our LAN/WAN spans across chicago, Sydney, London, and Singapore. the exchange server 2k7 is in sydney, and serves all users globally. the domain controller (w2k3 server) is centralized in chicago. all the various LANs access the internet via proxy server (MS ISA 2004 server) located in sydney. the problem is that (ONLY) users in singapore does experience network latency more frequently, specifically when they use MS Outlook (2003) or access MS SQL database (MS 2K5 SQL server) over the network. i will like to know in such setting, what are the common reasons why this latency issue should occur?

This sure looks like a homework question except I don't know of anyone who would design a wan like this. You would need to know what is on the link to sidney from singapore concerning outlook. You don't say where the sql server is.