Path overhead :
Out of the 783 bytes of payload, which occupy 87 columns, only 756 bytes in 84 columns actually carry payload capacity of STS-1. The first column, consisting of 9 bytes, is known as Path overhead (POH) . POH is transported point to point with the payload till the latter is demultiplexed. It carries service provider information and monitors end-to-end transport of the payload. Columns 30 and 59 are not used for payload.