Virtual Concatenation - Differential Delay

Differential Delay

VCAT helps in providing services at a lower cost and more quickly than contiguous concatenation. However, it creates differential delay whereby each path that is created, represented by a VT has a different propagational delay across the network. The difference in these delays is called "differential delay" (D). The major problem with differential delay is the requirement for high speed buffers at the receiving node to store incoming information while all paths converge. This buffer space, (B) can be equated to the bandwidth delay product such that B = n * D. Thus, each Virtually Concatenated connection requires B bits of buffer space. This need for buffer space eventually increases the network cost, so it is very important to select paths that minimize the differential delay, which is directly proportional to the buffer space required.

Several heuristics based algorithms exist, that attempt to minimize the differential delay to provide a solution. This is not a simple problem to tackle and is referred to mathematically as an NP-complete problem set, for which there exists no known algorithm that finds the optimum solution and terminates in a polynomial time constraint.