Editing LAPB (section)

===Frame format===
{| style="width: 90%; margin:auto; border: thick solid black; <!--
    --> border-collapse: collapse; text-align:center; background: white; line-height: 120%;"
|+
! scope="col" style="width:6em;  border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Flag
! scope="col" style="width:6em;  border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Address
! scope="col" style="width:6em;  border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Control
! scope="col" style="width:auto; border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Data
! scope="col" style="width:12em; border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Checksum
! scope="col" style="width:6em;  border-right: 1px solid black; <!--
    --> padding-top: 0.5em;" | Flag
|-
| style="border-right: 1px solid black;" | 0111 1110 
| style="border-right: 1px solid black;" |
| style="border-right: 1px solid black;" | 
| style="border-right: 1px solid black;" | 
| style="border-right: 1px solid black;" | 
| style="border-right: 1px solid black;" | 0111 1110
|-
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (8 bits) 
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (8 bits) 
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (8 bits) 
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (Variable) 
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (16 bits)  
| style="border-right: 1px solid black; padding-bottom: 0.5em;" | (8 bits)
|}

'''Flag''' – The value of the flag is always 0x7E. In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as [[Bit stuffing]] is used by both the transmitter and the receiver.

'''Address field''' – In LAPB, this field has no meaning since the protocol works in a point to point mode and the DTE [[network address]] is represented in the layer 3 packets. This byte is therefore put to a different use; it separates the link commands from the responses and can have only two values: 0x01 and 0x03. 01 identifies frames containing commands from DTE to DCE and responses to these commands from DCE to DTE. 03 is used for frames containing commands from DCE to DTE and for responses from DTE to DCE. Therefore, one side must be configured as a Layer 2 DTE and the other as a Layer 2 DCE (you must not confuse this with the more familiar Layer 1 DCE and DTE designations).

'''Control field''' – it serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.

'''Modes of operation''' – LAPB works in the Asynchronous Balanced Mode (ABM). This mode is balanced (i.e., no master/slave relationship) and is signified by the SABM(E)/SM frame. Each station may initialize, supervise, recover from errors, and send frames at any time. The DTE and DCE are treated as equals.

'''FCS''' – The Frame Check Sequence enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked.

'''Window size''' – LAPB supports an extended window size (modulo 128 and modulo 32768) where the maximum number of outstanding frames for acknowledgment is raised from 7 (modulo 8) to 127 (modulo 128) and 32767 (modulo 32768).<ref>[http://www.itu.int/rec/T-REC-X.25-199610-I/en/ ITU-T Recommendation X.25 (10/96)], p. 35.</ref>