoS Provisioning At The MAC Layer & A survey of quality of service in IEEE 802.11 networks 通工所一 693430019 馮士銓

Outline IEEE 802.11 MAC IEEE 802.11 service differentiation mechanisms IEEE 802.11e Enhanced DCF Admission control and bandwidth reservation Distributed admission control for EDCF 802.11e Direct link protocol 802.11e Group acknowledgment. Architecture of the wireless Internet Support for full mobility QOS AND MOBILITY MANAGEMENT IN HYBRID WIRELESS NETWORKS Integration of WLAN and 3G wireless networks

IEEE 802.11 MAC (1/3) Each superframe consists of a contention- free period (CFP) for PCF and a contention period (CP) for DCF. 802.11 medium access control (MAC) Distributed coordination function (DCF) Use carrier sense multiple access with collision avoidance Use exponential backoff Point coordination function (PCF)

IEEE 802.11 MAC (2/3) NAV : network allocation vector The time must elapse until current transmission session is complete. RTS/CTS : request to send/clear to send Solve the hidden terminal and capture effect problems CW : contention window Incremented exponentially with an increasing number of attempts to retransmit the frame.

IEEE 802.11 MAC (3/3) MPDU : MAC protocol data unit Contain header, payload, CRC Backoff time slot is chosen randomly in the interval [0,CW). Fragment : If it exceed Frag_threshold Advantage : if an error occurs during its transmission, a station does not have to wait to back off.

IEEE 802.11 MAC (DCF) A station with a frame to transmit monitors the channel activities until an idle period equal to a distributed interframe space (DIFS) is detected. After sensing an idle DIFS, the station waits for a random backoff interval before transmitting. The station transmits its frame when the backoff time reaches zero. After the destination station successfully receives the frame, it transmits an acknowledgment frame (ACK) following a short interframe space (SIFS) time.

DCF (Cont.) The hidden node problem may happen: transmissions of a station cannot be detected using carrier sense by a second station, but interfere with transmission from the second station to a third station. To reduce the hidden station problem, an optional four-way data transmission mechanism, RTS/CTS, is also defined in DCF. RTS and CTS frames reserve the channel for the data frame transmission that follows. All four frames (RTS, CTS, data, ACK) are separated by an SIFS time.

IEEE 802.11 MAC (PCF) It logically sits on top of the DCF and performs polling, enabling polled stations to transmit without contending for the channel. Transmits a beacon frame to initiate a CFP (i.e., to initiate a superframe). After a SIFS time, the PC sends a poll frame to a station to ask to transmit a frame. After receiving the poll frame from the PC, the station with a frame to transmit may choose to transmit a frame after a SIFS time.

PCF (Cont.) The PC waits a PIFS interval following the ACK frame before polling another station or terminating the CFP by transmitting a CF-End frame. If the PC receives no response from the polled station for a PIFS interval, the PC can poll the next station or terminate the CFP by transmitting a CF-End frame. The PCF cannot provide good QoS support since it lacks an admission function to control channel access from stations.

IEEE 802.11 service differentiation mechanisms 比較fair-scheduling-based scheme和 priority-based scheme ： priority-based 較能修改較少部分在原有802.11 DCF

IEEE 802.11e Enhanced DCF The EDCF is based on differentiating priorities at which traffic is to be delivered and works with four access categories (ACs). The EDCF supports eight different priorities, which are further mapped into four ACs. ACs are achieved by differentiating the arbitration interframe space (AIFS), initial window size, and maximum window size. 我覺得雖然他說有支援8個priority但我覺得實際只有4個不同的priority (AC數), 他這篇有個地方寫 AIFS[i] >= PIFS 我覺得怪怪的 我覺得是 AIFS[i] >= DIFS

Access categories 分成4個優先等級不同的佇列，他們有各自的AIFS時間和BACKOFF時間 將8個不同PRIORITY MAPPING到4個AC值上

IEEE 802.11e EDCF (Cont.) For 0 ≤ i < j ≤ 3, we have CWmin[i] ≥ CWmin[j], CWmax[i] ≥ CWmax[j], and AIFS[i] ≥ AIFS[j], and at least one of the above inequalities must be “not equal to.” If there is more than one queue finishing the backoff at the same time, it is called an internal collision. AC值越大, AIFS比之前的AIFS變小, CWmin和CWmax跟AIFS情形一樣 internal collision : Queue backoff完後, 要送出的時間跟其他的Queue時間相同, 所以會產生collision

EDCF timing diagram 跟原本的DCF差不多但是IFS時間跟BACKOFF時間隨PRIORITY不同而有所不同

P-DCF & DWDQ P-DCF : Persistent Factor DCF Each flow stops the backoff and starts transmission only if ( r > P ) in the current slot time. DWDQ : Distributed Weighted Fair Queue CW of any traffic flow is adjusted based between the actual and expected throughputs. Li′ = Ri /Wi Li is smaller than those of others, it will decrease its CW. P-DCF ： High-priority classes have smaller P Backoff interval is a geometrically distributed random variable with parameter P. DWDQ： CW will be decreased in order to increase the flow’s priority, and vice versa. Ri is the actual throughput and Wi the corresponding weight of the ith station.

