A seven-layer chart that is as entwined with the world of data communications as the alphabet is to language. “OSI” stands for “Open Systems Interconnect,” developed by the International Standards Organization (ISO). The seven layers define how different types of data interact with other types of data. The ultimate goal of OSI is that any vendors data system, connected to any network, can share information with any other suppliers system. In that sense, the OSI stack is a sort of recipe for interoperability.
Cable first dipped its toes into the world of OSI stacks when it, through CableLabs and DOCSIS, established an interoperable cable modem specification. So far, cable broadband generally sticks to the bottom two layers — Physical link and Data link. The layers, best visualized to correspond with the children’s jingle “the neck bone’s connected to the backbone” (because one layer doesnt exist well without the others,) are described below:
7 (top layer): Application: Defines common applications and how to link them, over a system, to a user.
6: Presentation: Establishes a common syntax between applications, so that apps can run independently, but in a coordinated fashion.
5: Session: Describes the control structure for intra-application communications (how an application talks to another application, like the billing interface). Defines how cooperating applications establish, manage and end communication.
4: Transport: Describes how to reliably transfer information from one end point to another, while describing end-to-end error recovery and flow control.
3: Network: Establishes how to switch and route packets; describes how to establish, maintain and terminate connections.
2: Data Link: Defines how information is reliably transferred over the physical link (wires), and describes how chunks of data (known as “frames”) should be synchronized, how transmission errors are controlled, and how data flows.
1: Physical: Defines how bits move over physical media (wires), and organizes mechanical, electrical, functional and procedural characteristics inherent to those wires.