Quick circuit question.

We have 1 DS-3, that if we get close to 45Mbps IN/OUT the other direction will be completely unusable. We have another DS3, that we have seen doing 45Mbps IN, and 15Mbps Out at the same time. This is going to sound dumb, but is duplex a consideration with lines of these types? I thought you could do either 45Mbps in, 45Mbps out, or a mix of both, not 45Mbps In, and 45Mbps out at the same time (90mbps).. I couldn't find this exact info on the net anywhere. -Drew