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LAPF Frame Format

LAPF Frame Format. Flag 1 octet. Address 2 - 4 octets. Information variable length. FCS 2 octets. Flag 1 octet. Frame Format. 8 7 6 5 4 3 2 1. Upper DLCI. C/R. EA 0. Lower DLCI. FECN. BECN. DE. EA 1.

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LAPF Frame Format

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  1. LAPF Frame Format Flag 1 octet Address 2 - 4 octets Information variable length FCS 2 octets Flag 1 octet Frame Format 8 7 6 5 4 3 2 1 Upper DLCI C/R EA 0 Lower DLCI FECN BECN DE EA 1 Address field 2 octets (default) Legend EA Address field extension bit C/R Command/response bit DE Discard eligibility bit FECN Forward explicit congestion notification BECN Backward explicit congestion notification DLCI Data link connection identifier

  2. LAPF Frame Format • No control field exists in the frame • The connection can only carry user data • Therefore no in-band signalling exists • No error control or flow control exists since there are no sequence numbers

  3. LAPF Frame Format • Address field carries DLCI • Address field length may be extended to 2, 3, or 4 octets • Length determined by EA bits - default is 2 octets • DLCI allows multiple logical connections to be multiplexed on single channel • DLCI can be 10, 17 or 24 bits depending on address field length

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