OSI Lower Layers

The lower OSI layers define how data moves through the network. Because Ethernet itself and the switching of Ethernet frames are classified in the lower OSI layers, most of the discussion in this book focuses on the lower layers. The lower layers of the OSI model are as follows:

• Transport— The layer responsible for error detection and correction, flow control, and data sequencing; also determines the size of the packet. Examples include Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

• Network— The layer responsible for the delivery of data packets. Network layer provides logical addressing and path determination. Examples include Internet Protocol (IP) and Internetwork Packet Exchange (IPX).

• Data Link— The layer responsible for access to media, hardware addressing, error detection, flow control, and encapsulation of data into frames. The two major components to Data Link layer are Logical Link Control (LLC) and Media Access Control (MAC). LLC handles error detection and flow control. MAC is responsible for communicating with the adapter card, and the type of media used. Examples include IEEE 802.3 CSMA/CD, 802.12 Demand Priority, and 802.5. Bridges and LAN switches also operate at this layer.

• Physical— The layer responsible for defining the electrical properties and physical transmission system. The physical layer is responsible in transmitting and receiving data. Examples include any type of cabling, hubs, repeaters, and fiber optics.