I suggest to address these issues that you apply an end to end approach. The parts are the workstation, communication link(s), server, database, and support systems (DNS for example).
Next, its important to remember that users do tasks. Any analysis must be based on the task concept. Also, the specific tasks in any application will likely differ by user type so its useful to look at the frequency of specific tasks by user group.
Each task can be looked at in terms of time. The total time is split among the parts. Before worrying about bandwidth, you should determine just where the time is being spent for each task. Don’t consider the user action in the task analysis - their keyboard time is best handled with scripts to eliminate that variable, and output is done when the screen is populated or the printout complete.
The communications aspect is impacted by volume of data moved, amount of communications overhead, background load, packet size, protocol, latency, and bandwidth.
While there are many “favorite” tools to speed the analysis, it can all be accomplished with a spreadsheet, a packet capture tool, and a knowledge of scripting.
There are some generals you can follow for this evolution.
First, while applications vary, most have yet to find the application that improves performance on any task when bandwidth goes above about 750Kb. Most see no improvement once bandwidth reaches 200Kb. Additional bandwidth then becomes an issue of user count. Next, most applications do not suffer performance drops until total average utilization goes above 80%.
The best time to define bandwidth requirements is during application development. The reason is that most applications can be tuned to perform with significantly less traffic while still in development, and the traffic is often a good indication of other problems like poor database structure or less than optimal distribution of work. The second best time is before purchasing an application. Often two similar applications will have significantly different WAN performance characteristics and this can be a key decision criteria.
So how much bandwidth is too much? If you can lease less than you currently have and lower your costs, you have too much.
I strongly suggest that you NOT enter directly into discussions with a bandwidth provider while deciding your bandwidth requirements. They’re more likely to be focused on “making a sale” than in helping you with your infrastructure decisions. Instead, seek the advice of an independent unbiased broker. They can walk you through the process to finding a solution which best makes business sense to you and your organization.
Some people opt to go with dial up instead.