OSI Model Layers

OSI Layer Purpose Examples
Application Provides services to network applications. This layer is responsible for determining resource availability, identifying communication peers, and synchronizing communication between the applications. Simple Mail Transport Protocol (SMTP)
File Transfer Protocol (FTP)
Trivial File Transfer Protocol (TFTP)
HyperText Transfer Protocol (HTTP)
Presentation Provides the coding and conversion functions that are applied to the data to/from the application layer. This layer ensures that there is a common scheme used to bundle the data between the two ends. ASCII (text)
EBCDIC (text)
JPEG (image)
GIF (image)
TIFF (image)
MPEG (sound/video)
Quicktime (sound/video)
Session Establishing, maintaining, and terminating communications sessions between upper layer applications. Session Control Protocol (SPC)
Remote Procedure Call (RPC) from Unix
Zone Information Protocol (ZIP) from AppleTalk
Transport Responsible for end-to-end data transmission. Can be either reliable (connection-oriented) or best effort (connectionless). This layer organizes data from various upper layer applications into data streams, handles end-to-end flow control, multiplexing, virtual circuit management, and error checking and recovery. Transmission Control Protocol (TCP) from IP
User Datagram Protocol (UDP) from IP
Network This layer allows both connection-oriented and connectionless data flows to access the network. The network layer addresses help define a network hierarchy. Network devices are normally grouped together based on their common network layer address. Internet Protocol (IP)
Data Link Provides either reliable or best effort transmission of data across a physical medium. Most networks use a best effort data link layer, such as Ethernet. The data Link Layer for LANs provides a physical address to each device called a Media Access Control (MAC) address. MAC addresses are typically burned into the network interface card (NIC). The LAN data link layer also uses a Logical Link Control (LLC) to indicate the type of network layer data that is encapsulated inside the frame. LAN:
Ethernet/IEEE 802.3 (include Fast Ethernet)
802.3z (Gigabit Ethernet)
Token Ring/IEEE 802.5
FDDI (from ANSI)
High-Level Data-link Control (HDLC)
Point-to-Point Protocol (PPP)
Frame Relay
Physical Defines the electrical, mechanical, and functional specifications for maintaining a physical link between network devices. This layer is responsible for such characteristics as voltage levels, timing and clock rates, maximum transmission distances, and the physical connectors used. LAN:
Category 3 cabling (LAN)
Category 5 cabling (LAN)

OSI Model versus TCP/IP Protocol Suite

OSI Model Layer Number OSI Model Layer TCP/IP Protocol Suite Layer Protocol Data Unit Network Device
7 Application Application Data
6 Presentation
5 Session
4 Transport Transport Segment
3 Network Internet Packet (or Data-gram) MultiLayer Switch or Router
2 Data Link Data Link Frame Switch or Bridge
1 Physical Physical Bits Hub

TCP/IP Transport Layer Protocols

Transmission Control Protocol (TCP) is a reliable, connection-oriented, protocol that uses sequence and acknowledgement numbers to provide reliability. TCP verifies that the remote end is listening prior to sending data, using a 3-way handshake: SYN, SYN/ACK, ACK.

User Datagram Protocol (UDP) is a best-effort, connectionless, protocol that does not have sequence or acknowledgement numbers, and does not do far-end verification.


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