Friday, 1 February 2013

The 7 layers of the OSI model

TCP/IP vs OSI and the corresponding protocols


The physical layer represents the physical elements used to transport the data on the network. This layer implements different technologies and some protocols like DSL and ISDN. RJ45, Ethernet, Circuit switching and multiplexing are things that can be placed on the physical layer. A hub or repeater is an example of devices that runs at the layer 1.


The data-link layer is the only layer of the osi model that is devised in two more sub-layer called the Logical Link Layer (LLC) and the Media access control layer (MAC). The data-link layers deal with protocols that works mainly with the mac address, like ARP (Address resolution protocol) and PPP (Point-to-point protocol).  Specified by the iso, this is the layer that should also manage flow control via CSMA (Carrier sense multiple access), where in reality most of this process happens on a higher level of the OSI. A switch is an example of a device that runs at the layer 2.


This is the layer that will take care of routing and forwarding of the packet. This is also the layer that implements TCP and UDP protocols.  A router is an example of a device which operates at the layer 3.


On the transport layer 4 is where the flow control and reliability of the transmission happens. As packets can be lost in the transport due to collisions and network congestion, the reliability is accomplished either by ACKs or checksums and requests a packet to be resent.
The congestion control is accomplished by CSMA which stands for Carrier sense multiple access. There is two principal way for CSMA to operates which are CA (Collision Avoidance) used mainly for wireless network. The other one being CD (Collision Detection) used for wired networks.


This layer establishes, manages and closes sessions for the process and applications.  This layer has some common tunneling protocols like L2TP (Layer 2 tunneling protocol) and L2F (Layer 2 forwarding protocol). Some authentication protocols like PAP (Password authentication protocol) are present at the session layer 5.


The presentation layer is where information is formatted following different standards. This is a translation layer that is responsible for delivery and formatting the information from the application layer. Common file formats like JPEG and PNG are part of this layer. Note that TLS, which supersedes the SSL protocol, is implemented at the presentation layer while SSL operates at the session layer.


This layer is used for all communication protocols. This is where the most common protocols reside, FTP, HTTP, SMTP and POP3 operates at this layer to name a few.

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