High Quality Content by WIKIPEDIA articles! PL-3 or POS-PHY Level 3 was the name of the interface that the Optical Internetworking Forum`s SPI-3 Interoperability Agreement is based on. It was proposed by PMC-Sierra to the Optical Internetworking Forum and adopted in June of 2000. The name means Packet Over SONET Physical layer level 3. PL-3 was developed by PMC-Sierra in conjunction with the SATURN Development Group.