The OSI Reference Model
Application Layer - (Layer 7) - Marks the spot where users actually communicate with the computer. Comes into play when access to the network is going to be needed. Examples are:Â file transfersemail clientsenabling remote accessnetwork/management activitiesclient/server interactions
Presentation Layer  - (Layer 6) - This layer of the OSI model gets its name from its purpose: to present data to the Application layer and is responsible for data translation and code formatting. The protocols in this layer define how standard data are formatted and include data:
compressiondecompressionencryptiondecryption
Session Layer - (Layer 5) - This layer is responsible for setting up, managing, and then tearing down sessions between Presentation entities. It also provides dialog control between devices or nodes, and coordinates communication between systems and offers to provide this coordination by means of three separate modes: simplex, half-duplex, and duplex. In summary, the Session Layer provides the means to keep different applications’ data apart from other applications’ data. Examples of Session Layer protocols and interfaces include:
Network File System (NFS)Structured Query Language (SQL)Remote Procedure Call (RPC)X-WindowAppleTalk Session Protocol (ASP)Digital Network Architecture Session Control Protocol (DNA SCP)
Transport Layer - (Layer 4) - This layer segments and reassembles data into a data stream. Services located here both segment and reassemble data from upper layer applications and unite it with data on the same data stream. They provide end-to-end data transport services and can provide a logical connection between the sending host and destination host on an internetwork. TCP/IP (Transport Control Protocol/Internet Protocol) resides at this level as well as UDP (Uniform Datagram Protocol). The former providing connection-oriented communication and the latter providing a connectionless communication environment. The connection-oriented communication provides flow control of data as well as redundancy.
Network Layer - (Layer 3) - This layer of the OSI reference model manages device or node addressing and determines the best means for moving data. Therefore, this layer must transport data between devices that aren’t locally connected. Routers (Layer 3 devices) are responsible for routing traffic across an internetwork at this layer using various routing protocols. Some of these are:
RIP - Routing Information ProtocolRIPv2 - Version 2 of RIP that permits Variable-length Subnet Masking (VLSM) on a network using CIDR (Classless Inter-Domain Routing)OSPF - Open Shortest Path First EIGRP - Enhanced Interior Gateway Routing ProtocolBGP - Border Gateway Protocol
Routers use update routing packets, network IP addressing, Interfaces, and Metrics to pass data between subnets on a LAN, between networks, and to the Cloud, or Internet.
Data Link Layer - (Layer 2) - Provides the physical transmission of data and handles error notifications, network topology, and flow control. This means that the Data Link Layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses, and translates messages passed down from the Network Layer into bits for the Physical Layer to transmit at wire speed. This layer consists of two sublayers:
Media Access Control (MAC) 802.1 sublayer which defines how packets are placed on the media, andLogical Link Control (LLC) 802.2 sublayer which is responsible for identifying the Network Layer protocols and encapsulating them
Network switches and bridges work at this layer of the OSI model and filter the network using hardware MAC addresses.
Physical Layer - (Layer 1) - Performs two vital functions at the bottom of the OSI reference model: it sends and receives data bits. Bits at this level are either 0s or 1s. The Physical Layer communicates with the various types of communications media. Some communicate as audio tones while others communicate through voltage changes on the wire that translate into the 0 or 1 bit values. The Physical Layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. It is at this layer where one differentiates the Data Terminal Equipment (DTE) from the Data Communication Equipment (DCE). Network Hubs (or data repeaters) operate at this layer of the OSI model.