Open Systems Interconnect Reference Model
Summary Overview

The OSIRM was developed to provide a reference model for the development of modular networking components. Composed of seven layers, it covers the region from interface to the wire to the application. Below is a summary of each layer you can remember the seven layers of the model with the acronym "Plant spa."

Physical
Link (i.e. data link)
Network
Transport
Session
Presentation
Application

The Physical Layer is the layer that is concerned with the signaling of the message and the interface between the sender or receiver and the medium. The physical layer is generally defined by one of the standards bodies and carries a designation that indicates the characteristics of the connection. Among frequently used physical layers standards are EIA-232-D, ITU V.35, and some of the X series (X.21/X.21bis, for example). Physical layer links.

The Data Link layer is comprised of a single layer in the OSI model but actually incorporates two sub-layers when viewed from the perspective of the IEEE Logical Link Control (LLC) model. The lower of the two sub-layers is called the media access control (MAC) and the upper sub-layer is the LLC layer. The MAC layer provides local addressing, error correction, and in some cases includes a field to distinguish the frame type from other frame types. The LLC provides Service Access points (SAP) for passing off the frame to higher level. Examples of data link standards include frame relay, ethernet, token ring, and FDDI. Data link layer links.

The Network Layer is concerned primarily with the delivery of packets. The addressing which makes this delivery possible can be thought of as a universal address as compared to the local addressing of the MAC sub-layer in the data link layer. This layer may also be concerned with end-to-end flow control and the segmentation and reassembly (SAR) functions for the transmission of large packets of data which exceed the maximum transmission unit (MTU) of the data link layer. The Internet Protocol (IP) is the most common example, and Novell's Internetwork Packet eXchange (IPX) is another popular implementation. Network layer links.

The Transport Layer serves as the layer responsible for insuring the integrity of the host-to-host session connection. In some protocol suites this layer also provides a means to provide multiple conversations (multiplex) between the same two end-points. Delivery may be 'guaranteed' (streams, like TCP) or 'best effort' (datagram, like UDP). Examples of this layer within the Internet are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Transport layer links.

The Session Layer is the layer that deals with the actual session between the user and the host. This layer is used to support transactions, provide some information on the application, and provide some flow control or dialogue management. The most common implementation today is probably the HyperText Transfer Protocol (HTTP) used with the World Wide Web. Session layer links.

The Presentation Layer provides a uniform means for a remote host to inform the local client how to present the data to the application or client. Still using our WWW example, this is the HyperText Markup Language (HTML) that defines how to present the data received by the client. Presentation layer links.

The Application Layer is the layer that provides the direct interface to the user--thereby shielding her from all of the underlying layers. The web browser you use to read this page is an example of the application layer. Application layer links.

Last updated: November 21, 2002 03:50 PM