Why OSI model was created?In beginning long stretches of PC organizing, sellers made restrictive systems administration models to help their own items. The most serious issue with these models was that they permitted correspondence just between the gadgets those were produced by a similar seller. For instance, a PC fabricated by the IBM had the capacity to connect and successfully communicate with just the PC’s or system administration gadget’s(network devices like hub ,switch ,etc.
) that was additionally made by the IBM.To get through this hindrance, leading equipment fabrication companies agreed to support a merchant unbiased system of communication between devices. To make a merchant unbiased system , two noteworthy efforts were made.The first was fabricated by ISO and then the second one was introduced by DoD. ISO (The International Organization for Standardization) came up the OSI (Open Systems Interconnection) model and DoD (U.S. Branch of Defense) with the TCP/IP display. Benefits of the OSI model Help network administrators to organize and decide the required equipment and programming to fabricate their system. Encourage equipment makers to make the systems administration items that can speak with one another over the universal system. Separate a complicated operation into straightforward parts. Make troubleshooting less demanding, as system administrators can investigate issue more quickly and viably by looking in a layer that is causing issue as opposed to discovering it in whole system.
Application layer is used by network applications Network application means computer applications that use internet like Google chrome Firefox Outlook Skype etc the web browser is a network application running in your PC it does not reside in the application layer but it uses application layer protocols HTTP or HTTPS to do web surfing not only web browser but all Network applications including outlook Skype etc all are dependent on application layer protocols to function there are dozens of application layer protocols that enable various functions at this layer all these protocols collectively for application layer these protocols form the basis for various network services like file transfer web surfing emails virtual terminals etc file transfer is done with the help of FTP protocol web surfing is done with the help of HTTP or HTTPS protocol for emails SMTP protocol is used and for virtual terminals telnet is used so application layer provides services for network applications with the help of protocols to perform user activities
Presentation layer receives data from application layer this data is in the form of characters and numbers presentation layer convert these characters and numbers to machine understandable binary format for example conversion of ascii to absolute this function of presentation layer is called translation before data is transmitted presentation layer reduces the number of bits that are used to represent the original data this bit reduction process is called data compression and it can be lossy or lossless data compression reduces the amount of space used to store the original file as the size of file is reduced it can be received at destination in a very less time that is data transmission can be done faster thus data compression is very helpful in real-time video and audio streaming to maintain the integrity of data before transmission data is encrypted encryption enhances the security of sensitive data at sender side data is encrypted and at receiver side data is decrypted SSL protocol or Secure Sockets Layer protocol is used in presentation layer for encryption and decryption so presentation layer performs three basic functions translation compression and encryption decryption
Suppose you have planned for a party you have hired if helpers ensuring that each activity runs smoothly helpers will help you in setting up assisting cleaning and then closing the party same is the case with session layer session layer helps in setting up and managing connections enabling sending and receiving of data followed by termination of connections or sessions like you hired some helpers for a party session layer two has its own helpers called api’s or application programming interfaces NetBIOS for networked basic inputoutput system is an example of api’s which allows applications on different computers to communicate with each other just before a session or a connection is established with the server server performs a function called authentication.
authentication is the process of verifying who you are for this server uses a username and a password once enter username and password are masked a session or a connection is established between your computer and the server after authenticating the user authorization is checked authorization is the process used by a server to determine if you have permission to access a file if not you will get a message saying you are not authorized to access this page both of these functions authentication and authorization are performed by session layer session layer keeps a track of the files that are being downloaded for example a web page contains text images etc these text and images are stored as separate files on the web server when you request a website in your web browser your web browser offers a separate session to the web server to download each of these text and image files separately these files are received in the form of data packets session layer keeps a track of which data packet belongs to which file either text file or image file and tracks where the received data packet go in this case it goes to the browser that is session layer helps in session management so session layer helps in session management authentication and authorization your web browser performs all functions of session presentation and application layer.
