OSI Reference Model Problem: In the past, networks were built
with different hardware and software; as a result, they were incompatible and difficult to communicate with each other. To make these tasks smooth, in 1977 the International Standards Organization (ISO) proposed the Open Systems Interconnection (OSI) network model. The Open Systems Interconnection (OSI) model breaks down the problems involved in moving data from one computer to another computer. Open Systems Interconnection (OSI) model categorizes these hundreds of problems to Seven Layers. A layer in Open Systems Interconnection (OSI) model is a portion that is is used to categorize specific problems. It is data communication model. It is logical and conceptual model. It is reference model or teaching model. Each layer can only talk to the one above it and below it. It is a theoretical model. It is not a technology technology,, it is not a protocol protocol,, it is not a program or software software.. It specifies how layers should talk to each other. It defines how information should be handled when being transported over a network. This model has 7 layers: 7 Application Layer Data 6 Presentation Layer Data 5 Session Layer Data 4 Transport Layer Segments 3 Network Layer Packets 2 Data Link Layer Frames 1 Physical Layer Bits 7. Application Layer 6. Presentation Layer 5. Session Layer 4. Transport Layer 3. Network Layer 2. Data link Layer 1. Physical Layer
7) Application Layer Deals with the communication of software on different machines. Such as HTTP, FTP, TFTP, SMTP, Telnet, SHH, DHCP, POP and DNS servers, all operating systems, web browsers, Firewalls, communication software (messengers, Skype etc.)
6) Presentation Layer Three activities are taking place at this layer: 1) Encryption: The process of converting the plain text in to cipher text for data confidentiality is called encryption. 2) Translation: Coverts protocol from one form to other. Like IPX to IP. 3) Compression: It simply works with the compression of data like win zip. Encoding, Decoding, Encryption, Decryption, Compression and Decompression 5) Session Layer A time period in which two machines communicate is called a session. This layer deals with: Session establishment: To establish a session sessio n before data communication. Session management: To manage the session throughout the communication. Session termination: To manually terminate the session sessio n or network down. 4) Transport Layer It uses protocols to transfer data from one machine to another machine. Two protocols work on this layer, TCP and UDP and port Numbers. Sequencing and Reassembling Error Correction Flow Control Total Ports: 0 – 65535 Server Ports: 1 – 1023 Client Ports: 1024 - 65535 TCP (Transmission Control Protocol) 1.TCP is connection-oriented. 2.Reliable communication with Acknowledgment. 3.Slower data transportation. 4.TCP allows for error detection. 5.TCP allows "windowing". SMTP, FTP, and HTTP use TCP.
UDP (User Datagram Protocol) 1.UDP is connection-less. 2.Unreliable communication without Acknowledgment. 3.Faster data transportation. 4.UDP allows best-effort delivery. 5.UDP delivers segments. 6.UDP allows no error detection. 7.UDP offers neither error detection nor error recovery. DHCP, SNMP, and TFTP use UDP. DNS uses both TCP and UDP ports. DNS uses TCP for zone exchanges between servers and UDP when a client is trying to resolve a hostname to an IP address.
3) Network Layer It deals with the function of path selection and logical addressing. Routed protocols and Routing protocols all works on this layer Routers, multilayer switches (L3) run at Network layer. 2) Data Link Layer ARP Media Access Control (MAC) address Switch, bridge, NIC card operate at Data Link layer. Protocols and services run at Data Link layer 1) Physical Layer It is pure hardware layer of OSI model. On this layer frame is converted into bits. Physical layer specifies cables, pins, connectors, voltage, wire-speed and moves bits between devices. Hubs, modems, repeaters and all cables operate at Physical layer.
Data on different layers Data on Transport layer is called a segment. Data on Network layer is called a packet . Data on Data link layer is called a frame frame.. Data on Physical layer is converted into bits bits..
