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Guaranteed Quality-of-Service Access to IEEE 802.11 Wireless LANs Dr-Jiunn Deng Department of Electrical Engineering National Taiwan University, Taipei, Taiwan, R.O.C. Email: [email protected] October 29, 2002 The next wave of the Internet The Net's founders predict its future: "Nomadic computing, providing access while you're on the road so that the Internet services you see when you're someplace else are no different than what you have back in your office.“ --Leonard Kleinrock "Radio-based links into the Net will be very typical. If you have a question, Oriented you'llTowards whip out your Multimedia Palm Pilot with a radio link and go on the Net and pull the data out.“ --Vinton Cerf Mobile Systems and providing “Anytime Anywhere Anyform” "Many sites in the research community will have access at gigabyte speed to the Internet. You'll see the increasing introduction of wireless access, Information Systems so people don't have to feel tethered to the Net. And we're going to see increasing content.“ --Robert Kahn "The Internet will become the pervasive network for the world's telecom traffic. Voice and video will transfer over to it in the next five to 10 years. Clearly, you're going to have video on demand, radio or TV, that can have millions of different sources or special subjects that (small numbers) care about.“ --Lawrence Roberts Status of specified wireless networking technologies IEEE 802.11 – keep growing Bluetooth – will see The rest – some need to be watched, most never take off 1% 17% 1% 2% 36% 31% Use today Will use within 18 mos. 83% Will not use within 18 mos. 51% 60% 5% 6% 6% 802.11 (WLAN/ IEEE) Bluetooth HyperLAN Don't know 802.11 WLAN Architecture Wired Network AP Basic Service Set AP DCF (CSMA/CA) DIFS Source Destination SIFS RTS DIFS Data SIFS CW SIFS CTS DIFS ACK CW DIFS NAV (RTS) NAV (CTS) NAV (Data) Other Defer access Backoff time: 2i ranf () 2 Slot_ Time CW Backoff time started MAC Architecture Contention-free Service MAC Extent Contention Service Point Coordination Function (PCF) Distributed Coordination Function (DCF) PCF Superframe CFP Superframe CP CFP CP Stretched DCF period Contention Free Period Repetition Interval Contention Free Period SIFS PIFS Beacon CF-Poll PIFS CF-Poll Sta-to-Sta ACK PIFS Contention Period SIFS CF-End Sta-to-Sta ACK SIFS SIFS NAV SIFS Motivation Packet-switched solutions that take advantage of silences in a given voice call by multiplexing voice data from other calls are more bandwidth-efficient than circuit-switched solutions In wireless networks, where bandwidth is more constrained DCF can not support service discipline of integrated multimedia traffic since it does not include any priority and access policy PCF mode offers a “packet-switched connection-oriented” service, which is well suited for telephony traffic In order to poll the stations an AP must maintain a polling list, which is implementation dependent Bandwidth management and QoS IETF groups are working on proposals including RSVP, Differentiated Services, and Integrated Services to provide better QOS control in IP networks Principle 1: Marking of packets is needed to distinguish between different classes Principle 2: provide protection (isolation) for one class from other classes Principle 3: It is desirable to use resources as efficiently as possible Principle 4: Application flow declares its needs, network may block call if it cannot satisfy the needs Should we supported these functionalities in.. CLIENT? CORE? EDGE? Wired Network AP AP Basic Service Set AP Delay in packet-switched networks 5000km 3 108 m / sec 16.6m ilisec onds 8kbits 9 10 bits / sec 8m icrosec onds transmission A propagation B nodal processing queueing (8 106 16.6 103 ) 109 103 16000.608packets Enforcing priority for RA Too support priority, we change the backoff time generation function ranf () 2 2i Consecutive times (i) Backoff slot numbers ranf () 2 k 2 mi 1st 2nd 3 rd ni 4 th Types of requests (k, m, n) Real-time handoff traffic ( 0, 1, 1 ) 0–3 Admitted inactivated video traffic4 – 7 ( 1, 1, 1 ) Non-real-time handoff traffic New request traffic 8 – 15 ( 2, 2, 1 ) 0 -7 0 – 15 0 – 31 8 - 15 16 – 31 32 - 63 16 - 31 32- 63 64 – 127 Adaptive contention window The collision avoidance strategy in DCF avoids long access delays when the load is light, but it causes a high collision probability and channel utilization in degraded in bursty arrival or congested scenarios In this paper, we propose an adaptive contention window mechanism to dynamically expand and contract the contention window size according to the current load and achieve the theoretical capacity limits. Define the utilization factor Sb 2mi The value of utilization factor provides a lower bound to the actual number of stations trying to access the channel during the last contention window Adaptive contention window M popt is a tight upper bound of in a system operating with the optimal channel utilization level By fixing a given value for the frame size, the value of M popt is almost constant M popt can be used as a measure of the network contention level when the network