serial0 is up, line protocol is down

[admin_2]

I have the above problem with my 2501 router. Everything is in place but still get the line protocol down

[#2]

Cisco routers rely on keepalive frames when deciding about the state of the line protocol. If four keepalives have been missed, the line protocol will be declared down. For troubleshooting, please refer to Table 15-1 ("Serial Lines: show interfaces serial Status Line Conditions") in "Troubleshooting Serial Line Problems" on:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1915.htm