SONET layers

1. photonic layer, dealing with transporting bits using optical OC-N signals on optical fiber, and conversion between STS-N (electrical) to OC-N (optical) signals and vice-versa, with decision to be made for
   1. pulse shape
   2. power level
   3. wavelength
2. section layer, transport of STS-N frame across the physical medium, and decisions to be made for
   1. Framing into STS frame, 
   2. scrambling
   3. Error Monitoring
   4. Section Maintenance
   5. Orderwire (voice connection E1 byte in section overhead)
3. Line layer, reliable transport of the path layer payload, and its overhead across the physical medium, adds/interprets line overhead for maintenance and protection switching, deals with
   1. synchronization for path layer
   2. Multiplexing for path layer
   3. error monitoring
   4. line maintenance
   5. protection switching
4. Path Layer, transport of services between path terminating equipments, payload mapping to and from SONET frame, main functions include
   1. map the signals into a format required by line layer, for example and STS PTE can assemble 25 DS1(1.544 Mbps) signals adding path overhead and forming a STS-1 (51.840 Mbps) signal
   2. reads interprets and modifies path overhead for performance monitoring and Automatic Protection Switching

 

References

http://www.techfest.com/networking/wan/dks1/dks1.htm

http://www.cisco.com/en/US/tech/tk482/tk607/technologies_tech_note09186a0080094313.shtml