utilizes the optimal contention window size corresponding to the ongoing network and traffic configuration We double the size of contention window when the utilization factor exceeds M popt , but we halve the size of contention window when the utilization factor becomes less than 0.25 M popt , rather then 0.5 M popt Packet scheduling policy in CFP Contention Free Period Repetition Interval Contention Period Contention Free Period Token buffer for voice traffic Token buffer for video traffic PIGGY-BACKING OF REQUEST Request Packet-Transmission Permission CSMA/CA with ENFORCING PRIORITY for RA Packet flow PACKET-TRANSMISSION CHANNEL Control flow Request flow Packet scheduling policy in CFP 1) The PBS first scans the token buffers of voice sources according to the preset priority order. If a token is found, it removes one token from this token buffer and transmits a packet for this voice source. Then, the PBS generates next token for this voice source after (1 rc ) t p second if the piggybacking was set while transmitting the packet, where t p is the time to transmit a packet. 2) If no tokens are found in the token buffers of voice sources, the PBS continues to scan the token buffers for video sources according to the preset priority order. If a token is found, it transmits a packet for this video source. And it will not remove the token if the piggybacking was set while transmitting this packet. If the piggybacking was not set and it is not the last packet (End-of-File) either, the PBS removes the token, and then generates next token for this video source after seconds. But if this admitted inactivated video source contended successfully within seconds, there is not any toke be generated by PBS automatically. 3) If there is no token found in all token buffers. The AP uses the CF-END frame to announce the end of the contention free period and the maximum time interval of following contention period. Admission Control for voice traffic r Let t p ck t p k 1 rci i i 1 , i 1,...,nc t p PIFS CFPoll SIFS Packet SIFS ACK If i 1 rci and i i for all i =1,2…, nc, then all the packets generated by new-call voice sources meet their jitter constraints. Furthermore, if i i 1 rci and i i i for i th sources which is handoffed from other cells, then the packet generated by the i th source after handoff meets its jitter constraint. Admission Control for video traffic nc Let 0 t p (nc 1) , rv 0 t p rci , j t p ( j 1) , rvj t p rvj , _ _ _ _ i 1 j and d *j j k 0 j 1 _ k t p ( rvk d k* ) k 1 j 1 _ 1 rvk , where j 1,...,nv . k 0 nv _ * rvk 1 j , then the delay d d If and for all j j k 0 constraints are satisfied for all the new-call video th * j d d sources. Furthermore, if j j j j for source which is handoff from other cells, then the packet generated by the j th source after handoff meets its delay constraint. Adaptive Bandwidth Allocation Strategy Contention Free Period Repetition Interval Contention Free Period Channel I for new-call/handoff voice/video traffic CFP-channel I Contention Period Channel II for handoff voice/video traffic CFP-channel II Channel III for data traffic CP-channel III Adaptive Bandwidth Allocation Strategy IF monitored dropping probability > threshold_D THEN IF bandwidth utilization < THEN size of allocated bandwidth II= min {max {size of allocated bandwidth I, size of allocated bandwidth II} up_ , total bandwidth } ELSE size of allocated bandwidth II= min {max {size of allocated bandwidth I, size of allocated bandwidth II} up_ , total bandwidth threshold_up_II } ELSE Adaptive Bandwidth Allocation Strategy (cont.) IF monitored blocking probability > threshold_B THEN IF bandwidth utilization < THEN size of allocated bandwidth I= min {size of allocated bandwidth I up_ , total bandwidth threshold.1_up_I } ELSE size of allocated bandwidth I= min {size of allocated bandwidth I up_ , total bandwidththreshold.2_up_I } ELSE IF bandwidth utilization < THEN size of allocated bandwidth II= max {size of allocated bandwidth II down_ , total bandwidththreshold_down_II } size of allocated bandwidth I= max {size of allocated bandwidth I down_ , total bandwidththreshold_down_I } Conclusions The design of priority-sensitive network protocols continues to be an important problem Broadband wireless links constitute a subclass where prioritization is key to optimizing overall performance We proposed a pragmatic non-preemptive priority based access control scheme built on well-know protocols and offered easily implemented and yet flexible criteria for traffic prioritization in a wireless environment. Various QoS requirements are needed in the future Multilevel priorities, bandwidth allocation, connection admission control, and traffic policing all need to be considered together in the future networks Reference D. J. Deng and R. S. Chang, “A Priority Scheme for IEEE 802.11 DCF Access Method,” IEICE Trans. Commun., vol. E82-B, no. 1, pp. 96-102, January 1999. D. J. Deng and R. S. Chang, “A Non-Preemptive Priority Based Access Control Scheme for Broadband Ad-Hoc Wireless ATM Local Area Networks,” IEEE Journal on Selected Areas of Communications, Vol. 18, no. 9, Sep. 2000, pp. 1731-1739.