240 ◾ IP Communications and Services for NGN
resides. For example, the gateway may have been used to connect a PSTN with
an IP-based network, possibly with a T-1 interface, operating at 1.544 Mbps on
the PSTN side, and an Ethernet interface operating at 10 or 100 Mbps on the IP
network side. Many other combinations are possible, including cellular wireless,
high-speed optical, cable television, ATM, and others [4]. Creating the physical
connection and the necessary interface and rate conversions is a fraction of the
responsibilities of the gateway. Depending on how the gateway is used and the
networks involved, it may also perform the following functions:
Converting ATM cells into packets used with IP networks ◾
Performing media processin