The Open Systems Interconnect (OSI) model has seven layers. this tex.pdf
- 1. The Open Systems Interconnect (OSI) model has seven layers. this text describes and explains them, starting with the 'lowest' within the hierarchy (the physical) and continuing to the 'highest' (the application). The layers area unit stacked this way: Application Presentation Session Transport Network Data Link Physical PHYSICAL LAYER The physical layer, very cheap layer of the OSI model, worries with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and useful interfaces to the physical medium, and carries the signals for all of the upper layers. It provides: Data encoding: modifies the easy digital signal pattern (1s and 0s) employed by the computer to higher accommodate the characteristics of the physical medium, and to help in bit and frame synchronization. It determines: What signal state represents a binary one How the receiving station is aware of once a "bit-time" starts How the receiving station delimits a frame Physical medium attachment, accommodating varied potentialities within the medium: Will associate degree external transceiver (MAU) be accustomed connect with the medium? How many pins do the connectors have and what's every pin used for? Transmission technique: determines whether or not the encoded bits are transmitted by baseband (digital) or broadband (analog) signal. Physical medium transmission: transmits bits as electrical or optical signals applicable for the physical medium, and determines: What physical medium choices will be used How many volts/db ought to be accustomed represent a given signal state, employing a given physical medium DATA LINK LAYER The data link layer provides error-free transfer of information frames from one node to a different over the physical layer, permitting layers on top of it to assume just about error-free transmission over the link. To do this, the info link layer provides:
- 2. Link institution and termination: establishes and terminates the logical link between 2 nodes. Frame traffic control: tells the transmittal node to "back-off" once no frame buffers area unit obtainable. Frame sequencing: transmits/receives frames consecutive. Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur within the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt. Frame delimiting: creates and acknowledges frame boundaries. Frame error checking: checks received frames for integrity. Media access management: determines once the node "has the right" to use the physical medium. NETWORK LAYER The network layer controls the operation of the subnet, deciding that physical path the info ought to take supported network conditions, priority of service, and different factors. It provides: Routing: routes frames among networks. Subnet traffic control: routers (network layer intermediate systems) will instruct a causing station to "throttle back" its frame transmission once the router's buffer fills up. Frame fragmentation: if it determines that a downstream router's most transmission unit (MTU) size is a smaller amount than the frame size, a router will fragment a frame for transmission and re-assembly at the destination station. Logical-physical address mapping: interprets logical addresses, or names, into physical addresses. Subnet usage accounting: has accounting functions to stay track of frames forwarded by subnet intermediate systems, to supply request info. Communications Subnet The network layer code should build headers in order that the network layer code residing within the subnet intermediate systems will acknowledge them and use them to route knowledge to the destination address. This layer relieves the higher layers of the necessity to understand something concerning the info transmission and intermediate shift technologies accustomed connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or many intermediate systems within the communication subnet). In the network layer and therefore the layers below, peer protocols exist between a node and its immediate neighbor, however the neighbor could also be a node through that knowledge is routed, not the destination station. The supply and destination stations could also be separated by several intermediate systems.
