>A Cisco router, connected to the same circuit, pings its peer
>successfully and reports the following:
>
>Serial0/1 is up, line protocol is up
>...
>  Encapsulation HDLC, loopback not set
>...

Just a guess, but Cisco's HDLC implementation is not 'standard'. I think it's
got an extra few bits for indicating which higher layer protocol is contained in
the packet. So if your peer is cisco you will probably have to do PPP instead of
HDLC to get them to talk.

Pete.