OSI Layers: An Introduction to the Open Systems Interconnection Model and 7 layers

For understanding of networks and before jumping into Cisco stuff, it’s very necessary to understand OSI layer architecture. ISO standardized the network sharing process into 7 different layers and that complete model is called OSI layer model. OSI stands for Open System Interconnection and was standardized by ISO(International Organization for Standardization).

OSI has a seven layer architecture and each layer function independently but they are dependent on each other for transfer of data from one system to other. OSI layer model is explanation of logical sequence of logical layers to

perform a data transfer. OSI layer model is an explanation of logical sequence of logical layers performed for a data transfer. Independent functioning means one layer is not related to the other for functioning of that particular layer. But they are dependent on each other for successful transmission of data, if one layer fails, whole architecture fails and data can not be transferred. It can be understood with an example, Jack and Ellis are part of a relay race. Jack runs the first round and then Ellis takes it over and she runs the second. They have to run independently in their rounds but if Jack does not complete his first round, Ellis can’t start her run. Hence they are independent in their individual rounds but when it comes to complete race, they are dependent on each other. In the same way, each layer of OSI is independent of other but when it comes to complete data transfer and its reading all layers have to function correctly.

Each layer has a different function and a list of protocols to perform that particular function. It will take years for someone to master even some of these layers, if not all. Because each layer has got a huge number of protocols and each protocol has a great depth of engineering in it. Most of the networker are interested in first 3 layers i.e. Physical, Data Link and Network layer. Please find below the summary of each layer functions and data unit:

	Layer	Summary of Function	Data Unit	Protocols Example
7	Application	Interface to user	Data	FTAM, X.400, X.500, NNTP, SIP, SSI, DNS, FTP
6	Presentation	Data representation, encryption and decryption		ISO/IEC 8823, X.226, TDI, ASCII, EBCDIC, MIDI, MPEG
5	Session	Session establishment, session tearing		Sockets. Session establishment in TCP, RTP
4	Transport	End to end connection and reliability	Segments	TCP, UDP, SCTP, DCCP
3	Network	Path Determination, Logical Addressing	Packet/Datagram	IP, IPsec, ICMP, IGMP, OSPF
2	Data Link	error check, physical address mapping	Frame	PPP, SLIP, PPTP, L2TP,802.3 (Ethernet), 802.11a/b/g/n MAC/LLC, 802.1Q (VLAN), ATM, HDP, FDDI, Fibre Channel, Frame Relay, HDLC, ISL, PPP, Q.921, Token Ring, CDP, ARP
1	Physical	Signal, bits, bytes, binary transmission	Bit	X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, UMTS Physical Layer or L1

Each layer has different data unit i.e. data on each layer is packed into different units and above is the list of their names. It is an industry standard to present layers in reverse order. Application layer is always written at the top and then presentation and it goes on till Physical layer.

Note: The standard documents that described the OSI model could be freely downloaded from the ITU-T as the X.200-series of recommendations.