The four sizes of virtual tributary frames. (From Ref. 2, courtesy of Hewlett-Packard.) There are two VT operating modes: floating mode and locked mode. The floating mode was designed to minimize network delay and provide efficient cross-connects of transport signals at the VT level within the synchronous network. This is achieved by allowing each VT SPE to float with respect to the STS-1 SPE in order to avoid the use of unwanted slip buffers at each VT cross-connect point. 5 Each VT SPE has its own payload pointer, which accommodates timing synchronization issues associated with the individual VTs. As a result, by allowing a selected VT1.5, for example, to be cross-connected between different transport systems without unwanted network delay, this mode allows a DS1 to be transported effectively across a SONET network. The locked mode minimizes interface complexity and supports bulk transport of DS1 signals for digital switching applications. This is achieved by locking individual VT SPEs in fixed positions with respect to the STS-1 SPE. In this case, each VT1.5 SPE is not provided with its own payload pointer. With the locked mode it is not possible to route a selected VT1.5 through the SONET network without unwanted network delay caused by having to provide slip buffers to accommodate the timing/synchronization issues. 19.2.2.5 The Payload Pointer. The STS payload pointer provides a method for allow- ing flexible and dynamic alignment of the STS SPE within the STS envelope capacity, independent of the actual contents of the SPE. SONET, by definition, is intended to be synchronous. It derives its timing from the master network clock. See Chapter 6, Section 6.12.2. Modern digital networks must make provision for more than one master clock. Examples in the United States are the several interexchange carriers that interface with local exchange carriers, each with their own master clock. Each master clock (stratum 1) operates independently. And each of these master clocks has excellent stability (i.e., better than 1 