It is a standard model that defines networking framework. It implements various protocols which are simply a set of rules in layers where control goes through one layer to the next. In OSI, it is the responsibility of the lower layer to provide data or services to its adjacent upper layer. Before providing data to its upper layer the provider layer must go for a valid check, so that any error may be trapped easily before providing data to its upper layer. The OSI model defines networking which is a stack of seven layers in vertical order. The upper layers of the OSI model represent software that implements network services like encryption, decryption, connection management etc. The lower layer of the OSI stack model implements various hardware-oriented functions such as routing addresses and flow control of datagrams or ip-packets. The OSI model segregates the architecture of the network into connected seven layers in a progression which is vividly logical.
In the model, lower layer deals with electrical signals and various chunks of data in the form of 0 and 1 i.e. binary data and the routing of these data across the network. In this flow, the higher levels deal with requests and responses across the network, representation of data and network protocols as seen from the point of view of the user.
Layer 1- Physical layer
At this layer of the model, the physical layer of the OSI model is responsible for ultimate transmission of data bits. Ethernet cables, token ring networks are the examples of the physical layer.
At the physical layer, data are transmitted using the type of signaling supported by the physical medium. The various medium used are electric voltages, pulse of infrared, radio frequencies, ordinary light etc.
Layer 2- Data link layer
As per data received from physical layer, the data link layer checks for physical transmission errors and packages bits into data frames. Physical addressing is also managed by data link layer schemes. Such example of physical addressing schemes are MAC addressing for Ethernet networks used for controlling access of any various network devices to physical medium. Due to high load of various responsibilities it is the most complex layer of the OSI model. Further this layer is divided into two sub layers known as media access control layer and logical link control layer which further takes the responsibilities separately.
Layer 3- Network layer
The third layer in the model holds the concept of routing above the data link layer. As the data arrives to the network layer, the source address and destination address contained inside each frame are examined to determine whether the data has reached its final destination. Further, this layer formats the data into datagram known as data packets which are delivered in the upward direction to the next layer known as transport layer for providing further services.
It is the duty of the network layer to maintain the logical addresses for devices on the network to support routing across the network. The network layer also manages the mapping between the logical and physical addresses too. In the IP networking, this mapping is accomplished through the Address Resolution Protocol.
Layer 4- Transport layer
It is the duty of the transport layer for delivering data across network connections. The TCP protocol is the most common example of a transport layer. Various data transfer related protocols also supports a range of optional capabilities such as recovery from errors and flow control. It also supports for re-transmission in case of need.
Layer 5- Session layer
This fifth layer has a duty to manage the sequences as well as flow of events that initiate and destroy down network connections in case of need for ending a session. At this fifth layer, support for multiple types of connections that can be created dynamically and run over individual networks.
Layer 6- Presentation layer
The presentation layer is generally a simplest layer of all the seven layers present in the OSI reference model. Presentation layer handles syntax processing of message data such as format conversion, encryption and decryption whenever needed to support the layer above it known as application layer.
Layer 7- Application layer
This last layer in the stack has the responsibility of that this layer provides network services to end-user applications. Network services are typically the protocols that works with users data. For example, the application layer protocol HTTP packages the data needed to send and receive web page content in a web browser application. Finally, this layer provides data to the presentation layer.
Cross-layer functions
Cross-layer functions are the services that are independent but are not tied to any given layer. On the other hand, it do also affect one or more layers. The following examples are the cases of cross-layer functions:
Management Service- Functions that permits to configure, monitor and terminate the communications among two or more networks entities are management systems. There is a specific protocol that is known as common management information protocol, CMIP in short. It serves this layer. Further, this service needs to interact with every layer in order to deal with their instances.
• Multiprotocol Label Switching- The OSI model further subdivides the third layer i.e. network layers in three roles. This division is done to provide a unified data-carrying service. It can also be used to carry various different kinds of network traffics.
• Domain Name Service- Domain name service or DNS in short is a service available at application layer service. DNS service is used to look up the IP address of a given domain name. In this level, when DNS server sends reply, it is then possible to form a four-layer connection or flow to the desired host.
The following table presents all the seven layers of OSI reference model along with basic services and protocols associated with each layer.
Comparison between OSI and TCP/IP Model-
In the TCP/IP model, design of protocols in each layer of the internet does not concern with strict layering of the protocols in these layers. TCP/IP follows a five layer scheme where OSI follows a seven layers scheme. The five layers of TCP/IP are here compared equally with its equivalent OSI layering scheme. The comparison of TCP/IP five layers along with the OSI seven layers is below:
• The internet application layer includes the OSI application layer, and most of the session layer.
• Its end-to-end transport layer includes the elegant close function of the OSI session layer as well as the OSI transport layer.
• The link layer includes the OSI data link layer and sometimes not only the physical layers but also some protocols of the OSI network layer.
The comparisons given above are based on the original seven-layer protocol model as defined in ISO 7498.