- 3. TRANSPORT LAYER The transport layer ensures that messages area unit delivered error-free, in sequence, and with no losses or duplications. It relieves the upper layer protocols from any concern with the transfer of information between them and their peers. The size and quality of a transport protocol depends on the sort of service it will get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is needed. If the network layer is unreliable and/or solely supports datagrams, the transport protocol ought to embody in depth error detection and recovery. The transport layer provides: Message segmentation: accepts a message from the (session) layer on top of it, splits the message into littleer units (if not already small enough), and passes the smaller units right down to the network layer. The transport layer at the destination station reassembles the message. Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments. Message traffic control: tells the transmittal station to "back-off" once no message buffers area unit obtainable. Session multiplexing: multiplexes many message streams, or sessions onto one logical link and keeps track of that messages belong to that sessions (see session layer). Typically, the transport layer will settle for comparatively giant messages, however there area unit strict message size limits obligatory by the network (or lower) layer. Consequently, the transport layer should hack the messages into smaller units, or frames, prepending a header to every frame. The transport layer header info should then embody management info, similar to message begin and message finish flags, to modify the transport layer on the opposite finish to acknowledge message boundaries. additionally, if the lower layers don't maintain sequence, the transport header should contain sequence info to modify the transport layer on the receiving finish to induce the items back along within the right order before handing the received message up to the layer on top of. End-to-end layers Unlike the lower "subnet" layers whose protocol is between straight off adjacent nodes, the transport layer and therefore the layers on top of area unit true "source to destination" or end-to- end layers, and don't seem to be involved with the small print of the underlying communications facility. Transport layer code (and code on top of it) on the supply station carries on a voice communication with similar code on the destination station by exploitation message headers and management messages. SESSION LAYER
- 4. The session layer permits session institution between processes running on completely different stations. It provides: Session institution, maintenance and termination: permits 2 application processes on completely different machines to determine, use and terminate a association, known as a session. Session support: performs the functions that enable these processes to speak over the network, performing arts security, name recognition, logging, and so on. PRESENTATION LAYER The presentation layer formats the info to be bestowed to the applying layer. It will be viewed because the translator for the network. This layer might translate knowledge from a format employed by the applying layer into a standard format at the causing station, then translate the common format to a format famous to the applying layer at the receiving station. The presentation layer provides: Character code translation: as an instance, ASCII to EBCDIC. Data conversion: bit order, CR-CR/LF, integer-floating purpose, and so on. Data compression: reduces the amount of bits that require to be transmitted on the network. Data write in codeion: encrypt knowledge for security functions. as an instance, word secret writing. APPLICATION LAYER The application layer is the window for users and application processes to access network services. This layer contains a range of ordinarily required functions: Resource sharing and device redirection Remote file access Remote printer access Inter-process communication Network management Directory services Electronic messaging (such as mail) Network virtual terminals Solution The Open Systems Interconnect (OSI) model has seven layers. this text describes and explains them, starting with the 'lowest' within the hierarchy (the physical) and continuing to the 'highest' (the application). The layers area unit stacked this way: Application Presentation
- 5. Session Transport Network Data Link Physical PHYSICAL LAYER The physical layer, very cheap layer of the OSI model, worries with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and useful interfaces to the physical medium, and carries the signals for all of the upper layers. It provides: Data encoding: modifies the easy digital signal pattern (1s and 0s) employed by the computer to higher accommodate the characteristics of the physical medium, and to help in bit and frame synchronization. It determines: What signal state represents a binary one How the receiving station is aware of once a "bit-time" starts How the receiving station delimits a frame Physical medium attachment, accommodating varied potentialities within the medium: Will associate degree external transceiver (MAU) be accustomed connect with the medium? How many pins do the connectors have and what's every pin used for? Transmission technique: determines whether or not the encoded bits are transmitted by baseband (digital) or broadband (analog) signal. Physical medium transmission: transmits bits as electrical or optical signals applicable for the physical medium, and determines: What physical medium choices will be used How many volts/db ought to be accustomed represent a given signal state, employing a given physical medium DATA LINK LAYER The data link layer provides error-free transfer of information frames from one node to a different over the physical layer, permitting layers on top of it to assume just about error-free transmission over the link. To do this, the info link layer provides: Link institution and termination: establishes and terminates the logical link between 2 nodes. Frame traffic control: tells the transmittal node to "back-off" once no frame buffers area unit obtainable. Frame sequencing: transmits/receives frames consecutive. Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur within the physical layer by retransmitting non-acknowledged frames and
