Speed Tests and Overhead

If you're familiar with speed testing services like fast.com, speedtest.net, or nperf.com, then you may be aware that test results from these websites don't match the advertised speed of your Internet service plan.

If there's a significant difference between test results and your service plan's speed, there may be a problem with your connection, computer, or other equipment. However, if speed tests are very close (90%+) to the rated speed of your plan, then the difference is likely explained by protocol overhead. In this case, your connection is probably performing at its rated speed, but a certain fraction of the bandwidth isn't being measured by the testing service due to overhead.

Letters and Packets

In broad terms, a computer network can be compared to a postal service. In both systems, messages are transported from senders to recipients. In a postal service, discrete communications are referred to as letters and in computer networks they're called packets. Just as a letter is comprised of an envelope and a message, a packet consists of headers and data.

Envelopes and headers serve similar purposes: each tell their respective systems about the sender and recipient. Messages and data are also analogous: both represent the contents of their communications, i.e. the information being sent.

Communication Overhead

If a postal service could eliminate the need for envelopes, they could transport letters more quickly and cheaply without the envelopes' added size and weight. For this reason, an envelope can be thought of as a form of overhead; it's a necessary but costly part of a system. The header of a packet can be thought of in the same way.

While an envelope might comprise about half of a letter's overall weight, a packet's header and checksum information only consumes about 6% of its total size.

Therefore, only about 94% of a connection's bandwidth is going to be usable on any Ethernet network.

Speed Tests and Overhead

Letters and Packets

Communication Overhead

Further Reading