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 Documentation/isdn/README.syncppp and
Documentation/isdn/syncPPP.FAQ for more information.