Traffic Management for the Available Bit Rate (ABR) Service in ...

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6.13.2 Switch Averaging Interval AI The switch averaging or measurement interval determines the accuracy of feedback. This interval is used to measure the load level, link capacity and the number of active VCs for an outgoing link. The length of the measurement interval establishes a tradeo between accuracy and steady state performance. This tradeo has been brie y discussed in section 6.5. ERICA measures the required quantities over an averaging interval and uses the measured quantities to calculate the feedback in the next averaging interval. Averag- ing helps smooth out the variation in the measurements. However, the length of the averaging interval limits the amount ofvariation which can be eliminated. It also determines how quickly the feedback can be given to the sources, because ERICA gives at most one feedback per source per averaging interval. Longer intervals produce better averages, but slow down the rate of feedback. Shorter intervals may result in more variation in measurements, and may consistently underestimate the measured quantities. The load factor and available capacity are random variables whose variance depends on the length of the averaging interval. In practice, the interval required to measure the number of active sources is su cient for the measurement of the load factor and available capacity. Both of these averaged quantities are fairly accurate, with an error margin of (one cell/averaging interval). Setting the target utilization below 100% helps drain queues due to errors in measurement of all the quantities. Whenever the scheme faces tradeo s due to high errors in measurement, the degree of freedom is to reduce the target utilization parameter, sacri cing some steady state utilization for convergence. 171
6.14 ERICA+: Queue Length as a Secondary Metric ERICA+ is a further modi cation of ERICA. In this and the following section, we describe the goals, target operating point, the algorithm, and parameter settings for ERICA+. ERICA depends upon the measurement of metrics such astheoverload factor and the number of active ABR sources. If there is a high error in the measurement, and the target utilization is set to very high values, ERICA may diverge, i.e., the queues may become unbounded, and the capacity allocated to drain the queues becomes insu cient. The solution in such cases is to set the target utilization to a smaller value, allowing more bandwidth to drain queues. However, steady state utilization (utilization when there is no overload) is reduced because it depends upon the target utilization parameter. A simple enhancement to ERICA is to have a queue threshold, and reduce the target utilization if the queue exceeds the threshold. Once the target utilization is low, the queues are drained out quickly. Hence, this enhancement maintains high utilization when the queues are small, and drains out queues quickly when they become large. Essentially, we are using the queue length as a secondary metric (input rate is the primary metric). In ERICA, we have not considered the queue length or queue delay as a possible metric. In fact, we rejected it because it gives no indication of the correct rates of the sources. In ERICA+, we maintain that the correct rate assignments depend upon the aggregate input rate, rather than the queue length. However, we recognize two facts about queues: a) non-zero queues imply 100% utilization, and, b) a system with very long queues is far away from the intended operating point. Hence, in ERICA+, 172
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Tra c Management for the Available
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ABSTRACT With the merger of telecom
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Tra c Management for the Available
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To my family iv
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VITA April 30, 1971 :::::::::::::::
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2.7 Switch Behavior . . . . . . . .
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5.8 Proof: Fairness Algorithm Impro
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8.15 Factors A ecting Bu ering Requ
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Bibliography . . . . . . . . . . .
