ISO’s(International Organization for Standardization) OSI (Open Systems Interconnection) Model

ISO’s(International Organization for Standardization) 

OSI (Open Systems Interconnection) Model 

Layer	PDUs		Remarks, Examples
7	Application		WWW, E-mail gateways, user interface. Also responsible for understanding the resources needed to communicate between two devices and establish their availability. SMTP, FTP.
6	Presentation		Translates and converts data into a known format such as ASCII, JPEG, MIDI, MPEG, encryption, compression. The only layer that can actually change data.
5	Session	Data	Keeps different applications’ data separate, NFS, SQL, RPC, NetBIOS names, X Window. Offers three modes – full-duplex, half-duplex and simplex. Maintains communication channels and provides dialogue control. Managing, setting up and tearing down sessions.
4	Transport	Segments	Reliable or unreliable delivery, error correction before retransmit, TCP/UDP. Performs flow control, end-to-end connection. Port numbers are used at this layer. Multiplexing, teardown of virtual circuits. Reassemble the data stream.
3	Network	Data and Route Update Packets or Datagrams	Logical addressing, routing, IP and IPX. Route update packets are sent at this layer, in addition to the data packets. Layer 3 devices such as routers break up broadcast domains and collision domains.
2	Data-Link	Frames	Layer 2 devices such as switches or bridges break up collision domains whereas hubs do not. Uniquely identifies each device on a local network. This layer uses service access points, identify network layer protocol used, flow control and sequencing of control bits (LLC – Logical Link Control - sublayer) and deals with the protocol access to the physical medium, network topology and error detection/notification (MAC – Media Access Control - sublayer). A MAC address on a NIC (Network Interface Card) is a 48 bits address formatted in 12 hexa digits grouped in twos as such: AF-98-C0-72-A3-2B
1	Physical	Bits	Moves bits between devices, specify voltages, wire speed and pin-out cables. Hubs are also known as multi-port repeaters and operate at this layer.