Network needs assessment

Layer 3 queuing subsystems operate at the network layer and are applied to Layer 3 packets at a logical level. Because IOS Layer 3 queuing subsystems are independent of the egress interface type, they can be applied to Asynchronous Transfer Mode (ATM), Frame Relay, High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), Multilink Point-to-Point Protocol (MLP), or tunnel interfaces. It is important to recognize that Layer 3 queuing typically considers only the IP overhead in its bandwidth provisioning. Layer 2 overhead such as the ATM cell tax is not factored into the equation and, therefore, must be provisioned explicitly for drop-sensitive applications such as Voice over IP (VoIP).

There is a long history of queuing algorithms in network needs assessment, not all of which are covered in this chapter because they are not applicable to QoS deployment for converged networks. Newer queuing and scheduling algorithms are simply combinations and enhancements of older queuing algorithms. For a historical perspective and an understanding of why these mechanisms are insufficient for today's converged networks, it is helpful to review some of these legacy queuing techniques.