These tell the multiplexers which bits are data and which are spare. Multiplexers on the same level of the hierarchy remove the spare bits and are synchronised with each other at that level only. Multiplexers on one level operate on a different timing from multiplxers on another level. Inserting and dropping out traffic from different customers can only happen at the level at which the customer is receiving the traffic. It includes capabilities for bandwidth on demand and is also made up of multiples of E1.
Network topologies are more flexible.
There is flexibility for growth. G708 - The SDH Network Node Interface. G709 - Synchronous Multiplexing structure. Stuff opportunities are identified by S1 and S2 and these are controlled by the control bits C1 and C2 respectively. 0s then the corresponding S bits contain data and if the C bits are 1s then the S bits are not defined.