Feedback control for providing QoS in IEEE e WLANs Saverio Mascolo Collaborators: G. Boggia, P. Camarda, L. A. Grieco Politecnico di Bari, italy WIP/BEATS/CUBAN Workshop May 23-25, Sidi Bou Said

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 QoS? Van Jacobson [interview March 7, 2005]: “QoS has been an area of immense frustration for me. We're suffering death by 10,000 theses. It seems to be a requirement of thesis committees that a proposal must be sufficiently complicated for a paper to be accepted. Look at Infocom, look at IEEE papers; it seems as though there are 100,000 complex solutions to simple priority-based QoS problems. The result is vastly increased noise in the signal-to-noise ratio. The working assumption is that QoS must be hard, or there wouldn't be 50,000 papers on the subject. The telephony journals assume this as a starting point, while the IP folks feel that progress in QoS comes from going out and doing something …[continue]”

More… …..[continue] “I hope that the circuit obsession is transitional. Anytime you try to apply scheduling to a problem to give latency strict bounds, the advantages are not worth the cost of implementation Because of the circuit-oriented background of ATM developers, they had bought into the telco religion that QoS equals scheduling. If you go down that path, it's a highway to disaster.”

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 QoS in WLAN: e Overview of IEEE e enhancements for QoS support Feedback based bandwidth allocation algorithms Call Admission Control (CAC) Ns-2 results Conclusions and future works

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Motivations The Medium Access Control (MAC) schemes of the standard is not well suited for multimedia transmissions because they are very sensitive to time delay. The interest for multimedia transmission using Wlan is increasing (home entertainment, IP TV, streaming etc.)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 IEEE Infrastructure WLAN BSS AP –All the traffic of the BSS (Basic Service Set) is channeled through the Access Point (AP) –The Distribution System connects two or more BSSs. ESS –This architecture is known as ESS (Extended Service Set) Access Point Wireless Client Basic Service Set (BSS) Wireless Client Access Point Distribution System Basic Service Set (BSS)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Basic DCF access scheme Basic MAC is the Distributed Coordination Function (DCF) CSMA/CA –it implements a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) algorithm; –it is mandatory; –it uses a backoff scheme for retransmissions. Station A Station B Station C Station D time RTS backoff time (8 slots) backoff time (11 slots) SIFS CTS SIFS DATA ACK SIFS transmission deferred; backoff timer=3 backoff time (7 slots) RTS SIFS CTS SIFS DATA transmission deferred … … … … DIFS residual backoff time (3 slots) DIFS

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Improved access method: HCF (Hybrid Coordination Function) EDCA –contention-based access, EDCA (Enhanced Distributed Coordination Access) HCCA –contention-free access, HCCA (HCF Controlled Channel Access) IEEE e enhancements QoS Station (QSTA) Station with QoS capabilities Hybrid Controller (HC) centralized controller at the AP Call Admission Control (CAC) QoS Service level negotiation: TSPEC for traffic stream specification –e.g., Delay Bound; MSDU and Burst size; Data Rate; etc.

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 EDCA method t Access CategoryEDCA is similar to DCF, but with different Contention parameters (AIFS) per Access Category (AC) 4 ACs to map the 8 Traffic Categories (TCs) of 802.1D AC_BK (Background) AC_VO (Voice) AC_BE (Best Effort) AC_VI (Video) TC: 1, 2 TC: 0, 3 TC: 4, 5 TC: 6, 7

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 TXOP TXOPsTXOPs (Transmission Opportunities): time interval during which a station has the right to transmit TXOP Starting Time Maximum Duration t

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 HCCA method It combines some EDCA characteristics The time is divided into repeated periods (Superframes) CAPHC starts a CAP (Contention Access Phase) during which only polled and granted QSTAs are allowed to transmit for TXOPs (CAP  dot11CAPlimit) AIFS TXOP ( Station n ) Stations HC time DATA Superframe QoS CF-Poll EDCA RTS backoff time … … PIFS Beacon Frame … … PCF EDCA CAP … … CAP (HCCA) … … … … T CA SIFS ACK TXOP ( Station m ) DATA QoS CF-Poll … PIFSSIFS ACK DIFS PIFS DIFS Beacon Frame

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Simple Scheduler The simple scheduler is described in the e Draft It does not exploit any feedback from stations It assigns fixed TXOPs based on static values declared in the TSPEC L i : nominal MSDU C i : physical data rate M : maximum MSDU size O : time overhead due to ACK frames, SIFS, and PIFS intervals T SI : Service Interval (minimum interval between two successive allocation to the same station)  i : Mean Source Data rate

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 New MAC frame format New QoS Control field in the header of the MAC frame MAC Qos Control DataFCS 30 Bytes2max Queue size (units of 256 octets) in the QoS Control Field Control BitsQueue Size bits 88 This field is useful to design novel HCCA-based dynamic scheduler using feedback control

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 QoS Signalling Message Sequence Chart for QoS Signalling QSTA SME QSTA MAC HC MAC HC SME 1) MLME-ADDTS Request 2) ADDTS Request ADDTS Timer 3) MLME-ADDTS Indication 4) MLME-ADDTS Response 5) ADDTS Response 6) MLME-ADDTS Confirm HC The Station Management Entity of the QSTA with the new Traffic Stream (TS) request sends a MAC Layer Management Entity-ADDTS request, with the TSPEC The QSTA MAC forwards the ADDTS request to the HC The ADDTS timer starts. The MAC layer of the HC generates a MLME_ADDTS indication for its SME layer The SME in the HC, based on the used CAC function, decides to accept or to refuse the new TS. Then it sends a Response The MAC of HC forwards the response to the QSTA’s MAC The ADDTS-timer is reset and, if the confirm message has a SUCCESS code, the TS enters into active state. Otherwise the whole process can be repeated.

