Overview of Windows 98 Network Architecture

Windows 98 provides multiple, simultaneous connections to a variety of networks (Windows NT, Novell NetWare, and others) and a variety of resources (files, programs, printers, host systems, and mail systems) over most popular media (Ethernet, Token Ring, X.25, ATM, and ISDN) from almost any location.

Windows 98 networking capabilities are implemented using a high-performance, reliable, and open architecture based on the Windows Open Services Architecture (WOSA) specification. This approach provides users with a consistent interface to different services on the front end, while giving system administrators the flexibility to mix and match multiple services on the back end.

Open Systems Interconnection Model

The modular networking architecture of Windows 98 is based on two industry standard models for a layered networking architecture, namely the International Standards Organization (ISO) model for computer networking, called the Open Systems Interconnection (OSI) Reference Model, and the Institute of Electrical and Electronics Engineers (IEEE) 802 model. Windows NT and Windows for Workgroups are also designed according to these standard models. The ISO OSI and IEEE 802 models define a modular approach to networking, with each layer responsible for some discrete aspect of the networking process. They are only models; they do not correspond exactly to any existing network. However, they can help you understand how networks function in general.

The OSI model describes the flow of data in a network, from the lowest layer (the physical connections) up to the layer containing the user’s applications. Data going to and from the network is passed from layer to layer. Each layer is able to communicate with the layer immediately above it and the layer immediately below it. In this way, each layer is written as an efficient, streamlined software component. When a layer receives a packet of information, it checks the destination address, and if its own address is not there, it passes the packet to the next layer.

When two computers communicate on a network, the software at each layer on one computer assumes it is communicating with the same layer on the other computer. For example, the transport layer of one computer assumes that it is communicating directly with the transport layer on the other computer. However, the actual connection occurs only at the physical layer, as Figure 29.1 shows. The transport layer on the first computer has no regard for how the communication actually passes through the lower layers of the first computer, across the physical media, and then up through the lower layers of the second computer.

Figure 29.1 shows the OSI model.

Figure 29.1 Layers in the OSI Model

The OSI model includes seven layers:

• The application layer represents the level at which applications access network services. This layer represents the services that directly support applications, such as software for file transfers, database access, and electronic mail.
• The presentation layer translates data from the application layer into an intermediary format. This layer also manages security issues by providing such services as data encryption, and compresses data so that fewer bits need to be transferred on the network.
• The session layer allows two applications on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, as well as when and how long it transmits.
• The transport layer handles error recognition and recovery. When necessary, it also repackages long messages into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving transport layer also sends receipt acknowledgments.
• The network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.
• The data link layer packages raw bits from the physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.
• The physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.

The IEEE 802 model defines low-level protocol standards at the physical and data link layers of the OSI model, dividing the data link layer into two sublayers: logical link control (LLC) and media access control (MAC). Figure 29.2 compares the IEEE 802 model and the lower layers of the OSI model.

Figure 29.2 IEEE 802 model compared to the OSI model

The LLC sublayer performs the following tasks:

• Link establishment and termination
• Frame control
• Frame sequencing
• Frame acknowledgment

The MAC sublayer performs the following tasks:

• Media access management
• Frame delimiting
• Frame error checking
• Frame address recognition

Windows 98 Networking Model

Figure 29.3 shows the layered components that make up the Windows 98 networking model.

Figure 29.3 Windows 98 network architecture

These layers correspond roughly to the layers in the OSI model, as the following list describes:

• The network providers, Installable File System (IFS) Manager, and redirectors provide functionality described in the application, presentation, and session layers of the OSI model.
• The transport protocols provide functionality described in the transport and network layers of the OSI model.
• Network driver interface specification (NDIS) and the network adapter drivers provide functionality described in the data link layer of the OSI model.

The following sections describe these elements of the Windows 98 network architecture, beginning with redirectors. Network providers are described in "Multiple Network Support" later in this chapter.