COMNET III CACI

CBR rt-VBR nrt-VBR ABR UBR
QoS Class 1 2 3 unspecified 4
Max CTD Yes Yes No No No
Mean CTD No No Yes No No
CDV Yes Yes No No No
CLR Yes Yes Yes Yes No
Table 22: required QoS by service category
This table further illustrates the distinction between the different service categories. Both
CBR and VBR provide a high level QoS and therefore make use of all parameters except
for the mean CTD. The lower priority categories on the other hand provide little or even
no support for the quality of service.
Once these parameters have been negotiated during the connection establishment phase,
the network commits to maintaining this QoS, provided the source itself does not violate
the values of the traffic descriptors. In other words, whenever a new call wished to
transmit, the network must ensure that the QoS of all existing calls are not violated by the
admittance of the new call.
A.2.3
Conformance Checking Rules
The third component of the traffic contract specifies the rules by which the network is
going to check the conformance of a user cell. One of the purposes of the traffic
descriptors and the QoS parameters is to negotiate the service level between the network
and the user in the traffic contract. Like any contractual agreement, penalty clauses are
specified to cover the case of non-compliance. In this case, non-compliance means that a
source agreed to transmit at a certain rate, for example the PCR, but in reality exceeds
this transmission rate at an unacceptable level for the network. For this reason, the source
and the network agree on the definition of a compliant connection. This includes a
precise definition of a conforming cell, a dropped cell, a delayed cell etc. This definition
is typically based on the generic cell rate algorithm (GCRA) with which the network
supervises the transmission rate of the source. Again, the definition differs between the
service categories. Details on the algorithm as well as the cell dropping procedures for
different service classes are given below in appendix B.
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