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CS 4594 Broadband ATM GCRA and Leaky Bucket. From the ATM Forum The GCRA is used to define conformance to the traffic contract. For each cell that arrives.

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Presentation on theme: "CS 4594 Broadband ATM GCRA and Leaky Bucket. From the ATM Forum The GCRA is used to define conformance to the traffic contract. For each cell that arrives."— Presentation transcript:

1 CS 4594 Broadband ATM GCRA and Leaky Bucket

2 From the ATM Forum The GCRA is used to define conformance to the traffic contract. For each cell that arrives at the specified point, the GCRA determines whether the cell conforms to the traffic contract of its connection.

3 Generic Cell Rate Algorithm GCRA = Generic Cell Rate Algorithm Defined by two equivalent methods –Virtual Scheduling –Leaky Bucket

4 GCRA – Virtual Scheduling Arrival of cell k at t =t a (k) TATt a (k)+L TAT=TAT+I TAT=t a (k) Discard nonconforming cell Yes No Yes No Accept conforming cell k=k+1 Initially: TAT=t a (1)

5 GCRA - Virtual Scheduling Arrival of cell k at t =t a (k) TATt a (k)+L TAT=TAT+I TAT=t a (k) Discard nonconforming cell Yes No Yes No Accept conforming cell k=k+1 TAT-LTAT fasttoo fastslow (slow) (fast) (too fast) (not too fast) CGRA(I,L)

6 TAT = Theoretical Arrival Time TATTAT-LTATTAT-L time I I slow fast too fast II

7 Burst Size GCRA(T, τ) N = maximum number of contiguous conforming cells T = targeted average time in GCRA τ = limit tolerance in GCRA δ = actual average arrival time

8 Leaky Bucket Smoothes traffic Regulates traffic Overflow = nonconforming Nonconforming can be tagged or discarded

9 GCRA – Leaky Bucket (Equiv to Virtual Scheduling) Arrival of cell k at t =t a (k) X’<0 X’>L X=X’+I LCT=t a (k) X’=0 Discard nonconforming cell Yes No Yes No Accept conforming cell k=k+1 (slow) (fast) (too fast) (not too fast) X’=X-(t a (k)-LCT) Initially: X=0 LCT = t a (1) Meanings: X = Value of counter X’ auxiliary variable LCT = Last Conformance Time L = bucket size

10 Explanation X is value of the counter. When a cell arrives at time t a (k), X’ is set to the value of what the counter would be: X’ = X – (t a (k) – LCT) (This is the value of previous counter X minus the amount would have leaked. If X’ is negative (bucket would be empty), the cell is compliant, X’ is set to 0, X is set equal to X’ + I, and LCT = t a (k). If X’ is nonnegative, check if bucket is full. If not full, cell is compliant and do as above. If bucket is full, the cell is not compliant, and nothing is updated.

11 Leaky (token) bucket as a gate ir = information rate bs = burst size ? yes bs ir Let token escape as cell goes by Tokens enter at given rate Cell goes through if bucket has enough tokens no inputoutput

12 Double Leaky Bucket Input Overflow output Overflow

13 Double Leaky Bucket (another arrangement) Input Overflow output Overflow

14 Double leaky buckets as a pair of gates pir = peak information rate pbs = peak burst size cir = committed information rate cbr = Committed burst size yes cbs pbs pir cir no

15 LBP schemes LBP=Leaky Bucket Policy

16 LBP Scheme type 1 Token pool server arriving cells dropped cells departing cells Arriving cells are dropped when no token is present in the token pool

17 LBP Scheme type 2 Token pool server arriving cells departing cells Arriving cells are tagged if not complient. tagged cells

18 LBP Scheme type 3 Token pool server arriving cells dropped cells departing cells Arriving cells are placed in the queue if no token is present in the token pool buffer

19 LBP Scheme type 4 Token pool server arriving cells departing cells Arriving cells are placed in the queue if no token is present in the token pool. Nonconforming cells are tagged. buffer tagged cells

20 GCRA and ATM Services GCRA is used to define usage parameters CBR (Class A) (one GCRA) –GCRA(1/PCR, CDVT) rt-VBR (Class B) (two GCRAs) –GCRA(1/PCR, CDVT) –GCRA(1/SCR, CDVT+BT) –ATM Forum defines 3 variations based on how the CLP bit is used. ABR –uses GCRA with changing values for parameters.

21 ATM Conformance Definitions VBR.1 – Leaky bucket 1 sends CLP(0+1) compliant cells to leaky bucket 2 and discards the noncompliant cells. Leaky bucket 2 sends CLP(0+1) compliant cells to the network and discards noncompliant cells. VBR.2 – Leaky bucket 1 sends CLP(1) compliant cells to the network, sends CLP(0) compliant cells to leaky bucket 2, and discards CLP(0+1) noncompliant cells. Leaky bucket 2 sends CLP(0) compliant cells to the network and discards noncompliant cells. VBR.3 – Leaky bucket 1 sends CLP(1) compliant cells to the network, sends CLP(0) compliant cells to leaky bucket 2, and discards CLP(0+1) noncompliant cells. Leaky bucket 2 sends CLP(0) compliant cells to the network and tags noncompliant cells and sends them to the network.


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