e construction reduction User data Application Application Data unit Presentation Presentation Data unit Session Session Data unit Transport Transport Data unit Network Network Data unit (information field) Data link Data link Serial bit stream Physical Physical Communication path on transmission media Figure 10.22 Buildup and breakdown of a data message based on the OSI model. OSI encapsulates at every layer except layer 1. 10.10.2.2 Functions of the First Four OSI Layers. Only the first four OSI layers are described in the following paragraphs. Layers 5, 6, and 7 are more in the realm of software design. Physical layer. The physical layer is layer 1, the lowest OSI layer. It provides the physical connectivity between two data terminals who wish to communicate. The services it provides to the data link layer (layer 2) are those required to connect, maintain the connection, and disconnect the physical circuits that form the physical connectivity. The physical layer represents the traditional interface between the DCE and DTE, described in Section 10.8. The physical layer has four important characteristics: 1. Mechanical 2. Electrical 3. Functional 4. Procedural The mechanical aspects include the actual cabling and connectors necessary to connect the communications equipment to the media. Electrical characteristics cover voltage and impedance, balanced and unbalanced. Functional characteristics include connector pin assignments at the interface and the precise meaning and interpretation of the various interface signals and data set controls. Procedures cover sequencing rules that govern the control functions necessary to provide higher-layer services such as establishing a connectivity across a switched network. Data-link layer. The data-link layer provides services for reliable interchange of data across a data link established by the physical layer. Link-layer protocols manage the 10.10 WHAT ARE DATA PROTOCOLS? establishment, maintenance, and release of data-link connections. These protocols control the flow of 