The Seven Layers of OSI Model – Their Protocols and Functions
OSI model is developed by ISO (International Standards Organization) , that cover all aspects of networks communications. Open system is a model that allows any two different systems to communicate regardless of their underlying architecture. The OSI model does not perform any functions in the communication process. The actual work is done by the appropriate S/W and H/W. The OSI model simply define which tasks need to be done and which protocols will handle those tasks , at each of the seven layers of the model.The Seven Layers of OSI Model – Their Protocols and Functions-
- Physical Layer
- Data Link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
The OSI Model Layered Architecture :-
The seven layers can be thought of as belonging to three subgroups. Layers 1,2 and 3- physical, data link and network are the network support layers; they deal with the physical aspect of moving data from one device to another . Layers 5,6 and 7 can be are thought of as the user support. Layer they allow interoperability among unrelated s/w system. Layer 4 the transport layer ensure reliable data transmission. the upper OSI layer are almost always implemented in software , lowers layers are a combination of hardware & software except for the physical layer which is mostly hardware .
Functions of Layers OSI Model : –
The Physical Layer is simply responsible for sending bits from one computer to another. It deals with the physical connection to the network and with transmission and reception of signals.
The following items are addressed at the physical layer:-
- Network connection types
- Physical topologies
- Analog and digital signaling
- Base band and Broadband transmission, which are different methods for using media band width
- Termination of signals.
Physical Layer Protocol – PLC, Wi-Fi, Ethernet Physical Layer, Modems etc
2.Data Link Layer –
The data link layer provides for the flow of data over a single link form one device to another. It accepts packets from the network Layer and packages the information data units into data units called frames to be presented to the physical Layer for transmission.
- The data link layer adds control information, such as frame type, routing and segmentation, information, to the data being sent.
- It also provides flow control.
- It adds reliability to the physical layer by adding mechanism to detect and re-transmit damage or lost frames.
Data Link Layer Protocol – PPP, HDLC, ISDN,FDDI, ARP etc
3. Network Layer –
The network layer is responsible for the source to destination delivery of a packet across multiple networks.
This layer is concerned with the following
- Addressing, including logical network address and service address.
- Circuit, message and packet switching.
- It also takes routing decisions.
Network Layer Protocol – IP Protocal, IPv4, IPv6, ICMP CIDR, NAT, ARP, RARP, BOOTP etc
4.Transport Layer –
The transport layer ensure that packets are delivered error free, in sequence and with no losses or duplication. The transport layer breaks large messages from the session layer into packets to be sent to the destination computer and reassembles packets into messages to be presented to the session layer.
- The transport layer typically sends an acknowledgment to the sender.
- Break data stream in smaller segments and provide reliable and unreliable data deliver.
Transport Layer Protocol – TCP, UDP,DCCP, SCTP, RSVP, ECN etc
5.Session Layer –
The session layer allows applications on separate computers to share a connection called a session. The session layer also provides for data synchronization and check pointing so that in case of network failure , only the data sent after the point of failure need be re- sent.
This layer also controls the dialog between two processes, determining who can transmit and who can receive at what point during the communication.Initiate and terminate session with remote system.
Session Layer Protocol -SMPP, BIOS,ADSP,L2TP, F2F, PAP, PPTF, RPC, SDP etc
The presentation layer translates data between the formats the network require and the formats the computer expects. The presentation layer does protocol conversion, data translation compression and encryption.
Encrypt, format and compress the data for transmission.
Presentation Layer Protocol – ASCII, EBCDIC, ICA, LPP, NCP, NDR, AFP etc
The application layer is the top most layer of OSI model, and it provides that directly support user applications such as database access. E-mail and file transfers.Provide user interface to send and receive the data.
Application Layer Protocol – HTTP, SMTP, POP3, FTP, TELNET, etc.