The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.Each container then has some control information known as the 'Path Overhead' (POH) and stuffing bits added to it. The path overhead bytes allow the system operator to achieve end to end monitoring of areas such as error indication, alarm indication and performance monitoring data. The container and the path overhead together form a 'Virtual Container' (VC).
Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'. The VC and its relevant pointer together form a 'Tributary Unit' (TU).
Tributary units are then multiplexed together in stages (Tributary User Group 2 (TUG-2) - Tributary User Group 3 (TUG-3) - Virtual Container 4 (VC-4)), to form an Administrative Unit 4 (AU-4). Additional stuffing, pointers and overheads are added during this procedure.This AU-4 in effect contains 63 x 2 Mbit/s channels and all the control information that is required.
Finally, Section Overheads (SOH) are added to the AU-4.These SOH's contain the control bytes for the STM-1 section comprising of framing, section performance monitoring, maintenance and operational control information.An AU-4 plus its SOH's together form an STM-1 transport frame.
Diagram below shows full SDH Multiplexing structure. PDH signals enter on the right into the relevant container and progress across to the left through the various processes to form the STM frame.
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