Introduction of OSI Networking Model

Introduction of OSI Networking Model this blog is written on OSI Networking Model with it’s layer description as well as function and compared OSI with TCP/IP model.

OSI Networking Model

At one point within the history of the OSI model, many of us thought that OSI would win the battle of the networking models discussed earlier. If that had occurred, rather than running TCP/IP on every computer within the world, those computers would be running with OSI. However, OSI didn’t win that battle. In fact, OSI not exists as a networking model that would be used rather than TCP/IP, although some of the original protocols referenced by the OSI model still exist. So, why is OSI even during this book? Terminology. During those years during which many of us thought the OSI model would become commonplace within the world of networking (mostly within the late 1980s and early 1990s), many vendors and protocol documents started using terminology from the OSI model. That terminology remains today. So, while you’ll never need to work with a computer that uses OSI, to know modern networking terminology, you would like to know something about OSI.

Comparing OSI and TCP/IP

The OSI model has many similarities to the TCP/IP model from a basic conceptual perspective. it’s (seven) layers, and every layer defines a group of typical networking functions. like TCP/IP, the OSI layers each refer to multiple protocols and standards that implement the functions specified by each layer. In other cases, even as for TCP/IP, the OSI committees didn’t create new protocols or standards, but instead referenced other protocols that were already defined. for example, the IEEE defines Ethernet standards, therefore the OSI committees didn’t waste time specifying a new sort of Ethernet; it simply referred to the IEEE Ethernet standards. Today, the OSI model are often used as a standard of comparison to other networking models. Figure 1-15 compares the seven-layer OSI model with both the four-layer and five-layer TCP/IP models.

Next, this section examines two ways during which we still use OSI terminology today: to explain other protocols and to explain the encapsulation process. Along the way, the text briefly examines each layer of the OSI model.

Describing Protocols by Referencing the OSI Layers

Even today , networking documents often describe TCP/IP protocols and standards by referencing OSI layers, both by layer number and layer name. for instance , a common description of a LAN switch is “Layer 2 switch,” with “Layer 2” referring to OSI layer 2. Because OSI did have a well-defined set of functions associated with each of its seven layers, if you recognize those functions, you’ll understand what people mean once they refer to a product or function by its OSI layer. for an additional example, TCP/IP’s original Internet layer, as implemented mainly by IP, equates most directly to the OSI network layer. So, most of the people say that IP is a network layer protocol, or a Layer 3 protocol, using OSI terminology and numbers for the layer. Of course, if you numbered the TCP/IP model, starting at rock bottom , IP would be either Layer 2 or 3, depending on what version of the TCP/IP model you care to use. However, even though IP is a TCP/IP protocol, everyone uses the OSI model layer names and numbers when describing IP or any other protocol for that matter.
The claim that a specific TCP/IP layer is similar to a specific OSI layer is a general comparison, but not an in depth comparison. The comparison may be a little like comparing a car to a truck: Both can get you from point A to point B, but they need many specific differences, just like the truck having a truck bed during which to carry cargo. Similarly, both the OSI and TCP/IP network layers define logical addressing and routing. However, the addresses have a different size, and therefore the routing logic even works differently. therefore the comparison of OSI layers to other protocol models may be a general comparison of major goals, and not a comparison of the specific methods.

Also Read : Overview of the TCP/IP Networking Model

OSI Layers and Their Functions

Today, because most of the people happen to be much more familiar with TCP/IP functions than with OSI functions, one of the best ways to find out about the function of various OSI layers is to think about the functions within the TCP/IP model and to correlate those with the OSI model. For the needs of learning, you’ll consider five of the OSI layers as doing the same kinds of things because the matching five layers within the TCP/IP model. for instance , the appliance layer of every model defines protocols to be used directly by the applications, and therefore the physical layer of every defines the electro-mechanical details of communicating over physical connections. Table 1-4 briefly describes each OSI layer.

Table 1-5 lists a sampling of the devices and protocols and their comparable OSI layers. Note that a lot of network devices must actually understand the protocols at multiple OSI layers, therefore the layer listed in Table 1-5 actually refers to the highest layer that the device normally thinks about when performing its core work. for instance , routers got to think about Layer 3 concepts, but they need to also support features at both Layers 1 and 2.

osi-protocol-eg-infosavvyBesides remembering the basics of the features of every OSI layer (as in Table 1-4), and a few protocol and device example at each layer (as in Table 1-5), you should also memorize the names of the layers. you’ll simply memorize them, but some people wish to use a mnemonic phrase to form memorization easier. within the following three phrases, the first letter of every word is that the same because the first letter of an OSI layer name.

Within the order laid out in parentheses:

– All People Seem to need data processing (Layers 7 to 1)
– Please don’t Take Sausage Pizzas Away (Layers 1 to 7)
– Pew! Dead Ninja Turtles Smell Particularly Awful (Layers 1 to 7)

OSI Layering Concepts and Benefits

While networking models use layers to assist humans categorize and understand the various functions during a network, networking models use layers for several reasons. for instance , consider another postal service analogy. a person writing a letter doesn’t have to think about how the postal service will deliver a letter across the country. The postal worker within the middle of the country doesn’t need to worry about the contents of the letter. Likewise, networking models that divide functions into different layers enable one software package or hardware device to implement functions from one layer, and assume that other software/ hardware will perform the functions defined by the other layers.

The following list summarizes the advantages of layered protocol specifications:

Less complex: Compared to not using a layered model, network models break the concepts into smaller parts.
Standard interfaces: the standard interface definitions between each layer allow multiple vendors to make products that fill a specific role, with all the advantages of open competition.
Easier to learn: Humans can more easily discuss and study the various details of a protocol specification.
Easier to develop: Reduced complexity allows easier program changes and faster product development.
Multivendor interoperability: Creating products to meet the same networking standards means computers and networking gear from multiple vendors can work in the same network.
Modular engineering: One vendor can write software that implements higher layers—for example, a web browser—and another vendor can write software that implements the lower layers—for example, Microsoft’s built-in TCP/IP software in its operating systems.

OSI Encapsulation Terminology

Like TCP/IP, each OSI layer asks for services from the next lower layer. to supply the services, each layer makes use of a header and possibly a trailer. The lower layer encapsulates the higher layer’s data behind a header. OSI uses a more generic term to refer to messages, instead of frame, packet, and segment. OSI uses the term protocol data unit (PDU). A PDU represents the bits that include the headers and trailers for that layer, also because the encapsulated data. for instance , an IP packet, as shown in Figure 1-14, using OSI terminology, is a PDU, more specifically a Layer 3 PDU (abbreviated L3PDU) because IP may be a Layer 3 protocol. OSI simply refers to the Layer x PDU (LxPDU), with x referring to the number of the layer being discussed, as shown in Figure 1-16.


Questions related to this topic

  1. What are the layers in TCP IP protocol?
  2. What layer in the Transmission Control Protocol Internet Protocol TCP IP model does IP use?
  3. What are the 4 layers of the TCP IP model?
  4. What are the five layers of Internet protocol stack?

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