In an effort to win customers, an Internet service provider (ISP) may offer a service level agreement (SLA) which guarantees specified packet error rates, up-time rate, and packet delivery time. For example, an SLA may guarantee no more than .1 percent of packets will result in errors, the average delivery time will be less than 300 milliseconds, and the network will be up at least 99.999% of the time. Penalties will be specified if these conditions are not met.
For isochronous applications like telephony or video conferencing, the variation in packet arrival rate is also critical. This is called jitter, and it may also be guaranteed in an SLA.
QOS tends to be more important for isochronous applications or synchronous applications like transaction processing than for asynchronous applications like file transfer or e-mail.
An Internet packet may travel across networks operated by several different organizations, making it difficult to guarantee QOS. Some applications require end-to-end coverage by a single ISP so they can guarantee QOS.
Since a network designer has control over a local area network (LAN), QOS is relatively easy to maintain. This makes isochronous applications like voice over IP and video conferencing easier to implement on LANs than wide area networks where multiple organizations are involved.