Transport layer controls the reliability of communication through segmentation flow control and error control in segmentation data received from session layer is divided into small data units called segments each segment contains a source and destination spot number and a sequence number port number helps to direct each segment to the correct application and sequence number helps to reassemble segments in the correct order to form correct message at the receiver in flow control transport layer controls the amount of data being transmitted consider our mobile is connected to a server supposed server can transmit data maximum at 100 Mbps and our mobile can process data maximum at 10 Mbps now we are downloading a file from the server but server starts sending data at 50 Mbps which is greater than the rate our mobile can process so mobile phone with the help of transport layer can tell the server to slow down data transmission rate up to 10 Mbps so that no data gets lost similarly if server is sending data at 5 Mbps mobile phone tells the server to increase data transmission rate to 10 Mbps to maintain system performance transport layer also helps in error control if some data does not arrive the destination transport layer uses automatic repeat request schemes to retransmit the lost or corrupted data a group of bits called checksum is added to each segment by the transport layer to find out the received can update segment protocols of transport layer are transmission control protocol or TCP and user Datagram protocol or UDP transport layer performed two types of services connection oriented transmission and connectionless transmission connection oriented transmission is done via TCP while connection less transmission is done via UDP, UDB is faster than TCP because it does not provide any feedback whether data was really delivered whereas TCP provides a feedback therefore lost data can be retransmitted in TCP UDP is used where it does not matter whether we have received all data for example online streaming movies songs games voice over IP TFTP dns etc on the other hand TCP is used we’re full data delivery is must for example world wide web email FTP etc so transport layer is involved in segmentation flow control error control connection oriented and connectionless transmission transport layer passes data segments through the network layer
Network layer works for the transmission of the received data segments from one computer to another located in different networks data units in the network layer are called packets it is the layer where routers recide the function of network layer our logical addressing routing and path determination IP addressing done in network layer is called logical addressing every computer in a network at a unique IP address network layer assigns sender and receivers IP address to each segment to form an IP packet IP addresses are assigned to ensure that each data packet can reach the correct destination the routing is a method of moving data packet from source to destination and it is based on the logical address format of ipv4 or ipv6 suppose computer a is connected to network one and computer B is connected to network two from computer B we have requested to access Facebook or comm and now there is a reply from Facebook server for them to be in the form of packet this packet needs to be delivered to computer B only since in a network each device has a unique IP address so these both computers will be having a unique IP address as well network layer of the Facebook server has already added sender and receivers IP address in the packet suppose mask use is 188.8.131.52 this mask tells that the first 3 combination represents network while the last combination represents host or computer B so based on IP address format received data packet will move first to network to and then to computer B so based on IP address and mask routing decisions are made in a computer network now path determination a computer can be connected to internet server for a computer in a number of waves choosing the best possible path for data delivery from source to destination is called fast determination layer devices use protocols such as open shortest path first(OSPF) border gateway protocol(BGP) intermediate system to intermediate system(IS-IS) to determine the best possible path for data delivery
Data link layer receives data packet from network layer data packets contain IP addresses of sender and receiver there are two kinds of addressing logical addressing and physical addressing logical addressing is done at network layer where sender and receivers IP addresses are assigned to each segment to form a data packet physical addressing is done at data link layer where MA addresses of sender and receiver are assigned to each data packet to form a frame MA address is a 12 digit alphanumeric number embedded in network interface card of your computer by your computer manufacturer data unit in data link layer is called frame data link layer is embedded as software in network interface card of the computer and provide means to transfer data from one computer to another via a local media local media includes copper wire optical fiber or air for radio signals please note here media does not correspond to audio video or animation it refers to the physical links between two or more computers or networks data link layer performs two basic functions it allows upper layers of OSI model to access media using techniques such as framing it controls how data is placed and received from media using techniques such as media access control and error detection consider two distant hosts a laptop and a desktop communicating with each other as laptop and desktop are connected to different networks so they will be using Network layer protocols IP for example to communicate with each other in this example desktop is connected to router r1 via an Ethernet