Performance measurement: quality of service

The following performance measures are often used in assessing quality of service (QOS):
  1. Ping time (latency): The time to send a single packet from one host to another and get back an acknowledgement that the packet was received.
  2. Average data transfer rate: The average rate at which a data is transmitted between two hosts. (Note that transfer rate may be asymmetric with transfers in one direction being faster than in the other direction.
  3. Maximum (burst) data transfer rate: The maximum data transfer rate if there is no competing traffic.
  4. Variability in ping time (jitter): The variability of Ping time between two hosts.
  5. Packet loss rate: The percent of packets which fail to arrive or arrive with an error.

Since routers only make a best effort to deliver packets and congestion and errors can occur, QOS varies. In an effort to win customers, an Internet service provider (ISP) may offer a service level agreement (SLA) which guarantees minimum performance. For example, an SLA may guarantee a data transfer rate of no less than 1 Mbps, maximum latency of 300 milliseconds, and a packet loss rate under .001 percent. Penalties will be specified if these conditions are not met.

Since a packet may travel across networks operated by several different organizations it is difficult to guarantee quality of service. Some applications require end-to-end coverage by a single ISP so they can guarantee QOS.

Network Diagnostic Tool (NDT) is an excellent tool for measuring data transfer rate. (NDT requires you to have a java runtime installed. If you do not have it, download and install it).

Some ISPs post performance statistics on line as a marketing tool.

Stanford University has been monitoring Internet performance for many years.

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