Although these two standards are not directly compatible they have been harmonised to maximise ease of interworking between the two. Agreement harmonizing the two standards was reached in 1988. Table 2 shows the proposed carrier rates for the two schemes. OC stands for Optical Carrier and defines the optical signal, STS stands for Synchronous Transport Signal and defines the equivalent electrical signal, and STM stands for Synchronous Transmission Module. The STM-1 signal is formatted into frames of 2430 bytes, which repeat at 125 s intervals.
Frames have a large header which carries pointers to the payload and management information.
The payload consists of containers, each with its own header. Containers may themselves carry other containers and the position of a container is allowed to float, being indicated by a pointer within the header of the enclosing container or the frame. This scheme uses a synchronous formatted medium in the form of two unidirectional buses. Every station is connected to both buses, receiving from upstream nodes and transmitting to downstream nodes on each bus. Access control to the buses for asynchronous traffic is handled by the distributed queueing mechanism.