OSI Stack


OSI Protocol Description

The ISO {International Organization for Standardization} developed the OSI architectural model [www.iso.org].
The model is divided in seven layers, with layers 1 to 4 called the lower layers and 5 to 7 the upper layers.

Layer Responsibilities:

Layer 7. Application Layer: The highest layer. Generates or interprets data, may also provide encryption or decryption.
Applications using the network learn how to send a request, how to specify a filename over the net, how to respond to a request.
Protocol Data Unit [PDU] is called Data at this layer. This layer interfaces directly to and performs common application services for the application processes; it also issues requests to the Presentation Layer.

The common application services provide semantic conversion between associated application processes.
Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.


Layer 6. Presentation Layer: Determines how computers represent data [ASCII, GIF..]. Protocol Data Unit [PDU] is called Data at this layer. This layer responds to service requests from the Application Layer and issues service requests to the Session Layer. The Presentation Layer relieves the Application Layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

Layer 5. Session Layer: Establishing a communication session, Security, Authentication. NetBIOS is a layer 5 protocol. Protocol Data Unit [PDU] is called Data at this layer. This layer responds to service requests from the Presentation Layer and issues service requests to the Transport Layer. The Session Layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either duplex or half-duplex operation and establishes check-pointing, adjournment, termination, and restart procedures.

Layer 4. Transport Layer: Provides transfer correctness, data recovery, and flow control. TCP is a layer 4 protocol. Protocol Data Unit [PDU] is called a Segment at this layer. This layer responds to service requests from the Session Layer and issues service requests to the Network Layer. The purpose of the Transport Layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.

Layer 3. Network Layer: Provides address assignment, and Packet's forwarding methods. Protocol Data Unit [PDU] is called a Packet at this layer. This layer responds to service requests from the Transport Layer and issues service requests to the Data Link Layer. The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing, flow control, segmentation/desegmentation, and error control functions.

Layer 2. Data Link layer: Frame format, Transmitting frames over the net [additional bit/byte stuffing, start / stop flags, checksum, and CRC]. CAN bus, ATM, StarLAN, LocalTalk and HDLC are layer 2 protocols. Different network and protocol characteristics are defined by different data link layer specifications. The Data Link layer is subdivided into the Media Access Control (MAC) which controls access and encodes data into a valid signaling format [for the physical layer], and the Logical Link Control (LLC), which provides the link to the network [for the Network layer] Protocol Data Unit [PDU] is called a Frame at this layer. This layer responds to service requests from the Network Layer and issues service requests to the Physical Layer. The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Note: Examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC for local area networks.





Layer 1. Physical Layer: Defines the physical [hardware] implementation and the electrical [signal level] implementation of the bus; network cabling, connector type, pin-out, physical data rates, maximum transmission distances, and data transmission encoding. At this layer information is placed on the physical network medium. RS-232, and RS422 are examples of a physical layer specification. Protocol Data Unit [PDU] is called a Bit at this layer. The Physical layer performs services requested by the Data Link Layer. The major functions and services performed by the physical layer are: (a) establishment and termination of a connection to a communications medium; (b) participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control; and, (c) conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. Other physical layer specifications are listed on the Electronic Bus page.


OSI Protocol Stack
OSI Protocol Stack

Related Topics: Protocol Acronyms, Protocol Definitions


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Modified 3/05/12
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