The international telephone community has committed to a future network design that combines elements of both circuit and packet switching to enable the provision of integrated services. The ITU (formerly CCITT, an international standards body for telecommunications) has adopted a ``cell-switching'' technology called ATM (asynchronous transfer mode) for future high-speed networks. Cell switching closely resembles packet switching in that it breaks a data stream into packets which are then placed on lines that are shared by several streams. One major difference is that cells have a fixed size while packets can have different sizes. This makes it possible in principle to offer bounded delay guarantees (since a cell will not get stuck for a surprisingly long time behind an unusually large packet).
An ATM network also resembles a circuit-switched network in that it provides connection-oriented service. Each connection has set-up phase, during which a ``virtual circuit'' is created. The fact that the circuit is virtual, not physical, provides two major advantages. First, it is not necessary to reserve network resources for a given connection; the economic efficiencies of statistical multiplexing can be realized. Second, once a virtual circuit path is established switching time is minimized, which allows much higher network throughput. Initial ATM networks are already being operated at 155 Mbps, while the non-ATM Internet backbones operate at no more than 45 Mbps. The path to 1000 Mbps (gigabit) networks seems much clearer for ATM than for traditional packet switching.