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To assist you in quickly comparing the layers of the OSI Reference Model, and understanding where they are different and how they relate to each other, I have created the summary chart shown in Table 18. It shows each layer's name and number, describes its key responsibilities, talks about what type of data is generally handled at each layer, and also what the scope of each layer is in approximate terms. I also show some of the more common protocols that are associated with each layer.
I will say here that “standard disclaimers apply”. I don't want to repeat all of the caveats I mentioned elsewhere in discussions of the OSI model, but know that they apply here as well. Namely: the layers aren't always hard-fast, I haven't listed every single protocol here, some may really fit into more than one layer, and so on. In particular, note that many of the technologies listed as being in the data link layer are there because that is the layer where their primary functionality resides. In reality, most of these technologies include components in other layers, especially the physical layer.
Table 18: OSI Reference Model Layer Summary
Group
#
Layer Name
Key Responsibilities
Data Type Handled
Scope
Common Protocols and Technologies
Lower Layers
1
Physical
Encoding and Signaling; Physical Data Transmission; Hardware Specifications; Topology and Design
Bits
Electrical or light signals sent between local devices
(Physical layers of most of the technologies listed for the data link layer)
2
Data Link
Logical Link Control; Media Access Control; Data Framing; Addressing; Error Detection and Handling; Defining Requirements of Physical Layer
Frames
Low-level data messages between local devices
IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI and CDDI; IEEE 802.11 (WLAN, Wi-Fi); HomePNA; HomeRF; ATM; SLIP and PPP
3
Network
Logical Addressing; Routing; Datagram Encapsulation; Fragmentation and Reassembly; Error Handling and Diagnostics
Datagrams / Packets
Messages between local or remote devices
IP; IPv6; IP NAT; IPsec; Mobile IP; ICMP; IPX; DLC; PLP; Routing protocols such as RIP and BGP
4
Transport
Process-Level
Addressing; Multiplexing/Demultiplexing; Connections; Segmentation and
Reassembly;
Acknowledgments and Retransmissions;
Flow Control
Datagrams / Segments
Communication between software processes
TCP and UDP; SPX; NetBEUI/NBF
Upper Layers
5
Session
Session Establishment, Management and Termination
Sessions
Sessions between local or remote devices
NetBIOS, Sockets, Named Pipes, RPC
6
Presentation
Data Translation; Compression and Encryption
Encoded User Data
Application data representations
SSL; Shells and Redirectors; MIME
7
Application
User Application Services
User Data
Application data
DNS; NFS; BOOTP; DHCP; SNMP; RMON; FTP; TFTP; SMTP; POP3; IMAP; NNTP; HTTP; Telnet