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From ANSI X3.139-1987 Section 4.3.1 Preamble (PA). The PA of a frame shall be transmitted by the frame originator as a minimum of 16 symbols of Idle. Physical Layers of subsequent repeating stations may change the length of the Idle pattern consistent with Physical Layer clocking requirements. Thus, repeating stations may see a variable length preamble that is shorter or longer than the originally transmitted preamble. A given MAC implementation is not required to be capable of copying frames received with less than 12 symbols of preamble; however if it cannot correctly repeat such a frame, then it shall not repeat any part of the frame (including the starting delimiter).
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