SDH stands for Synchronous Digital Hierarchy and it refers to a multiplex technology used in the telecommunication. Synchronous Optical Network is internationally used and is taken equal to SDH. Both technologies provide quite fast and low-priced network interconnection than PDH which stands for Plesiochronous Digital Hierarchy.
SDH allows the data stream to pass having low bit rates to be combined into high rate data streams. As the entire network is synchronous, individual bitstreams can be embedded into and are extracted from high rate data streams quiet easily. Initially, it was developed back in 1985 in the USA under the name of SONET.
Functions :
SDH used to combine n signals with a bit rate of b to form data streams with bit rates of (n × b) on the synchronously clocked networks. Unlike PDH, the transmission paths that transfer individually have minimal clock discrepancies.
The synchronous mode of operation allows low order multiplex system such as communication links for telephone systems, to be inserted in higher hierarchy levels and then remove again via add and drop. SDH has following Synchronous Transport Modules (STM) and their rates are as follows :
1. STM-1 (155 Mbps) 2. STM-4 (622 Mbps) 3. STM-16 (2.5 Gbps) 4. STM-64 (10 Gbps)
Advantages :
- The structure is synchronous and is quite flexible.
- It is capable of conveying Broadband signals.
- The format is fully digital and fulfills the criteria of world standard.
- It provides network transport services such as LAN in video conferencing and various interactive multimedia.
- Speed of SDH is high as compared to PDH.
- SDH allows quick recovery from any kind of failures.
- It supports different kinds of operators.
Disadvantages :
- The bandwidth utilization ratio is quite low as compared to PDH as because of OH bytes used for OAM.
- There are various software’s being used and so it is more prone to viruses.
- Pointers are being used for the adding or dropping of low rate signals and this causes the circuit to become more complex.
- It requires complicated SDH materials due to a variety of management traffic types.
- It cannot carry E2 due to the unavailability of the container.
Applications :
- It is being used in electrical converters.
- It is widely used in optical transports.
- It can be used as carriers in ATM cells.
- It can be used for bandwidth in demand.
- It can be used to cable TV networks.
- It can be used for T1 and T3 carriers.