ronous Transport Module" at level m, where m = 1, 4, 16, 64, or 256. STM-1 operation is at 155.52 Mbit/s and the reason for this value follows from the data frame structure used (Ramaswami & Sivarajan, 2002; Sexton & Reid, 1997). It includes the data payload plus a number of bytes that function as administrative unit point- ers and regenerator and multiplex section overheads, which are vital for error monitoring Chapter 6 Optical Fiber Transmission Table 6.1 Datarates Used for Synchronous Digital Hierarchy Operation SDH Terminology Exact Datarate (Mbit/s) Common Abbreviation STM-1 155.52 155 Mbit/s STM-4 622.08 622 Mbit/s STM-16 2488.32 2.5 Gbit/s STM-64 9953.28 10 Gbit/s STM-256 39813.12 40 Gbit/s Not standardized at the time of writing 160 Gbit/s and above STM = synchronous transport module. and network management. The higher SDH datarates are simply factors of four above 155.52 Mbit/s, as specified in Table 6.1. As an approximate guide, at the time of writing metropolitan area networks operate at all rates up to 10 Gbit/s; wide-area terrestrial networks use 2.5 and 10 Gbit/s and (more recently) 40 Gbit/s; older subsea networks are at 2.5 Gbit/s; and nearly all new subsea networks work at 10 Gbit/s. Although many laboratory experiments have been performed with 160-Gbit/s channels, operation at such high rates has yet to be standardized or commercialized. The final aspect of SDH communications that we mention relates to the important func- tion of clock recovery in the receiver, performed to ensure that the decision process necessary for low error bit recognition occurs at the correct instant in the bitslots. Clock recovery is much more difficult when there are long strings of 1s, and for this reason systems normally include electronic bit scramblers at the transmitter and descramblers at the receiver. Reducing the probability of many consecutive 1s also reduces certain types of nonlinear crosstalk, as discussed in later sections on nonlinear crosstalk and system design and implementation. Some networks function