The Packet-Over-SONET specification is primarily concerned with the use of the PPP encapsulation over SONET/SDH links. Since SONET/SDH is by definition a point-to-point circuit, PPP is well suited for use over these links. Point-to-Point Protocol (PPP) was designed as a standard method of communicating over point- to-point links.The Packet OC-3c Interface Processor (POSIP) provides a single 155.520-Mbps, OC-3c network interface, which provides a direct connection between the high-speed Cisco Extended Bus (CxBus or CyBus) and your external networks. The physical layer interface on the POSIP is OC-3c.