Serial link utilization

Hi guys

This is a question for the NFA guys and everyone else here please.

I'm running NFA monitoring serial interfaces on my core routers, each hooked up to a 2Mbit pipe to an MPLS cloud. I'm intrigued by the concept of utilization here, as my links rarely get above 25% utilization.

What confuses me is why, when I can send data to the router on the LAN at 100Mbit (or even any speed above 2Mbit/s), it doesn't hike up the throughput to 100% until my data has cleared the link before dropping back down again. Instead, that data seems to trickle out. As an example, I had a user transfer 100MB of data across a link today, to a site on the same MPLS cloud, with <25% current utilization, and the transfer took 30+ minutes, but only pushed the average utilization up to 45%ish.

Sorry this is vague, and I'm not looking for a solution per se. I was just hoping someone understood how these things work and could explain it to me, or point me at some information.

Thanks, in advance

Stuart