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1、Wireless Link: Service Quality,EE206A (Spring 2002): Lecture #6,This Lecture,Channel state dependence QoS and fairness in wireless,Reading,Mandatory Bharghavan99 Bharghavan, V.; Songwu Lu; Nandagopal, T. Fair queuing in wireless networks: issues and approaches. IEEE Personal Communications, vol.6, (
2、no.1), IEEE, Feb. 1999. p.44-53. Vaidya00 Vaidya, N.H.; Bahl, P.; Gupta, S. Distributed fair scheduling in a wireless LAN. MobiCom 2000. Proceedings of the Sixth Annual International Conference on Mobile Computing and Networking, Boston, MA, USA, 6-11 Aug. 2000. p.167-78. Recommended Bhagwat96 Bhagw
3、at, P.; Bhattacharya, P.; Krishna, A.; Tripathi, S.K. Enhancing throughput over wireless LANs using channel state dependent packet scheduling. Proceedings IEEE INFOCOM 96, San Francisco, CA, USA, 24-28 March 1996. p.1133-40 vol.3.,Bursty Wireless Channel Errors,Burst errors due to fading, frequency
4、collision etc.,Location-dependent Channel Capacity and Errors,Contention and effective channel capacity are location dependent Channel errors are location dependent Due to interference, fading etc. Bad interaction with how MAC schedules packets for transmission,Problems with FIFO Scheduling in MAC B
5、hagwat96,Burst errors may be spatially selective e.g. link to only one receiver may be under interference or in fade During burst, all retransmission attempts to specific MH will fail burst errors observed to be 50-100 ms long in WLANs FIFO is basically causing head of line blocking! other MHs starv
6、e even though link to them may be good TCP to all MHs will increase RTT estimates, further increasing timeouts poor resource utilization fairness problem: MHs with bad link claiming more link time & b/w a “fair” MAC is not enough in the presence of errors on the link,Channel State Dependent Scheduli
7、ng,Primary culprits: CSMA/CA MAC makes repeated attempts even when channel is bad FIFO dispatcher continues to send packets without regard to channel state Solution: defer scheduled transmissions until next good period transmit packets for other destinations (those marked good) meanwhile burst perio
8、ds for different MHs are independent potential risk: TCP sender may timeout but TCP timers average burst durations bad periods detected by radio feedback or multiple MAC transmit attempts channels remain marked bad for an estimated burst interval length round-robin scheduler (two sets: good & bad) w
9、orked best,Providing QoS,Communication Link,QoS provided by a combination of: resource reservation at the flow level “fair” resource allocation / packet scheduling at the packet level Easy to do in point-to-point links,Providing QoS in Wireless Links is Much Harder,Distributed tied to the multiple a
10、ccess problem User mobility makes resource reservation hard Channel errors make resource reservation meaningless (no guarantees!) make packet scheduling and fair resource allocation hard what does fair mean in an error prone channel? Time varying channel,QoS Scheduling for Communication Links,Schedu
11、ling Admission control (for “schedulability”) Policing (for “isolation”) Goals: meet performance and fairness metrics high resource utilization (as measured by resource operator) easy to implement small work per data item, scale slowly with # of flows or tasks easy admission control decisions Schedu
12、lable region: set of all possible combinations of performance bounds that a scheduler can simultaneously meet,Fairness,Intuitively each connection gets no more than what it wants the excess, if any, is equally shared Fairness is intuitively a good idea Fairness also provides protection traffic hogs
13、cannot overrun others automatically builds firewalls around heavy users reverse is not true: protection may not lead to fairness,A,B,C,A,B,C,Transfer half of excess,Unsatisfied demand,Max-min Fairness,Maximize the minimum share of task or flow whose demand is not fully satisfied Resources are alloca
14、ted in order of increasing demand, normalized by weight No task or flow gets a share larger than its demand Task or flows with unsatisfied demands get resource shared in proportion to their weights,Example,Given Four flows with demands 4, 2, 10, 4 and weights 2.5, 4, 0.5, and 1, and a link with capa
15、city C=16 Steps Normalize weights so that smallest is 1: 5, 8, 1, 2 In each round give a flow a share to its weight Round 1: allocation is 5, 8, 1, 2 Results in 1 and 6 units extra for flows 1 & 2 = 7 Allocate this 7 to flows still in deficit according to re-normalized weights Round 2: allocation is
16、 7*1/3 and 7*2/3 to flows 3 & 4 Results in 2.666 excess for flow 4 while flow 3 is still short Allocate this 2.666 to flows still in deficit according to re-normalized weights Round 3: allocation is 2.666 for flow 3 Results in flow 3 with a total of 6, i.e. a deficit of 4,Policing,Three criteria: (Long term) Average (Sustained) Rate 100 packets per sec or 6000 packets per min? crucial aspect is the interval length Peak Rate e.g., 6000 p p minute Avg and 1500 p
