<P> The OSI standards documents are available from the ITU - T as the X. 200 - series of recommendations . Some of the protocol specifications were also available as part of the ITU - T X series . The equivalent ISO and ISO / IEC standards for the OSI model were available from ISO . Not all are free of charge . </P> <P> The recommendation X. 200 describes seven layers, labeled 1 to 7 . Layer 1 is the lowest layer in this model . </P> <Table> <Tr> <Th_colspan="5"> OSI Model </Th> </Tr> <Tr> <Th_colspan="2"> Layer </Th> <Th> Protocol data unit (PDU) </Th> <Th> Function </Th> </Tr> <Tr> <Th> Host layers </Th> <Td> 7 . Application </Td> <Td> Data </Td> <Td> High - level APIs, including resource sharing, remote file access </Td> </Tr> <Tr> <Td> 6 . Presentation </Td> <Td> Translation of data between a networking service and an application; including character encoding, data compression and encryption / decryption </Td> </Tr> <Tr> <Td> 5 . Session </Td> <Td> Managing communication sessions, i.e. continuous exchange of information in the form of multiple back - and - forth transmissions between two nodes </Td> </Tr> <Tr> <Td> 4 . Transport </Td> <Td> Segment (TCP) / Datagram (UDP) </Td> <Td> Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing </Td> </Tr> <Tr> <Th> Media layers </Th> <Td> 3 . Network </Td> <Td> Packet </Td> <Td> Structuring and managing a multi-node network, including addressing, routing and traffic control </Td> </Tr> <Tr> <Td> 2 . Data link </Td> <Td> Frame </Td> <Td> Reliable transmission of data frames between two nodes connected by a physical layer </Td> </Tr> <Tr> <Td> 1 . Physical </Td> <Td> Bit </Td> <Td> Transmission and reception of raw bit streams over a physical medium </Td> </Tr> </Table> <Tr> <Th_colspan="5"> OSI Model </Th> </Tr>

Which layer of osi model does data compression