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The active monitoring method obtains the current status of the network by setting up the test
machine from the point which one wishes to measure, and then sending extra traffic from
one machine to another during a specific time. Passive monitoring can monitor
the network status without additional traffic. But limited NPMs can be measured
comparing to the active monitoring method. By using SNMP agents, we can measure the
status of the network device. For example, RMON monitors traffic information with
SNMP agent. Therefore, Loss, Delay and Connectivity are the NPMs (Network
performance metrics) that can be mainly measured by active monitoring. Utilization and
Throughput are the NPMs which can be measured by passive monitoring. Functionality
and Throughput can be measured using SNMP agent.
Although we may obtain various NPMs using these network monitoring methods,it is
difficult to apply these values to SLA parameters directly. The contract between service
providers and customers is performed using SLA Parameters and the network quality is
measured using NPMs. To measure the SLA Parameters, first the NPMs for each
SLA parameter should be decided. A QoS parametercan be mapped to one NPM or
many. This mapping depends on the type of services and can be very complicated. And the
quality information of service should be presented in customer friendly form,
SLAParameters, not NPMs. Therefore it is necessary to translate the measured NPMs to
SLA parameter.In this report, SLA is used interchangeably with QoS parameters at
the application layer. And the QoS parameters at the network level is used
interchangeably same as NPMs (network performance metrics).
B. Quality of Service (QoS)
As said in [FH98], QoS is a vaguely defined term. But, here in our context, we are defining
QoS as a 'predictable traffic quality' that satifies following criterian:
1) Availability
2) Delay
3) Jitter
4) Packet loss probability
5) Throughput
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