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Layer 1—Physical

The Physical Layer is Layer 1 of the OSI model. Layer 1 describes units of data such as bits represented by energy (such as light, electricity, or radio waves) and the medium used to carry them (such as copper or fiber optic cables). WLANs have a physical layer, even though we cannot physically touch it.

Cabling standards such as Thinnet, Thicknet, and Unshielded Twisted Pair (UTP) exist at layer 1, among many others. Layer 1 devices include hubs and repeaters.

The Data Link Layer handles access to the physical layer as well as local area network communication. An Ethernet card and its MAC (Media Access Control) address are at Layer 2, as are switches and bridges.

Layer 2 is divided into two sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). The MAC layer transfers data to and from the physical layer. LLC handles LAN communications. MAC touches Layer 1, and LLC touches Layer 3.

Layer 3—Network

The Network Layer describes routing: moving data from a system on one LAN to a system on another. IP addresses and routers exist at Layer 3. Layer 3 protocols include IPv4 and IPv6, among others.

Layer 4—Transport

The Transport Layer handles packet sequencing, flow control, and error detection. TCP and UDP are Layer 4 protocols.

Layer 4 makes several features available, such as resending or re-sequencing packets. Taking advantage of these features is a protocol implementation decision. As we will see later, TCP takes advantage of these features, at the expense of speed. Many of these features are not implemented in UDP, which chooses speed over reliability.

Layer 5—Session

The Session Layer manages sessions, which provide maintenance on connections. Mounting a file share via a network requires several maintenance sessions, such as Remote Procedure Calls (RPCs); these exist at the session layer. A good way to remember the session layer’s function is “connections between applications.” The Session Layer uses simplex, half-duplex, and full-duplex communication.

Note
The transport and session layers are often confused. For example, is “maintenance of connections” a transport layer or session layer issue? Packets are sequenced at the transport layer, and network file shares can be remounted at the session layer: you may consider either to be maintenance. Words like “maintenance” imply more work than packet sequencing or retransmission: it requires “heavier lifting,” like remounting a network share that has been unmounted, so session layer is the best answer.