IT Consulting from the Trenches

AN EASY AND COMPREHENSIVE EXPLANATION OF THE OSI MODEL

"All People Seem To Need Data Processing."

"Please Do Not Throw Sausage Pizza Away."

Either way, these are the two mnemonics for the OSI model of the ISO’s standard networking protocols. In the 1990s when I was studying for my various certifications (Novell’s CNE, Microsoft’s MCSE, and Cisco’s CCNA), I learned those two mnemonics. I'd like to pass the knowledge along to the next generation. You might not find this information presented this way anywhere else.

The International Standards Organization (ISO) developed a model to serve as a standard point of reference for networking protocols. It uses seven layers to break down the networking process into seven independent processes. OSI stands for Open Systems Interconnection.

These are the seven OSI layers, listed from the outermost to the innermost layer.

APPLICATION
This is where network services and applications reside. These services and applications utilizes the formats established in the Presentation layer.

PRESENTATION
This is where file and file access formats reside; examples of file formats are PostScript, ASCII, Microsoft Word; examples of file access formats are Sun Microsystem's Network Filing System (NFS) or Microsoft's New Technology Filing System (NTFS).

SESSION
This layer serves two functions. First, it establishes and maintains the inter-nodal connections (i.e., connections between nodes). And second, it maintains the logical sequence of message exchanges.

TRANSPORT
This layer ensures the reliable delivery of messages. If one portion of the message transmission is lost or garbled, this layer re-transmits the necessary portion.

NETWORK
This layer addresses the messages for delivery. It functions like an envelope wrapper for the message. The envelope has the recipient's address and the sender's return address. These are logical addresses that are specific to the networking protocol that is used (e.g., IP). The envelope is known as a datagram.

DATA LINK
This layer refers to the specific kind of communication medium being employed. It places the datagram inside the network's delivery vehicle - generically known as a frame. Network delivery frames are specific to the particular kind of communication medium (e.g., Ethernet or Token Ring).

PHYSICAL
This describes the actual physical media being employed (e.g., network interface card). This layer often has its own form of addressing in addition to the protocol-specific logical addressing. For example, each Ethernet node (e.g., the network interface card or NIC) has a permanent unique 48-bit hardware address. The first 24 bits identify the device's manufacturer. The second 24 bits identify the specific device. Example of an Ethernet device's hardware address: 08-22-12-32-12-12.

To be continued...