The OSI Model Layers Table provides an overview of the primary functions of each layer of the OSI model. It also presents the unit of information and address type where appropriate.

Physical Layer

The Physical Layer provides the service of transmitting a signal, across a physical communication medium, that represents binary bits. That medium can be a copper cable (coaxial or twisted pair), a fiber optic cable, or a radio channel. Thus, the Physical Layer includes the following types of hardware devices that send and receive signals over each type of physical medium:

Physical Layer processes are concerned only with the physical signals that represent data bits. Thus, they are only "aware" of signals over the medium, and are not concerned with any device that may be at the other end of the wire or channel. This also means that Layer 1 processes cannot detect errors in data transmission. Most error detection, and all error correction, are the responsibility of higher layers.

Data Link Layer

The Data Link Layer addresses groups of bits to a device located across a single physical transmission path, called a link. Each group of bits that the Data Link Layer transmits is called a frame.

To form a frame, the Data Link Layer encapsulates a Network Layer packet within a header and trailer. The header contains the hardware address of the destination node. The trailer contains a Frame Check Sequence (FCS) value that the receiving node uses for error detection. The Data Link Layer is the only OSI layer that adds a trailer to the data it transmits.

Each frame carries a packet of data across a single physical link. The encapsulated packet does not change, but a new frame is built around the packet for the trip across each link. Thus, we often say that the Data Link Layer is concerned with transmitting data to the next node in the network.

Popular Data Link protocols include:

The Network Layer is responsible for transmitting data packets between source and destination nodes that may not be connected by the same physical link. The Network Layer addresses a data packet to the logical description of a computer that may be located several links away from the source. If the source and destination nodes are not directly connected, then the Network Layer must use intermediate nodes, when necessary, to get a packet to its destination.

Unlike a Data Link address, which is globally unique, a Network Layer address is a logical identifier. Each Network Layer address is only unique within a single network. If a packet's source and destination are in different networks, the Network Layer may have to resolve different addressing conventions and duplicated node addresses used in different types and versions of networks.

The Network Layer also moves packets to and from node types that may use different Data Link protocols. For example, when a router forwards a packet from an Ethernet LAN to a frame relay network, it removes the Ethernet header and trailer and builds a new frame formatted for the frame relay network.

Common Network Layer protocols include:

Transport Layer

The Transport Layer, or Layer 4, addresses data to a particular process running on a destination computer. Peer software processes at either end of a connection use the Transport Layer to carry on a conversation. Processes in the Transport Layer act as if their nodes are adjacent. They rely on lower layers to handle the details of passing data through intermediate nodes across the network. Thus, Layer 4 insulates the higher levels from all concerns about the transportation of data.

Basic services provided by the Transport Layer include:

The most common Transport Layer protocols are:

Upper Layers: Session, Presentation, and Application

The job of the upper layers, taken collectively, is to provide user-oriented services through a set of widely available standard applications, and through specialized applications written for the users by programmers. The Session Layer, and the Presentation Layer above it, provide reusable services for the applications that reside in the Application Layer.

The Session Layer facilitates a step-by-step interaction between two entities. It establishes the session, manages the dialog to prevent simultaneous transmission, and ends the session gracefully. A single session may continue across one or more connections. Similarly, a single connection may support one or more sessions.

The Presentation Layer deals with the format, or representation, of computer information. It resolves differences between different types of character encoding systems, such as Extended Binary Coded Decimal Interchange Code (EBCDIC) and the American Standard Code for Information Interchange (ASCII) character code. It provides security by encrypting and decrypting data. It also compresses data before transmitting it, to use the communication channel more efficiently.

The Application Layer contains programs that invoke the underlying services of the network. Some of these applications are written specifically for one network, while others are widely used standard applications. When these applications need to communicate with peers over the network, they can use their own protocols, plus the services of the lower layers.

Application Layer programs include:

   Activities

See the Activities and Extended Activities section in Unit 1 Lesson 3 in your textbook Local Area Networks to test what you have learned so far.

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