ESF(redirected from D4)
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ESF(1) (Extended SuperFrame) An enhanced T1 format that allows a line to be monitored during normal operation. It uses 24 frames grouped together (instead of the 12-frame D4 superframe) and provides room for CRC bits and other diagnostic commands.
(2) (External Source Format) A specification language for defining an application in IBM's CSP/AD application generator.