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Transcript
In Seven Layers of Open Systems Interconnection (OSI) Model lesson, you will learn about the seven layers of OSI model and their functions. Student A read introduction and Layers 1,2 and 3 Student B read about layers 4,5,6, and 7 Student C read about Layers 4 and 3 in the Four layers of TCP/IP model text below Student D read about layers 2 and 1 in the Four layers of TCP/IP model text below Student A: If network communications need to happen without any trouble, many problems must be solved. Coordinating all these problems is complex and not easy to manage. 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. This model categorizes these hundreds of problems into seven layers. A layer in Open Systems Interconnection (OSI) model is a portion that is used to categorize specific problems. Open Systems Interconnection (OSI) Seven Layered reference model is only a reference model. All the problems which are related to the communications are answered by specific protocols operating at different layers. The following image shows the seven layers described. Seven Layers of Open Systems Interconnection (OSI) Model Layer 1. Physical Layer The first layer of the seven layers of Open Systems Interconnection (OSI) network model is called the Physical layer. Physical circuits are created on the physical layer of Open Systems Interconnection (OSI) model. Physical layers describe the electrical or optical signals used for communication. Physical layer of the Open Systems Interconnection (OSI) model is only concerned with the physical characteristics of electrical or optical signalling techniques which includes the voltage of the electrical current used to transport the signal, the media type (Twisted Pair, Coaxial Cable, Optical Fibre, etc.), impedance characteristics, physical shape of the connector, Synchronization etc. The Physical Layer is limited to the processes needed to place the communication signals over the media, and to receive signals coming from that media. The lower boundary of the physical layer of the OSI model is the physical connector attached to the transmission media. The physical layer of the does not include the transmission media. Transmission media stays outside the scope of the Physical Layer and are also referred to as Layer 0. Layer 2. Datalink Layer The second layer of the seven is called the Datalink layer. The Data Link layer resides above the Physical layer and below the Network layer. Datalink layer is responsible for providing end-toend validity of the data being transmitted. The Data Link Layer is logically divided into two sublayers, The Media Access Control (MAC) Sublayer and the Logical Link Control (LLC) Sublayer. Media Access Control (MAC) Sublayer determines the physical addressing of the hosts. The MAC sub-layer maintains MAC addresses (physical device addresses) for communicating with other devices on the network. MAC addresses are burned into the network cards and constitute the low-level address used to determine the source and destination of network traffic. MAC Addresses are also known as Physical addresses, Layer 2 addresses, or Hardware addresses. The Logical Link Control sublayer is responsible for synchronizing frames, error checking, and flow control. Layer 3. Network Layer The third layer is the Network layer. The Network layer is responsible for managing logical addressing information in the packets and the delivery of those packets to the correct destination. Routers, which are special computers used to build the network, direct the data packet generated by Network Layer using information stored in a table known as a routing table. The routing table is a list of available destinations that are stored in memory on the routers. The network layer is responsible for working with logical addresses. The logical addresses are used to uniquely identify a computer on the network, but at the same time identify the network that system resides on. The logical address is used by network layer protocols to deliver the packets to the correct network. The Logical addressing system used in Network Layer is known as IP address. IP addresses are also known as Logical addresses or Layer 3 addresses. Student B Layer 4. Transport Layer The fourth layer is the Transport layer. The Transport layer handles transport functions such as reliable or unreliable delivery of the data to the destination. On the sending computer, the transport layer is responsible for breaking the data into smaller packets, so that if any packet is lost during transmission, the missing packets will be sent again. Missing packets are determined by acknowledgments (ACKs) from the remote device, when the remote device receives the packets. At the receiving system, the transport layer will be responsible for opening all of the packets and reconstructing the original message. Another function of the transport layer is TCP segment sequencing. Sequencing is a connectionoriented service that takes TCP segments that are received out of order and place them in the right order. The transport layer also enables the option of specifying a "service address" for the services or application on the source and the destination computer to specify what application the request came from and what application the request is going to. Many network applications can run on a computer simultaneously and there should be some mechanism to identify which application should receive the incoming data. To make this work correctly, incoming data from different applications are multiplexed at the Transport layer and sent to the bottom layers. On the other side of the communication, the data received from the bottom layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using "Port Numbers". The protocols operating at the Transport Layer, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) uses a mechanism known as "Port Number" to enable multiplexing and de-multiplexing. Port numbers identify the originating network application on the source computer and destination network application on the receiving computer. Layer 5. Session Layer The position of Session Layer is between Transport Layer and the Presentation Layer. Session layer is the fifth layer. It is responsible for establishing, managing, and terminating connections between applications at each end of the communication. In the