If you’ve had enough of the OSI model, you don’t need to read this. You already have more than enough material to pass your exam. Skip this page. But if layer overlap is something that’s confused you in the past, read on…
Once in a while you will come across a protocol that does not fit neatly into the OSI model, and things start to get a little messy. Remember, the OSI model is a concept that allows you to understand internetworking. Once you do understand it, you can now deal with any protocols that break the model.
Example 1: TCP – Transmission Control Protocol
This protocol lives primarily at the Transport Layer therefore its functions are:
· Chopping up data from upper layers and creating a single data stream using sequence numbers and port numbers to label segments
· Ensuring reliable data transfer by acknowledging any data sent and re-transmitting any unacknowledged data
· Flow control
TCP also creeps into the Session layer and therefore has another additional (session layer) function:
· Establishing a connection before transferring any data using a three-way handshake therefore ensuring a connection-oriented data transfer.
This makes TCP both a connection-oriented AND reliable protocol.
Example 2: Ethernet II
Ethernet was originally developed by Xerox Corporation who later teamed up with DEC and Intel to create today’s most widely used form of Ethernet, which is called Ethernet II. This standard spans both sub-layers of the Data Link Layer and keeps going to include the physical layer.
Its functions therefore include
· Encapsulating upper layer protocols and labelling them with a “type” field (Logical Link Control sub-layer)
· Creation of a frame and using a MAC address to address it (Media Access Control sub-layer)
· Defining the technology that is used to transmit the frames (CSMA/CD) (Media Access Control sub-layer)
· Defining physical topology, maximum transmission distances, interface specifications, etc. (physical layer).
Example 3: SMTP, FTP, Telnet
I know, I know, I did say that these belonged to the application layer but the truth is that they creep into the presentation layer as well, and can even sometimes do session layer stuff. These protocols will therefore allow the user to interface with the end-device and the end-devices to interface with each other (application layer) but they are also responsible for converting data into a standard format (presentation layer). For the exam however, just say that they live in the application layer.
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