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 desired data transfer, certain data packets must be treated preferentially. It is necessary for this, that at first a LANCOM recognizes which data packets should be preferred at all.
There are two possibilities to signal the need for a preferential treatment of data packets in the LANCOM:
- The application, as e.g. the software of certain IP telephones, is itself
able to mark the data packets appropriately. This marking, the “tag”,
is set within the header of the IP packets. The two different variants
of this marking “ToS” and
“DiffServ” can
simply described assume the following states:
- ToS “Low Delay“
- ToS “High Reliability“
- DiffServ “Expedited Forwarding“
- DiffServ “Assured Forwarding“
Note: The IP header bits of the ToS resp. DiffServ field are copied in case of a VPN route also into the enclosing IP header of the IPSec VPN packet. Thus QoS is available also for VPN routes over the Internet, as long as your provider treats according packets preferentially also in the WAN. - When the application itself has no possibility to mark the data packets
appropriately, the LANCOM can ensure the correct treatment. For this,
it uses the existing functions of the firewall, which can classify e.g.
data packets according to subnets or services (applications). Due to
these functions it is e. g. possible to treat individually data packets
of a FTP connection or those of a certain department (in a separate subnet).
For treatment of data packets classified by the firewall the following
two possibilities can be chosen:
- Guaranteed minimum bandwidth
- Limited maximum bandwidth