I'm having some trouble setting up, or at least maintaining, a ppp link over a serial RF radio. What happens is, I get the PPP connection but it's unable to sustain heavy usage - the symptoms are very reminiscent of bad flow control. It's OK with light usage, but doing something that would be likely to require flow control - say, transferring a file over PPP using netcat or ftp - causes it to grind to a halt.

I look in my syslog, and I see the following:
Code:
Nov 19 17:36:57 xyzzy pppd[236]: pppd 2.4.1 started by root, uid 0
Nov 19 17:36:57 xyzzy pppd[236]: Couldn't restrict write permissions to /dev/ttyS48: Read-only file system
Nov 19 17:36:57 xyzzy pppd[236]: Using interface ppp0
Nov 19 17:36:57 xyzzy pppd[236]: Connect: ppp0 <--> /dev/ttyS48
Nov 19 17:36:57 xyzzy pppd[236]: kernel does not support PPP filtering
Nov 19 17:36:58 xyzzy pppd[236]: not replacing existing default route to eth0 [192.168.0.1]
Nov 19 17:36:58 xyzzy pppd[236]: Cannot determine ethernet address for proxy ARP
Nov 19 17:36:58 xyzzy pppd[236]: local  IP address 192.168.1.93
Nov 19 17:36:58 xyzzy pppd[236]: remote IP address 192.168.1.213
Nov 19 17:43:24 xyzzy pppd[236]: Terminating on signal 15.
Nov 19 17:43:24 xyzzy pppd[236]: Connection terminated.
Nov 19 17:43:24 xyzzy pppd[236]: Connect time 6.5 minutes.
Nov 19 17:43:24 xyzzy pppd[236]: Sent 20341 bytes, received 20296 bytes.
Nov 19 17:43:24 xyzzy pppd[236]: Exit.
Nov 19 17:43:26 xyzzy pppd[350]: pppd 2.4.1 started by root, uid 0
Nov 19 17:43:26 xyzzy pppd[350]: using channel 2
Nov 19 17:43:26 xyzzy pppd[350]: Using interface ppp0
Nov 19 17:43:26 xyzzy pppd[350]: Connect: ppp0 <--> /dev/ttyS48
Nov 19 17:43:26 xyzzy pppd[350]: sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x3e4493cc> <pcomp> <accomp>]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [LCP ConfReq id=0x18 <mru 1522> <asyncmap 0x0> <magic 0x64bfd529> <pcomp> <accomp>]
Nov 19 17:43:29 xyzzy pppd[350]: sent [LCP ConfAck id=0x18 <mru 1522> <asyncmap 0x0> <magic 0x64bfd529> <pcomp> <accomp>]
Nov 19 17:43:29 xyzzy pppd[350]: sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x3e4493cc> <pcomp> <accomp>]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x3e4493cc> <pcomp> <accomp>]
Nov 19 17:43:29 xyzzy pppd[350]: kernel does not support PPP filtering
Nov 19 17:43:29 xyzzy pppd[350]: sent [IPCP ConfReq id=0x1 <addr 192.168.1.93> <compress VJ 0f 01>]
Nov 19 17:43:29 xyzzy pppd[350]: sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15> <bsd v1 15>]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [LCP ProtRej id=0x1a 80 fd 01 01 00 0f 1a 04 78 00 18 04 78 00 15 03 2f]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [IPCP ConfReq id=0x72 <compress VJ 0f 00> <addr 192.168.1.213>]
Nov 19 17:43:29 xyzzy pppd[350]: sent [IPCP ConfAck id=0x72 <compress VJ 0f 00> <addr 192.168.1.213>]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [IPCP ConfNak id=0x1 <compress VJ 0f 00>]
Nov 19 17:43:29 xyzzy pppd[350]: sent [IPCP ConfReq id=0x2 <addr 192.168.1.93> <compress VJ 0f 00>]
Nov 19 17:43:29 xyzzy pppd[350]: rcvd [IPCP ConfAck id=0x2 <addr 192.168.1.93> <compress VJ 0f 00>]
Nov 19 17:43:29 xyzzy pppd[350]: not replacing existing default route to eth0 [192.168.0.1]
Nov 19 17:43:29 xyzzy pppd[350]: Cannot determine ethernet address for proxy ARP
Nov 19 17:43:29 xyzzy pppd[350]: local  IP address 192.168.1.93
Nov 19 17:43:29 xyzzy pppd[350]: remote IP address 192.168.1.213
Nov 19 17:43:29 xyzzy pppd[350]: Script /etc/ppp/ip-up started (pid 353)
Nov 19 17:43:30 xyzzy pppd[350]: Script /etc/ppp/ip-up finished (pid 353), status = 0x1
Nov 19 17:43:34 xyzzy pppd[363]: pppd 2.4.1 started by root, uid 0
Nov 19 17:43:34 xyzzy pppd[363]: Device ttyS48 is locked by pid 350
Nov 19 17:44:04 xyzzy last message repeated 20354 times
Nov 19 17:45:05 xyzzy last message repeated 40835 times
Nov 19 17:46:06 xyzzy last message repeated 40766 times
Nov 19 17:47:08 xyzzy last message repeated 40995 times

I'm worried about those 150000 messages at the end there...

Can anybody suggest any avenues I should use to investigate?
Thanks!