INTRODUCTION
The ISO (International Standards Organization) has created a layered model called the OSI (Open Systems Interconnect) model to describe defined layers in a network operating system. The purpose of the layers is to provide clearly defined functions to improve internetwork connectivity between "computer" manufacturing companies. Each layer has a standard defined input and a standard defined output.
Understanding the function of each layer is instrumental in understanding data communication within networks whether Local, Metropolitan or Wide.
ISO OSI The International Standards Organization (ISO) Open Systems Interconnect (OSI) is a standard set of rules describing the transfer of data between each layer. Each layer has a specific function. For example the Physical layer deals with the electrical and cable specifications. The OSI Model clearly defines the interfaces between each layer. This allows different network operating systems and protocols to work together by having each manufacturere adhere to the standard interfaces. The application of the ISO OSI model has allowed the modern multiprotocol networks that exist today. There are 7 Layers of the OSI model:
7. Application Layer (Top Layer) 6. Presentation Layer 5. Session Layer 4. Transport Layer 3. Network Layer 2. Data Link Layer 1. Physical Layer (Bottom Layer) The OSI model provides the basic rules that allow multiprotocol networks to operate. Understanding the OSI model is instrument in understanding how the many different protocols fit into the networking jigsaw puzzle. The OSI model is discussed in detail in
OSI Model Explained
This is a top-down explanation of the OSI Model, starting with the user's PC and what happens to the user's file as it passes though the different OSI Model layers. The top-down approach was selected specifically (as opposed to starting at the Physical Layer and working up to the Application Layer) for ease of understanding of how the user's files are transformed through the layers into a bit stream for transmission on the network.
There are 7 Layers of the OSI model:
· Application Layer (Top Layer) · Presentation Layer · Session Layer · Transport Layer · Network Layer · Data Link Layer · Physical Layer (Bottom Layer)
The OSI Seven-Layer Model In the 1980s, the European-dominated International Standards Organization (ISO), began to develop its Open Systems Interconnection (OSI) networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or -- seven-layer model --), and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.
Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in OSI 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it. Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host.
The seven layers of the OSI Basic Reference Model are (from bottom to top):
The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. This layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method. The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. Ex: this layer defines the framing, addressing and checksumming of Ethernet packets. The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Ex: this layer defines the addressing and routing structure of the Internet. The Transport Layer describes the quality and nature of the data delivery. Ex: this layer defines if and how retransmissions will be used to ensure data delivery. The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call. The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating point numbers can be exchanged between hosts with different math formats. The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations. The original Internet protocol specifications defined a four-level model, and protocols designed around it (like TCP) have difficulty fitting neatly into the seven-layer model. Most newer designs use the seven-layer model.
OSI's biggest problem is that it doesn't really offer anything new. The strongest case for its implementation comes from its status as an "international standard", but we already have a de facto international standard - the Internet. OSI protocols will be around, but its most significant contribution is the philosophy of networking represented by its layered model.
Protocol Layering Protocol layering is a common technique to simplify networking designs by dividing them into functional layers, and assigning protocols to perform each layer's task.
For example, it is common to separate the functions of data delivery and connection management into separate layers, and therefore separate protocols. Thus, one protocol is designed to perform data delivery, and another protocol, layered above the first, performs connection management. The data delivery protocol is fairly simple and knows nothing of connection management. The connection management protocol is also fairly simple, since it doesn't need to concern itself with data delivery.
Protocol layering produces simple protocols, each with a few well-defined tasks. These protocols can then be assembled into a useful whole. Individual protocols can also be removed or replaced.
The most important layered protocol designs are the Internet's original DoD model, and the OSI Seven Layer Model. The modern Internet represents a fusion of both models.
DoD Four-Layer ModelThe Department of Defense Four-Layer Model was developed in the 1970s for the DARPA Internetwork Project that eventually grew into the Internet. The core Internet protocols adhere to this model, although the OSI Seven Layer Model is justly preferred for new designs.
The four layers in the DoD model, from bottom to top, are:
The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login. DoD Four-Layer ModelThe Department of Defense Four-Layer Model was developed in the 1970s for the DARPA Internetwork Project that eventually grew into the Internet. The core Internet protocols adhere to this model, although the OSI Seven Layer Model is justly preferred for new designs.
The four layers in the DoD model, from bottom to top, are:
The Network Access Layer is responsible for delivering data over the particular hardware media in use. Different protocols are selected from this layer, depending on the type of physical network. The Internet Layer is responsible for delivering data across a series of different physical networks that interconnect a source and destination machine. Routing protocols are most closely associated with this layer, as is the IP Protocol, the Internet's fundamental protocol. The Host-to-Host Layer handles connection rendezvous, flow control, retransmission of lost data, and other generic data flow management. The mutually exclusive TCP and UDP protocols are this layer's most important members. The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.
