OSI Model Explained (Beginners’ Guide)
OSI Model stands for Open System Interconnection and it is a reference model for communications. Any protocol which we use these days such as TCP/IP, UDP or Netware have got a reference from OSI model. In other words OSI committee did not create any new protocol standards but instead referenced other protocols that were already defined such as TCP/IP or UDP etc. OSI model consist of 7 layers where each layer performs a specific task and provides services to the layer above it.
[Please Note: For 'Networking Terminology' refer to this post.]
OSI Layers and their functions
Just to give a brief overview of OSI model’s layers please note that upper three layers such application, presentation and session layer define functions related to application. The lower four layers such as transport, network, data link and physical define functions related to end to end delivery of the data. Let’s elaborate these layers and learn more about these layers.
Layer 7 : Application Layer
Functional description- Application Layer provides network services to the end users such as E-Mail, NFS, ftp, DNS etc. This is the layer that actually interacts with the operating system or application whenever the user chooses to transfer files, read messages or perform other network-related activities. It also defines processes for user authentication. This Layer contains variety of more functions directory services, remote file access, remote printer access and network virtual terminals.
Protocols- HTTPS, FTP, Telnet, VoIP, SNMP, SMTP.
Layer 6 : Presentation Layer
Functional description- Presentation layer acts as a data translator for the Network. It is mainly responsible for delivery and formatting of information so that Application layer can further process it and can display it to the end user. It is also responsible for data encryption/decryption, coding/decoding, compression/decompression and expansion of graphics based commands. In vice versa, it takes the data provided by the Application layer and converts it into a standard format that the other layers can understand.
Protocols- Work along with Application layer protocols.
Layer 5 : Session Layer
Functional Description- Session Layer opens, establishes and close sessions across the network. It is also responsible for recognizing identity so that only designated parties can participate in that session. It performs three main tasks: Authentication, Permissions and session restoration.
Examples: File transfer connections, the session would connect and re-connect if there was an interruption.
Layer 4 : Transport Layer
Functional description- Transport Layer provides end to end communication to the application. It provides services such as connection – oriented data stream and connection – less data stream. These services are associated with 2 well known transport layer protocols which are TCP (transmission control protocol) and UDP (user data gram protocol). If you want reliability, end to end delivery, error – checking and delivery confirmation then you would use TCP in that case. But if you just want to send data in fast pace and you are not worried about the delivery confirmation and error checking then you would UDP. Emails, Remote – administration, file transfer and WWW uses TCP. Whereas UDP used in VoIP, online games, streaming media, service discovery and for broadcasting.
Protocols- TCP, UDP.
Layer 3 : Network Layer
Functional description- Network layer defines three main features such as logical addressing, path determination and routing (forwarding). Routing comes into play when you want to send data from one network to another network mainly knows as WAN using a special device known as Router. Logical addressing or IP addressing mainly known as IP address of the computer which consists of 32 bits. Routing process uses this IP address to forward and receive packets over the network. Path determination refers to the work done by routing protocols such as RIP, EIGRP, OSPF or BGP by which all available routes are learned, but the best route is chosen for use.
Protocols- IP, RIP, OSPF, EIGRP.
Layer 2 : Data Link Layer
Functional description- Data Link layer defines the format of data on the network. A network data frame includes checksum, source and destination address, and data. MTU (Maximum Transmission Unit) the largest packet that can be sent through a data link layer. The data link layer handles the physical and logical connections to the packet’s destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself. Ethernet addresses also known as MAC (Media Access Control) address and consist of 48 bits. This MAC address is unique and is associated with a particular Ethernet device.
Protocols- L2TP, PPP, ARP, HDLC.
Layer 1 : Physical Layer
Functional description- Physical layer This is the level of actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing. It also defines the cable or physical medium itself for example: – unshielded twisted pairs (UTP), Coaxial cable, Fibre Optic cables etc. Conversion from one media to another operates at this physical layer. It also defines how data would travel on physical medium
Hope this post was helpful to you. Please post your comments if you would like me to discuss/post on any particular topic. Your comments are always helpful. Thanks for reading.
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