OSI Model is a 7 layer model created to define a standard for networking. Even though it was meant to be a protocol, nowadays ISO/OSI is just a model. Whereas TCP/IP is the protocol (or collection of protocols) modern networks follow.
7. Application: (DataUnit: Data )
Here are your applications. E-mail, browsing the web (HTTP), FTP and many more.
6. Presentation: (DataUnit: Data )
This one will make sure that information is readable for the application layer by formatting and structuring the data. Most computers use the ASCII table for characters. If another computer would use another character like EBCDIC than the presentation layer needs to “reformat” the data so both computers agree on the same characters.
Data representation and Encryption is done in this layer
5. Session: (DataUnit: Data )
The session layer takes care of establishing, managing and termination of sessions between two hosts. When you are browsing a website on the internet you are probably not the only user of the web-server hosting that website. This web-server needs to keep track of all the different “sessions”.
4. Transport: (DataUnit: Segments )
The transport layer takes care of transport. While browsing Internet webpages are sent as segments and transported to your computer. segments great chunks of data received from the upper layer protocols. Establishes and terminates connections between two computers. Used for flow control and data recovery.
- TCP lives here; it’s a protocol which send data in a reliable way. (TCP Windowing, Three way handshake )
- UDP lives here; it’s a protocol which sends data in an unreliable way.
3. Network: (DataUnit: Packets )
This layer takes care of connectivity and path selection (routing). This is where IPv4 and IPv6 live. Every network device needs a unique address on the network. defines device addressing, routing, and path determination. Device (logical) addressing is used to identify a host on a network (e.g. by its IP address).
2. Data link: (DataUnit: Frames )
This layer makes sure data is formatted the correct way, takes care of error detection and makes sure data is delivered reliably. This is where “Ethernet” lives. MAC Addresses and Ethernet frames are on the Data Link layer. encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header, most commonly, contains a source and a destination MAC address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors.
The data link layer has two sublayers:
1. Logical Link Control – used for flow control and error detection
2. Media Access Control – used for hardware addressing and controlling the access method
1. Physical: (DataUnit: Bits )
This layer describes stuff like voltage levels, timing, physical data rates, physical connectors and so on. Everything you can “touch” since it’s physical. defines how to move bits from one device to another. It details how cables, connectors and network interface cards are going to work and how to send and receive bits.
Diary of a sysadmin 