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8.12 Maximum Queues for Satellite N
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5.1 Transmitted cell rate (instanta
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6.13 Results foratwo sources con gu
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7.11 E ect of UILI on Medium Bursts
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C.3 Flow Chart of Bi-Directional Co
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header information which limits the
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switches since a standard does not
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congestion control deals with the c
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hand side of the equation is the su
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analyses which are used to validate
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CHAPTER 2 THE ABR TRAFFIC MANAGEMEN
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time (RTT). As we explain later, AB
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feedback from nearby switches to re
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Note that in-rate and out-of-rate d
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Data cells also have an Explicit Fo
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opportunity the source may nd that
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Figure 2.7: Scheduling of forward R
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as the timeout value) which reduces
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It has been incorrectly believed th
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NI CI Action 0 0 ACR Min(ER, ACR +
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Source Rule 13: Sources can optiona
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Switch Rule 1: This rule speci es t
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Observe that the ABR tra c manageme
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complex method like per-VC queuing
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graphs have a steady state with con
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Figure 3.2: Operating point between
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The following example illustrates t
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mention this concept of fairness fo
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control problem was also simpler. S
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of such an occurrence is expected t
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y giving only one feedback per meas
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CHAPTER 4 SURVEY OF ATM SWITCH CONG
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The adaptive FCVC algorithm [63] co
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np. Thus, long path VCs have fewer
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networks like ATM can have complete
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scheme followed by a discussion of
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exibility of decoupling the enforce
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4.6.1 Key Techniques In EPRCA, the
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If the mean ACR is not a good estim
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is the ratio of the input rate to t
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which shares the link equally as al
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proportional to the the unused ABR
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The key technique in the scheme is
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the bandwidth allocations to satis
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4.10 DMRCA scheme The Dynamic Max R
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on its CCR. Further MAX times out i
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other words, a di erent set of para
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A combination of several ideas in a
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4.13 SP-EPRCA scheme The SP-EPRCA s
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RTD and shuts o the source (for sta
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4.14.1 Common Drawbacks Though the
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The ATM Tra c Management standard a
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5.1.1 Control-Cell Format The contr
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The last two elds are used in the b
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Figure 5.3: Flow chart for updating
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LAF in cell Max(LAF in cell, z) The
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the time of departure (instant mark
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the steady state. The system operat
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5.2.3 Use Measured Rather Than Decl
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said about the maximum queue length
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The key problem with some unipolar
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Figure 5.6: Space time diagram show
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As noted, these heuristics do not g
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Transmitted Cell Rate ABR Queue Len
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Page 151 and 152: Transmitted Cell Rate Link Utilizat
Page 157 and 158: Figure 5.17: The parking lot fairne
Page 159 and 160: Figure 5.19: Network con guration w
Page 165 and 166: 5.8 Proof: Fairness Algorithm Impro
Page 167 and 168: Region 2: y s and x s and U(1 + ) x
Page 169 and 170: eing divided into eight non-overlap
Page 171 and 172: Proof for Region 2 Triangular regio
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Page 175 and 176: The OSU scheme is, therefore, incom
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Page 179 and 180: (a) Regions used to prove Claim C1
Page 181 and 182: 6.1 The Basic ERICA Algorithm The s
Page 183 and 184: A ow chart of the basic algorithm i
Page 185 and 186: efore competing sources can receive
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Page 193 and 194: ABR Capacity Input Rate Overload Fa
Page 195 and 196: of arithmetic averaging used above.
Page 197: level of control. These parameters
Page 201 and 202: 6.16 ERICA+: Maintain a \Pocket" of
Page 203 and 204: hence, introduces a new parameter,
Page 205 and 206: Figure 6.4: Linear functions for ER
Page 207 and 208: and Figure 6.6: The queue control f
Page 209 and 210: small averaging intervals. If the a
Page 211 and 212: The maximum value of T 0 also depen
Page 213 and 214: 6.22 Performance Evaluation of the
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Page 217 and 218: 6.22.4 Fairness Figure 6.9: Parking
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Page 221 and 222: From the gures, it is clear that ER
Page 223 and 224: counts the bursty source as active
Page 225 and 226: 6.23 Summary of the ERICA and ERICA
Page 227 and 228: (a) Transmitted Cell Rate (c) Link
Page 233 and 234: (a) Transmitted Cell Rate (basic ER
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(a) Transmitted Cell Rate (basic ER
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RM cells. In this chapter, we devot
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low. This tradeo was discovered and
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in the speci cation. b) ACR shall n
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7.1.6 December 1995 Proposals There
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Figure 7.2: Multiplicative vsAdditi
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is never triggered. However, the PR
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simulation results of bursty source
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1. The time-based proposal also ind
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The switch maintains a local alloca
