- is an error that occurs when data networks are overly congested. When pieces of data ("packets") are unable to be transmitted, they are sometimes "thrown out" by the network. Packet loss may or may not be disruptive to the recipient of the data, depending on the severity of loss.
- is "a metric measured for packets traversing the network segment between the source reference point and destination reference point. The Packet Loss metric is reported as the number of lost packets at the destination reference point divided by the number of packets sent at the sender reference point to that destination."
VoIP is exceptionally intolerant of packet loss. Packet loss can result from excess latency, where a group of packets arrives late and must be discarded in favor of newer ones. It can also be the result of jitter, that is, when a packet arrives after its surrounding packets have been flushed from the buffer, making the received packet useless.
VoIP-specific packet loss issues exist in addition to the packet loss issues already associated with computer networks; these are the cases where a packet is not delivered at all. The good news is that VoIP packets are very small, containing a payload of only 10-50 bytes, which is approximately 12.5-62.5 ms, with most implementations tending toward the shorter range. The loss of such a minuscule amount of speech is not discernable or at least not worthy of complaint for a human VoIP user. The bad news is these packets are usually not lost in isolation. Bandwidth congestion and other such causes of packet loss tend to affect all the packets being delivered around the same time. So although the loss of one packet is fairly inconsequential, probabilistically the loss of one packet means the loss of several packets, which severely degrades the quality of service in a VoIP network.
- NIST Special Publication 800-58, at 21.