Mapping Practice to Theory: NT Networking and the OSI Model

The Open Systems Interconnect (OSI) model is a theoretical framework the International Organization for Standardization (ISO) developed to organize networking functions into a series of layers. This model is important because it fosters modular protocol development and open systems (i.e., protocols from various sources that can communicate with each other for effective networking). Not every network uses all the model’s layers; ISO’s intent in creating the OSI model wasn't to describe every network but to give protocol designers a map to follow to aid in interoperable protocol design.

The OSI model doesn’t reflect a perfect division of networking functions; you can’t map existing protocols into the model and have those protocols fit neatly into each division. However, this model is useful for conceptualizing networking components—how the pieces of your system, from the cable through Network Neighborhood, fit together to help the computers on your network communicate.

So how does the OSI model relate to Windows NT network communications? In this article, I’ll review each level of the OSI model. Then, I’ll examine protocols you’re likely to encounter in NT networking and discuss where those protocols fit in the OSI model and how they cooperate to create a working NT network.

A Tour of the OSI Model
Starting from the bottom layer, the OSI model describes how the pieces of a network work together from the physical connections up to the applications. Figure 1 shows the OSI model. Information from each layer passes up to the next layer, so that a protocol operating at a given layer can access all the information the protocols operating below it collect or prepare. For example, you don’t have to tell a file-sharing protocol that the network runs on Category 5 cable.

Physical: Network Cable
The lowest common denominator in a network is the kind of cable it uses, such as copper, fiber, or wireless communications. The cable addresses the problem of how to get raw bits from point A to point B. The type of cable you use determines the top speed physically available on the network. However, the top speed can be logically limited, which is why Category 5 unshielded twisted pair works for both 10Mbps Ethernet and 100Mbps Ethernet. The type of cable also determines how far the network segment can extend before the signal wears out.

Data Link: Basic Network Communications
Networks use the data link layer to establish virtual channels between a sender and a recipient. At the data link level, the protocol packages the data into frames that contain source and destination addresses. These frames refer to the physical hardware address (i.e., the address burned into the network card, rather than the address the user assigns to the software) of each network card attached to the network cable. Ethernet, Token Ring, and ARCnet are examples of data link protocols.

If your network is a simple LAN, it’s probably running Ethernet as its data link protocol. Ethernet is a complex protocol that requires an in-depth description; however, to save space, I’ll outline only Ethernet’s basic characteristics:

• Ethernet broadcasts data to the entire network, but only the node to which the data are addressed hears the broadcast transmissions.
• Only one node can transmit data at a time. Before beginning a transmission, a node listens to the network to ensure that it’s clear. If the network is clear, the node begins transmission. If the network is busy, the node waits a few microseconds, then listens again.
• If two nodes try to transmit data at the same time, a collision occurs and each node stops, waits a predetermined period, then retries the transmission. The system randomly generates the length of the wait period so the chance of a second collision is reduced.

Ethernet isn’t the only data link protocol available for NT networks. If you’re running applications from an NT 4.0 Terminal Server Edition machine, your network uses either Terminal Server’s Remote Desktop Protocol (RDP) or Citrix MetaFrame’s Independent Computing Architecture (ICA) protocol. These protocols download instructions on how GUI images are created from the application server to the client and how users upload input from the client to the application server. The system encapsulates this information data stream within the data link display protocol your network uses; however, the network requires RDP and ICA for flow control and error checking.

If communication extends beyond the LAN onto the Internet, the network might use other data link protocols, such as Point-to-Point Protocol (PPP) or Serial Line Internet Protocol (SLIP). NT supports both protocols, but PPP is the preferred protocol for online communication because it’s faster than SLIP, natively supports error correction, and is self-configuring.

PPP is a connection-oriented protocol that encapsulates multiple protocol packets so that all the packets can travel across the same link at the same time. PPP contains three parts: one part encapsulates the protocol into generic PPP packets with a header that identifies the network protocol type; a second part creates the connection; and a third part handles any special conditions unique to that protocol, such as resolving IP addresses to hardware addresses.

To create a PPP connection, both ends of the connection send each other link control packets to establish the link. After they establish a link, both ends handle authentication (e.g., a password) if required for that connection. When both ends have authenticated the connection, the appropriate network control protocol for the network-layer protocol type (e.g., IP or IPX) configures the PPP packets and opens the link. After the network control protocol opens the link, PPP carries packets to their destination until the network protocol closes the link.

Network: Address Mapping and Data Routing
ISO put network-layer protocols, such as IP and IPX, at the network level of the OSI model. In this layer, the network assigns software addresses and sends packets to those software addresses. The network also makes routing decisions at the network layer.

Mapping Physical Addresses to IP Addresses. Network-layer protocols use logical addresses, rather than the physical addresses the manufacturer burns into the network card. However, lower-level protocols such as Ethernet, Token Ring, ICA, and Fiber Distributed Data Interface (FDDI), use physical addresses to move information from point A to point B. How does the network reconcile these two addresses?

One option is for the systems administrator to maintain a static table of IP addresses and their related hardware addresses. However, updating and editing static lists is time-consuming: whenever you change an IP address, you have to update the list on each machine.