ignal Label MF AS 16 ms Extended Reserved Signal Label 16 ms Reserved Multiframe Four Bytes 512 ms Fig. 11.12 Low-order virtual concatenation overhead. The frame structure for a low-order VC-m-Xv is shown in Figure 11.12. In the existing POH of VC-ms, there were not enough reserved bits available to be used for the low-order VCAT overhead, and a two-stage process had to be defined to circumvent this problem. This was achieved by defining that when the "signal label bits" of the V5 byte have a value of 101, an extended signal label is to be used. The first bit of K4 is defined as the Multiframe Alignment Signal (MFAS) and is a fixed value of 0111 1111 110 followed by an extended signal label byte, a fixed bit equal to 0, and 12 reserved bits set to 0. The 32-bit multiframe repeats itself every 16 ms (because each bit equals 500 us). The start of the MFAS is synchro- nized with the low-order VCAT overhead in bit 2 of the K4 byte, which consists of a 5-bit Multiframe Indicator (MFI), a 6-bit sequence number, and 21 reserved bits set to 0. In this way, a 32-bit control sequence for low-order virtual concatenation is achieved. Because of the multiframe nature of the channel, the granularity of the differential delay measurement achievable using the MFI is 16 ms. Link Capacity Adjustment Scheme Virtual concatenation extends the payload transport capability of SDH networks by the ability to match the concatenated container size with the client signal bandwidth. However, if the client signal is packet based, the required transport bandwidth will vary with time, and a mechanism is necessary to adjust the size of the provisioned bandwidth in a manner that does not disrupt the transported service. The link capacity adjustment scheme was standardized (ITU-T G.7042) to provide such a capability in conjunction with virtual Chapter 11 Next Generation Synchronous Digital Hierarchy concatenation. LCAS relies on a two-way handshake signaling protocol that enables addi- tion and removal of high-order and low-order SDH path(s) into 