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SONET/SDH is not a complete communications protocol in itself, but a transport protocol (not a "transport" in the OSI Model sense). Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the choice for transporting the fixed length Asynchronous Transfer Mode (ATM) frames also known as cells. It quickly ...
The STM-1 frame is on the basic transmission format for SDH (Synchronous Digital Hierarchy). An STM-1 frame has a byte-oriented structure with 9 rows and 270 columns of bytes, for a total of 2,430 bytes (9 rows * 270 columns = 2430 bytes). Each byte corresponds to a 64 kbit/s channel. [3] TOH: Transport Overhead (RSOH + AU4P + MSOH)
Unlike SDH/SONET, the line rate is increased by maintaining the G.709 frame structure (4 rows x 4080 columns) and decreasing the frame period (in SDH/SONET the frame structure is increased and the frame period of 125 μs is maintained).
The former is used to transport client data, while the latter is used to transport point-to-point management information like loss of signal, etc. Client management frames can be differentiated from the client data frames based on the payload type indicator. The GFP control frame currently consists only of a core header field with no payload area.
The OTN signals at all data-rates have the same frame structure but the frame period reduces as the data-rate increases. As a result, the Time-Slot Interchange (TSI) technique of implementing SONET/SDH switch fabrics is not directly applicable to OTN switch fabrics. OTN switch fabrics are typically implemented using Packet Switch Fabrics.
Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes (LEDs) over optical fibre at high line rates.
OC-768 SONET interfaces have been available with short-reach optical interfaces from Cisco since 2006. Infinera made a field trial demonstration data transmission on a live production network involving the service transmission of a 40 Gbit/s OC-768/STM-256 service over a 1,969 km terrestrial network spanning Europe and the U.S.
After traversing SDH paths, the traffic is processed in the reverse fashion: virtual concatenation path processing to recreate the original synchronous byte stream, followed by decapsulation to converting the synchronous data stream to an asynchronous stream of Ethernet frames. The SDH paths may be VC-4, VC-3, VC-12 or VC-11 paths.