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Description of OSI layers

The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model.

OSI Model
LayerProrotocol Data Unit(PDU)FunctionExamples
Host
layers
7. ApplicationDataHigh-level APIs, including resource sharing, remote file access, directory services and virtual terminals
6. PresentationTranslation of data between a networking service and an application; including character encoding, data compression and encryption/decryption
5. SessionManaging communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodesRPC, SCP, NFS, PAP, TLS, FTP, HTTP,HTTPS, SMTP, SSH, Telnet
4. TransportSegment (TCP) /Datagram (UDP)Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexingNBF, TCP, UDP
Media
layers
3. NetworkPacketStructuring and managing a multi-node network, including addressing, routing and traffic controlAppleTalk, ICMP, IPsec, IPv4, IPv6
2. Data linkFrameReliable transmission of data frames between two nodes connected by a physical layerIEEE 802.2, L2TP, LLDP, MAC, PPP
1. PhysicalBitTransmission and reception of raw bit streams over a physical mediumDOCSIS, DSL, Ethernet physical layer, ISDN,USB

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