>I recently maintain one NT domain.
>This domain encompasses two subnets separated by a T1 connection.
>My subnet masks are 255.255.255.0 on both.
>I use DHCP and TCP/IP.
>About 8 PCS on office A and about 30 on office B.
>PDC and BDC are in office B, authentication therefore happens
>in office B.
>The two offices are connected through routers with a T1 line.
>Use ethernet within every office.
>
>My problem: If I want to copy files from office A to office B
>it takes very long. It makes sense as the T1 represents a bottleneck.
>T1 at its best is 1.5MBS so I understand.

Your understanding is wrong - T1 is 24B+D, or 1.5Mb - megabits, not megabytes per second. Given framing, latency and protocol overhead, that's approximately 1/8th the speed of a 10Mbps EtherNet connection, and at least 10x worse compared to a 100Mbps Fast EtherNet; the longer the T1 run, the greater the latency, and as a result, the worse the overall throughput on protocols that do not use a windowed transfer protocol, allowing multiple packets to be in transit simultaneously without confirmation.

>When I try to copy a 1MB file from one PC in office B to another PC
>in office B it is instantenous, I don't even get a transfer file window.
>
>When I try to copy a file from an office A PC to another office A PC
>it also takes a long time, the 1MB file takes 10 seconds. I don't understand.

This is because every confirmation, address resolution and authentication request must cross the T1 line to reach the PDC or BDC, and the reply must go back across the T1, a latency delay of ~2x packet transmittal delay per control packet. Setting up a second BDC in office A would vastly reduce the traffic required to cross the T1.

>Maybe I have it set up wrong. Because as I understand it when PC1 in
>office A tries to send a file to PC2 in office A it theoretically should
>go from PC1 to Router in Office A and rerouted to PC2 in office A. That
>would be instantenous. Yet what it seems to be happening is that the packets
>travel all the way to office B and back to office A.
>Can I avoid this?

You need a way of resolving the Windows address without crossing the router; I'd suggest using DNS rather than WINS, and have each router act as a DNS Server for it's local subnet, as well as adding the second BDC. I use a similar approach for some of my clients' multisite implementations.