DFS & DDRR DFS : Distributed Fair Scheduling The backoff interval (BI) based on the packet length and traffic class, and the station with smaller BI transmits first. BIi = ρi × scaling × factor × Li /ϕi DDRR : Distributed Deficit Round Robin In order to minimize the collision between stations with the same deficit counter, randomization of IFSi,j will be further adopted if a backoff scheme is eliminated. DFS : Li is the packet length, ϕi is the weight(the weights of high throughput classes arelarger than that of low classes) ρi is introduced to minimize the collision caused by multiple stations with the same backoff interval. ρi is a random variable uniformly distributed in [0.9,1.1]. DDRR : The ith throughput class at the jth station is assigned with a service quantum rate (Qi,j) equal to the throughput Deficit counter (DCi,j) that accumulates at the rate of Qi,j and is decreased by the packet length whenever a packet is transmitted. DCi,j is used to calculate the interframe space ( IFSi,j )

Admission control and bandwidth reservation admission control schemes比 bandwidth reservation修改較少的802.11 standard. measurement-based schemes, admission control decisions are made based on the measurements of existing network status, such as throughput and delay. calculation-based schemes construct certain performance metrics or criteria for evaluating the status of the network. Virtual MAC : It passively monitors the channel by virtual MAC frames and estimates local service level (i.e., throughput and delay). virtual source (VS) algorithm allows application parameters to be tuned according to dynamic channel conditions by utilizing virtual MAC. Probe packet : using a sequence of probe packets for ad hoc networks. Data probe packet : Using data packets to measure the network load. Permissible throughout : Using the Ad Hoc On Demand Distance Vector (AODV) routing protocol and discovering the bottleneck along the path. Saturation-based : Stations can effectively determine whether they are approaching saturation condition based on piggybacked information, including the number of active stations, their corresponding traffic bit rates, and average packet lengths. Flow reservation and priority allocation : optimizing the usage of priority resources. ARME : Use a token-bucket-based algorithm to detect whether the network is in overloading condition, and improve the performance of the system by adjusting the CW appropriately. AACA : Use the RTS/CTS access method on a common channel solely for reservation purposes. The AACA protocol was designed to effectively solve the hidden terminal and exposed terminal problems in multihop networks.

Distributed admission control for EDCF (1/4) The QoS parameter set element (QPSE) includes CWmin[i], CWmax[i], AIFS[i], TXOPLimit[i], TXOPBudget[i], Load[i], and SurplusFactor[i] for (i = 0,…3) SurplusFactor[i] : the ratio of over-theair bandwidth reserved to the bandwidth of the transported frames required for successful transmission TXOPBudget[i] : the additional amount of time available during the next beacon interval ATL[i] is the maximum amount time that may be spent on transmissions of AC I per beacon interval. 由QAP在每次傳BEACON時傳QPSE這些參數給QSTA TXOPBudget[i] ： 在下一次的BEACON INTERVAL將需要多的時間 SurplusFactor[i] ： 可以成功傳送的BANDWIDTH所占可以傳送的BANDWIDTH的機率 ATL[i] ： 最大能花在傳輸資料的時間

Distributed admission control for EDCF (2/4) TXOPLimit[i] : the time limit on TXOPs Load[i] : the amount of time used during the previous beacon interval The QAP shall maintain a set of counters TxTime[i], which shall be set to zero immediately following transmission of a beacon. For each data frame transmission , the QAP shall add to the TxTime counter. TXOPBudget[i] = Max(ATL[i] – TxTime[i]*SurplusFactor[i],0). TXOPLimit[i] ：在QSTA所擁有的優先等級能傳的最大時間 Load[i] ： 在前一次BEACON INTERVAL所用的時間 由QAP去MAINTAIN TxTime[i] 這個計數器的值 ， 當QAP送出BEACON後，立即將這個計數器的值設為0 當有資料在傳輸時，就設這個計數器的值為累積QAP所傳的資料量

Distributed admission control for EDCF (3/4) Each QSTA has to maintain the following variables for each AC: TxCounter[i], TxUsed[i],TxLimit[i], TxRemainder[i], and TxMemory[i]. TxUsed[i] counts the amount of time occupied on air by transmissions. TxCounter[i] counts successful transmissions. TxRemainder[i] = TxLimit[i] – TxUsed[i]; 所有的QSTA都需MAINTAIN這些值TxCounter[i], TxUsed[i],TxLimit[i], TxRemainder[i], and TxMemory[i]. TxUsed[i] ： QSTA所花在傳輸的時間，不管成不成功，其中包括ACK，SIFS，且此時間不能超過TXOPLimit[i] TxCounter[i] ： QSTA花在資料成功傳輸的時間 TxLimit[i] ： QSTA最大能傳的時間 TxRemainder[i] ： 上一次BEACON INTERVAL可傳但沒傳的時間 TxRemainder[i] = TxLimit[i] – TxUsed[i];

