Also known as ESF, Extended Super Frame is an enhanced version of D4 frame formatting. It is used most often in conjunction with B8ZS line coding in building point-to-point clear-channel T1 data lines. ESF has 24 frames of 192 bits each and provides 16 signaling states in the 193rd bit to ensure sychronization, supervisory control, and maintenance capabilities.
Thanks to Dwight Baker for this information