Virtual concatenation

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Virtual concatenation (VCAT) is an inverse multiplexing technique used to split SONET/SDH bandwidth into logical groups, which may be transported or routed independently. Alternate SONET/SDH concatenation techniques are contiguous concatenation and arbitrary concatenation.

Virtual concatenation is considered the primary enhancement to voice optimized SONET, in order to support the transport of variable bit data streams. Other recent SONET enhancements include Link Capacity Adjustment Scheme (LCAS), and the Generic Framing Procedure (GFP).

In conjunction with LCAS and GFP, Virtual Concatenation gives the advantage of splitting the required bandwidth equally among a set number of sub paths called Virtual Tributaries (VT).

Basically, several Virtual Tributaries, form part of a Virtual Concatenation Group (VCG). The pragmatic use of spawning Virtual Tributaries to transport data across a VCAT enabled network is the fact that in many cases, particularly when the underlying network is relatively congested, then splitting the traffic over several distinct paths allows us to provide lower cost solutions than if we had to find just one path that meets the required capacity. Chances are this splitting of paths will also allow us to find shorter paths to channel our traffic across.

Basically, the Virtual Concatenation protocol performs its content delivery through a process called byte-interleaving. For example, given that we wish to provision a Gigabit Ethernet (n, 1Gb/s) service then we would provision it across (7) STS-nc VT’s, where each of the VCG members carry a bandwidth equivalent of V = n/k [bits/second], where in this case n = 1Gb and k = 7. What typically happens is that the data is sent such that the r^th byte is put onto VT1, and then the (r+1)^th byte is sent out on VT2, and so on, until a loopback is created sending the next byte back on VT1.

Effectively, this helps a lot with being able to provide services at a lower cost and much faster than contiguous concatenation, it however is intrinsically bound to the problem of differential delay where each path that is created, represented by a VT has a different propagational delay across the network and the difference in these delays is what is known as differential delay (D). The major problem with differential delay is the fact that we require 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. So for each Virtually Concatenated connection we would require 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.