IBM SC30-3681-08 manual Ncp Bcp Sets the Bridging Control Protocol BCP parameters, Ccp options

Con®guring PPP Interfaces (Talk 6)

ncp...

bcp Sets the Bridging Control Protocol (BCP) parameters.

Example:

set bcp

TINYGRAM COMPRESSION [no]:

Tinygram Compression

Speci®es whether or not Tinygram Compression is used. This option is useful for protocols that are prone to problems when bridged over low-speed (64 Kbps and below) lines. These protocols add zeroes between the data and the frame checksum to pad the Protocol Data Unit (PDU) to the minimum size. Tinygram compression removes the zeroes and preserves the frame checksum at the transmitting end. At the receiving end, it restores the packet to the minimum length.

ccp options

Prompts you for the con®gurable options of the compression algorithms. Some of the options may be modi®ed later by PPP negotiations with the peer router on the WAN link. For additional information, see ªUsing the Data Compression Subsystemº in Using and Con®guring Features.

Example:

set ccp options

STAC: # histories [1]?

STAC: check mode (0=none, 1-LCB, 2=CRC, 3=Seq) [3]?

STAC: # histories

This sets the number of compression ªcontextsº or ªhistoriesº that are used by the STAC compression engine.

A nonzero value means that the compression engine maintains the speci®ed number of histories where it keeps information about previous data sent in packets. This historical data is used to improve the effectiveness of the compression.

The receiver maintains a similar history and as long as the transmitter and receiver keep their histories in sync, the receiver can properly decompress the packets it receives. If the histories get out of sync, packets are discarded as unusable data. Normally, you should set the number of histories to 1 unless the link quality is very poor.

A value of zero means that each packet sent is compressed without regard to any past packets sent and may always be reliably decompressed by the receiver. However, because the compressor cannot exploit any information derived from examining prior packets, the effectiveness of the compression usually is not as good.

Some implementations support more than one history, subdividing the data stream into separate streams that are compressed independently. The router does not support the use of more than one history on a PPP link.

STAC: check mode (0=none, 1=LCB, 2=CRC, 3=Seq)

STAC compressed datagrams normally include a check value used by the two ends of the link to recognize when a compressed packet has been lost or corrupted, and some action is needed to re-synchronize the sender's and receiver's histories.

Chapter 33. Con®guring and Monitoring Point-to-Point Protocol Interfaces 475