In this post i will highlight the two different Terminal Services Modes available in Windows Server 2k/2k3.
Remote Administration ModeTerminal Services Remote Administration mode allows any server running Windows 2000Server, for instance, a domain controller, or BackOffice Server, to be administered remotelywith full access to all the built-in graphical user interface-based (GUI-based) administrative tools, as if the administrator was sitting right at the server. Within Windows Server 2003,remote administration mode is built-in and does not need to be installed. This ability to administer the server can be made available from any client device, including a legacy version of Windows, or even non-Windows-based clients. This server management feature is an invaluable tool for quick and easy administration of large- and small-scale networks. TerminalServices has two built-in per-server connections specifically for remote administration. ATerminal Services Client Access License (CAL) is not required to connect to Terminal Services in remote administration mode.
Application Server ModeIn Application Server mode, applications can be deployed and managed from a central location, saving administrators initial development and deployment time as well as the time and effort required for maintenance and upgrades. Once an application is deployed using Terminal Services, many clients can connect whether through a remote access connection,local area network (LAN), or wide area network (WAN). The clients can still be Windows-based,Windows CE-based, or even non-Windows-based.Licensing is required when deploying a Terminal Services-enabled server as an application server.
You can install Windows 2003 Server Terminal Services in either of two modes: Remote Administration or Application Server. Remote Administration mode installs only the remote access components of Terminal Services and performs with very little overhead, so it's ideal for mission-critical servers. Terminal Services in Remote Administration mode permits a maximum of two concurrent remote administration connections. No additional licensing is required for those two connections, and you don't need to run the Terminal Services license server.
Application Server mode installs the application-sharing components of Terminal Services in addition to the remote access components. This mode lets users run applications remotely. However, running Terminal Services in Application Server mode requires you to purchase licenses and set up a Terminal Services licensing server within 120 days of installation. For administration purposes, you should install Terminal Services in Remote Administration mode. Remote Administration mode minimizes the impact on server performance while still facilitating remote administration.
The following links will give you an insight on terminal services
1)How registry and profile information is handled differently between the 2 modes ?
As far as profiles and registry, in remote admin mode there are no changes to the core functionality of Windows. In Application Server Mode (what’s called Terminal Server in 2k3) application installations are monitored by the OS, which attempts to remap user-specific settings (files and registry keys) into global areas. For example, during install HKCU info gets written to HKLM locations and mapped into all other users profiles also. When you’re not in install mode, ini files and similar that an application tries to write to the default Windows directory get remapped into the user’s profile to isolate that user’s settings, etc. No all applications will be Terminal Server-friendly.http://support.microsoft.com/kb/297379http://support.microsoft.com/kb/186498/en-us
2)How the server is tuned for each distinct mode ?
Terminal Servers in remote admin mode are not tuned any differently than out of the box server. If you install other applications like SQL, Exchange, etc, you might get some tuning changes from those, but the default memory management and thread scheduling are the same. In Application Server mode (what’s called Terminal Server in 2k3) you get your thread scheduling tuned like a workstation with short variable quanta and foreground boots (i.e. tuned for foreground applications) and your memory management is set to tuned to favor processes. Either of these options can be tuned back via the UI or the registry after you install TS.http://support.microsoft.com/kb/259025/en-us
Please check the following link for a Terminal Server Scaling .
3)If there are any license issues that would arise i.e differences in the licensing
There is no additional licensing required for Remote Administration / Remote Desktop mode. Application Server mode (Terminal Server) requires that you purchase licenses and deploy a Terminal Server licensing server. Depending on how you license the server you are required to purchase a license per user or a license per device. These are not concurrent licenses, if you have 100 users logging onto the server, even if only 10 will be on at the same time, you must purchase 100 user TS CALs if you are in user mode. Similar requirement for devices if you license in Per Device mode instead. http://www.microsoft.com/windowsserver2003/techinfo/overview/termservlic.mspx
4) Does the concept of Session 0 exist in Windows 2000 ?
No the concept does not exist. In windows 2003 this is the session which you take using the mstsc/console session.