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PNI = f0, 1g : f1 ) No rule 5b, 0 )
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after reaching the goal. The time-b
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Figure 7.8: Closed-Loop Bursty Tra
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The algorithm measures the load and
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Medium Bursts Medium bursts are exp
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count-based technique may be insu c
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Figure 7.13: An event trace illustr
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CHAPTER 8 SUPPORTING INTERNET APPLI
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u ering which does not depend upon
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4 make it to the destination are ar
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Figure 8.3: At the ATM layer, the T
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issues and e ect of bursty applicat
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from the loss and they trigger the
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8.8 Performance Metrics We measure
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the performance is fair. Also, the
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Figure 8.7(b) shows the rates (ACRs
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Window Size in bytes vfive-tcp/opti
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The e ect of large bu ers on CLR is
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8.13 Summary of TCP/IP performance
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Feedback delay: Twice the delay fro
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on a link, two cells are expected a
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state only after the switch algorit
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queue length is less susceptible to
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to equalize rates for fairness, and
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QB = Link bandwidth (RT T ; T ) and
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u ered at the end-system, and not i
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Part b): When ABR load goes away, t
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All our simulations presented use t
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Averaging RTT(ms) Feedback Max Q Th
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a modi ed version of the ERICA algo
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ABR is better than UBR in these (en
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problems. During the ON time, the V
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hand, the frequency of the VBR is h
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like ERICA+ which uses the queueing
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minimum fairshare is low. This may
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8.17 E ect of Long-Range Dependent
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terminology) are called \Presentati
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The key point is that the MPEG-2 ra
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the MPEG-2 Transport Stream, and st
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8.17.4 Observations on the Long-Ran
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For the video sources, we choose me
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Video Sources ABR Metrics # Mean St
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However, with modi cations to ERICA
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Video Sources ABR Metrics # Avg Src
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Video Sources ABR Metrics # Avg Src
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On the other hand, if the applicati
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of managing bu ers, queueing, sched
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hence control the total load on the
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call such a switch a \VS/VD switch"
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Figure 9.2: Per-class queues in a n
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which arises is where the rate calc
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9.2 The ERICA Switch Scheme: Renota
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The unknowns in the above equations
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Figure 9.9: Two methods to measure
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9.4 VS/VD Switch Design Options 9.4
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# VC Rate VC Input Rate Input Rate
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8 uses source rate measurement, we
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The allocated rate update and the e
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sources in chapter 6. We expect the
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con guration mentioned in the table
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can be very di erent for di erent V
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CHAPTER 10 IMPLEMENTATION ISSUES At
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With an enhanced UBR service, appli
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2. Some switch schemes have a proce
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4. Large legacy switches have a pro
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section 9. Further, WAN switches wo
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CHAPTER 11 SUMMARY AND FUTURE WORK
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good transient performance. Since r
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variant background tra c conditions
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APPENDIX A SOURCE, DESTINATION AND
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7. After following behaviors #5 and
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set the QL and SN elds to zero, pre
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d) VS/VD Control: The switch may se
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5. Setting of other parameters at V
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4. The averaging interval timer exp
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1. Initialization: Target Cell Rate
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THEN IF (OCR In Cell Fair Share Rat
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APPENDIX C ERICA SWITCH ALGORITHM:
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Number Active VCs In Last Interval
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IF (NOT(Averaging VCs Option)) THEN
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IF (Load Factor = In nity) THEN Loa
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; (Contribution[VC] = Decay Factor)
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Name Explanation Flow Chart (FC) or
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Figure C.2: Flow Chart for Achievin
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Figure C.4: Flow Chart of averaging
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Figure C.6: Flow chart of averaging
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C.3 Pseudocode for VS/VD Design Opt
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(* | Bottleneck rate of next loop |
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Follow SESRules 1-4 (see appendix A
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CRM - Missing RM-cell Count DIR bit
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TUB -Target Utilization Band Trm -
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[12] J. Bennett and G. Tom Des Jard
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[38] M. Grossglauser, S.Keshav, and
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[64] H. T. Kung. Flow Controlled Vi
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