ANYONE KNOW OF A WEBSITE WITH RELIABLE INFO ON THE 7 LAYERS OF THE OSI MODEL ?

[leeland]

well i have a networking class and the book is worthless as far as describing what happens at each layer...so i am asking you wonderful people if there are any good sites with definitions and possible examples of the osi model to reference from


any info is greatly appreciated

thanks

leeland

 

[InlineFive]

I'M NOT EXCATLY SURE BUT WHY DON'T YOU TRY DOING A GOOGLE SEARCH?

-POR

 

[SaigonK]

Originally posted by: leeland
well i have a networking class and the book is worthless as far as describing what happens at each layer...so i am asking you wonderful people if there are any good sites with definitions and possible examples of the osi model to reference from any info is greatly appreciated thanks leeland

The 7 Layers of the OSI Model
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Application
(Layer 7) This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Presentation
(Layer 6) This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session
(Layer 5) This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport
(Layer 4) This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network
(Layer 3) This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Data Link
(Layer 2) At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical
(Layer 1) This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

 

[SaigonK]

Originally posted by: leeland
well i have a networking class and the book is worthless as far as describing what happens at each layer...so i am asking you wonderful people if there are any good sites with definitions and possible examples of the osi model to reference from any info is greatly appreciated thanks leeland

The 7 Layers of the OSI Model
The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Application
(Layer 7) This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Presentation
(Layer 6) This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session
(Layer 5) This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport
(Layer 4) This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network
(Layer 3) This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Data Link
(Layer 2) At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical
(Layer 1) This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

 

[MrDudeMan]

yeah, i second the google idea. i have a text book with it all in there, but that doesnt do you any good.

 

[Buddha Bart]

seriouisly one of my favorite things about the internet is half the time you can guess the domain name... case in point:
http://www.howtheosimodelworks.com/

 

[cmetz]

leeland, Google and textbooks will tell you about the OSI model more than you ever wanted to know. Two things they won't tell you that you need to know: First off, the OSI protocol suite which faithfully implemented the model was a spectacular failure, and the IP protocol suite predates the OSI model and doesn't map exactly into it - so when professors talk about the OSI model and use IP protocols as an example, remember that the fit is not as exact as you are led to believe. Second off, layering is a great abstraction to help you build things starting off, but you will find in practice that a lot of important and useful things require you to ignore layering and the model and be pragmatic. One of many examples are intelligent managed Ethernet switches, which can have IP and TCP/UDP ACLs (even through application protocols in some instances) even though they are an Ethernet level device device. Firewalls, IDSs, and load balancers are also huge layering violators. I personally find that many people have the OSI model so firmly ingrained in their thinking that it prevents them from seeing what might be otherwise obvious solutions to real-world problems.

Put more simply - it's important to learn the theory, but also important to understand that the theory isn't always the practice.

 

[GoldChain]

One of many examples are intelligent managed Ethernet switches, which can have IP and TCP/UDP ACLs (even through application protocols in some instances) even though they are an Ethernet level device device. Firewalls, IDSs, and load balancers are also huge layering violators.

Although many of these devices have multilayer functions, they still pretty much follow the OSI/IP layers. My co-workers sometimes get confused about this. For instance, when logged into a switch (with an administrative IP address in one VLAN) that has a directly connected end device in another VLAN, they don't understand why a ping from the switch has to go back through the network to the gateway router (assuming that the switch isn't also the L3 switch that IS the gateway for those subnets), then back to that very same switch (at L2) to be switched to the end device.

You are correct in that theory is great to learn, but knowing the real-world is what counts. However, I can't say I've come across many situations where the OSI/IP protocol layers didn't accurately describe and predict the processes that were going on.

I don't understand why you say Firewalls and load-balancers are huge layering violators either (I left out IDS's as most IDS is permiscuous, and don't really modify or forward the traffic it sees). In my experience, firewalls and load-balancers not only conform to the layers, but have to do it exceedingly well just to work. They just play some "tricks" with the information (which I guess you could call a violation )

GoldChain

 

[suklee]

Great Site