1.1 Windows NT Network Architecture and the OSI Model

The network architecture of Microsoft® Windows NT™ and Windows 95™ is based conceptually on the seven-layer networking model developed by the International Standards Organization (ISO). Introduced in 1978, the ISO Open Systems Interconnection (OSI) reference model describes networking as a series of protocol layers with a specific set of functions allocated to each layer. The purpose of each layer is to offer specific services to the higher layers, shielding the upper layers from the details of how the services are actually implemented. Between each pair of adjacent layers there is a well-defined interface. The interface specifies which services the lower layer offers to the upper one and how those services will be accessed.

Network drivers under Windows NT and Windows 95 fit comfortably within this conceptual framework. Windows network drivers are associated with the protocols and interfaces used at the bottom four layers of this network architecture.

The Physical Layer is the lowest layer of the OSI model. This layer is concerned with the reception and transmission of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium. It is the physical layer that carries the signals for all of the higher layers. Under Windows NT, the physical layer is implemented by the network interface card (NIC), its transceiver, and the medium to which it is attached. For network components that use the serial ports, the physical layer may also include low-level network software that defines how the serial stream of bits is divided into packets of data.

OSI divides the Data Link Layer into two sublayers. The Logical Link Control (LLC) sublayer is responsible for providing the error-free transfer of data frames from one node to another. This sublayer includes the establishment and termination of logical links, frame flow control, frame sequencing, frame acknowledgement, and retransmission of unacknowledged frames. Using frame acknowledgement and retransmission, the Logical Link Control sublayer allows the layers above to assume virtually error-free transmission over the link. The Media Access Control (MAC) sublayer is responsible for managing access to the network media, frame error checking, and address recognition of received frames. Under the Windows NT network architecture, the functions allocated to the Logical Link Control sublayer are implemented in the transport driver, while those allocated to the Media Access Control sublayer are implemented in the network interface card.The network interface card or "NIC" is controlled by a software device driver called the "NIC driver" (sometimes referred to as a "MAC driver"). Windows NT ships with NIC drivers for a number of popular network interface cards.

The Network Layer is responsible for controlling the operation of the subnet. It decides which physical path the data should take based on network conditions, priority of service, and other factors. This includes routing, traffic control, frame fragmentation and re-assembly, logical to physical address mapping, and usage accounting.

The Transport Layer ensures that messages are delivered error-free, in sequence, and with no losses or duplication. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. A minimal Transport Layer is required in protocol stacks that include a reliable Network or Logical Link Control sublayer that provides virtual circuit capability. For example, since the Windows NT NetBEUI transport driver includes an OSI-compliant LLC sublayer, its transport layer functions are minimal. If the protocol stack does not include an LLC sublayer, and if the Network Layer is unreliable and/or supports datagrams (as with TCP/IP's IP layer or NWLink's IPX layer), the transport layer should include frame sequencing and acknowledgment, as well as retransmission of unacknowledged frames.

In the Windows NT model, the Logical Link Control, Network and Transport layers are implemented by software drivers known as "Transport Drivers" (sometime referred to as "protocols", "protocol drivers", or "protocol modules"). Windows NT comes standard with TCP/IP, IPX/SPX, NetBEUI, and AppleTalk Transport Drivers.