Slow network

[Gerben Wierda]

I've got two machines on a 100Mbps ethernet hub (part of a ZyXEL DSL 
modem), an MacBook (10.4.9 client) and a G4 (with 100Mbps ethernet, 
10.3.9 Server). All are running firewalls.

When I scp a large file from one to the other with scp, the copy starts 
at 2-2.5MB/s (16-20 Mbps) but after some time (around 512MB) it drops 
to 60-100 kB/s. Using rsync (with -e ssh) gets me 2 MB/s.

I am curious:
- Why am I not seeing larger top speeds? Is this packet filtering 
somewhere? Or is 20Mbps to be expected? (I plan to do a separate test 
with a separate ethernet hub to see if the ZyXEL is to blame)
- What could be the reason for this drop in performance by scp after 
copying several hundreds of MB? Also maybe packet filtering? Any way to 
test?

Thanks in advance for any hints at resolution,

G