as the differential delay. The receiving interface has to be aware of this SO that the delay can be compensated for before reconstituting the original contiguous payload. In addition, if a protection switch occurs that affects the path of individual members, this delay compensation must be dynamic. The realignment process is achieved by using variable-length buffers for each member of the VCG, and the recommendations state that it should be possible to compensate for a maximum differential delay of 256 ms. Because light propagates through fiber at 5 ms/1000 km, this is equivalent to a differential delay of 50,000 km. The maximum differential delay achievable is vendor specific and typically less than this figure. The Multi-Frame Indicator (MFI) in the path overhead is defined to detect the differential delay and is discussed in detail in the next section. To reassemble the con- stituent VCG members, which might arrive in a different order from how they were trans- mitted, a Unique Sequence Number (SQ) is assigned to each VC-n member of the VCG by the Network Management System (NMS). VCAT has been defined for both low- and high-order SDH. Figure 11.6 serves to dem- onstrate the flexibility of capacity assignment to SDH clients using VCAT for both low- and high-order containers. In comparison with contiguous concatenation, virtual concatenation offers an efficient method of transporting packet services, as illustrated in Figure 11.7. Whereas contiguous concatenation offers poor container utilization, virtual concatenation allows containers to closely match the required amount of bandwidth for the client service, with efficiencies typically greater than 90 percent. A new SDH service enabled through VCAT is a fractional Ethernet service in which a customer connects through a Gigabit Ethernet interface but can Container Individual Capacity Virtual Capacity VC-11-Xv 1 to 64 1600 Kbit/s 1600 to 102,400 Kbit/s Lower Order VC-12-Xv 1 to 64 2176 Kbit/s 2176 to 139,264 Kbit/s VC-2-Xv 1 to 64 6784 Kbit/s 6784 to 434,176 Kbit