Installing and Administering PPP

78 Chapter 4
Common pppd Options
Unique PPP Implementations
Unique PPP Implementations
Although most implementations of PPP occur over aysnchronous dial up
connections, PPP can be used for synchronous transmission over high
speed serial interfaces. It can also be used on dedicated lines and
constantly open telephone lines. The latter is a dial-up connection, but it
is not on-demand.
Synchronous PPP
PPP can run in synchronous mode using a high speed serial interface at
line speeds up to T1 (1.544Mb/s). To prepare your system to use a high
speed interface, follow the instructions in the hardware installation
guide.
Dedicated Lines
Use pppd's dedicated argument if the PPP implementation uses
asynchronous serial connections that are always available. These
connections often use high-speed asynchronous short-haul modems over
a building or campus wiring plant. The dedicated argument instructs
pppd to never give up on the connection. If the peer tells pppd to
disconnect, pppd will continuously attempt to reconnect and connectivity
is reestablished as soon as possible if one end of the link goes down.
If there is a fatal disconnect, through LQM failures or loss of the Carrier
Detect signal, pppd closes the device and consults the Systems file to
find another matching entry. If none is available, pppd waits for the call
retry delay to pass and tries the original connection again. Normally, no
getty or login process is run on a dedicated line device and both ends of
the circuit actively try to connect to their peer. Each machine’s
Autostart script should contain a line like the following:
pppd local:remote auto dedicated
The Systems file should specify a device name like cuh00 in the device
field. ACU, for “Any Call Unit”, should not be used. For example:
remote Any cuh00 38400 0
The Devices file should contain a line like the following:
Direct cuh00 38400