Which layer is responsible for establishing a connection?
The Open Systems Interconnection (OSI) model describes how data communications should take place. When protocols or other standards are developed, they are placed into a layer of the model, which helps communication protocol integration and conceptual understanding. This is why it is important as an IT professional to understand the aspects of the OSI model as it provides the fundamental concepts of communications between systems and
applications. The OSI model, published by the International Standard Organisation (ISO) in 1984, relates to connecting systems that are open for communication with other systems. It does this by categorizing the network communication into seven abstract layers, grouping similar functions into the same layer. The model is more than 3 decades old, but the fact that it is still
relevant proves the power of its concepts. The OSI model defines standards for: The OSI model divides the network communication into a vertical stack that consists of the seven layers that are depicted in the figure below: Each layer takes care of a specific job and is intended to function with the layer above and the layer below. A summary of each layer's responsibilities:
The top 4 layers are called the Host layers and the bottom 3 layers are referred to as the Media layers. The following chapters will describe these layers in more detail. In the OSI model, control is passed from one layer to the next. When transmitting data from one machine to another, the data flow starts at the Application layer proceeding down to the Physical layer. Then over the channel to the receiving machine and back up the hierarchy. Layer 7: ApplicationHigh-level API's, including resource sharing, remote file access The Application layer is the highest layer of the OSI model. It provides the interface between the network protocol and the software running on the computer. The Application layer is the location where users and application processes access network services. Some commonly needed functions are provided at this layer:
Application
layer - protocols Layer 6: PresentationData Formatting, encoding, compression and encryption The Presentation layer's primary responsibility is to define how the data is communicated by the network hosts. When transmitting data from the sender to the receiver, the application of the sender moves the data to the Presentation layer. This layer then translates the data to a common format which can be read by both computers. On the receiving end, the Presentation layer translates the data to a format which the application can read. Compression, encryption, serialization, protocol conversion, character set conversion all fall in the functions of this layer. Presentation layer - protocols Layer 5: SessionManaging communication sessions The Session Layer provides process to process communications between two or more networked hosts. This layer is responsible for maintaining proper communication by establishing, managing and terminating sessions (a property of TCP) between two nodes. It takes care of gracefully closing sessions and for session check pointing and recovery. The following important functions are performed at this layer to establish, maintain and terminate communication sessions between applications:
Session layer - protocols Layer 4: TransportEnd-to-end connections and reliability The Transport Layer ensures that messages are delivered error-free, in sequence and with no loss or duplication. This layer verifies that the application transmitting the data is actually allowed to access the network and verifies that both ends of the connection can start the data transfer process. The following important functions are performed at the Transport layer:
The common Transport protocols utilized at this layer are:
Transport layer - protocols Layer 3: NetworkRouting, addressing and traffic control The network (or Internet) layer is primarily responsible for establishing the paths used for transfer of data packets between nodes on the network. This is the layer that routers operate on. The functions performed at the Network layer are:
This layer determines via which path data should be sent and does this based on the following:
The network layer also takes care of mapping logical (IP) addresses to physical (MAC) addresses that are used in the Data Link layer. If the message is too large to be transmitted from one node to another on the data link layer, the message may be split into several fragments at the network layer on one node. These fragments are sent independently and reassembled at the receiving node. It may, but does not need to, report delivery errors. The Internet Protocol (IP) operates in this layer. Network layer - protocols Layer 2: Data LinkReliable transmission of data frames between two nodes connected by a physical layer At the Data Link layer, data packets are encoded into bits. It defines the protocol for flow control and to establish and terminate a connection between two physically connected nodes on the network. It handles errors in the physical layer and is concerned with local delivery of frames between devices on the same LAN. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local network. Inter-network routing and global addressing are functions provided by higher layers. This allows the data-link protocols to focus on local deliver, addressing and media arbitration. The IEEE 802 is a family of standards dealing with Local and Metropolitan area networks and divides the Data link layer into two sub layers:
Most network technologies based on IEEE 802 use MAC addresses that are used in the MAC-layer for addressing purposes. Data Link Layer - protocols Layer 1: PhysicalHardware, signaling and binary transmission The physical layer is the lowest layer of the OSI model and consists of the functionality that interacts with the actual hardware and signaling mechanism. It handles the transmission and reception of the unstructured raw bit stream over a physical medium and carries the signals for all of the higher layers. Think of the wiring, cabling, frequencies, pulses used to represent binary signals, etc. The physical layer provides:
Physical Layer - protocols Which layer is responsible for connection establishment?Session Layer (Layer 5) :
This layer is responsible for the establishment of connection, maintenance of sessions, authentication, and also ensures security.
What layer is responsible for establishing the connection when sending the file?The transport layer. The transport layer is responsible for transferring data across a network and provides error-checking mechanisms and data flow controls. It determines how much data to send, where it gets sent and at what rate. TCP within the TCP/IP suite is the best-known example of the transport layer.
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