Network Reference Models:
A computer network connects two or more devices together to share information and services. Multiple networks connected together form an internetwork. Internetworking present challenges – interoperating between products from different manufacturers requires consistent standards. Network reference models were developed to address these challenges. A network reference model serves as a blueprint, detailing how communication between network devices should occur. The two most recognized network reference models are:
• The Open Systems Interconnection (OSI) model
• The Department of Defense (DoD) model
Without the framework that network models provide, all network hardware and software would have been proprietary. Organizations would have been locked into a single vendor’s equipment, and global networks like the Internet would have been impractical, if not impossible. Network Reference Models are organized into layers, with each layer representing a specific networking function. These functions are controlled by protocols, which are rules that govern end-to-end communication between devices. Protocols on one layer will interact with protocols on the layer above and below it, forming a protocol suite or stack. The TCP/IP suite is the most prevalent protocol suite, and is the foundation of the Internet. A network model is not a physical entity – there is no OSI device. Manufacturers do not always strictly adhere to a reference model’s blueprint, and thus not every protocol fits perfectly within a single layer. Some protocols can function across multiple layers.
OSI Reference Model
The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network.
The OSI model consists of seven layers, each corresponding to a specific network function:
ISO further developed an entire protocol suite based on the OSI model; however, the OSI protocol suite was never widely implemented. The OSI model itself is now somewhat deprecated – modern protocol suites, such as the TCP/IP suite, are difficult to fit cleanly within the OSI model’s seven layers. This is especially true of the upper three layers. The bottom (or lower) four layers are more clearly defined, and terminology from those layers is still prevalently used. Many protocols and devices are described by which lower layer they operate at.
OSI Model – The Upper Layers
The top three layers of the OSI model are often referred to as the upper layers:
• Layer-7 – Application layer
• Layer-6 – Presentation layer
• Layer-5 – Session layer
Protocols that operate at these layers manage application-level functions, and are generally implemented in software. The function of the upper layers of the OSI model can be difficult to visualize. Upper layer protocols do not always fit perfectly within a layer, and often function across multiple layers.