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The frame has a repetitive structure with a period of 125 microseconds—the same as for pulse code modulation (PCM)—and consists of nine equal-length segments. At the gross transport rate of 155.52 Mbps for the base synchronous transport module (STM–1), there is a burst of nine overhead bytes at the start of each segment, as shown at the top of Figure 5. This figure also depicts how the SDH frame at STM–1 is conventionally represented, with the segments displayed as from nine rows and 270 columns. Each byte is equivalent to 64 kbps, so each column of nine bytes is equivalent to 576 kbps.
Figure 5. SDH Frame Structure
The first nine columns contain the section overhead (SOH) for transport-support features such as framing, management-operations channels, and error monitoring, with the first segment containing the frame word for demultiplexer alignment. The remaining columns can be assigned in many ways to carry lower bit-rate signals, such as 2 Mbps; each signal has its own overhead. For transporting PDH traffic signals, payload capacity is allocated in an integral number of columns, inside of which are management overheads associated with the particular signal, as Figure 6 depicts.
Figure 6. Payload Capacity
The first level of division is the administrative unit (AU), which is the unit of provision for bandwidth in the main network. Its capacity can be used to carry a high bit-rate signal, such as 45 Mbps or 140 Mbps (for the two sizes of AU, AU–3 and AU–4, respectively). Figure 6 shows an AU–4, which occupies all of the payload capacity of an STM–1. An AU can be further divided to carry lower-rate signals, each within a tributary unit (TU), of which there are several sizes. For example, a TU–12 carries a single 2–Mbps signal, and a TU–2 carries a North American or Japanese 6–Mbps signal.
A specific quantity of one or more TU's can be notionally combined into a tributary unit group (TUG) for planning and routing purposes. No overheads are attached to create this item, so its existence relies on network management tracking its path. For example, in Europe, ETSI proposes that a TUG–2 should carry 3 x 2 Mbps in the form of 3 x TU–12s.
