The Packet-Over-SONET OC3 port adapters provide a single 155.520-Mbps, OC-3 physical layer interface for packet-based traffic. This OC-3 interface is fully compatible with SONET and Synchronous Digital Hierarchy (SDH) network facilities and is compliant with RFC 1619, "PPP over SONET/SDH," and RFC 1662, "PPP in HDLC-like Framing." The Packet-Over-SONET specification is primarily concerned with the use of the PPP encapsulation over SONET/SDH links.