The OSI model network layers

Last revision July 28, 2004

1. Physical - cable or media standards. Electrical characteristics, length limitations, how computers or other devices mechanically and electrically connect.
2. Data Link - format of data on the network and how it flows. Standards describe how data is packaged into "packets", including things like maximum length and what type of address is included for the sender and receiver (this level of addressing is at the "hardware" level - dealing with the actual devices directly connected to this physical network). Other standards describe how a device can gain access to the physical layer, what to do if there is contention, how to tell if there is a transmission error, how to mark packets to keep them in sequence, etc.
3. Network - provide routing and related functions that enable multiple physical network segments to be combined into an internetwork. This layer provides the standards for logical naming and addressing of devices so you can route to them even if they are not physically connected to the same network.
4. Transport - provide reliable process to process communication. Most important layer from the standpoint of the actual application, which doesn't care how the data gets to the other computer, but does need to be sure that communication is occurring. This layer implements "connections", which require that data flow in sequence, that errors be detected and corrected (for example, by requesting retransmission of data), and that data transmissions be acknowledged (if desired - some applications like web servers don't check if you actually got the data). At this level, pangea also provides "addresses" for different types of services to make sure that data received by a computer goes to the correct application process.
5. Session - not well defined. Concept of tying together multiple transport streams into a single "session". In practice, collapsed into transport level.
6. Presentation - issues of data format conversion, compression, encryption, etc. In practice, collapsed into the application level.
7. Application - what the user is trying to accomplish, for example, email, file transfer, web browsing, etc.

More detail on the OSI model can be found in documentation provided by Cisco Systems.

A computer network requires adherence to common standards for all seven layers. If your computer and another do not use the same standards at all layers of the networking model, then they are not part of the same network, even if they are next to each other and connected to the same physical cable or network hub, and they cannot communicate.

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