hi aparna,
i dont get you properly but it seems that you are looking for setting up voip on LAN. This is pretty simple as compared to setting up voip on wan. Well you will have to first look at the typical bandwidth required for voip which wud depend on a lot of factors like :
no of users on voip
the codec used
traffic on the LAN generated by other applications, etc. Let me know what are u looking for exactly .
regds
vickie