Definitions of Terms

To provide clarity for people of all expertise levels, here we provide definition of terms used in M-Lab documentation or with respect to Internet research, measurement, etc.


Access ISP or Access Network

An access ISP is the Internet Service Provider that you interact with the most. They connect people’s homes and charge a monthly fee for Internet service. To connect their networks and customers to the rest of the Internet, they either directly interconnect with transit ISPs or pay one or more transit ISPs to carry their traffic.

Inter-eXchange Points (IXPs)

The physical locations where ISPs exchange Internet traffic (interconnect) between their networks.


An interconnection is where different ISPs connect their respective networks to one another. All of these interconnection taken together make it possible to access content anywhere on the Internet. Many interconnections are private, where only two ISPs meet, while others are shared Inter-eXchange Points (IXPs) where many ISPs connect to each other.

Measurement Point

An M-Lab node, consisting of three specially configured servers connected to a Transit ISP. When you run an M-Lab test, your computer connects to the closest M-Lab measurement point, which coordinates the test you’re running and collects the data for that test.

Transit ISP or Transit Network

A transit ISP is a kind of “meta-ISP”, which provides long-distance carrying of packets, usually to other ISPs instead of to consumers directly. Transit ISPs are the organizations which lay undersea cables and dig trenches across mountain ranges, and then charge ISPs to carry traffic through those links. Transit ISPs are similar to the long distance shipping companies that move big containers – they aren’t the company that brings the Internet into your house, but they do the transportation between cities, regions, countries, and across the ocean. Sometimes transit ISPs are also called “Tier 1 ISPs”, because those top-tier ISPs can usually get traffic to anywhere in the world.

Tools and Tests

Network Diagnostic Tool (NDT)

A sophisticated speed and diagnostic test suitable for both the novice and the network researcher, NDT reports upload and download speeds, attempts to determine what problems limited those speeds, and provides detailed diagnostic reporting on what it found.


Download Throughput (Megabits per second, abbreviated Mbps)

How much data can be downloaded (server to user computer) per unit of time. Note that networking capacity is generally measured in bits per second, while application file sizes are generally measured in bytes, which are 8 bit each. So for example downloading a 1 megabyte photo image in 10 seconds would be 8 Megabits per second or 8 Mbps.

Packet Retransmission Rate

The fraction of packets (bundles of Internet data) that need to be sent more than once to deliver complete data. A big part of the Internet’s robustness comes from its ability to repair missing data by having it retransmitted. Data can be lost due to congestion or other problems in the network. The repair process normally has no explicit symptoms except it does take time and hurts performance. The retransmission rate is a measure of how much difficulty the network is having delivering the data in the first place, and provides clues as to how much the attached computers had to slow down to repair the losses.

Round Trip Time (Milliseconds, ms)

Also sometimes referred to as Latency, Round Trip Time is a measure of the time it take for a packet to go from point A to point B and back. The shorter the time, the better. Minimum: the minimum latency measured in transmissions from the server to the client, reported in milliseconds. Normally, this is a good indication of physical path distance, except when there is high load. ISPs with smaller RTTs are probably better connected to other ISPs, meaning that they have more interconnections in more widely distributed geographic locations. Average: the average latency of data transfers from the server to the client. This is calculated as the sum of round trip times sampled during the test against the number of samples, reported in milliseconds.Comparing Average and Minimum RTTs provides an estimate of the average delay caused by queuing traffic in the network.

Upload Throughput (Megabits per second, Mbps)

How much data can be uploaded (user computer to Internet server) per unit of time.

Back to Top