- Frame relay
- SMDS (Switched Multi-Megabit Data Service)
- ATM (asynchronous transfer mode)
- High speed LAN services such as FDDI
- A hierarchy of multiplexers at either end of the link can lead to reduced reliability and resilience, minimum flexibility, long reconfiguration turn-around times, large equipment volume, and high capital-equipment and maintenance costs.
- PDH links are generally limited to point-to-point configurations with full demultiplexing at each switching or cross connect node.
- Incompatibilities at the optical interfaces of two different suppliers can cause major system integration problems.
- To add or drop an individual channel or add a lower rate branch to a backbone link a complete hierarchy of MUXs is required as shown in figure 3.
- Because of these limitations of PDH, the introduction of an acceptable world-wide synchronous transmission standard called SDH is welcomed by all.
BENEFITS OF SDH TRANSMISSION
- Software Control allows extensive use of intelligent network management software for high flexibility, fast and easy re-configurability, and efficient network management.
- Survivability. With SDH, ring networks become practicable and their use enables automatic reconfiguration and traffic rerouting when a link is damaged. End-to-end monitoring will allow full management and maintenance of the whole network.
- Efficient drop and insert. SDH allows simple and efficient cross-connect without full hierarchical multiplexing or de-multiplexing. A single E1 2.048Mbit/s tail can be dropped or inserted with relative ease even on Gbit/s links.
- Standardisation enables the interconnection of equipment from different suppliers through support of common digital and optical standards and interfaces.
- Robustness and resilience of installed networks is increased.
- Equipment size and operating costs are reduced by removing the need for banks of multiplexers and de-multiplexers. Follow-on maintenance costs are also reduced.
- Backwards compatibly will enable SDH links to support PDH traffic.
- Future proof. SDH forms the basis, in partnership with ATM (asynchronous transfer mode), of broad-band transmission, otherwise known as B-ISDN or the precursor of this service in the form of Switched Multimegabit Data Service, (SMDS).