EXPERT RESPONSE
Dear Preston,
You've certainly got a big project ahead of you and I strongly believe you can make the correct decisions if you carefully plan everything out.
Due to the size of your project, you can understand it's almost impossible for me to get a clear picture of your LAN/WAN requirements, available equipment and budget, to make a suggestion that will provide you with the necessary answers you are looking for.
However, there are a few guidelines you can make use of that can help you stay on the right track.
T1 lines can provide a maximum of 1.544 Mbps. You mention you've got three such lines, giving you a capacity of 4.632 Mbps which run at 30-40% during the day, but peak up to 80-90% at times.
Since you are doubling the amount of workstations, you will most certainly need to upgrade your lines to serve the new clients because according to your current statistics, you can expect a 70-80% load average when the clients are installed.
The problem here though isn't how many additional lines you should install. I find it far more important to firstly understand where the traffic is going, then, based upon your findings, make the necessary decisions.
For example, you mention Microsoft's Active Directory is running throughout your WAN alongside with other server-centric applications. You'll need to figure out what the bandwidth requirements are for each of these services. Once this is done, you can easily calculate the required bandwidth for your clients. Obviously, any excess traffic would be classified as Internet traffic (email, Web etc). Considering the importance of each application, you can then classify the traffic and provide a higher priority to your network applications ensuring they are not starved by Internet (or other) traffic.
When you've successfully managed to obtain a complete view of your traffic needs, taking into consideration possible redundant connections, you can then decide if you should.
Again, because you are talking about 6,000+ clients, I strongly advise you layout your traffic needs, break them into categories and then priorities. From that point, you'll know exactly what you need to do.
In closing, keep in mind that we are currently preparing a very interesting article on the ISDN protocol where you'll be able to obtain a lot of information regarding the protocol, T1, T3 lines and much more.
Good luck!
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