The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model.
| OSI Model | ||||
|---|---|---|---|---|
| Layer | Data unit | Function[3] | Examples | |
| Host layers | 7. Application | Data | High-level APIs, including resource sharing, remote file access, directory servicesand virtual terminals | Mail, Internet Explorer, Firefox |
| 6. Presentation | Translation of data between a networking service and an application; includingcharacter encoding, data compression and encryption/decryption | ASCII, EBCDIC, JPEG | ||
| 5. Session | Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes | RPC, PAP, HTTP, FTP, SMTP, Secure Shell | ||
| 4. Transport | Segments | Reliable transmission of data segments between points on a network, includingsegmentation, acknowledgement and multiplexing | TCP, UDP | |
| Media layers | 3. Network | Packet/Datagram | Structuring and managing a multi-node network, including addressing, routing andtraffic control | IPv4, IPv6, IPsec, AppleTalk, ICMP |
| 2. Data link | Bit/Frame | Reliable transmission of data frames between two nodes connected by a physical layer | PPP, IEEE 802.2, L2TP | |
| 1. Physical | Bit | Transmission and reception of raw bit streams over a physical medium | DSL, USB | |
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