Open Systems Interconnections (OSI)

The OSI Reference model for networking is separated into seven layers. This hierarchy was placed into this “reference model” by two organizations. These organizations are known as the International organization for Standardization (ISO) and the American National Standards Institute(ANSI). The ISO is well known for their contribution to the network protocol standards. The ANSI works closely with businesses, government agencies, and internal groups to achieve agreements on standards for all sorts of equipment, from bicycle helmets to network equipment.

By using the OSI reference model, it can aid you in choosing the best equipment for the job at hand. Create the most effective network designs and design networks that will communicate with other networks. Also, with this as a reference, you can more easily troubleshoot network problems more effectively. Without such models, we would be like a blind man in an orgy, “Have to feel our way around.”
The OSI was developing in the late 1970’s and applies to both LAN and WAN communications and takes the effort to standardize the network software and hardware implementation. Also, it has aided in the growth in network communications by allowing communications between different types of LANS, WANS, and MANS. By providing standardization of network equipment, it allows many different brands; or vendors, to communicate seamlessly with one another. Another beneficial factor of this is by helping people retain their investments by allowing older equipment to communicate with the newer devices, reducing the need to replace everything. Another wonderful feature is the possibility of worldwide network communications, with the internet as a prime example.

The OSI model predates anything we would see today, but it has set the stage for cooperative networking and is constantly evolving to accomadate the latest in network developments. It should be emphasized that the OSI model is strictly a theoretical model and not a specific hardware device or a set of software routines. Rather, it is a set of guidelines for vendors to consider and follow when they design communication hardware or software. The guidelines are to protocol and network device development as grammer is to the spoken language. The OSI Model specifies how network devices contact each other, and how devices using different protocols to communicate. Also designs how the physical network devices are arranged and connected, as well as methods to ensure that network devices maintain a consistent rate of data flow.

The OSI model consists of seven distinct layers stacked on one another: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has its own tasks and uses specific protocols to communicate with the next layer in the stack. Communications between two devices go up and down the layered stack at each device. For example, when a workstation communicates with a server, communication starts at the workstations application layer. It then forms specific information from the lower layers until data is reached at the physical layer and transmitted to the server. The server then receives the data at the physical layer of its stack and sends it up each layer for interpretation until it reaches the application layer. Each layer is called by its actual name or by its placement in the stack.

Without standards we would soon notice when products turn out to be of poor quality, do not fit, or are incompatible with equipment that we already have, are unreliable or dangerous. Today we take standards for granted because anything we see or do is most likely up to our expectations. Take foodservice as an example, if standards didn’t exist, we could be served some rotten old food, or perhaps the whole kitchen wasn’t up to code, this would have a drastic effect on our daily health. We take standards for granted.

The bottom layers perform functions, such as constructing frames and transmitting packet-containing signals, related to physical communications. The middle layers coordinate network communications between nodes ensuring that a communication session continues without interruptions or errors. The top layers perform work that directly affect the software applications and data presentation, including data formatting, encryption and data and file transfer management. When you put them all together, the set of layers is called a stack.
Physical Layer is the lowest on the totem pole. Also known as layer one, is responsible for all data transfer mediums, such as wire, cable, fiber optics, radio waves and microwaves. Then comes the network connectors and the network topology. It also handles the signaling and encoding methods and data transmission rates. Finally the physical layer encompasses the network interfaces and the detection of signaling errors.

The Data Link layer in a LAN is to organize bits so that they are formatted into frames. It is also known as Layer Two. Each frame is formatted into a specified way so that the data transmissions are synchronized for reliable transmission from node to node. This layer formats the data so that it is encoded as a frame into an electrical signal, and once it is formatted, the frame transferred to the physical layer so that it can be placed onto the communications medium (such as cable) by the transmitting node. The receiving node then picks up the frame via the physical layer, decodes the electrical signal into bits, organizes the bits into frame and checks the frame for errors.

The third layer up is the Network Layer. This layer controls the passage of packets along the routes on the network. In a network transmission, each packet is composed up of data bits and fields of information containing transmission control instruction, with source and destination information, data, and error checking information. That brings us to the Transport Layer. This fourth layer performs functions that ensure that data us sent reliably from the sending node to the destination node. When a transmission is made, the receiving node sends out a acknowledgment sometimes referred to as an “ack”. Another function of this layer is to break or fragment messages into smaller units when networks use different protocols requiring different packet sizes.

Layer five, known as the Session Layer is responsible for establishing and maintaining the communications link between two nodes. It also provides for orderly communication between nodes; for example, it establishes which node transmits first. It also determines how long a node can transmit and how to recover from transmission errors. This layer also links each unique address to a given node, the same way zip codes allow mail to be associated with a particular postal region.

Moving on, Layer six the Presentation Layer. This layer manages data formatting, because software applications often use different data-formatting schemes. The Presentation layer ensures that numbers and text are sent in a form that can be read by the Presentation layer at the receiving end. This layer is also responsible for the data encryption. For example, the Secure Socket Layer (SSL) that encrypts your credit card information on a WAN. One other function of the presentation layer is data compression. When data is formatted, there may be empty space that gets formatted between text and numbers. Data compression removes this space and compacts data so it is much smaller to send. After it is transmitted, it is decompressed by the Presentation layer on the receiving node.

The seventh layer known as the Application Layer is the highest level of the OSI model. This layer governs the computer user’s most direct access to applications and network services. These services include file transfer, file management, remote access to files and printers, message handling for electronic mail, and terminal emulation. This is the layer programmers use to connect workstations to network services, such as linking an application into e-mail or perhaps a database over a network.