Cisco IOS Wide-Area Networking Configuration Guide - Free Books

LAPB Configuration Task ListConfiguring X.25 and LAPBWhen using compression, adjust the maximum transmission unit (MTU) for the serial interface and theLAPB N1 parameter as in the following example, to avoid informational diagnostics regarding excessiveMTU or N1 sizes:interface serial 0encapsulation lapbcompress predictormtu 1509lapb n1 12072For information about configuring X.25 TCP/IP header compression and X.25 payload compression, seethe sections “Setting X.25 TCP/IP Header Compression” and “Configuring X.25 Payload Compression”later in this chapter.Modifying LAPB Protocol ParametersLAPB specifies methods for exchanging data (frames), detecting out-of-sequence or missing frames,retransmitting frames, and acknowledging frames. Several protocol parameters can be modified tochange LAPB protocol performance on a particular link. Because X.25 operates the Packet LevelProtocol (PLP) on top of the LAPB protocol, these tasks apply to both X.25 links and LAPB links. Theparameters and their default values are summarized in Table 9. Detailed descriptions of each parameterare given after the table.Table 9LAPB ParametersCommand Purpose (LAPB Parameter) Values or Ranges Defaultlapb modulo modulus Sets the modulo. 8 or 128 8lapb k window-size Sets the window size (K). 1– (modulo minus 1) 7frameslapb n1 bits Sets the maximum bits per frame (N1). Bits (multiple of 8) Based on hardware MTUand protocol overheadlapb n2 tries Sets the count for sending frames (N2). 1–255 tries 20lapb t1 milliseconds Sets the retransmission timer (T1). 1–64000 milliseconds 3000lapb interface-outage Sets the hardware outage period.0 (disabled)millisecondslapb t4 seconds Sets the idle link period (T4). 0 (disabled)The following sections provide more information about the LAPB parameters in the Table 9:• LAPB modulo—The LAPB modulo determines the operating mode. Modulo 8 (basic mode) iswidely available because it is required for all standard LAPB implementations and is sufficient formost links. Modulo 128 (extended mode) can achieve greater throughput on high-speed links thathave a low error rate (satellite links) by increasing the number of frames that can be sent before thesending device must wait for acknowledgment (as configured by LAPB parameter K).• LAPB parameter K—LAPB K must be at most one less than the operating modulo. Modulo 8 linkscan send seven frames before an acknowledgment must be received by the sending device; modulo128 links can send as many as 127 frames. By default, LAPB links use the basic mode with a windowof 7.4