What are OSI Protocols? – Definition from Techopedia

In some networks, protocols are still popular using only the data link and network layers of the OSI model.

Each layer of the ISO model has its own protocols and functions. The OSI protocol stack was later adapted into the TCP/IP stack.

These protocols were developed and designed by the International Organization of Standardization (ISO). In 1977 the ISO model was introduced, which consisted of seven different layers. This model has been criticized because of its technicality and limited features and never met the goal of becoming universal standard.

OSI protocols are a family of standards for information exchange. They consist of a set of rules that should represent a standard for physical connections, cabling, data formats, transmission models, as well as means to ensure correction of errors and missing data.

Techopedia Explains OSI Protocols

The OSI protocol stack works on a hierarchical form, from the hardware physical layer to the software application layer. There are a total of seven layers. Data and information are received by each layer from an upper layer.

After the required processing, this layer then passes the information on to the next lower layer. A header is added to the forwarded message for the convenience of the next layer. Each header consists of information such as source and destination addresses, protocol used, sequence number and other flow-control related data.

The following are the OSI protocols used in the seven layers of the OSI Model:

Layer 1, the Physical Layer

This layer deals with the hardware of networks such as cabling. It defines the mechanical and electrical standards of interface devices and the types of cables used to transmit digital signals (e.g. optical fiber, coaxial cable, wireless, etc.).

The major protocols used by this layer include Bluetooth, PON, OTN, DSL, IEEE.802.11, IEEE.802.3, L431 and TIA 449.

Layer 2, the Data Link Layer

This layer receives data from the physical layer and compiles it into a transform form called framing or frame. The principal purpose of this layer is to detect transfer errors by adding headers to data packets.

The protocols are used by the Data Link Layer include: ARP, CSLIP, HDLC, IEEE.802.3, PPP, X-25, SLIP, ATM, SDLS and PLIP.

Layer 3, the Network Layer

This is the most important layer of the OSI model, which performs real time processing and transfers data from nodes to nodes. Routers and switches are the devices used for this layer that connects the notes in the network to transmit and control data flow.

The network layer assists the following protocols: Internet Protocol (IPv4), Internet Protocol (IPv6), IPX, AppleTalk, ICMP, IPSec and IGMP.

Layer 4, the Transport Layer

The transport layer works on two determined communication modes: Connection oriented and connectionless. This layer transmits data from source to destination node.

It uses the most important protocols of OSI protocol family, which are: Transmission Control Protocol (TCP), UDP, SPX, DCCP and SCTP.

Layer 5, the Session Layer

The session layer creates a session between the source and the destination nodes and terminates sessions on completion of the communication process.

The protocols used are: PPTP, SAP, L2TP and NetBIOS.

Layer 6, the Presentation Layer

The functions of encryption and decryption are defined on this layer. It ensures that data is transferred in standardized formats by converting data formats into a format readable by the application layer.

The following are the presentation layer protocols: XDR, TLS, SSL and MIME.

Layer 7, the Application Layer

This layer works at the user end to interact with user applications. QoS (quality of service), file transfer and email are the major popular services of the application layer.

This layer uses following protocols: HTTP, SMTP, DHCP, FTP, Telnet, SNMP and SMPP.