LAPB parameters
LAPB frame numbering modes
Two LAPB frame numbering modes are available: modulo 8 and modulo 128. Data frames (I frames) are numbered cyclically in sequence in the range of 0 to modulo – 1.
All standard LAPB implementations support modulo 8 (the basic mode), which is sufficient for most links.
LAPB window size K
LAPB window size K indicates the maximum number of outstanding frames for acknowledgement on the DTE or DCE within any period.
LAPB N1 parameter
The LAPB N1 parameter specifies the maximum number of bits that an I frame can hold. The maximum N1 value is calculated by using the following formula:
(hardware MTU + protocol header) × 8
LAPB N2 parameter
N2 sets the maximum number of times a data frame can be sent from DCE to DTE or vice versa.
LAPB timers
LAPB defines the following timers:
T1—The retransmission timer. It determines how long a transmitted frame can remain unacknowledged before LAPB polls for an acknowledgement. When the T1 timer expires, the DTE or DCE retransmits the frame.
T2—The explicit acknowledge deferral timer. It determines the time the DTE or DCE waits before sending an explicit acknowledgement. The acknowledgement is piggybacked with data, unless no data exists and then an explicit acknowledgement is sent when the timer expires. T1 must be no smaller than 2 × T2.
T3—The idle channel state timer. It is set on the DCE. When the T3 timer expires, the DCE reports an excessively long idle channel state condition to the packet layer. T3 must be greater than the DCE T1 timer. To disable the T3 timer, set it to zero.
T4—The timer to detect unsignaled link failures. The T4 timer is reset every time a frame is received from the partner on the link. If the T4 timer expires, a Receiver Ready frame with the Poll bit set is sent to the partner, which is required to respond. If the partner does not respond, the standard polling mechanism determines whether the link is down. You must set T4 to be greater than T1, the LAPB retransmission timer. When the number of transmission attempts exceeds the LAPB N2 parameter, the link is renegotiated. To disable the T4 timer, set it to zero.