OSI MODEL

The OSI Model (Open System Interconnection)

The main troubleshooting structure in my opinion that is designed to:

1. Help software vendors create software that is inter-operable with any network.
2. Makes it easier for me and you to learn networking concepts and to create a basis for us to understand how data traverses the network.
3. Gives us a basic model of which we can troubleshoot from.

The reason for #1 and in me saying that in my opinion the OSI model is the main troubleshooting structure is that it breaks the layers down into several more layers than the TCP/IP model.  Though, it can be said that us as Networking professionals, really don't worry too much with troubleshooting above layer 3 anyway.  I will discuss the layer mappings between OSI and TCP/IP models a little further down in the post.  

The OSI model has 7 layers and is structured in a way to demonstrate data flow and to give us a structured approach to troubleshooting.  Now, the layers of the OSI burrito!

7 - Application Layer: This layer is where all the end user interaction happens and any protocol needing user attention, happens here.  Authentication also happens at this layer and the communication between a remote partner so that resources exist and that both ends agree on data integrity, privacy, and error recovery.  Some special things to note about this layer are: Authentication, Firewalls, SMTP/POP3, Telnet, HTTP, FTP/TFTP, RIP, and SNMP all happens at this layer. 

- Interfaces with the application
- Provides network access to applications

6 - Presentation Layer: This layer deals with how data is presented, the compatibility with the operating system, and to make sure that everything is in proper formatting, encrypted, compressed, and translated.  This layer also makes sure that everything is properly encapsulated for transmission.  Some special things to note about this layer are: Encryption, JPEG, ASCII, GIF, MPEG, MIDI, EBCDIC, TIFF all happen here. 

5 - Session Layer:  This layer is the manager of the two way communication between two remote hosts.  The session layer handles the creation, the maintenance, and tear-down of communication between those two hosts.  That's why this is called the session layer, because the overall communication is called a "SESSION".

- Logically keeps sessions separate

4 - Transport Layer:  This layer is responsible in establishing a logical end to end connection between two hosts and segments data received from the upper three layers.  This is where the magic happens!  The transport layer is responsible for making sure that data gets to the destination in the correct order and free of errors, so that it can be reassembled on the other host and make sense.  Some special things to note about this layer are: TCP/UDP, SPX, Positive acknowledgement and retransmission (PAR), Flow control, Windowing, SYN, ACK.
- Dictates how the data is sent
- Defines well-known services and ports

3 - Network Layer:   The network layer is sometimes called the routing layer.  This is the first layer of the three bottom layers that network engineers spend their time in.  There are two things that we are really concerned about in this layer and that is:

1. What valid paths exist from the local router to the destination?
2. What is the best and optimal path to get there?

Some special things to note about this layer are: IP, ARP, IGMP, ICMP, the logical path between A and B, Routers, and layer 3 switches.
- Provides logical addressing
- Finds the best path to a destination

2 - Data Link Layer:  Error detection is performed here through the frame check sequence (FCS), but is only checked and detected via the receiving host and is dropped if it doesn't match up, so there is no correction.  Some special things to note about this layer are: Switches, WAP, Cable modems/DSL, Ethernet, High Data Link Control (HDLC), Point-to-point protocol (PPP), Frame Relay, MAC address, and Bridges.

- Provides Physical addressing
- Ensures data is error-free (error detection, not correction)

1 - Physical Layer:   This is where the actual physical transmission of data occurs and the last step of encapsulation for the packets to be sent across the wire.  some special things to note about this layer are: Physical cabling, Pins, Connectors, Electrical current, Hubs, and repeaters.
- Provides access to the cable
- Electrical Signals, ones and zeros
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The mapping between the two models (OSI and TCP/IP)

So as you can see, the OSI layer is separated into seven distinct layers while the TCP/IP model has chosen four.  The reason why I think the OSI model is more useful for troubleshooting is that it breaks down the troubleshooting process into an easier model with more levels and ways to better plan, separate, and attack each issue.