In general, the aim of the Quality of Service is to ensure that particular data packets are transmitted either with high certainty or as quickly as possible.
- When transmitting data, it may well happen that data packets do not arrive at the recipient. However, some applications strongly depend on all of the sent data packets actually arriving. For example, an e-mail that is divided into numerous small data packets can only be reassembled at the receiver when all of its parts have arrived. The occasional packet arriving with a slight time delay is uncritical. These applications usually rely on the connection-oriented Transmission Control Protocol (TCP). This protocol ensures that the data is transported over the network correctly and in the right sequence. It reduces the transmission rate by itself if the confirmations of the sent data packets take longer to arrive, and automatically repeats transmission in the event of packet loss.
- For other applications, such as Internet telephony (Voice over IP, VoIP), it is vital that the data packets arrive at the recipient with a minimum of delay. The occasional data packet getting lost on the way is not so important here. The subscriber at the other end of the line can still understand the caller even if small parts of the speech get lost. This application therefore places priority on data packets being transmitted as quickly as possible. These applications frequently rely on the connectionless User Datagram Protocol (UDP). The administration overhead for this protocol is very small. However, there is no guarantee that the data packets are delivered in the correct order; they are simply sent off. There is no acknowledgment of receipt, so lost packets are not re-sent.
