This configuration is pretty normal and 5250 telnet traffic is pretty small and no traffic traverses the wire unless there are keystrokes or the remote host is updating the screen. However, some questions you also need answers to:
1. What other applications are using this connection? Will these applications create slow telnet session responses? For example, when you type a character in a telnet session, it gets sent to the server and echoed back to the workstation. How much lag response is appropriate? Are the users in China and the host server in the US? This could be an issue.
2. What about printing? Is the remote host sending back print jobs to the users local printers? This is likely going to be a much larger contribution to network usage than the telnet sessions themselves.
3. Are these 400+ users using the system all at the same time or are they doing some type of shiftwork where only a percentage will be online and active at any one time? This will also contribute to how large your network links need to be.
My recommendation is to have at least bonded T1′s (2 x 1.5Mbps) links at each end. As I noted in question #1, you need to understand what other applications are also using this link, and question #2 – how is printing done.
You can get some metrics for user sessions by using a packet capture tool like <a href=”http://www.wireshark.org/”>Wireshark</a> and capturing traffic during specific transactions. Calculate the number of transactions per user / per hour and this will give you an estimate of how much bandwidth you need during high usage windows.