cause transport capacity can be allocated dy- namically for voice or data. In current networks, transport must be transferred from voice circuit switches to data packet switches or ATM switches or routers to support frame relay and other customer data services. 19.4.3 VolP on the PPDN Major companies have been leasing dedicated transport from telephone companies and other service providers for both voice and data for decades. The term public packet data network (PPDN) is new, but the functionality is simply the expansion of leased-line ser- vice plus the infrastructure of the Internet. With the advent of VoIP, companies now have a new way of sharing capacity between voice and high-speed data on leased lines. These lines are available from long-distance carriers and also from local exchange car- riers, where they serve metropolitan areas on metropolitan area networks (MANs). For example, a company could lease IP services on a MAN to connect two offices. The com- pany could then use this virtual connection to support both corporate data and corpo- rate voice calls, reducing its cost for local calls. The service could be allocated dynami- cally for voice and data, utilizing capacity effectively. In addition, would possible to monitor usage levels at the packet level. In the future, MAN and long-distance packet service providers could offer per-packet rates based on actual usage rather than rates based on maximum line capacity. 19.4.4 VolP on the cellular backhaul network There is one other place where VoIP is coming into use: the cellular telephone net- works. Over the air interface, low available bandwidth already requires that the voice signal be compressed, and variable compression is optimal. As a result, the duplex voice channel is already a variable-rate channel. Moving that signal on a fixed-bandwidth circuit hardly makes sense. Some cellular networks already use packet pipes for back- haul. Those that do not, by and large, are converting to IP pipes as they deploy equip- ment to support 3G services. As 2.5G 