AIS user manual AI2524 X.25 Configuration Steps

Chapter 12: AI2524 X.25 Configuration Steps

zLAPB Modulo and LAPB K. The LAPB modulo determines the operating mode. Modulo 8 (basic mode) is widely available, be- cause it is required for all standard LAPB implementations and is sufficient for most links. Modulo 128 (extended mode) can achieve greater throughput on high-speed links that have a low er- ror rate (some satellite links, for example) by increasing the num- ber of frames that can be transmitted before waiting for acknowledgment (as configured by the LAPB window parameter, k). By its design, LAPB's k parameter can be at most one less than the operating modulo. Modulo 8 links can typically send seven frames before an acknowledgment must be received; modulo 128 links can set k to a value as large as 127. By default, LAPB links use the basic mode with a window of 7.

zLAPB N1. When connecting to an X.25 network, use the N1 pa- rameter value set by the network administrator. This value is the maximum number of bits in an LAPB frame, which determines the maximum size of an X.25 packet. When you are using LAPB over leased lines, the N1 parameter should be eight times the hardwar Maximum Transmission Unit (MTU) size plus any protocol over- head.

The LAPB N1 range is dynamically calculated by the Cisco IOS software whenever an MTU change, an L2/L3 modulo change, o a compression change occurs on a LAPB interface.

Caution: The LAPB N1 parameter provides little benefit beyond the interface MTU and can easily cause link failures if misconfigured. This parameter should be left at its default value.

zLAPB N2. The transmit counter (N2) is the number of unsuccess- ful transmit attempts made before the link is declared down.

zLABP T1. The retransmission timer (T1) determines how long transmitted frame can remain unacknowledged before the Cisco IOS software polls for an acknowledgment. For X.25 networks, the retransmission timer setting should match that of the network.

For leased-line circuits, the T1 timer setting is critical because the design of LAPB assumes that a frame has been lost if it is not ac- knowledged within period T1. The timer setting must be larg enough to permit a maximum-sized frame to complete one round trip on the link. If the timer setting is too small, the software will poll before the acknowledgment frame can return, which may re- sult in duplicated frames and severe protocol problems. If the tim- er setting is too large, the software waits longer than necessary before requesting an acknowledgment, which reduces bandwidth.

August 1997	Page 12-51
2524UM