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Network Models

OSI Model

OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. 

Its purpose is to guide product implementers so that their products will consistently work with other products. 

The seven layers are:

Layer 7: The application layer
Layer 6: The presentation layer
Layer 5: The session layer
Layer 4: The transport layer
Layer 3: The network layer
Layer 2: The data-link layer
Layer 1: The physical layer

Application
(Layer 7)

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation
(Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session
(Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport
(Layer 4)

Usually TCP (the top half of TCP/IP). This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network
(Layer 3)

Typically IP (the bottom half of TCP/IP).  This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link
(Layer 2)

Ethernet, ATM, Frame Relay, etc.  At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization.

The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical
(Layer 1)

This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

The TCP/IP model

OSI layer model and TCP/IP model

TCP/IP is based on a four-layer reference model. 

All protocols that belong to the TCP/IP protocol suite are located in the top three layers of this model.

As shown in the following illustration, each layer of the TCP/IP model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) reference model proposed by the International Standards Organization (ISO).

The types of services performed and protocols used at each layer within the TCP/IP model are described in more detail in the following table :

Layer Description Protocols
Application Defines TCP/IP application protocols and how host programs interface with transport layer services to use the network. HTTP, Telnet, FTP, TFTP, SNMP, DNS, SMTP, X Windows, other application protocols
Transport Provides communication session management between host computers. Defines the level of service and status of the connection used when transporting data. TCP, UDP, RTP
Internet Packages data into IP datagrams, which contain source and destination address information that is used to forward the datagrams between hosts and across networks. Performs routing of IP datagrams. IP, ICMP, ARP, RARP
Network interface Specifies details of how data is physically sent through the network, including how bits are electrically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted-pair copper wire. Ethernet, Token Ring, FDDI, X.25, Frame Relay, RS-232, v.35

Difference between OSI and TCP/IP models



Last updated on: 08/10/2019

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