connection establishment phase, the service and the rules (who transmits and when, how much data can be sent at a time etc.) for communication between the two devices are proposed. The participating devices must agree on the rules. Once the rules are established, the data transfer phase begins. Connection termination occurs when the session is complete, and communication ends gracefully. In practice, Session Layer is often combined with the Transport Layer. Layer 6. Presentation Layer The position of Presentation Layer is just below the Application Layer. When the presentation layer receives data from the application layer, to be sent over the network, it makes sure that the data is in the proper format. If it is not, the presentation layer converts the data. On the other side of communication, when the presentation layer receives network data from the session layer, it makes sure that the data is in the proper format and once again converts it if it is not. Formatting functions at the presentation layer may include compression, encryption, and ensuring that the character code set (ASCII, Unicode, EBCDIC (Extended Binary Coded Decimal Interchange Code, which is used in IBM servers) etc.) can be interpreted on the other side. For example, if we select to compress the data from a network application that we are using, the Application Layer will pass that request to the Presentation Layer, but it will be the Presentation Layer that does the compression. Layer 7. Application Layer The Application Layer is the seventh layer in the OSI network model. This is the top-most layer. Real traffic data will be often generated from the Application Layer. This may be a web request generated from HTTP protocol, a command from telnet protocol, a file download request from FTP protocol etc. In this lesson (Seven Layers of Open Systems Interconnection (OSI) Model), you have learned what are the Seven Layers of Open Systems Interconnection (OSI) Model and the functions of these seven layers. The top-most layer of the Seven Layers of Open Systems Interconnection (OSI) Model is the Application Layer and the bottom-most layer of the Seven Layers of Open Systems Interconnection (OSI) Model is Physical Layer. Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models Student C: Like OSI network model, TCP/IP also has a network model. TCP/IP was on the path of development when the OSI standard was published and there was interaction between the designers of OSI and TCP/IP standards. The TCP/IP model is not the same as the OSI model. OSI is a seven-layered standard, but TCP/IP is a four layered standard. The OSI model has been very influential in the growth and development of TCP/IP standard, and that is why much OSI terminology is applied to TCP/IP. The following figure compares the TCP/IP and OSI network models. As we can see from the above figure, presentation and session layers are not there in TCP/IP model. Also note that the Network Access Layer in TCP/IP model combines the functions of Datalink Layer and Physical Layer. Layer 4. Application Layer The Application layer is the top most layer. It is present on the top of the Transport layer. It defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network. Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc. Layer 3. Transport Layer Transport Layer is the third layer. The position of the Transport layer is between Application layer and Internet layer. The purpose of the Transport layer is to permit devices on the source and destination hosts to carry on a conversation. It defines the level of service and status of the connection used when transporting data. The main protocols included in the Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Student D: Layer 2. Internet Layer The internet Layer is the second layer. Its position is between the Network Access Layer and Transport layer. The Internet layer packs data into data packets known as IP datagrams, which contain a source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for the routing of IP datagrams. A packet switching network depends upon a connectionless internetwork layer. This layer is known as the Internet layer. Its job is to allow hosts to insert packets into any network and have them deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to the proper network applications operating at the Application layer. The main protocols included at Internet layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol) and IGMP (Internet Group Management Protocol). Layer 1. Network Access Layer The Network Access Layer is the first layer. It defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire. The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc. The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media when Ethernet operates in a shared media. An Access Method determines how a host will place data on the medium. IN CSMA/CD Access Method, every host has equal access to the medium and can place data on the wire when the wire is free from network traffic. When a host wants to place data on the wire, it will check the wire to find whether another host is already using the medium. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be retransmitted. After collision, each host will wait for a small interval of time and again the data will be retransmitted. Text Swapping: 1. Now get together with another student who has read the same text as you and discuss what is was about. 2. Now students A and B get together and tell each other about your text Students C and D do the same 3. Now C’s and A’s make pairs and tell each other about both your text and the one your partner told you about Students B and D do the same Vocabulary The Seven Layer Model Find an underlined word in the text that means a. When many signals are put together into a single complex one. b. How well a concept or function works, how much it does what it is supposed to do. c. The act of sending something d. A small piece of something e. A set of rules that specify how different things should interact or communicate together. f. At the same time g. The resistance of an electric circuit or a component in one. The Four Layer Model a. when two systems or programmes interact b. a pause c. two wires wrapped around each other d. bump together e. Allow or let f. a network in which data is broken down into small units so it can be sent along many different paths.