cable router r1 and r2 are connected wire that link and laptop is connected to router r2 via a wireless link now Dexter wants to send some data to laptop based on the medium used to connect desktop and router r1 data link layer adds some data in the head and tail of IP packet and converts it to a prim Ethernet frame in this case router r1 receives this Ethernet frame big up slit into an IP packet and then encapsulate it again to a frame so that it can cross satellite link to reach the router r2 router r2 will again be capsulate the received frame and encapsulate it again to form a wireless data link frame laptop receives this wireless data link frame D capsulate it and then forward IP packet to network layer finally data arrives application layer application layer protocols then make the received data visible on computer screen so network layer or higher-level layers are able to transfer data over media with the help of data link layer data link layer provides access to media for higher layers of OSI model data link layer also controls how data is placed and received from the media the technique used to get the frame on and of the media is called media access control there may be a number of devices connected to a common media if two or more devices connected to same media send data at the same time then there may be a possibility of collision of the two messages resulting in a useless message that neither recipient can understand to avoid these situations data link layer keeps an eye on when the shared media is free so that device can transmit data for the receiver this is called carrier sense multiple access so data link layer with this media access control methods controls data transmission tale of each frame contains bits which are used to detect errors in the received frame errors occur due to certain limitations of the media used for transmitting data
Physical layer till now data from application layer has in segmented by transport layer plays into packets by network layer and framed by datalink layer which is a sequence of binary zeros and ones physical layer convert these binary sequence into signal and transmitted over local media it can be an electrical signal in case of copper cable or LAN cable light signal in case of optical fiber and radio signal in case of air so signal generated by a physical layer depends on the type of media used to connect two devices at the receiverphysical layer receives signals convert it to bits and pass it to data link layer as a frame frame is further decap selected as data moves through higher layers finally data is moved to application layer application layer protocols makes the sender’s message visible in the application in the receivers computer screen in this way OSI model is helping to transfer data between distant hosts so these seven layers of OSI model are the smooth functioning of Internet.
For application data to travel uncorrupted from one host to another, header (or control data), which contains control and addressing information, is added to the data as it moves down the layers. The process of adding control information as it passes through the layered model is called encapsulation.Each layer adds control information at each step. The generic term for data at each level is protocol data unit (PDU), but a PDU is different at each layer. Sending and Receiving Process The data starts on application layer. On the sending side. Then moves down to presentation layer. As data moves into presentation layer data is encoded or compressed to a standard format. Then it move into session layer. At the Session layer session ID is attached to the data. At this point data is still one whole data block. Then data moves into transport layer. At the Transport layer data is broken up into different small blocks or pieces. Each block is added with header, which contains destination port, source port, sequence number and other information. This is called a segment. At the Network layer IP header is added into segment. IP Header contains source IP address, destination IP address and other information. This called a packet. At the Data-Link layer adds new Header and a Trailer. This is called a frame. Frame header contains source MAC address, destination MAC address and other information. Frame trailer used for error checking. At the physical layer frame is converted into bits. The process is reversed on receiver.
Application Layer ” FTP ( File transfer ) HTTP/S ( Web Surfing )SMTP ( Emails)Telnet ( Virtual Terminals )FMTP, SNMP and etc. Presentation Layer ” TLS (Transport Layer Security)AFP (Apple Filling Protocol) SSL (Secure Socket Layer). With help of SSL protocol data is encrypted in presentation layer. Session Layer ” PAP (Passport Authentication Protocol) PPTP (Point-to-Point Tunneling Protocol) RTCP (RTP Control Protocol) SDP (Sockets Direct Protocol) and etc. Transport Layer ” TCP (Transmission Control Protocol) is connection-oriented transmission. UDP (User Datagram Protocol) is connectionless transmission. UDP is faster than TCP. Because UDP doesn’t provide feedback. However, TCP provides feedback and can retransmit missed data. UDP is mainly used in online streaming videos, songs, games and etc. Network Layer ” IP (Internet Protocol) OSPF, BGP and IS-IS protocols are used to determine best path way for data delivery. Data-Link Layer ” DCAP, LACP, PPP, LLDP, CHAP and etc. Physical Layer ” CSU/DSU, DCE, DTE and etc.
Application Layer ” PC’s PhonesServers. Presentation Layer ” GatewaysFirewallsPC’s. Session Layer ” GatewaysFirewallsPC’s. Transport Layer ” GatewaysFirewallsPC’s. Network Layer ” RoutersRouters3-layer switches. Data-Link Layer ” BridgesModemsNetwork cards2-layer switches. Physical Layer ” HubsRepeatersCablesFibersWireless.
Getting to Know the OSI Model for the CCNA Exam. Retrieved from Guide to OSI and TCP/IP models. Retrieved from Computer Networking Notes. Retrieved from