SDH is a transport protocol suited to conveying streams of bits at a number of discrete bit rates across the network. It is extensively used world-wide and will continue to be used as part of 21CN. SDH provides transmission pipes that run between the network nodes. Those pipes are typically at transmission speeds of 2.5Gbit/s or 10Gbit/s although lower speeds of 155Mbit/s and 622Mbit/s are also used. These pipes can be sub-divided into smaller pipes (pipes within pipes) of 2, 34, 45, 150 and multiples of 150Mbit/s and these smaller pipes can be switched at network nodes using SDH cross-connects to form an end-to-end connection across the network. Once set up in the cross connects, the capacity of the smaller pipes is always available and does not have to compete or contend for capacity with other services. This makes SDH very suitable for leased lines. SDH networks and equipments can be configured in ring or mesh topologies and have a range of protection and restoration mechanisms to improve network resilience.
