Over digital connections such as ISDN, there is no need to synchronize sender and recipient's clocks with start and stop bits as is done over analog telephone lines. Instead, one can use "synchronous PPP". Saying Y here will include this protocol. This protocol is used by Cisco and Sun for example. So you want to say Y here if the other end of your ISDN connection supports it. You will need a special version of pppd (called ipppd) for using this feature. See <file:Documentation/isdn/README.syncppp> and <file:Documentation/isdn/syncPPP.FAQ> for more information.
This enables Van Jacobson header compression for synchronous PPP. Say Y if the other end of the connection supports it.
With synchronous PPP enabled, it is possible to increase throughput by bundling several ISDN-connections, using this protocol. See <file:Documentation/isdn/README.syncppp> for more information.
Say Y here if you want to be able to filter the packets passing over IPPP interfaces. This allows you to control which packets count as activity (i.e. which packets will reset the idle timer or bring up a demand-dialled link) and which packets are to be dropped entirely. You need to say Y here if you wish to use the pass-filter and active-filter options to ipppd.
Support for the BSD-Compress compression method for PPP, which uses the LZW compression method to compress each PPP packet before it is sent over the wire. The machine at the other end of the PPP link (usually your ISP) has to support the BSD-Compress compression method as well for this to be useful. Even if they don't support it, it is safe to say Y here.
If you say Y here, the modem-emulator will support a subset of the EIA Class 8 Voice commands. Using a getty with voice-support (mgetty+sendfax by <gert@greenie.muc.de> with an extension, available with the ISDN utility package for example), you will be able to use your Linux box as an ISDN-answering machine. Of course, this must be supported by the lowlevel driver also. Currently, the HiSax driver is the only voice-supporting driver. See <file:Documentation/isdn/README.audio> for more information.
If you say Y here, the modem-emulator will support a subset of the Fax Class 1 and 2 commands. Using a getty with fax-support (mgetty+sendfax, hylafax), you will be able to use your Linux box as an ISDN-fax-machine. This must be supported by the lowlevel driver also. See <file:Documentation/isdn/README.fax> for more information.
This feature provides the X.25 protocol over ISDN connections. See <file:Documentation/isdn/README.x25> for more information if you are thinking about using this.
This driver provides a virtual ISDN card. Its primary purpose is testing of linklevel features or configuration without getting charged by your service-provider for lots of phone calls. You need will need the loopctrl utility from the latest isdn4k-utils package to set up this driver.
This option allows you to use some supplementary diversion services in conjunction with the HiSax driver on an EURO/DSS1 line. Supported options are CD (call deflection), CFU (Call forward unconditional), CFB (Call forward when busy) and CFNR (call forward not reachable). Additionally the actual CFU, CFB and CFNR state may be interrogated. The use of CFU, CFB, CFNR and interrogation may be limited to some countries. The keypad protocol is still not implemented. CD should work in all countries if the service has been subscribed to. Please read the file <file:Documentation/isdn/README.diversion>.