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Two 1 Mbps audio connections transmit their packets over the 1.5 Mbps link. The combined data rate of the two flows
exceeds the link capacity. Even with classification and marking (Principle 1), isolation of lows (Principle 2), and
sharing of unused bandwidth (Principle 3), of which there is none, this is clearly a losing proposition. There is simply
not enough bandwidht to accommodate the application's needs. If the two applications equally share teh bandwidth, each
would receive only 0.75 Mbps.
For a flow that needs a minimum QoS in order to be considered usable, the network should either allow or block
the flow.
Implicit with the need to provide a guaranteed QoS to a flow is the need for the flow to delcare its QoS requirements.
This process of having a flow declare its QoS requirement, and then having the network either accept the flow or block the
flow is referred to as the call admission process. The need for call admission is the fourth underlying
principle in the provision of QoS guarantees:
Principle 4: If sufficient resources will not always be available, a call admission process is needed in
which flows declare their QoS requirements and are then either admitted to the network or blocked from the network.
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