... support hundreds of thousands of telephone calls (called DS–0s, running at 64 Kbps per line) to parallel the scalability of the existing PSTN switches. • Support for several types of access networks—Needed ... variety of optical interfaces (including OC 3, OC–12, OC–48, and OC–192 speeds). • Carrier-class reliability—Also known as five nines (99.999 percent) reliability (i.e., less than five minutes of ... dynamics of voice communication, in the absence of noticeable echo, whereas in the presence of noticeable echo, increasing delay makes echo effects worse. When delay reaches above 30 ms, echo...