Packet over SONET

Packet over SONET or Packet over SDH (POS) is a term coined to describe the transport of data over SDH or SONET. Although this seems to be a self explanatory statement as all information is already transmitted as data it refers to the transport of Packetised data generated from devices that would otherwise not interface with the lower layers of the OSI stack that SDH and SONET occupies as a ubiquitous transport protocol for long distances.

The best example is to take the output from a Data Device, a Router, that would on a local scale interface directly with another Router over Multimode Fibre optics. The modulated light that transports the data is framed i.e. packetised using the Ethernet protocol (IEEE 802.3 and related). To connect two routers that were not local (defined as needed another transport system to complete the link) one could not use SDH or SONET directly as the SDH/SONET devices would not recognise the Ethernet protocol. Thus the Ethernet protocol is itself converted into another protocol that can be recognised by the SDH/SONET devices, transported and then decoded back to the original Ethernet format. That is Packet over SDH/SONET, POS transport.