- 6. handling duplicate frame receipt. Frame delimiting: creates and acknowledges frame boundaries. Frame error checking: checks received frames for integrity. Media access management: determines once the node "has the right" to use the physical medium. NETWORK LAYER The network layer controls the operation of the subnet, deciding that physical path the info ought to take supported network conditions, priority of service, and different factors. It provides: Routing: routes frames among networks. Subnet traffic control: routers (network layer intermediate systems) will instruct a causing station to "throttle back" its frame transmission once the router's buffer fills up. Frame fragmentation: if it determines that a downstream router's most transmission unit (MTU) size is a smaller amount than the frame size, a router will fragment a frame for transmission and re-assembly at the destination station. Logical-physical address mapping: interprets logical addresses, or names, into physical addresses. Subnet usage accounting: has accounting functions to stay track of frames forwarded by subnet intermediate systems, to supply request info. Communications Subnet The network layer code should build headers in order that the network layer code residing within the subnet intermediate systems will acknowledge them and use them to route knowledge to the destination address. This layer relieves the higher layers of the necessity to understand something concerning the info transmission and intermediate shift technologies accustomed connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or many intermediate systems within the communication subnet). In the network layer and therefore the layers below, peer protocols exist between a node and its immediate neighbor, however the neighbor could also be a node through that knowledge is routed, not the destination station. The supply and destination stations could also be separated by several intermediate systems. TRANSPORT LAYER The transport layer ensures that messages area unit delivered error-free, in sequence, and with no losses or duplications. It relieves the upper layer protocols from any concern with the transfer of information between them and their peers. The size and quality of a transport protocol depends on the sort of service it will get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport
- 7. layer is needed. If the network layer is unreliable and/or solely supports datagrams, the transport protocol ought to embody in depth error detection and recovery. The transport layer provides: Message segmentation: accepts a message from the (session) layer on top of it, splits the message into littleer units (if not already small enough), and passes the smaller units right down to the network layer. The transport layer at the destination station reassembles the message. Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments. Message traffic control: tells the transmittal station to "back-off" once no message buffers area unit obtainable. Session multiplexing: multiplexes many message streams, or sessions onto one logical link and keeps track of that messages belong to that sessions (see session layer). Typically, the transport layer will settle for comparatively giant messages, however there area unit strict message size limits obligatory by the network (or lower) layer. Consequently, the transport layer should hack the messages into smaller units, or frames, prepending a header to every frame. The transport layer header info should then embody management info, similar to message begin and message finish flags, to modify the transport layer on the opposite finish to acknowledge message boundaries. additionally, if the lower layers don't maintain sequence, the transport header should contain sequence info to modify the transport layer on the receiving finish to induce the items back along within the right order before handing the received message up to the layer on top of. End-to-end layers Unlike the lower "subnet" layers whose protocol is between straight off adjacent nodes, the transport layer and therefore the layers on top of area unit true "source to destination" or end-to- end layers, and don't seem to be involved with the small print of the underlying communications facility. Transport layer code (and code on top of it) on the supply station carries on a voice communication with similar code on the destination station by exploitation message headers and management messages. SESSION LAYER The session layer permits session institution between processes running on completely different stations. It provides: Session institution, maintenance and termination: permits 2 application processes on completely different machines to determine, use and terminate a association, known as a session. Session support: performs the functions that enable these processes to speak over the network, performing arts security, name recognition, logging, and so on.
- 8. PRESENTATION LAYER The presentation layer formats the info to be bestowed to the applying layer. It will be viewed because the translator for the network. This layer might translate knowledge from a format employed by the applying layer into a standard format at the causing station, then translate the common format to a format famous to the applying layer at the receiving station. The presentation layer provides: Character code translation: as an instance, ASCII to EBCDIC. Data conversion: bit order, CR-CR/LF, integer-floating purpose, and so on. Data compression: reduces the amount of bits that require to be transmitted on the network. Data write in codeion: encrypt knowledge for security functions. as an instance, word secret writing. APPLICATION LAYER The application layer is the window for users and application processes to access network services. This layer contains a range of ordinarily required functions: Resource sharing and device redirection Remote file access Remote printer access Inter-process communication Network management Directory services Electronic messaging (such as mail) Network virtual terminals