Terminating on signal 15?

[Hans <hansfong@zonnet.nl>]

I get this message when dialing in my ISP with my PCMCIA modem.

Apr 25 20:56:06 knack@orchard pppd[752]: pppd 2.3.11 started by root, uid 0
Apr 25 20:56:06 knack@orchard pppd[752]: Removed stale lock on ttyS2 (pid 727)
Apr 25 20:56:32 knack@orchard pppd[752]: Serial connection established.
Apr 25 20:56:32 knack@orchard pppd[752]: Using interface ppp0
Apr 25 20:56:32 knack@orchard pppd[752]: Connect: ppp0 <--> /dev/ttyS2
Apr 25 20:56:33 knack@orchard pppd[752]: Terminating on signal 15.
Apr 25 20:56:33 knack@orchard pppd[752]: Connection terminated.
Apr 25 20:56:33 knack@orchard pppd[752]: Hangup (SIGHUP)
Apr 25 20:56:33 knack@orchard pppd[752]: Exit.

What does signal 15 stand for. I searched the net, but to no avail. Anyone?
--hans