Analog voice is multiplexed with the packet stream by the DSL technology over
the copper wire pairs. Asynchronous transf er mode (ATM) switches are installed
in the RT and route individual customer packets over ATM virtual circuits to the
ISP edge switch (i.e., typically an Internet Protocol (IP) rout er, which is not
shown). The ATM links are routed over SONET rings/chains at a lower layer,
whose links in turn route over the feeder fiber layer. In our layered model, the
ATM links are implemented as connections in the SONET layer [in u nits of
synchronous transport signal (STS)-n, where an STS-1 signal is 51.84 Mb/s and
an STS-n signal has a data rate of n 51.84 Mb/s]. There is consideration for
using coarse wavelength division multiplexing (CWD