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CNPA B

Nasser S. Abouzakhar

Resource Allocation 2

Week 6 – Lecture 22nd November, 2009

Content

Evaluation Criteria

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Evaluation Criteria Effective Resource Allocation.

– Throughput allowing as many packets into the network as possible. drive the utilisation of all the links up to 100%. however, increasing the throughput increases the length of the

queues at each router. This means that packets are delayed longer.

Power = Throughput/Delay

a function of how much load that can be placed on the network.

3Source: Peterson & Davie, 2007 p 465

Evaluation Criteria, cont.

The power metric limitations.– The power is based on an M/M/1 queuing network that

assumes infinite queues. However, real networks have to drop packets.

– Assumes a single connection (flow).

In an M/M/1 queue, 1 refers to a single server and both Ms mean that both the packet arrival and service time have an exponential “Markovian” distribution.

4Source: Peterson & Davie, 2007 p 465

Evaluation Criteria, cont.

The load is set by the resources allocation mechanism.

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Many packets are allowed to traverse the network. This leads to increasing the delay due to long queues.

Not many packets being allowed into the network to keep the links busy.

Source: Peterson & Davie, 2007 p 465

Ratio of throughput to delay as a function of load

Evaluation Criteria, cont.

Fair Resource Allocation.– an equal share of BW does not mean a fair share of BW.

– Given a set of flow throughputs (x1, x2,....., xn) in bps, the fairness index to the flows is

1 is representing greatest fairness.6

1) index fairness (0 ,.....,,

1

2

2

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ii

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xn

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xxxf

Source: Peterson & Davie, 2007 p 467

Evaluation Criteria, cont.

Consider the case where all n flows get a throughput of 1 unit of data per second. In this case the fairness index is

Suppose one flow gets a throughput of . The fairness index is

Whether the odd flow out was more or less than all other flows, the fairness index would be < 1.

7Source: Peterson & Davie, 2007 p 467

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n

1

1

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Example

Suppose a congestion-control scheme results in a collection of competing flows that achieve the following throughput rates: 100 KBps, 60 KBps, 110 KBps, 95 KBps, and 150 KBps.

(a)Calculate the fairness index for this scheme.

(b)Now add a flow with a throughput rate of 1,000 KBps to the above, and recalculate the fairness index.

8Source: Peterson & Davie, 2007 p 528

Solution

(a) The fairness index is 0.926.

x1+· · ·+x5 = 515 and

(b) The index falls to 0.361.

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21 xx

1) index fairness (0 ,.....,,

1

2

2

111

n

ii

n

ii

n

xn

x

xxxf

Reference

Computer Networks: A systems approach by Larry Peterson and Bruce Davie, published by Morgan Kaufmann (Fourth edition ISBN: 0 12 370548 7).

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