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From ANSI X3.139-1987 Section 4.2.1 The preamble is transmitted by the token 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. Tokens shall be recognized and acted upon when received with a preamble of zero or greater length, independent of previously established symbol boundaries. If a valid token is received but cannot be repeated (owing to ring timing or latency constraints), the station shall issue a new token.
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