Control Field Format - three frame types

• Information frame (ARQ piggybacked in data)

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  0 send-seq# P/ F rec-seq#
  (3 or 7 bits) (3 or 7 bits)

• Supervisory (ARQ if not piggybacked)

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  1 0 sup-fcn P/ F rec-seq#
  

• Unnumbered (other functions)

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  1 1 unn-fcn P/ F unn-fcn

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