Bandwidth monitoring with PRTG

While there is no way to measure the total available bandwidth, there are many ways to define measured bandwidth, depending on the need.

Theoretical maximum

The highest transmission rate under ideal circumstances. The theoretical maximum transfer rate cannot be achieved in actual installations and is usually only used for comparison purposes, for example, to determine how well a connection is functioning compared to its theoretical maximum potential.

Effective bandwidth 

The highest reliable transmission rate. It is always lower than the theoretical maximum and sometimes considered the best usable bandwidth. The effective bandwidth is necessary for understanding the amount of traffic a connection can support.

Throughput 

The average rate of successful data transfer. It is useful for understanding the typical or usual speed of a connection. Throughput is the size of the transmitted data divided by the time it takes for the transmission to finish. Measured in bytes per second, throughput can be compared to the effective bandwidth and the theoretical maximum as a way of determining how well the connection is performing.

Goodput 

The amount of useful data that is transferred, excluding undesirable data such as packet retransmissions or protocol overhead. Goodput is calculated by dividing the size of the transmitted data by the amount of time the transmission took.

Total transfer method 

Counts all traffic over a set period of time, typically a month. This is most useful for billing based on how much bandwidth is used.

95th percentile method 

To avoid having bandwidth measurements skewed by spikes in usage, carriers often use the 95th percentile method. The idea is to continuously measure bandwidth usage over time and then remove the top 5 percent of use. This is useful for billing based on how much bandwidth is usually used in a set period.

In real-world networks, bandwidth varies over time depending on usage and network connections. As a result, a single bandwidth measurement says very little about the actual bandwidth usage. A series of measurements can be more useful when determining averages or trends.