Windows 2000 and Later Network Architecture and the OSI Model
Microsoft® Windows® 2000 and later operating systems use a network architecture based 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. Each layer offers specific services to higher layers while shielding these layers from the details of how the services are implemented. A well-defined interface between each pair of adjacent layers defines the services offered by the lower layer to the higher one and how those services are accessed."
OSI Reference Model
In Windows 2000 and later versions of the operating system, network drivers implement the bottom four layers of the OSI Reference Model:
- Physical Layer
- This is the lowest layer of the OSI model. This layer is involves 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. The physical layer carries the signals for all the higher layers. Under Windows 2000 and later versions, the physical layer is implemented by the network interface card (NIC), its transceiver, and the medium to which the NIC is attached. For network components that use serial ports, the physical layer can also include low-level network software that defines how the serial stream of bits is divided into packets of data.
- Data Link Layer
- This OSI layer is further divided by Institute of Electrical and Electronics Engineers (IEEE) into two sublayers: LLC and MAC. The LLC sublayer provides error-free transfer of data frames from one node to another. The LLC sublayer establishes and terminates logical links, controls frame flow, sequences frames, acknowledges frames, and retransmits unacknowledged frames. The LLC sublayer uses frame acknowledgement and retransmission to provide virtually error-free transmission over the link to the layers above. The MAC sublayer manages access to the network media, checks frame errors, and manages address recognition of received frames. In the network architecture used by Windows 2000 and later versions of the operating system, the Logical Link Control sublayer is implemented in the transport driver, while the Media Access Control sublayer is implemented in the network interface card (NIC). The NIC is controlled by a software device driver called the NIC driver. Windows 2000 and later versions of the operating system ship with NIC drivers for a number of popular NICs.
- Network Layer
- This layer controls the operation of the subnet. It determines the physical path the data should take, based on the following:
- network conditions
- priority of service
- other factors, including routing, traffic control, frame fragmentation and reassembly, logical-to-physical address mapping, and usage accounting
- Transport Layer
- This layer ensures that messages are delivered error-free, in sequence, and with no loss 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, because the NetBEUI transport driver for Windows 2000 and later versions 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 network architecture used in Windows 2000 and later versions of the operating system, the Logical Link Control, Network, and Transport layers are implemented by software drivers known as transport drivers, which are sometimes referred to as protocols, protocol drivers, or protocol modules. Windows 2000 and later versions ship with TCP/IP, IPX/SPX, NetBEUI, and AppleTalk transport drivers.
