POS enabled SONET equipment and the two cables have to run along different paths, SO that it is improbable that both are cut at the same moment. In this protection scheme, the router is a single point of failure. This problem can be mitigated by terminating the working and the protection links on different routers cards, or even by using two different routers. These solutions improve network survivability at the expenses of resource allocation. The problem can be mitigated in the case of the use of two routers by applying load balancing. Since POS uses the so-called concatenated MS frame, the slower possible channel is at 155 Mbit/s and if the data flux of the router toward that port is sensibly smaller, bit stuffing allows the OC-1 channel to be filled and utilized for transmission. From a physical and topological point of view, a POS network will appear as schemati- cally depicted in Figure 3.48 where the network topology is assumed to be a logical IP mesh over a physical SONET interconnected ring network. Since the hardware that performs the POS encapsulation is in the router line cards, tran- sit ADMs that are not connected to a router must only have the capability to recognize concatenated container created by POS and to manage them as a whole without demulti- plexing, SO that it is possible to use the most modern among the legacy machines. POS is a powerful and relatively easy to implement solution to transport IP services over a TDM transport platform-it over performs the solution requiring ATM at level two, but it has some rigidities that causes some low efficiency. The main point is the need of using concatenated high-speed SONET paths; that means that the lowest capacity POS path is a concatenated 155 Mbit/s STS. Concatenated means that the STS cannot be further demultiplexed by a SONET machine, being completely filled by client data: IP payload and bit stuffing up to the full available capacity. Frequently, all this capacity is not needed for a single POS path and stuffing is abun- dantly used, loading