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Call Admission Control new Traffic Stream HC New Traff.Stream request (TSPEC) Call Admission Response Decision taken by the Admission Control Unit in the HC With k admitted flows, the flow k +1 is accepted if T : Superframe duration T CP : time used for EDCA traffic during the superframe

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 FBDS (Feedback Based Dynamic Scheduler) Basic assumptions –HCCA method – T CA (time between two successive CAPs) constant –Within each CAP, the HC is aware of all M traffic queue levels, q i, in the network (feedback in frame header) Dynamic of the i th queue: q i (k+1)=q i (k)+d i (k)T CA +u i (k) T CA q i (k) : i th queue level at the beginning of the k th CAP u i (k): average depletion rate of the i th queue level d i (k)=d i S (k)-d i CP (k) difference between the average input rate at the i th queue during the k th T CA interval and the amount of data transmitted during the k th CP (i.e., EDCA) divided by T CA

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 FBDS Control Law Objective:Objective: drive the queuing delay  i experienced by each frame of the i th queue to a desired target value  i T. For each queue the target queue level q T is 0 –

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 The Control Law The control law is as simple as

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Stability Analysis: Proportional Controller (Gc i =k i ) Z-transforms of queue level q i and depletion rate u i System Poles Stability condition

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Queueing Delays After a little algebra, the steady state delay is To satisfy the target delay  i T the following inequality must be satisfied: Thus, we need :

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Stability Analysis: PI Controller Stability conditions Controller Transfer Function Due to the integral action the steady state queuing delay is zero

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 TXOP assignments From the computed u i (k), the HC assigns the following TXOP The extra quota of the assigned TXOP depends on the overhead due to ACK frames, SIFS and PIFS time intervals. The overhead could be estimated assuming that all MSDUs have the same nominal size, specified in the TSPEC

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Channel Saturation (1) If the the WLAN is not overloaded, than the sum of assigned TXOPs is smaller than the maximum CAP duration dot11CAPLimit. If the channel is saturated i.e.: each computed TXOP is decreased by an amount  TXOP so that

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Channel Saturation (2) The generic  TXOP is given by  TXOP is proportional to the physical data rate

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 CAC proposal Similar to the CAC proposed by the IEEE standard Use TXOPs dynamically assigned instead of fixed values The proposed CAC test takes into account the bandwidth actually used by traffic streams T : Superframe duration T CP : time used for EDCA traffic during the superframe

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 ns results Ns-2 simulations (54Mbps) 3  G.729 Voice Flows with VAD (Markov ON/OFF model)  H.263 flows (library traces)  MPEG-4 Flows (library traces)  FTP Flows AP Voice MPEG-4 H.263 FTP Voice Type of flow Nominal (Maximum) MSDU Size Mean (Maximum) Data Rate Target Delay H.263 VBR 1536 (2304) byte 450 (3400) kbps 40 ms MPEG-4 HQ 1536 (2304) byte 770 (3300) kbps 40 ms G.729 VAD 60 (60) byte 8.4 kbps 30 ms

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results MPEG-4 flows  = 5  CDFs of the one-way packet delay DCFEDCA Simple Scheduler FBDS

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results H.263 flows  = 5  CDFs of the one-way packet delay DCFEDCA Simple Scheduler FBDS

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results G.729 flows  = 5  CDFs of the one-way packet delay DCFEDCA Simple Scheduler FBDS

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results MPEG-4 flows  = 10  CDFs of the one-way packet delay

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 ns Results H.263 flows  = 10  CDFs of the one-way packet delay

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results G.729 flows  = 10  CDFs of the one-way packet delay

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results CAC Scheme –If a station does not receive a response to an admission request, it repeats the same request after  TO =1.5 s –An admission request is repeated up to  adm =10 times –A new request is initiated after an exponential distributed random time with average value equal to 1 min. AP Voice MPEG-4 H.263 FTP Voice    

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results (MPEG4 flows)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results (H.263 flows)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results (G.729 flows)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Conclusions A simple feedback based scheduler provides bounded delays to real-time flows in a wide range of traffic conditions and frame loss probabilities Using a PI regulator, the CAC scheme admits the same flows than the proposed standard scheme, but still providing bounded delays (i.e., QoS guarantee) to each admitted flow We are extending this approach to power saving

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 References G. Boggia, P. Camarda, L. A. Grieco, and S. Mascolo, “Dynamic bandwidth allocation with call admission control for providing delay guarantees in IEEE e networks," to appear on Computer Comm., special issue. L. A. Grieco, G. Boggia, S. Mascolo, and P. Camarda, “A control theoretic approach for supporting quality of service in IEEE e WLANs with HCF," in Proceedings of 42nd IEEE Conference on Decision and Control, CDC'03, Hawaii, USA, Dec G. Boggia, P. Camarda, C. D. Zanni, L. A. Grieco, and S. Mascolo, “A dynamic bandwidth allocation algorithm for WLANs with HCF access method," in Proceedings of Fourth COST263 Int. Workshop on Quality of Future Internet Services, QoFIS'03, Stockholm, Sweden, Oct

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 ns Results Ratio of admitted traffic flows

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 ns Results Average one-way packet delay

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 ns Results Simple Scheduler + CAC  5)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results FDBS + modified CAC  5)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns Results Simple Scheduler + CAC  5)

Saverio Mascolo WIP/BEATS/CUBAN Workshop May 23-25, 2005 Ns results FDBS + modified CAC  5)