1. Routing Metrics Neil Tang 11/03/2009
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2. Outline Weaknesses of Hop-count Based Metric Delay Based Metric
Utilization Based Metric
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3. Weaknesses of Hop-Count Metric
It does not distinguish between links on a delay basis.
It does not distinguish between links on a capacity basis.
It does not distinguish between links on a load basis.
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4. Delay Based Metric
Delay = queuing delay + transmission delay + prop delay
queuing delay = depart time – arrival time
transmission delay = message size/bandwidth
propagation delay = distance/speed of light
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5. Delay Based Metric Problems
Instability: traffic switch back and forth from certain links frequently.
A large range of link cost values: e.g., a heavily loaded 9.6kpbs link may be 127 more costly than a lightly loaded 56kbps link.
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6. Utilization Based Metric
Link utilization is measured based on the average of the previous reported utilization.
Link cost is given be a function which takes the measured link utilization, link type (e.g., wireless link, wired link) and link capacity as parameters.
A hard limit is usually set for the range of link values.
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7. Utilization Based Metric
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8. Routing Metrics
None of metrics are instantaneous. Link cost is usually given based on an average value over a period of time.
In order to reduce the overhead, link cost update messages are normally sent out only when the change exceeds some threshold.
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