Description of OSI layers OSI model

1 description of osi layers

1.1 layer 1: physical layer
1.2 layer 2: data link layer
1.3 layer 3: network layer
1.4 layer 4: transport layer
1.5 layer 5: session layer
1.6 layer 6: presentation layer
1.7 layer 7: application layer

description of osi layers

the recommendation x.200 describes 7 layers, labeled 1 7. layer 1 lowest layer in model.

at each level n, 2 entities @ communicating devices (layer n peers) exchange protocol data units (pdus) means of layer n protocol. each pdu contains payload, called service data unit (sdu), along protocol-related headers or footers.

data processing 2 communicating osi-compatible devices done such:

some orthogonal aspects, such management , security, involve of layers (see itu-t x.800 recommendation). these services aimed @ improving cia triad - confidentiality, integrity, , availability - of transmitted data. in practice, availability of communication service determined interaction between network design , network management protocols. appropriate choices both of these needed protect against denial of service.

layer 1: physical layer

the physical layer defines electrical , physical specifications of data connection. defines relationship between device , physical transmission medium (for example, electrical cable, optical fiber cable, or radio frequency link). includes layout of pins, voltages, line impedance, cable specifications, signal timing , similar characteristics connected devices , frequency (5 ghz or 2.4 ghz etc.) wireless devices. responsible transmission , reception of unstructured raw data in physical medium. bit rate control done @ physical layer. may define transmission mode simplex, half duplex, , full duplex. defines network topology bus, mesh, or ring being of common.

the physical layer layer of low-level networking equipment, such hubs, cabling, , repeaters. physical layer never concerned protocols or other such higher-layer items. examples of hardware in layer network adapters, repeaters, network hubs, modems, , fiber media converters.

layer 2: data link layer

the data link layer provides node-to-node data transfer—a link between 2 directly connected nodes. detects , possibly corrects errors may occur in physical layer. defines protocol establish , terminate connection between 2 physically connected devices. defines protocol flow control between them.

ieee 802 divides data link layer 2 sublayers:

medium access control (mac) layer – responsible controlling how devices in network gain access medium , permission transmit data.
logical link control (llc) layer – responsible identifying , encapsulating network layer protocols, , controls error checking , frame synchronization.

the mac , llc layers of ieee 802 networks such 802.3 ethernet, 802.11 wi-fi, , 802.15.4 zigbee operate @ data link layer.

the point-to-point protocol (ppp) data link layer protocol can operate on several different physical layers, such synchronous , asynchronous serial lines.

the itu-t g.hn standard, provides high-speed local area networking on existing wires (power lines, phone lines , coaxial cables), includes complete data link layer provides both error correction , flow control means of selective-repeat sliding-window protocol.

layer 3: network layer

the network layer provides functional , procedural means of transferring variable length data sequences (called datagrams) 1 node connected in different networks . network medium many nodes can connected, on every node has address , permits nodes connected transfer messages other nodes connected merely providing content of message , address of destination node , letting network find way deliver message destination node, possibly routing through intermediate nodes. if message large transmitted 1 node on data link layer between nodes, network may implement message delivery splitting message several fragments @ 1 node, sending fragments independently, , reassembling fragments @ node. may, not need to, report delivery errors.

message delivery @ network layer not guaranteed reliable; network layer protocol may provide reliable message delivery, need not so.

a number of layer-management protocols, function defined in management annex, iso 7498/4, belong network layer. these include routing protocols, multicast group management, network-layer information , error, , network-layer address assignment. function of payload makes these belong network layer, not protocol carries them.

layer 4: transport layer

the transport layer provides functional , procedural means of transferring variable-length data sequences source destination host via 1 or more networks, while maintaining quality of service functions.

an example of transport-layer protocol in standard internet stack transmission control protocol (tcp), built on top of internet protocol (ip).

the transport layer controls reliability of given link through flow control, segmentation/desegmentation, , error control. protocols state- , connection-oriented. means transport layer can keep track of segments , re-transmit fail. transport layer provides acknowledgement of successful data transmission , sends next data if no errors occurred. transport layer creates packets out of message received application layer. packetizing process of dividing long message smaller messages.

osi defines 5 classes of connection-mode transport protocols ranging class 0 (which known tp0 , provides fewest features) class 4 (tp4, designed less reliable networks, similar internet). class 0 contains no error recovery, , designed use on network layers provide error-free connections. class 4 closest tcp, although tcp contains functions, such graceful close, osi assigns session layer. also, osi tp connection-mode protocol classes provide expedited data , preservation of record boundaries. detailed characteristics of tp0-4 classes shown in following table:

an easy way visualize transport layer compare post office, deals dispatch , classification of mail , parcels sent. remember, however, post office manages outer envelope of mail. higher layers may have equivalent of double envelopes, such cryptographic presentation services can read addressee only. speaking, tunneling protocols operate @ transport layer, such carrying non-ip protocols such ibm s sna or novell s ipx on ip network, or end-to-end encryption ipsec. while generic routing encapsulation (gre) might seem network-layer protocol, if encapsulation of payload takes place @ endpoint, gre becomes closer transport protocol uses ip headers contains complete frames or packets deliver endpoint. l2tp carries ppp frames inside transport packet.

although not developed under osi reference model , not strictly conforming osi definition of transport layer, transmission control protocol (tcp) , user datagram protocol (udp) of internet protocol suite commonly categorized layer-4 protocols within osi.

layer 5: session layer

the session layer controls dialogues (connections) between computers. establishes, manages , terminates connections between local , remote application. provides full-duplex, half-duplex, or simplex operation, , establishes checkpointing, adjournment, termination, , restart procedures. osi model made layer responsible graceful close of sessions, property of transmission control protocol, , session checkpointing , recovery, not used in internet protocol suite. session layer commonly implemented explicitly in application environments use remote procedure calls.

layer 6: presentation layer

the presentation layer establishes context between application-layer entities, in application-layer entities may use different syntax , semantics if presentation service provides mapping between them. if mapping available, presentation service data units encapsulated session protocol data units , passed down protocol stack.

this layer provides independence data representation translating between application , network formats. presentation layer transforms data form application accepts. layer formats data sent across network. called syntax layer. presentation layer can include compression functions. presentation layer negotiates transfer syntax.

the original presentation structure used basic encoding rules of abstract syntax notation 1 (asn.1), capabilities such converting ebcdic-coded text file ascii-coded file, or serialization of objects , other data structures , xml. asn.1 makes application protocol invariant respect syntax.

layer 7: application layer

the application layer osi layer closest end user, means both osi application layer , user interact directly software application. layer interacts software applications implement communicating component. such application programs fall outside scope of osi model. application-layer functions typically include identifying communication partners, determining resource availability, , synchronizing communication. when identifying communication partners, application layer determines identity , availability of communication partners application data transmit. important distinction in application layer distinction between application-entity , application. example, reservation website might have 2 application-entities: 1 using http communicate users, , 1 remote database protocol record reservations. neither of these protocols have reservations. logic in application itself. application layer per se has no means determine availability of resources in network.