[Next] [Previous] [Top]

Frame Structure

Sonet

STS-1 Frame Format

• Sonet is based on the STS-1 frame

• STS-1 consists of 810 octets
  • 9 rows of 90 octects

  • 27 overhead octects formed from the first 3 octets of each row
    • 9 used for section overhead

    • 18 used for line overhead

  • 87x9 = 783 octets of payload
    • one column of the payload is path overhead - positioned by a pointer in the line overhead

  • Transmitted top to bottom, row by row from left to right

• STS-1 frame transmitted every 125 us: thus a transmission rate of 51.84Mbps

SDH

STM-N Frame Format

• STM - "Synchronous Transmission Module"

• STM-N general format

• Basic frame STM-1 consists of
  • 270x9=2430 octets

  • 9x9=81 octets section overhead

  • 2349 octets payload

• Higher rate frames are deriveved from multiples of STM-1 according to value of N