Distributed admission control for EDCF (4/4) If TXOPBudget[i] = 0 –TxMemory[i] shall be set to zero for new QSTAs and 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] f is the damping factor, which does not affect the entrance of a new flow into the system when enough budget is available, because the decreased TXOPBudget is offset by an increased TxCounter instantaneously, so TxMemory does not change a lot. Decreased TXOPBudget is not offset by an increased TxCounter; consequently, the TxMemory converges to the lower target value. The value TxCounter + TXOPBudget is the target to which TxMemory converges. TxLimit is equal to TxMemory plus a possible capped remainder. This prevents new flows from entering the specific AC when it is saturated. A suitable initial value for this variable could be between 0 and TXOPBudget[i]/SurplusFactor[i].

802.11e Direct link protocol Direct Link Protocol (DLP) allows QSTAs to transmit frames directly to other QSTAs. A direct link can be built by the following sequences: QSTA-1 sends a DLP-request frame to QAP QAP forward DLP-request to QSTA-2 QSTA-2 send response frame to QAP QAP forward DLP-response to QAP-1 Frame can be sent from QSTA-1 to QSTA-2 and QSTA-2 to QSTA-1 After DLPIdleTimeout, frames with destination QSTA-2 shall be sent via the QAP 通常STA1要傳給進入POWER SAVING 的STA2，就先由AP暫存此FRAME， 但DLP他可以讓AP不用暫存資料FRAME，他可以讓STA2 WEAK UP 後再由STA1傳給STA2 STEP1： QSTA-1傳DLP RESUEST給QAP STEP2： 若QSTA-2跟AP做ASSOCIATE 此時QAP就轉送DLP-REQUEST給QSTA-2 STEP3： 若QSTA-2 ACCEPT時，就傳DLP-RESPONSE給QAP STEP4： QAP再把DLP-RESPONSE 轉送給QSTA-1 STEP5： 此時QSTA-1跟QSTA-2就互傳FRAME 當DLPIDLETIMEOUT時就回復原來的機制就是再靠AP來暫存資料

802.11e Group acknowledgment In order to reduce the acknowledgment overhead, a new mechanism, called group acknowledgment (GA) GA allows a group of frames to be transmitted before any acknowledgment. After sending a burst of frames, the sender sends a group acknowledgment request (GroupAckReq) frame, and the receiver must respond by sending the group acknowledgment (GroupAck) frame, in which the correctly received frames’ information is included. 802.11e提供一個機制為GA，他只要為降低一些ACK所造成的OVERHEAD GA ALLOW 可SENDER傳一群FRAME後再傳一個GroupAckReq， RECEIVER回應一個GroupAck來表示這些接收的一群FRAME正確無誤。 如果接收端送出的GroupAck指出接收遺失，SENDER必須再重送，且重送的順序為原來傳送的順序 所以接收端必須MAINTAIN一個RECORD紀錄接收FRAME的SEQUENCE NUMBER

Group acknowledgment

Architecture of the wireless Internet Two kinds of mapping between DiffServ code point (DSCP) : direct mapping and hierarchical mapping Direct mapping : they are placed in priority queues without preemption. Hierarchical mapping, IP packets are classified and shaped according to the priority of the DSCP values before being forwarded to 802.1e priority queues. Integrating RSVP and WRESV for the support of IntServ in heterogeneous wired-cum-wireless networks.

Support for full mobility This roaming capability is achieved through MSs’ beacon scanning in a channel sweep. Recent efforts have been made to extend 802.11 WLANs into outdoor cellular networks to provide fully mobile broadband service with ubiquitous coverage and high-speed connectivity. When an MS enters a new basic service area, it first scans across all channels, and then acquires the channels from the AP.

QOS AND MOBILITY MANAGEMENT IN HYBRID WIRELESS NETWORKS Roaming and horizontal handoff among 802.11 WLANs, supporting QoS anytime, anywhere, and by any media requires seamless vertical handoffs between different wireless networks. 希望達到Qos anytime, anywhere, seamless 在不同的無線網路之間

QOS AND MOBILITY MANAGEMENT IN HYBRID WIRELESS NETWORKS (Cont.) Integration of WLAN and MANET: routing within MANETs is handled by the Optimized Link State Routing (OLSR) protocol, and handoff between WLANs and MANETs is supported through automatic mode detection and node switching capabilities of the mobiles. Integration of WLAN and Bluetooth: evaluation of the interference between IEEE 802.11 and Bluetooth. WLAN 和 BLUETOOTH有相互interference的問題 The performance is evaluated by packet error probability in terms of the relative distances between the two systems for different conditions.

Integration of WLAN and 3G wireless networks A mobile node can maintain two connections in parallel (i.e., data connection through WLAN and voice connection through UMTS). With the decreasing size of cells in next-generation multimedia-enabled wireless networks, the number of handoffs during a call’s lifetime increases.