• Terminal Services Virtual Channels API
  • Configuring Printer Redirection
  • Configuring Client Settings
  • Disabling Virtual Channels
  • Printer Queues
  • Automatic Printer Queues
  • Automatic Printer Naming Convention
  • Operating System Windows 2000 Terminal Services Printer Redirection

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    RDP Thin Client Fundamentals

    Thin clients can connect to a Terminal Server with less of the hardware and software overhead associated with a full-featured PC. Thin clients can be purchased without hard drives, CD ROMs, or removable media, and can boot from Read Only Memory (ROM). Many of these clients run the Windows CE operating system. Some may have COM and parallel ports, while others may not. The lack of moving parts and subsequent price of thin clients can make them very attractive alternatives to full-featured PCs on the client side.

    Thin clients use a subset of the drivers found in NTPRINT_PARSED.TXT on the thin client. When these clients have a printer installed, it will be installed from this list. They will redirect their queues in exactly the same way as a full-featured client. As a result, with thin clients, you are always guaranteed to have a driver that will redirect and work on your Terminal Server for redirected printing.

    Terminal Services Virtual Channels API

    Virtual Channels transmit channel-specific commands and data between the Terminal Server and its clients. Because Virtual Channels run under the RDP stream, they use the same encryption level as the RDP connection itself. Virtual Channels in RDP allow for lossless communication between client and server components over the main RDP data connection. Virtual Channel data is application-specific. Virtual Channels are negotiated between client and server at connection time, and can be extended by third parties to include a variety of additional features.

    Software developers can use the Terminal Server Virtual Channels Interface API to customize existing applications and even develop completely new applications contained within the RDP protocol. More information on extending Virtual Channels and how they operate is available in the Remote Desktop Protocol section of MSDN at the following location, or by searching for “Virtual Channel” on MSDN:


    Configuring Printer Redirection

    There are two ways to configure printer redirection. By configuring the client settings on the RDP-TCP protocol (Windows printer mapping is unchecked or enabled by default) and by allowing or disabling Virtual Channels in the Advanced permissions for RDP.

    Configuring Client Settings

    To access the Client Settings tab, follow these steps:

    1. Open the Terminal Services Configuration snap-in under Administrative Tools.

    2. Right-click on the RDP-Tcp connection under Connections in the right-hand pane.

    3. Choose Properties and then choose the Client Settings tab. Figure 1 shows the Windows printer mapping checkbox used for enabling or disabling printer redirection.

    Figure 1 Windows printer mapping checkbox

    Disabling Virtual Channels

    CAUTION: The Virtual Channels setting affects all redirection from the client to the server. Disabling the Virtual Channels setting will also prevent clipboard redirection and any other Virtual Channel features from working, in addition to disabling printer redirection.
    To access Virtual Channels permissions, follow these steps:

    1. Open the Terminal Services Configuration snap-in under Administrative Tools.

    2. Right-click on the RDP-Tcp connection under Connections in the right-hand pane.

    3. Choose the Permissions tab.

    4. Select Advanced.

    5. Select the group or account you want to control and then choose View/Edit.

    6. Allow or Deny Virtual Channels as shown in Figure 2:

    Figure 2 RDP-Tcp Permissions dialog

    Printer Queues

    A printer queue is a logical representation, created by the Spooler, of a printer that is physically attached to the computer or a printer server. This is slightly different from a physical printer attached to the computer. A queue can be created regardless of whether a printer is attached to the physical ports of the client computer. Only those with Administrator or Power User rights can create queues on the Terminal Server.
    The Add Printer Wizard is accessed by clicking Start, Settings, Printers, and then clicking on the Add Printer icon, shown in Figure 3:

    Figure 3 Add Printer icon in Printers

    In a printing environment, a printer defines a physical object, while a printer queue defines the assocation between a driver, a port, and a shell-based object that exposes the ability to print via the user interface. However, in this document, we will use printers and printer queues interchangeably.

    Automatic Printer Queues

    As part of the RDP client’s connection sequence, the TS client enumerates local print queues to detect locally attached printers. All the printers enumerated with this technique are always called automatic printer queues.

    When the client enumerates and announces printers to the server, it also retrieves both the driver string name from appropriately connected devices and the client-side print queue name for each printer that it finds, and sends this information to the server. The server’s TS Printer Redirection User-mode component uses this information to automatically create a corresponding server-side queue for each client-side queue (if a corresponding driver is present; see Common Problems for more information).

    Automatic Printer Naming Convention

    Automatic TS redirected printers use the following naming convention:


    Configuration settings (for example, paper orientation) for automatic printer queues are preserved in the registry (see Automatic Queue Configuration Changes). These settings are restored by the server for this printer queue on installation.

    When a client disconnects or ends the session, the printer queue is deleted and any incomplete or pending print jobs are lost. Information about the client's local printers and settings is saved on the client computer. During subsequent logon sessions, the printer queue is created using the information stored on the client computer.

    If a printer driver is not found on the server, an event is logged and the client printer is not created. To make the printer available, the driver must be manually installed on the server that matches the string name noted in the event, which also matches the string name found on the incoming client that failed to redirect.

    Note: Printer administrators can see all redirected printers, both those on the server and those redirected from their session. Normal users can see normal printers on the server, but can see only their own redirected printer queues.

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