Data Link Control (DLC)

DLC

Generally, the services that the data-link layer of the Open Systems Interconnection (OSI) reference model provides to adjacent layers of the OSI protocol stack. Specifically, a Data Link Control (DLC) is a specialized network protocol that is used primarily for two purposes:

* To provide connectivity with IBM mainframe or AS/400 environments, such as Systems Network Architecture (SNA), which are configured to run DLC. DLC complements SNA because SNA operates only at higher levels of the OSI model.
* To provide connectivity for network print devices (such as certain Hewlett Packard printers that have their own network cards and are connected directly to the network).

DLC is not used as a network protocol in the usual sense of enabling communication among computers on the network. It is not used by the redirector in the Microsoft Windows NT and Windows 2000 operating systems and so cannot be used for session-level communication over a network. DLC is not routable; it is designed only to give devices direct access to the data-link layer. DLC is supported by most Windows operating systems, including Windows 95, Windows 98, Windows NT, and Windows 2000. Windows 95 OSR2 includes both a 16-bit and a 32-bit version of DLC.
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To use DLC on Windows NT or Windows 2000 to connect to a Hewlett-Packard network print device, perform the following steps:

1. Connect the printer to the network, and run the self-test routine to obtain the MAC address of the printer. Also think of a friendly name for the printer.
2. Install the DLC protocol on the Windows NT or Windows 2000 server that will be used as a print server for the network print device. (Use the Network utility or the Windows 2000 Network and Dial-up Connections utility in Control Panel.)
3. Run the Add Printer Wizard on the print server, choosing My Computer, Add Port, Hewlett Packard Network Port, and New Port. Enter the friendly name for the printer and select its MAC address from the list (or type it if the print device is offline). In Windows 2000, run the Add Printer Wizard, then right-click on the printer in the Printers folder and choose Properties. In the Property sheet for the printer, click the Ports tab, click Add Port, select Hewlett Packard Network Port, and then click New Port. Enter the friendly name for the printer and select its MAC address from the list (or type it if the print device is offline).

Generic Data Link Control Environment

Generic data link control (GDLC) is a generic interface definition that allows application and kernel users a common set of commands to control data link control (DLC) device managers within the operating system.

For problem determination, see GDLC Problem Determination in AIX 5L™ Version 5.3 Communications Programming Concepts.

Generic data link control (GDLC) is a generic interface definition that provides application and kernel users a common set of commands to control DLC device managers within the operating system.

The GDLC interface specifies requirements for entry point definitions, functions provided, and data structures for all DLC device managers. DLCs that conform to the GDLC interface include:

• 8023 (IEEE 802.3 for Ethernet)
• ETHER (Standard Ethernet)
• SDLC (Synchronous Data Link Control)
• TOKEN (Token-Ring)
• FDDI (Fiber Distributed Data Interface)

DLC device managers perform higher-layer protocols and functions beyond the scope of a kernel device driver. However, the managers reside within the kernel for maximum performance and use a kernel device driver for their I/O requests to the adapter. A DLC user is located above or within the kernel.

Synchronous data link control (SDLC) and IEEE 802.2 Data Link Control are examples of DLC device managers. Each DLC device manager operates with a specific device driver or set of device drivers. SDLC, for example, operates with the multiprotocol device driver for the system's product and its associated adapter.

The basic structure of a DLC environment is shown in the "DLC Device Manager Environment" figure. Users within the kernel have access to the communications memory buffers (mbufs) and call the add entry points through the fp kernel services. Users above the kernel access the standard interface-to-kernel device drivers, and the file system calls the dd entry points. Data transfers require a move of data between user and kernel space.

Figure 1. DLC device manager environment

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