Function of OSI layer and comparison it with TCP/IP layers:

The OSI Reference Model:

• OSI stands for Open System Interconnection model. It is the primary model for network communication. It was created by ISO and provided vendors with a set of standards for compatibilty and interoperability between different types of network technologies.
• It has 7 numbered layers each of them with a specific network function. The separation of network functions is called Layering.The OSI reference model defines the network functions that occur at each layer. It helps to understand of how information travels throughout a network.
• Since the network is divided into 7 layers it provide advantages such as: Reduces complexitiy, provides a standard for interfaces, ensures interoperable technology, helps and simplifies teaching and learning.

Functions of each OSI Layer:

Application Layer (7):

• The application layer is the OSI layer closest to the user. It provides network services to the users applications and its processes. It does not provide services to any other OSI layer.
• It provides services only to applications outside the OSI model. Layers 4 to 7 are upper layers.
• Its main functions are user interface, data generation, resource sharing, directory services, network management.
• It provides as end user layer and network process to application. User applications examples are electronic email, SMTP, Telnet, FTP, WWW broswers, network management etc.

Presentation Layer (6):

• It ensures that the information sent by one application layer is read by the application layer on another system.
• It translates between multiple data formats by using a common format.
• It performs data representation like ensures data is readable by receiving system, formats data, structures data, negotiates data transfer for application layer. The unit of this layer is data and is part of the Host layer. Examples are ASCII, TIFF, PICT, JPEG, PNG, MIDI etc.

Session Layer (5):

• It provides interhost communication. It manages multiple applications, establishes session, performs security, name recognition, logging etc.
• It synchronizes dialogue between presentation layers of 2 hosts and manages their data exchange.
• Examples are RPC, SQL, NFS, NetBIOS names. The unit is data and it is a part of Host Layer.
• It also provides efficient data transfer, class of service and reporting of session, presentation and application layer problems.

Transport Layer (4):

• It segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system.
• It provides a data transport service that shields the upper layers from transport implementation details.
• Application, presentation and session layers are concerned with application issues while the lower 4 layers are concerned with data transport issues.
• Error detection, recovery and information flow control are used to provide reliable service. It provides end to end connections, handles transport issues between hosts, delivery and sequencing, message acknowledgement, traffic control, segmentation. Examples are TCP/SPX/UDP protocols. The unit is segment and is part of Host Layer.

Network Layer (3):

• It provides data delivery by providing connection and path selection between 2 host systems. It provide switching and routes data packets and selects the best path to deliver data.
• It also performs frame fragmentation, logical to physical address mapping, subnet traffic control, error handling, packet sequencing and congestion control..
• It sends data as packets which contains IP address. Routers and switches are devices and IP, IPX, ICMP are examples of this layer. Its unit is packet, Datagram and part of Media Layer.

Data Link Layer (2):

• It provides access to media. It defines how data is formatted for transmission and access to the network is controlled.
• It transmits packets from node to node as per MAC address, creates frames from data bits, provides error detection but no correction. The data is in form of frames which contains MAC address.
• Media Access Control (MAC) and Logical Link Control. (LLC) are part of this layer. Examples are devices like switch and bridge, protocols like WAP, PPP, FDDI, ATM, Frame Relay, SLIP etc. The data unit is frames, cell and part of Media Layer.

Physical Layer (1):

• It performs binary transmission. It defines the electrical, mechanical, procedures and functional specifications for activation, maintenance and deactivation of the physical link.
• It contains physical structure like cables, cards, hubs, Fast Ethernet, RS232, ATM protocols, transmission technique like baseband or broadband, physical medium transmission etc. devices like hub, concentrators, multiplexers are examples.
• Its unit is frame, bit and part of Media Layer. Layers 1 to 3 are lower layers.

TCP/IP Model:

• TCP/IP model is more based on protocols and protocols are not flexible with other layers. TCP/IP follows vertical approach.
• In TCP/IP it is not clearly separated its services, interfaces and protocols.
• It is built around the TCP/IP protocol suite. TCP/IP has 4 layers

1. Network Interfacer Layer
2. Internet Layer
3. Transport Layer
4. Application Layer

• Application layer of TCP/IP encompasses same functions as Application Layer, Presentation and Session Layer of the OSI model layers.
• Transport Layer of TCP/IP model functions the same as the Transport layer in OSI model and part of Session Layer.
• In TCP/IP model the transport layer does not guarantee delivery of packets.
• TCP and other similar protocols take on some of the function of the session layer.
• It synchronize source and destination computers to setup the session between the respective computers.
• Internet layer of TCP/IP model performs same function as OSI model network layer.
• Many of the functions of the logical lin control sublayer of the OSI model's data link layer.
• Primary protocol is Internet protocol (IP). It also uses Address Resolution Protocol (ARP) which performs much of the LLC sublayer's job in the area of physical addressing.
• Interface Layer of the TCP/IP model performs much of the job of the MAC portion of the data Link and Physical layers of the OSI model.
• TCP/IP protocol does not dictate what happens on network interface layer. The Network layer in TCP/IP model provides connectionless service.
• TCP/IP protocol suite relies on standards created by the various standards organizations concerning how to encode bits onto media to do the work on this layer.