This standard specifies a communication layer, Flexible Optical Service Unit (OSUFlex) to provide small service channels in optical transport networks. The main features of the OSUflex communication layer are providing massive OSUflex connections to directly carry power services (e.g. E1 or IEEE C37.94) into an optical transport network, enabling the flexible bandwidth (e.g. n × 2 Mb/s) and hitless bandwidth adjustment for each OSUflex connection, direct mapping of services into OSUflex with the capabilities of high-bandwidth efficiency, low latency, low latency jitter and high-precision clock synchronization for power system applications.