Class-based queuing allows a greater degree of control than PRIQ does. CBQ groups traffic into different classes, and gives each class a percentage of total bandwidth. This grouping of classes has a hierarchy, with classes further being divided into sub-classes. As with PRIQ, we can use many different criteria for classifying traffic. We can classify traffic based on protocol, the application used, the IP address of the source, and so on. CBQ operates at the IP network layer (Layer 3 of the OSI model). In addition, CBQ is in the public domain.
With CBQ, classes are allocated a certain amount of bandwidth. Packets in each class are processed until the bandwidth limit for the class is reached. With this system, even low-priority packets that would be starved of bandwidth in PRIQ receive some bandwidth. The first goal of CBQ is quantitative bandwidth sharing—every class should get some bandwidth. A secondary goal, however, is that when a class is not using all of its bandwidth, allocation of this excess bandwidth should not be arbitrary. On the contrary, the distribution of this bandwidth should follow set guidelines.
To illustrate how PRIQ and CBQ would work in practice, consider a small college that has 1.5 Gbps of download bandwidth and 500 Mbps of upload bandwidth. They also have four networks: one for administration, one for faculty, one for students, and a DMZ for a web server and other resources that need to be made available to the public. It is decided that two-thirds of the download bandwidth and half the upload bandwidth should go to the administration and faculty networks. In addition, 60% of this share of the bandwidth is allotted to the faculty network. Thus, faculty should get 600 Mbps of download bandwidth and 150 Mbps of upload bandwidth, administration should get 400 Mbps of download bandwidth and 100 Mbps of upload bandwidth, with the remainder allocated to the students and the DMZ.
PRIQ does not give us a means of guaranteeing that the administration and faculty receive a fixed share of bandwidth. We could assign higher priority levels to the faculty and administration networks, so that faculty has a higher priority than administration, which, in turn, has a higher priority than students and the DMZ. However, this could result in the students and the DMZ being starved of bandwidth, as a packet from either the faculty or administration networks will always win out over one from the students or DMZ networks.
CBQ solves all of that. Each of the subnets can be assigned a percentage of the total bandwidth. Moreover, we can arrange the classes in a hierarchical manner, with faculty and administration occupying a higher level on the hierarchy of classes, and with the faculty being assigned a greater portion of the shared bandwidth.