ISO/OSI MODEL

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summary of ISO/OSI Model and comparison with TCP/IP protocol suite

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ISO/OSI MODEL

  1. 1. OSI MODEL
  2. 2. Introduction  An open system is a model that allows any two different systems to communicate regardless of their underlying architecture.  OSI is not a protocol, it’s a model for understanding and designing a network architecture which should be robust, flexible and interoperable  It’s a layered framework for the design of network that allows communication across all types of computer systems
  3. 3. The Layers
  4. 4.  Seven layers can be classified into three sub groups. 1. Network support layers- layers 1,2,3(physical, data link and network) 2. User support layers- layers 5,6,7(session, presentation, and application) 3. Transport layer
  5. 5. Functions of Physical Layer  Deals with the mechanical and electrical specification of the interface and transmission medium.  Also defines the procedures and functions that physical devices have to perform for transmission  Its mainly concerned with: ◦ Physical characteristic of interfaces and media ◦ Representation of bits ◦ Data rate ◦ Line configuration ◦ Physical topology and transmission media
  6. 6. Functions of data link layer  Responsible for node to node delivery and its main responsibilities are: 1. Framing: converts the stream of bits into small units called as frames 2. Physical addressing: it adds a header to the frame to define the physical address of the sender and/or the receiver of the frame 3. Flow control: implements a flow control mechanism to prevent loading of receiver 4. Error control: it detects and retransmits damaged or lost frames. This is usually done with addition of trailer 5. Access control: it defines the control over link by a device
  7. 7. In Figure, a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection
  8. 8. Functions of Network Layer  Responsible for source to destination delivery of a packet across multiple links  Major role of network layer is: 1. Logical addressing 2. Routing
  9. 9. If we want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.
  10. 10. Functions of transport layer  Responsible for source to destination delivery of entire message (end to end)  Major functions include: 1. Segmentation and reassembly 2. Flow control 3. Error control 4. Connection control
  11. 11. Functions of Session layer  Establishes, maintains and synchronizes the interaction between communicating systems.  Its role includes 1. Dialog control: allows the communication between two process to take place either in half duplex or full duplex 2. Synchronization: allows to add checkpoints in to stream of data
  12. 12. Functions of Presentation Layer  Mainly concerned with syntax and semantics of the information exchanged between two systems  Specific functions include: 1. Translation: provides for interoperability between two different encoding methods 2. Encryption: to assure privacy of the message 3. Compression
  13. 13. Function of Application Layer  Provides user interface and support for services such email, remote file access, shared database management and other types of distributed information services
  14. 14. Summary of OSI Layers
  15. 15. TCP/IP PROTOCOL SUITE The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similarly to the ones in the OSI model.
  16. 16. Figure 2.8 TCP/IP and OSI model
  17. 17. Addressing  Four levels of addresses are used in an internet employing the TCP/IP protocols: physical address, logical address, port address, and application-specific address. Each address is related to a one layer in the TCP/IP architecture.
  18. 18. TCP/IP Protocol Suite 23 Figure 2.15 Addresses in the TCP/IP protocol suite
  19. 19. In Figure a node with physical address 10 sends a frame to a node with physical address 87. As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver. The data link layer at the sender receives data from an upper layer. It encapsulates the data in a frame. Each station with a physical address other than 87 drops the frame because the destination address in the frame does not match its own physical address. Example
  20. 20. TCP/IP Protocol Suite 25 Example: physical addresses Data87 10 1 packet accepted Data87 10 4
  21. 21. As we will see in Chapter 3, most local area networks use a 48- bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below: 07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address
  22. 22. Figure shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks. So each router has three pairs of addresses, one for each connection. The computer with logical address A and physical address 10 needs to send a packet to the computer with logical address P and physical address 95. Example 2.5
  23. 23. TCP/IP Protocol Suite 28 Figure 2.17 Example 2.5: logical addresses DataA P20 10 DataA P20 10 Physical addresses changed DataA P33 99 DataA P33 99 Physical addresses changed DataA P95 66 DataA P95 66
  24. 24. TCP/IP Protocol Suite 29 The physical addresses will change from hop to hop, but the logical addresses remain the same. Note
  25. 25. Figure shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Example 2.6
  26. 26. A Sender Receiver P Internet Figure 2.18 Example 2.6: port numbers a DatajA PH2 a DatajA P a Dataj Data a DatajA PH2 a DatajA P a Dataj Data
  27. 27. As we will see in future chapters, a port address is a 16-bit address represented by one decimal number as shown. Example 2.7 753 A 16-bit port address represented as one single number

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