Frame Relay Congestion Control

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1 Frame Relay Congestion Control Professor of Computer and Information Sciences Columbus, OH

2 Overview Congestion avoidance vs recovery Discard control Explicit forward/backward congestion notification Implicit notification 2

3 Frame Relay Congestion Techniques Discard Control (DE Bit) Backward Explicit Congestion Notification Forward Explicit Congestion Notification Implicit congestion notification (sequence numbers in higher layer PDUs) 3

4 Discard Control Committed Information Rate (CIR) Committed Burst Size (Bc): Over measurement interval T T = Bc/CIR Excess Burst Size (Be) Between Bc and Be Mark DE bit Over Be Discard 4

5 Bits Be B c Discard Access rate CIR DE = 1 DE = 0 T 0 T + T 0 Time Frame 1 Frame 2 Frame 3 All frames with CIR 5

6 Bits B c + B e B c Discard Access rate CIR DE = 1 DE = 0 T 0 T + T 0 Time Frame 1 Frame 2 Frame 3 Frame 4 One Frame marked DE 6

7 Bits B c + B e B c Discard Access rate CIR DE = 1 DE = 0 T 0 T + T 0 Time Frame 1 Frame 2 Frame 3 Frame 4 One Frame marked DE; one frame discarded 7

8 Leaky Bucket Algorithm B e = Excess burst size Limit C to B + B e c Discard any incoming data while C is at its threshold C = counter; increment with incoming data Decrement C by MIN [C,B c ] every T Time units CIR = B /T c c 8

9 Performance Parameters Defined in I.233 Annex A and in T1.606 Section 4 Used by commercial networks in customer contracts Throughput: Successful information bits/second. Transit Delay: first-bit in to last bit out. Over one or all networks. Residual error rate (RER): 1 - Frames with no error/total frames Delivered Errored frames: Frames with undetected errors Delivered duplicated frames Delivered out-of-sequence frames Lost frames: Not delivered within a specified time 9

10 Performance Parameters (Cont.) Misdelivered frames: Delivered to the wrong destination Switched virtual call establishment delay Switched virtual call clearing delay Premature disconnect Switched virtual call clearing failure 10

11 Forward Explicit Congestion Notification S FECN Frame Relay Switch D Source sets FECN = 0 Networks set FECN if avg Q >1 Destination tells source to increase/decrease the rate (or window) Start with R = CIR (or W=1) If more than 50% bits set decrease to R (or 0.875W) If less than 50% bits set increase to R (or min{w+1, Wmax}) If idle for a long time, reset R = CIR (or W=1) 11

12 Backward Explicit Congestion Notification S Frame Relay Switch BECN D Set BECN bit in reverse traffic or send Consolidated Link- Layer Management (CLLM) message to the source On first BECN bit: Set R = CIR On further "S" BECNs: R=0.675 CIR, 0.5 CIR, 0.25 CIR On S/2 BECNs clear: Slowly increase R = R If idle for long, R = CIR 12

13 BECN (Cont.) For window based control: S = One frame interval Start with W=1 First BECN W = max(0.625w,1) Next S BECNs W = max(0.625w,1) S/2 clear BECNs W = max(w+1, Wmax) CLLM used if no reverse traffic CLLM = XID message on maintenance DLCI = 1007 (decimal) CLLM contains a list of congested DLCIs 13

14 Implicit Congestion Control Decrease window on frame loss Increase window slowly Decrease by 1, Decrease to Wmin, Decrease by a factor α Increase by 1 after N frames Increase by 1 after W frames 14

15 Summary Discard strategy: Leaky bucket Forward explicit congestion notification Backward Explicit congestion notification Implicit congestion control 15

16 Homework Read chapter 12 of Stallings book 16

17 Leaky Bucket in ATM Networks Overflow Non-Conforming Conforming Provides traffic shaping: Input bursty. Output rate controlled. Excess traffic can be discarded or admitted with CLP = 1 17

18 Generic Cell Rate Algorithm: GCRA(I, L) I = Increment = Inter-cell Time = Cell size/pcr L = Limit Leaky bucket of size I + L and rate 1 Last Theoretical Cell Time No OK Arrival Time Yes I-L I L Time I L 18

19 GCRA(I, L): Virtual Scheduling Algorithm Cell Arrival at t TAT<t? Late? Yes (late) Non Conforming Cell Yes TAT = Theoretical Arrival Time TAT>t + L? Too early? TAT = TAT + I Conforming Cell 19 No (early) No TAT = t

20 GCRA(I, L): Leaky Bucket Algorithm F = X-(t-LCT) F<0? Yes Non Conforming Cell Yes F>L? No F = 0 LCT = Last Compliance Time X = Bucket contents at LCT F = Bucket contents now 20 No X = F + I LCT = t Conforming Cell

21 GCRA Examples δ = cell time = 2.73 µs at 155 Mbps GCRA(4.5 δ, 0.5 δ): Arrivals TAT Time GCRA(4.5 δ, 7 δ): Arrivals TAT Time

22 Maximum Burst Size (MBS) δ = cell time at PCR, I = cell time at SCR, L=Limit N = Maximum burst size GCRA(I, L ): 0 δ (N-1)δ Arrivals TAT Time 0 I (N-1) I (N-1) I - (N-1)δ < L MBS= N= Int[1+ L /(I -δ)] L = (MBS-1)(I -δ) 22

The transmission of MPEG-2 VBR video under usage parameter control

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS Int. J. Commun. Syst. 2001; 14:125}146 The transmission of MPEG-2 VBR video under usage parameter control Lou Wenjing, Chia Liang Tien*, Lee Bu Sung and Wang