ive process carried out at repeaters and nodes. Bit errors do accumulate, and these noise impairments are one of the several causes of errors. One way to limit error accumulation is to specify a stringent bit error rate (BER) requirement for each repeater 12 Slips are a major impairment in digital networks. Slips and slip rate are discussed in Section 12.7. DIGITAL NETWORKS section. Up to several years ago, repeater sections were specified with a BER of 1 X 10-9. Today a BER of 1 X 10-10 to X 10-12 is prevalent in the North American network. It is interesting to note that PCM provides intelligible voice performance for an error rate as low as 1 in 100 (1 X 10-2. However, the bottom threshold (worst tolerable) BER is one error in one thousand (1 X 10-3 at system end-points. This value is required to ensure correct operation of supervisory signaling. The reader should appreciate that such degraded BER values are completely unsuitable for data transmission over the digital network. 6.9.5 Crosstalk Crosstalk is a major impairment in PCM wire-pair system, particularly when "go" and "return" channels are carried in the same cable sheath. The major offender of single- cable operation is near-end crosstalk (NEXT). When the two directions of transmission are carried in separate cables or use shielded pairs in a common cable, far-end crosstalk (FEXT) becomes dominant. One characteristic has been found to be a major contributor to poor crosstalk coupling loss. This is the capacitance imbalance between wire pairs. Stringent quality control during cable manufacture is one measure to ensure that minimum balance values are met. 6.10 DIGITAL LOOP CARRIER Digital subscriber loop carrier is a method of extending the metallic subscriber plant by using one or more DS1 configurations. As an example, the SLC-96 uses four DS1 configurations to derive an equivalent of 96 voice channels. The digital transmission facility used by a DLC system may be repeatered wire-pair cable, optical fibers, either or both combined with digital multiplex