Page "Synchronous optical networking" Paragraph 14

In the case of an STS-1, the frame is 810 octets in size, while the STM-1 / STS-3c frame is 2, 430 octets in size.

For STS-1, the frame is transmitted as three octets of overhead, followed by 87 octets of payload.

This is repeated nine times, until 810 octets have been transmitted, taking 125 µs.

In the case of an STS-3c / STM-1, which operates three times faster than an STS-1, nine octets of overhead are transmitted, followed by 261 octets of payload.

This is also repeated nine times until 2, 430 octets have been transmitted, also taking 125 µs.

For both SONET and SDH, this is often represented by displaying the frame graphically: as a block of 90 columns and nine rows for STS-1, and 270 columns and nine rows for STM1 / STS-3c.

This representation aligns all the overhead columns, so the overhead appears as a contiguous block, as does the payload.