A slightly older framing standard for T1s. Also called D4 or D3/D4 framing.

In order to determine where each channel is in the stream of data being received, each set of 24 channels is aligned in a frame. The frame is 192 bits long (8 * 24), and is terminated with a 193rd bit, the framing bit, which is used to find the end of the frame.

In order for the framing bit to be found by receiving equipment, a pattern is sent on this bit. Equipment will search for a bit which has the correct pattern, and will align its framing based on that bit.

The pattern sent is 12 bits long, so every group of 12 frames is called a Super Frame. The pattern used in the 193rd bit is 1000 1101 1100.

Preferred over SF is Extended Super Frame, which provides additional features.