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...

THERE ARE ONLY TWO TYPES OF COMPUTER NETWORKS

Circuit-switched and packet-switched. What type of network do you think is the Internet?

CIRCUIT-SWITCHED NETWORKS

Circuit-switched networks use point-to-point links between the nodes of the network. A node is an endpoint. For example, refer to the map and imagine a regional network of five cities: Chicago, Pittsburgh, Pittsburgh, Boston, and Miami. Each city is a node since it is an endpoint.

A point-to-point link means that each city

 (a point) connects, or is linked, directly to another city (another point). How many point-to-point links are possible in a network of five nodes? The answer can always be expressed mathematically.

It is n (n-1) where n is the number of nodes. Don’t let that intimidate you.

FULL-MESH TOPOLOGY

A circuit-switched network consists of point-to-point link. These links create a full-mesh network. That formula calculates the number of links in a full-mesh network.

In this instance, five nodes create a full mesh network that contains 20 links. The following illustration shows a classroom of students.

Since they can’t talk loudly, two students created a tin-can telephone.

Each student holding a tin can is a node. The circuit is represented by the string that connects the tin cans.

If another pair of students wants to communicate, they borrow the tin-can telephone. The circuit has now switched to another pair of nodes.

PACKET-SWITCHED NETWORKS

Packet-switched networks use a single common road that all nodes connect to. Some nodes may be on side roads but ultimately all side roads connect to the main road. Members communicate by sending messages through that road. A message consists of packets. An individual packet is a unit of information that has, among other things, the network address of the sender and recipient.

Today there is no longer a single main road. Instead there are numerous main roads and their number grows each year. This explanation will keep it simple and assume that the network has only one main road. Each node is connected to the main road—some directly but most indirectly.

Note the actual road network in Belgium. Only the main roads are visible but, of course, there are numerous side roads that feed into the main ones.

The following illustration shows the same classroom.

Their teacher decided that, to be fair, each student should be allowed to communicate with any other.

In lieu of the tin-can telephone, the students are now passing notes to each other. Each note contains the addressee’s and sender’s name. This is a packet-switching network.

Each note is a packet and each packet is being switched from one node to another until the packet reaches the addressee.

Notice how much more communication is possible with a packet-switched network. Notice also that the notes will still reach the addressee even if some students are absent.

To be continued…