IBM SC30-3681-08 manual Addr/Cntl Field Compression Acfc, Protocol Field Compression PFC

Con®guring PPP Interfaces (Talk 6)

Specify a 32-bit bit mask in hexadecimal. If a bit in position 'N' of the mask is set, the corresponding ASCII character 'N' must be escaped (the LSB is bit number 0, corresponding to the ASCII NUL character).

The default value for this option is '0A0000', indicating that XON and XOFF (control-Q and control-S) need to be escaped. This is for the bene®t of modems that use XON/XOFF to perform software handshaking. If this is not an issue, then it is recommended that you change the ACCM to zero (no characters escaped).

LCP is always willing to negotiate the ACCM, even on synchronous lines, and the list lcp command in the PPP monitoring process will display the negotiated value. However, synchronous lines employ a ªbit-stuffingº mechanism rather than an ªescapingº mechanism, so the ACCM is not normally meaningful on synchronous lines. It may be meaningful if the router is connected to a modem that performs sync-to-async conversion, in which case its value should re¯ect the requirements of the attached modem on the asynchronous side.

Addr/Cntl Field Compression (ACFC)

Speci®es whether the peer can employ address and control ®eld compression.

If the ACFC option is successfully negotiated by LCP, it means that the Address and Control ®eld bytes which start off each packet may be omitted in the datagrams sent back and forth on the link. These bytes are always 0xFF 03, so there is no real information provided by them, and enabling ACFC means that the datagrams that are transmitted will be two bytes shorter.

To be precise, if you enable ACFC, you are indicating a receive-side capability. If you enable ACFC and LCP successfully negotiates it, the other end can employ ACFC in the packets it transmits to the local end (most PPP options work like this). The local end will only transmit packets without the address and control ®elds if the other end also indicates its ability to handle such packets.

Enabling ACFC does not obligate the other end to send packets without the address and control ®elds, even if it accepts the option. Enabling ACFC merely tells the peer that it optionally may use ACFC, and the router will be able to handle the incoming packets. If the peer indicates that it can handle ACFC, then the router always performs ACFC on the packets it transmits regardless of whether ACFC is enabled locally.

LCP packets always are sent with address and control ®elds present. This guarantees that LCP packets will be recognized even if there is a loss of link synchronization.

Protocol Field Compression (PFC)

Speci®es whether the peer is to employ protocol ®eld compression.

When you specify ªyesº, if the PFC option is negotiated successfully by LCP, the leading zero byte may be omitted from the ªProtocolº ®eld for those protocol values in the range '0x0000'±'0x00FF', for a one byte savings in the packets being transmitted. This range includes the majority of layer-3 protocol datagrams.

480MRS V3.2 Software User's Guide