There are a number of critical network services that support business applications. If these services have performance problems then the user experience will be at risk. For example, Domain Name Services (DNS) is crucial.
So, what does this mean for network measurement...?
Speed testing is a measurement of connection performance. Service testing measures the services that applications will use. If the network is affecting those services it will have an effect on the application.
MCS provides a number of service related tests, such as DNS, FTP and SNMP, that can also be affected by the network. This is important because they can have just as big of an impact on the application as bandwidth. One of the most useful service tests is SNMP, because it allows the assessment of utilization and demand on compliant routers, switches and firewalls. This enriches the bandwidth measurement data of how the network is performing and helps to find the cause of any problems.
Every data (TCP) packet that goes out onto the network is given a fixed amount of time to reach its destination.
If that packet fails to reach its destination another packet will be sent out onto the network, which is called a retransmit event.
When a retransmit event occurs the sender of the packets assumes it's the cause of the problem so it dramatically reduces the flow of packets, even though it may not be the problem.
The problem doesn't end here. If the reason for the retransmit event was packet loss then all the packets after the loss can't be processed.