Traffic management involves assigning priorities to data moving through the network. Higher-priority traffic is sent as fast as possible, while lower-priority traffic is queued or dropped when channels are too busy.

Current-generation NPUs are generally poor at traffic management. Most support external DRAM for queuing packets, but at best, they can handle hundreds of flows. This level is adequate for an enterprise router but not for WAN applications.

Even achieving this level of traffic management is tough. In many NPUs, enabling management increases the number of cycles needed to process a packet. As a result, most NPUs can't maintain wire speed with traffic management turned on.

One exception is Agere's Payload Plus, one of the few NPUs that supports ATM as well as packet traffic. Traffic management is a core feature of ATM, and Agere has carried this expertise into the packet world.

To address NPU shortcomings, coprocessor vendors such as Acorn, Azanda and Vitesse are developing add-on traffic managers that handle up to a million flows. Many of these vendors, like Agere, are leveraging ATM experience to build multiprotocol traffic-management chips.

NPU makers, however, will not simply cede this area to the coprocessor vendors. Most of the NPU vendors with whom I speak are working to solve the shortcomings of the current generation.

Some NPU vendors are taking a page from the coprocessor book, adding fixed-function managers to their chip sets. Others are taking a more programmable approach, letting OEMs add value with their own traffic-management algorithms.

OEMs often implement traffic management in ASICs, but the demand for more complex and fine-grained traffic management will persuade many to switch to standard silicon. With flexible solutions becoming available from various vendors, why start a costly ASIC design now?

Not all network processors will have the same traffic-management capabilities, however, and subtle differences can make or break an OEM's application. Thus, traffic management will be a differentiator in the next generation of NPUs.

Founder and Principal Analyst of the Linley Group, Linley Gwennap is the coauthor of "Classification and Traffic Management Coprocessors" (www.linleygroup.com).