Quality of Service
BAT54-Rail/F..
Release
7.54
06/08
9.1
Why QoS?
311
9 Quality of Service
This chapter dedicates itself to quality: Under the generic term Quality of Ser-
vice (short: QoS) those LCOS functions are summarized, which are con-
cerned with the guarantee of certain service availabilities.
9.1 Why QoS?
The main objective of Quality of Service is to transfer certain data packets ei-
ther particularly safe or as immediately as possible:
D It may happen during a data transfer that data packets are not delivered
to the addressee. But for some applications it is very important that all
sent packets really do arrive. An e-mail, for example, divided into several
small data packets, can only be assembled together again, when all parts
have arrived completely. Whether one or an other packet arrives with little
time delay does not make any difference. These applications often count
on the connection-orientated Transmission Control Protocol (TCP). This
protocol ensures that data will be transferred correctly and chronologically
via the net. It automatically adjusts the sending rate downwards if the con-
firmation of sent data packets is outstanding for longer times, and also
takes care of repeated transmission in case of packet losses.
D In other applications, e.g. telephony via the Internet (Voice-over-IP,
VoIP), it is - differently to the case above - very important that the data
packets arrive at the addressee with only little time delay. But it really
doesn’t matter if once a data packet gets lost in this case. The participant
at the other end of the connection will understand the caller, even if small
parts of the speech got lost. This application aims at the fastest sending
of data packets as possible. The connectionless User Datagram Protocol
(UDP) is often used for this kind of application. Also this protocol has very
little administrative overhead. But chronological delivery of packets is not
guaranteed, data packets are simply sent out. Because no confirmation
receipt exists, lost packets never get delivered again.
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