Last Updated on June 3, 2015 by Daniele Besana
Today I’m going to get back to basics and discuss the 7 layer OSI reference model and how it applies to networking. Most experienced network engineers know the OSI model all to well and most if not all certifications will expect you to know it. So lets unwrap this 7 layer burrito and see whats inside.
The first thing we need to understand is what is the OSI reference model anyway? Well, Wikipedia defines it as – The ‘Open Systems Interconnection Basic Reference Model’ (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design.
Basically it is structure of guide lines for developers and equipment manufacturers to be able to follow when designing new network cards, create new network drivers, new network software, etc and feel confident that the widget will work once it hits the wire.
So the 7 layers of the OSI Model is broken down as follows: Application, Presentation, Session, Transport, Network, Data Link, and Physical. Lets look at each one to get a better idea of how each one works and what each layer is responsible for.
Application – This is the layer that is closest to the user. In a web application this would be the browser.
Presentation – This layer would define how the application is presented to the user. Again using the web example. The html of a web page would fall into the presentation layer.
Session – This layer established the communication between computers. From client to server. In our web application the http protocol would fit nicely in the preentations layer.
Transport – This layer provides reliable transport services to the upper layers between users. At this point the TCP session of our web application is functioning here. The TCP syn-synack-ack occurs at the transport layer.
Network – This is the layer that ensures a functional means of having variable data lengths transfer from source to destination between different networks. Here is where IP is defined and routing occurs.
Data Link – This layer provides a functional means to transfer data between network entities. This is where the MAC address lives and switching and bridging occurs.
Physical – This layers defines the physical and electrical specifications for devices. This is where the rubber meets the road. Here is where the one’s and zero’s get put out onto the wire. This is also where cable specs are maintained, network drivers and the transition from physical to electrical occurs.
Now not everyone may agree with my definitions and there has been volumes written about exactly what occurs within the OSI Model. In my example I am merely trying to give a brief overview and understanding of what the OSI reference model is.
In order to help you memorize the OSI model its often helpful to use an analogy. There are a couple common ones you can use. One popular one is – All People Seem To Need Data Processing. Or you can make one up. Either way the, more you understand the OSI Model the more it will help you in your networking career.
Until next time
Get your Freak On!
9 Responses
hi ,, i am shakeel , i am taking ccna classes for last 1 year now, and trust me the difficult class for me in the complete course is this osi class. Joe can u give me some simple tips or some simple notes for this topic ,,,,,,,,,,,, plz
Sorry for the delay, was off for vacation.
Thanks for clarifying Joe!
Thanks for the reply Joe.
So although most of the implementation is tcp/ip way but we use OSI for reference (not implementation though) as its talks at more granular level.
Is my understanding correct?
No, Sorry for my confusion. What I meant to say is that there are engineers who may use the tcp/ip model. But I would say 99% of network designers, engineers, architects, and CCIE's all use the OSI model when either designing or troubleshooting their networks. I believe this is the case simply because of the OSI model is more granular. By breaking things down into 7 layers instead of 4, a more clear delineation can be made about the duties of each layers. Plus Cisco teaches the OSI so there is more engineers familiar with this model.
This also assists developers when creating network card drivers to interface between layer 2 and layer 1, or say a programmer creating a new web browser who needs to understand layers 5, 6, and 7.
Really the two models exist because they were created by two different standards bodies. The tcp/ip model was created by the U.S. DoD in approximately 1970. The OSI model was created by the Open Systems Interconnect. The OSI model was create as an attempt at vendor neutrality in the network market place.
Some do say that the tcp/ip is "more real world". And both the tcp/ip model and the OSI reference model both map fairly closely to each other. However, if you start breaking out a packet sniff on a really difficult network problem, being able to separate things into 7 segments is extremely helpful and allows you to fix a problem more quickly.
I hope this makes more sense. Honestly the last time I saw a reference to the tcp/ip model was when I was in the Air Force or studying for my MCSE. Since both exist its good to be familiar with both as you will no doubt run into both.
A basic question, as far as i know all present implementations are mostly use tcp/ip reference architecture which has 4 layers, couldn’t understand why we still refer to internet/network layer as L3 and transport as L4 even in this context.
Thanks for your comments! Some people do use the tcp/ip reference model. But for most implementations and architecture design we always use the OSI reference models. Its a much more granular model and from a Cisco point of view and underestanding networking the OSI is used most frequently.
Most switches today are multi-layer switches and understand layer 2, layer 3 and layer 4 so it is easy to see how this relates more toward the tcp/ip model.. but the OSI model is still the reference model in use everywhere.
Thanks again!
Hello Dwarka,
Routers in the past have only operated at the network layer of the OSI model yet could filter packets based on layer 4 port and protocol. For example, an Extended Access list within a router will filter a tcp packet however the router itself still only "routes" at layer 3 based on ip address.
With many of the advancements in networking technology the lines are starting to blur between routers and switches. New multi-layer routers can switch as well as route. These devices will route once and then switch. If the source and destination ip address, port, protocol, and mac address all do not change then it makes sense that we can make one routing decision and reference the mac-address-table for switching . This allows the router to perform at much higher speeds.
I hope this answered your question.
Joe
hello,
respected all of anybody
I want to kown router is a network device work on network layer of OSI reference module but why router work on network layer
second router is support port TCP and UDP so it does not work on transport layer and application
Plz tell me about my confuse