F (framing) bit

The F or framing bit in the DS-1 (T-1) digital multiplexing system is usually described as the first bit in the frame pattern that comprises 193 bits. The following 192 bits in the frame comprise 24 channels or time slots, each time slot comprising 8 bits from one channel. In the very first historical 1961 version of DS-1 technology, called the D-1 system (now obsolete, the current version being D-4), the F bit was alternately set to 1 and then 0 and this simple pattern repeated in each two consecutive frames. The D-2 and all later versions of DS-1 required a multi-frame or super-frame sequence of 12 frames so that certain bits in the traffic channels could be used for robbed bit signaling in the 6th and 12th frames. In a 12-frame sequence this is the F bit pattern: 100011011100. Bear in mind that in the actual bit stream there are 192 bits of channel data (not shown here) between each two adjacent F bits shown in this list. In the 1980s, a new 24 frame sequence called Extended Super Frame (ESF) was introduced. It has several advantages and is now very widely used. ESF uses a sequence of 24 F bits as follows:

m e1 m 0 m e2 m 0 m e3 m 1 m e4 m 0 m e5 m 1 m e6 m 1

Note that only 6 of these are fixed values: 001011. The 12 m bits constitute a message channel...