default or selected kernelversion does not have config value CONFIG_ISDN_PPP.
Result is shown for kernelversion 6.8.2
Support synchronous PPP
configname: CONFIG_ISDN_PPP
Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Support synchronous PPP
In linux kernel since version 2.6.20 (release Date: 2007-02-04)
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.
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.