Drew Weaver wrote: is duplex a consideration with lines of these types?

DS3s are full-duplex AFAIK. There are two coax cables (one in one out) and I don't ever remember seeing anything that looked like a collision detection mechanism. Compare to Ethernet: on 10base-2 or 10base-5, you have half-duplex as the physical medium (single coax) is the same for transmit and receive. Same on a 10base-T or 100base-T hub, where even if the TX and Rx pairs are separate they are mixed inside the hub. When connected to a switch, a NIC that either is configured to full-duplex or negociates full-duplex actually deactivates its built-in collision detection circuit.

We have 1 DS-3, that if we get close to 45Mbps IN/OUT the other direction will be completely unusable.

The condition you describe is rather common: your upstream is maxed out and your downstream becomes very slow. This has nothing to do with duplex: your downstream sits unused. The perceived slowness is likely due to DNS requests, TCP SYNs and the like being slowed or discarded in the egress queue. If you traffic-shape your egress (by tweaking queuing, among other things) you could get close to 45Mpbs in and out. Usual tricks include granting small packets a higher priority, and lowering the priority of packets that max out your pipe. Things such as wRED and dCAR are to be looked at too. Michel.