Netmon, Statistics, Traffic, Analisys, Performance

I have a 512 Kbps link with a distant location and month after month, the
speed of the link is degradating. I have no idea what kind of traffic is
"eating" the bandwidht.

I've tried EtheReal and it is very very good, but the statistics does not
show the per protocol statistics (i think the culprit is MS Exchange Server
2000, but i wann prove that)

I don't have access to the router, the owner of the router does not
provide any method of analisys and statistical data

I have to monitor the internal network with a "promiscous mode" adapter?
How can i obtain a per-protocol statistics? I wann know what kind of traffic
is "eating" the internal network bandwidht and try to "guess" what is going
on