17-05-2013, 02:23 PM
SDH Frame Structure
SDH Frame.pdf (Size: 147.33 KB / Downloads: 103)
SDH Frame Structure
• A single SDH frame is called a Synchronous Transmission Module (STM-1).
Transmitted over a duration of 125 μs, the frame consists of 2430 octets organized
as 9 rows of 270 octets each. A single octet in an SDH frame represents a 64 kbps
channel (8 bits every 125 μs), several octets can be aggregated to form containers
for larger data rates.
SDH Section Overhead (SOH)
• A relatively large number of 72 octets in an STM-1 frame have been reserved for
various management and monitoring purposes. This so-called section overhead
(SOH) is further divided into a regenerator section overhead (RSOH) and multiplex
section overhead (MSOH).
Administrative Unit (AU-4) and Virtual Container (VC-4)
• The actual payload carried in an STM-1 frame is encapsulated in an administrative
unit (AU-4). The AU-4 consists of a VC-4 virtual container comprising 261 columns
plus a 9 octet wide AU-4 pointer that points to the first octet of the VC-4 payload
container.
AU-4 Pointer
• The SDH pointer mechanism is a very elegant way of multiplexing multiple data
containers without the need to align the containers to a common frame start.
Thus the frame buffers in SDH multiplexing equipment can be kept small and the
transmission delay due to buffering is minimized.
• The VC-4 container is allowed to float freely within the AU-4. The H1 and H2 pointer
bytes form a word with a range of 0 to 782 which indicates the offset, in three byte
increments, between the pointer and the first byte of the VC-4. If the offset has the
value 0 then the J1 byte of the VC-4 follows immediately after the H3 bytes of the
AU-4 pointer