The digital trunks most commonly used today are either T1 or E1 facilities (as described in the "Physical Layer—MTP L1" section of Chapter 4). With digital trunks, bits are robbed from specific frames and are used for signaling purposes. This discussion focuses on T1 digital trunks.
T1 has two types of framing formats: Super Frame (SF) and Extended Superframe (ESF). The least significant bits are robbed from frames 6 and 12 for SF and frames 6, 12, 18, and 24 for ESF. These bits are referred to as the Sa and Sb bits for SF, and the Sa, Sb, Sc, and Sd bits for ESF. Robbing these bits has a negligible effect on voice quality.
The SF signaling bits—Sa and Sb—are equal to each other and provide two-state, continuous supervision signaling. Bit values of zero are used to indicate on-hook, and bit values of 1 are used to indicate off-hook.
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