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LANCOM Reference Manual LCOS 3.50 ̈ Chapter 9: Quality of Service
168
Quality of Service
9 Quality of Service
This chapter dedicates itself to quality: Under the generic term Quality of Serv-
ice (short: QoS) those LCOS functions are summarized, which are concerned
with the guarantee of certain service availabilities.
9.1 Why QoS?
The main objective of Quality of Service is to transfer certain data packets
either particularly safe or as immediately as possible:
̈ 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 chronologi-
cally via the net. It automatically adjusts the sending rate downwards if
the confirmation of sent data packets is outstanding for longer times, and
also takes care of repeated transmission in case of packet losses.
̈ 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.
9.2 Which data packets to prefer?
The necessity of a QoS concept results only from the fact that the available
bandwidth is not always sufficient for transferring all pending data packets
reliably and on time. Load peaks result easily from running simultaneously
large FTP downloads, while exchanging e-mails and using IP telephones over
the data line. In order to meet also in these situations the demands of the