SummaryThe Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the 'lowest' in the hierarchy (the physical) and proceeding to the 'highest' (the application). The layers are stacked this way: Show
PHYSICAL LAYERThe physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
DATA LINK LAYERThe data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:
NETWORK LAYERThe network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
Communications SubnetThe network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address. This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet). In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems. TRANSPORT LAYERThe transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery. The transport layer provides:
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame. The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above. End-to-end layersUnlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages. SESSION LAYERThe session layer allows session establishment between processes running on different stations. It provides:
PRESENTATION LAYERThe presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station. The presentation layer provides:
APPLICATION LAYERThe application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
What OSI layer is responsible for data representation?Presentation
The sixth layer of the OSI model converts data formats between applications and the networks. Responsibilities of the presentation layer include: Data conversion. Character code translation.
At what layer of the OSI model is data formatted into packets?The network layer is the third level of the open systems interconnection model (OSI Model) and the layer that provides data routing paths for network communication. Data is transferred in the form of packets via logical network paths in an ordered format controlled by the network layer.
Which layer in the OSI model is concerned with finding the best path for the data from one point to the next within the network?Layer 4 of the OSI Model: Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation and desegmentation, and error control.
What is the OSI layer that ensures that the information is in a format that can be read by both ends?The sixth layer, Presentation layer, is responsible for ensuring data is encoded in a fashion that can be understood at the other end. It converts data from one data format into another so that two devices can communication.
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