Flow detection makes it possible to apply policy-based networking directly to Internet Protocol traffic, and shows that a combination of technologies-ATM, IP and DSL-can provide solutions for multiservice delivery in the access network.

For proper network functioning, operators must analyze the identity of the subscribers and their traffic. For example, voice calls can tolerate significant information loss without becoming unintelligible, whereas file transfers are extremely sensitive to data loss but less susceptible to transmission delays.

Also, a high-value business customer may well take priority over a free-service Internet customer at times of congestion. Or, network operators can vary an individual's subscription or priority over the course of each day, perhaps offering a high-performance, prime-shift service with a lower-priority, off-hours service.

Policy-based networks use rules provided by the network operator that determine how a subscriber's traffic should be managed. First, all traffic must be classified according to type and sender. Second, network policies must be inspected to select the appropriate service class for this traffic. Third, a switching technology capable of providing quality-of-service (QoS) management must be employed.

Traffic classification is simple to implement in a connection-oriented network (frame relay, ATM), since the call establishment process can identify both subscriber and traffic type. But in a connectionless network environment like IP, this classification is impossible because no call establishment takes place.

That can be overcome by flow detection-analyzing the traffic in an IP network to detect flows of traffic belonging to a single subscriber, class of subscriber or application. Those flows allow traffic classification to implement policy-based networking.

While several switching technologies claim to provide QoS management, most can only achieve good results in lightly loaded networks. ATM offers true QoS management in a heavily used network, but it's a connection-oriented protocol, and most subscribers want connectionless IP services.

With flow detection, connectionless IP services can be carried over connection-oriented ATM. The flows within IP traffic are treated as connections and mapped directly into ATM virtual circuits. So flow detection is the last piece in the jigsaw, enabling a mix of technologies to provide the services that are in demand.

Ian Moir is Vice President of Technology for Tut Systems Inc. (Pleasanton, Calif.).