What is the OSI Model? The 7 Layers Explained

By | July 1, 2021

In 1978, Open Systems Interconnect (OSI), model was introduced by the International Standards Organization (ISO) to provide a conceptual model for networking.

What is OSI Model?

OSI model servers the purpose in which the end user need not worry about using devices, protocols or services from different manufacturers as it lays out the guidelines for interoperability between network manufacturers.

For example a network interface card manufacturer need not think about how the cables are manufactured, how protocols work or if an application would be compatible with the network interface card.

The OSI Model defines how data communication occurs on networks. Most of the network communication protocols used today have a structure based on the OSI model.

The OSI model defines the communications process into 7 layers, which divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups.)

A task or group of tasks is assigned to each of the seven OSI layers. Each layer can independently implement the tasks assigned. This enables solutions offered by one layer to be updated without adversely affecting the other layers.

Each layer addresses a specific set of functions as illustrated in the figure above.

  • Layers 7 through 4 deal with end to end communications between data source and destinations.
  • Layers 3 to 1 deal with communications between network devices.

The seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1).

The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user.

The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software.

The lowest layer, the physical layer, is closest to the physical network medium (for example wires) and is responsible for placing data on the medium.

Data flow in the OSI Model

data-flow-in-osi-model

Data flows two ways in the OSI model, The first one is Down means (data encapsulation) and The second one is Up means (data decapsulation).

For any data that needs to be sent from one computer to another, the OSI model ensures that everyone follows some guidelines and hence each computer is able to communicate with every other computer, regardless of whether one computer is a Macintosh and the other is a PC.

When data flows down the OSI model, every layer adds a header that (“encapsulation”) and this is removed by the same layer on the other end of the session (“decapsulation”). These headers are layer specific.

The sending process passes the data to the application layer. The application layer attaches an application header and then passes the frame to the presentation layer.

The presentation layer transforms data (if needed, such as by translating it) and adds a header. It gives the result to the session layer.

The presentation layer is not concerned with which portion (if any) of the data received from the application layer is the application header and which portion is actually user data, because that information not necessary for the presentation layer’s role.

The process of adding headers is repeated from layer to layer until the frame reaches the data link layer. Here, in addition to a data-link header, a data-link trailer is added. The data-link trailer contains a checksum and padding. The frame is then passed down to the physical layer, where it is transmitted to the receiving computer.

On the receiving computer, the various headers and the data trailers are stripped off one by one by each layer and passed on to the next upper layer till the packet finally reaches the receiving process.

What are the seven layers of OSI Model?

seven-layers-of-osi-model

Physical Layer

Physical Layer is the first (lowest) layer in the OSI Model and deals with media, signal and binary transmission. This layer:

Establishes and terminatès connections to communication medium,controls modulation & demodulation of signals transmitted over wired (copper or fiber optic) or wireless media (such as radio wave).

Interfaces between the network medium and sending/receiving devices. (Hubs & Repeaters operate at this layer.)

Defines electrical, mechanical, and procedural interface to the transmission medium. Cables, connectors, voltages, topologies, etc. fall under the physical layer.

Protocols at this layer include 10BASE-T, 10BASE2, 10BASE5, 100BASE -TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX, DSL, ISDN,SONET, etc.

Unit of measurement (called as Protocol Data Units, PDU) at physical layer is bits such as Kbps. Mbps or Gbps.

Data Link Layer

Data Link Layer is the second (Layer 2) layer in the OSI Model. Data link layer is concerned with local delivery of frames between devices on the same LAN. Devices that operate at this layer include Network Interface Cards, Bridges and Switches. Communication happens through physical addressing; an address that is hardcoded into the network interface card (also referred to as MACaddress).

Data Link Layer has the following two sub layers:

Logical Link Control (LLC) provides multiplexing mechanisms that make t possible for several network protocols (IP, IPX, Decnet and Appletalk) to coexist within a multipoint network and to be transported over the same network media.

Media Access Control (MAC) provides addressing and channel accesscontrol mechanisms for several network nodes to communicate in a shared medium such as the Ethernet.

The Data Link Layer

  • Defines procedures for operating the communication links by physical The Data Link layer: addressing, framing, flow control, error control, media access control etc.
  • Frames packets
  • Detects and corrects packets transmit errors
  • Protocols at this layer include Ethernet, PPP, SLIP, Token Ring, etc.
  • Unit of measurement in data link layer is frames (data packet).

Network Layer

Network Layer, is also referred to as Layer 3. This layer:

  • Determines how data moves between network devices by using logical addresses and routing functions in logical networks.
  • Routes packets using unique network device addresses as the IP Internet Protocol) address. Routers operate at this Layer.
  • Protocols in this layer include IPv4, IPv6, IPX, RIP, OSPF, ICMP, IGMP.
  • Unit of measurement in network layer is packets or datagram etc.

Transport Layer

Transport Layer, referred to as Layer 4, handles end-to-end communication anderror-free transmission in conjunction with Layer 3. This layer:

  • Ensures end-to-end communication and error-free transmission
  • Provides reliable and sequential packet delivery through error recovery and flow control mechanisms
  • Provides connection/connectionless oriented packet Delivery
  • Protocols in this layer include TCP, UDP, etc.
  • The unit of measurement in transport layer is segments.

Session Layer

Session Layer, referred to as Layer 5, is responsible for establishing, between network applications. This layer:

  • Manages user sessions and dialogues (Logon/Logoff is handled here).
  • Controls establishing, maintaining (synchronizing) and terminating sessions (conversations)/logic links between users/ network applications
  • Reports upper layer errors
  • Protocols in this layer include NetBIOS, PAP, PPTP, L2TP, etc.

Presentation Layer

Presentation Layer is also referred to as Layer 6. This layer:

  • Masks the differences of data formats between dissimilar systems
  • Specifies architecture-independent data transfer format Encodes/decodes, encrypts/decrypts, compresses/decompresses data
  • Protocols in this layer include ASCII, EBCDIC, MIDI, SSL, TLS, etc.

Application Layer

Application Layer, referred to as Layer 7; protocols in this layer are responsible for process-to-process communication across an IP network. For example, if you initiate a web browser to visit a website, this layer initiates the HTTP protocol which in turn sends the data request to the underlying layers for communicating with the web server (HTTP) at the other end. To summarize, this layer:

  • Defines interface to user processes for communication and data transferrin network
  • Provides standardized services such as virtual terminal, file and job transfer and operations (provides services to end-users such as browsing, email, file transfers, etc.)
  • Protocols in this layer include HTTP, FTP, SMTP, POP3, DNS, DHCP, NNTP, etc.
  • Unit of measurement at Session, Presentation & Application layers is data.

IEEE 802 Standards IEEE stands Institute of Electrical and Electronics Engineers and is an international non profit organization that set standards, IEEE 802 deals with LAN and WAN related technologies. Services and protocols specified in IEEE 802 map to the lower two layers (Data Link and Physical) of the seven-layer OSI networking reference model. In fact, IEEE 802 splits the OSI Data Link Layer into two sub-layers named Logical Link Control (LLC) and Media Access Control (MAC).

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