which allows applications to request QoS guarantees from a network. In data networks, if resources are not available somewhere along the way, then data are queued or delayed, usually with little noticeable effect on application performance. With voice calls, delay must be minimised to the extent necessary to permit voice conversations. Unless sufficient bandwidth is available, it is better to keep voice traffic off the network entirely, rather than allowing the traffic on and delaying or dropping the packets to an extent which reduces voice quality. In cases where end-to-end resources for a VoIP session are not immediately avail- able, network operators have several configuration options. Voice traffic can be redir- ected to the PSTN, for example, or users can receive an "equipment-busy" tone. VoIP WANs must account for the fact that the customer access link - which is generally the slowest network segment - is most likely to become congested. 13 Traffic Management An enhancement to the RSVP protocol in Cisco equipment ensures that network resources are available end-to-end for the duration of voice sessions. This is accom- plished using RSVP as a call admission control mechanism in Cisco H.323 VoIP net- works. The technology synchronises RSVP procedures with H.323 Version 2 setup procedures to ensure that the required QoS for a call is maintained across the IP network. Cisco's RSVP implementation has been enhanced to address scalability concerns. Cisco IOS contains RSVP scalability enhancements, which integrates Integrated Services (IntServ) and Differentiated Services (DiffServ) QoS mechanisms. IntServ allows a strong bandwidth guarantee for RSVP traffic, whereas DiffServ provides specific treatments to different classes of traffic. RSVP typically performs admission control, classification, policing and scheduling of data packets on a per-flow basis. It also keeps a database of information for each flow. If a carrier has 200 00 RSVP flows in the network core, then scalability could be a concern. On the other ha