 there is actually no STM-0 signal designation. However, because an STM-1 can be considered to consist of three STS-1 signals, we can use the designator STM-0 to refer to an STS-1 signal in the SDH hierarchy. Thus, SDH begins operations at 155.52 Mbits/s, which represents 3 times the lowest operating rate of SONET. However, the five levels of operation of SDH can be equated to the five higher layers in the SONET hierarchy. Another item that warrants a bit of elaboration is the STM-256 signal. Similar to a SONET's OC-768, this signal is common- ly rounded and described as a 40 Gbit/s data transmission rate. Another difference between SONET and SDH that warrants attention is one of terminology. We can note this difference by examining the SDH frame. THE STM-0 FRAME Figure 6.9 illustrates the basic structure of the SDH frame. If you compare Figure 6.9 to the basic structure of the STS-1 frame illustrated in Figure 6.5, you will note that they are nearly identi- cal but employ different nomenclatures. For example, the repeater sec- tion is referred to as the section overhead area in an STS-1 frame. Another difference with respect to nomenclature involves the payload area. In an STM-0 frame the payload area is known as administrative unit 3 (AU3). Since the payload floats, the H pointers used in the STM-1 frame were renamed as the AU3 pointer area. Chapter Six TABLE 6.2 Signal Buildup Operating rate, Mbits/s The SDH Transmis- sion Hierarchy STM-0 51.84 STM-1 3 X STM-0 155.52 STM-4 4 X STM-1 622.08 STM-16 16 X STM-1 2488.32 STM-64 64 X STM-1 9953.28 STM-256 256 X STM-1 39813.12 90 columns Figure 6.9 Section The STM-0 frame Overhead structure. Repeater Section AU3 Pointer Area Administrative Unit 3 (AUs) main area of 783 bytes Multiplex Section Overhead The STM-0 frame represents a virtual signal that forms an STM-1 frame. Once this SDH frame is formed, four STM-1 frames then form an STM-4 frame, and SO on. Each STM frame represents an increase in the two-dimensional matrix used for representation similar to the manner 