The terms IP over SONET/SDH or packet over SONET/SDH as they are used in the industry today, refer essentially to the addition of SONET/SDH interfaces to a router that terminates the PPP. Faster routing represents a natural migration for original network-routing equipment, and, with the widespread adoption of PPP as a framing protocol for IP traffic, it made sense to combine the two standards to create devices for the rapid transport of IP traffic from point to point.

IP traffic on a SONET/SDH router is treated as a serial datastream that travels hop by hop through the network using PPP for its framing and encapsulation functions. The datastreams are mapped into STS frames in accordance with Request For Comment 1619 (RFC 1619), PPP over SONET/SDH. Frames travel at optical carrier level three/synchronous transport module one (OC–3/STM–1), OC–12/STM–4 and OC–48/STM–16 rates.¹ At each node, the IP packet is unwrapped from its PPP frame, the destination address is examined, the routing path is determined, and the packet is rewrapped in a new PPP frame and sent on its way.

Transporting IP over SONET has the following advantages:

• efficient/low overhead point-to-point transport of IP traffic

• provision of large amounts of bandwidth for nondifferentiated services

¹To ensure compatibility with deployed SONET/SDH infrastructures, RFC 1619 is being revised by the Internet Engineering Task Force (IETF) to include scrambling of data before it is handed to the SONET/SDH layer.