tenated payloads. Operational, Administration, Management, and Prorisioning (OA&M) Path Overhead (POH) of the individual VC-4s are located in the X columns of the VC-4-Xc. The first column is used as the POH for the whole VC4-Xc, and, as with any AU pointer, an AU-4-Xc pointer indicates the position of J1, which is the position of the first byte of the VC-4-Xc container. The remaining X-1 columns contain fixed byte values to indicate concatenation and thus link all the units together. The remaining X X 260 columns provide the payload area of the VC-4-Xc that has exactly the same size as a C-4-Xc. Because the whole contiguous container is transported as a unit across the network, the integrity of the bit sequence is guaranteed. After the upgrade or installation of equipment supporting contiguous concatenation, an issue regarding bandwidth fragmentation appeared. As a result of the continuous setup and teardown of trails through the network, the total payload area in an STM-N might become fragmented and lead to an inefficient use of network bandwidth. As with the problem experienced with computer hard disks, a virtual container fragmentation problem is exhibited when physically available SDH paths exist but are not usable because of the noncontiguous nature of the available containers. Consider the example in Figure 11.3 in which two SDH network elements (NE-X and NE-Y) are interconnected by an STM-16 link. Initially, 8 X VC-4s are connected between clients the A-and-D and 8 X VC-4s between B and E. After a period of time (t = T1), the A- to-D connection releases 2 X VC-4s and the B-to-E connection releases 3 X VC-4s; this results in a total of five VC-4s becoming available between nodes X and Y. To illustrate the frag- mentation problem, consider the case when a VC-4-4c connection is requested between C and F at time T2. Even though there is enough bandwidth between the two nodes, the restri- ction of keeping the four VC-4 containers together as one concatenated container means that the connection cannot be establ