Editing Quality of service (section)

==Factors==
In [[packet-switched network]]s, quality of service is affected by various factors, which can be divided into human and technical factors. Human factors include: stability of service quality, availability of service, waiting times and user information. Technical factors include: reliability, scalability, effectiveness, maintainability and network congestion.<ref name="peuhkuri">{{cite web |author=Peuhkuri M. |title=IP Quality of Service |publisher=Helsinki University of Technology, Laboratory of Telecommunications Technology |url=https://www.netlab.tkk.fi/~puhuri/htyo/Tik-110.551/iwork.ps |date=1999-05-10}}</ref>

Many things can happen to packets as they travel from origin to destination, resulting in the following problems as seen from the point of view of the sender and receiver:

=== Goodput ===
Due to varying load from disparate users sharing the same network resources, the maximum throughput that can be provided to a certain data stream may be too low for real-time multimedia services.

=== Packet loss ===
The network may fail to deliver (''drop'') some packets due to network congestion. The receiving application may ask for this information to be retransmitted, possibly resulting in [[congestive collapse]] or unacceptable delays in the overall transmission.

=== Errors ===
Sometimes packets are corrupted due to [[bit error]]s caused by noise and interference, especially in wireless communications and long copper wires. The receiver has to detect this, and, just as if the packet was dropped, may ask for this information to be retransmitted.

=== Latency ===
It might take a long time for each packet to reach its destination because it gets held up in long queues, or it takes a less direct route to avoid congestion. In some cases, excessive latency can render an application such as VoIP or online gaming unusable.

=== Packet delay variation ===
Packets from the source will reach the destination with different delays. A packet's delay varies with its position in the queues of the routers along the path between source and destination, and this position can vary unpredictably. Delay variation can be absorbed at the receiver, but in so doing increases the overall latency for the stream.

=== Out-of-order delivery ===
When a collection of related packets is routed through a network, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than they were sent. This problem requires special additional protocols for rearranging out-of-order packets. The reordering process requires additional buffering at the receiver, and, as with packet delay variation, increases the overall latency for the stream.