Computer Networks and Data Comuncation-LEC4 PDF

Open Systems Interconnection Reference Model OSI RM  LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there...

Open Systems Interconnection Reference Model OSI RM  LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. Topics discussed in this section: Sender, Receiver, and Carrier Hierarchy 2.2  Figure 2.1 Tasks involved in sending a letter 2.3  Sender, Receiver, and Carrier At the Sender Site Let us first describe, in order, the activities that take place at the sender site. Higher layer. The sender writes the letter, inserts the letter in an envelope, writes the sender and receiver addresses, and drops the letter in a mailbox. Middle layer. The letter is picked up by a letter carrier and delivered to the post office. Lower layer. The letter is sorted at the post office; a carrier transports the letter. 2.4  Sender, Receiver, and Carrier The Way The letter is then on its way to the recipient. On the way to the recipient's local post office, the letter may actually go through a central office. In addition, it may be transported by truck, train, airplane, boat, or a combination of these. 2.5  Sender, Receiver, and Carrier At the Receiver Site o Lower layer. The carrier transports the letter to the post office. o Middle layer. The letter is sorted and delivered to the recipient's mailbox. o Higher layer. The receiver picks up the letter, opens the envelope, and reads it. 2.6 OSI Reference Model ⬡ OSI is a set of Layers that allows any two different systems to communicate regardless of their underlying architecture. The purpose of the OSI ⬡ The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. ⬡ The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable. BENEFITS OF USING A LAYERED MODEL ⬡ It divides the network communication process into Layers, so easier to troubleshooting. ⬡ It allows multiple-vendor development through standardization of network components ⬡ It allows various types of network hardware and software to communicate. ⬡ Changes in one layer do not affect other layers because of layer separation –Layers interact with each other. Why a Layered Network Model? ⬡ Reduces complexity ⬡ Standardizes interfaces ⬡ Facilitates modular engineering ⬡ Simplifies teaching and learning  OSI (7-Seven Layers) Application Presentation Session Transport Network Data Link Physical Figure 2.4 An exchange using the OSI model 2.12  7. Application Layer The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as – Network virtual terminal. A network virtual terminal is a software version of a physical terminal, and it allows a user to log on to a remote host. – File transfer, access, and management. – Mail services. This application provides the basis for e-mail forwarding and storage. – Directory services. This application provides distributed database sources and access for global information about various objects and services.  6. Presentation Layer Specific responsibilities of the presentation layer include the following: – Translation: the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format. – Encryption: Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form. – Compression: Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video  5. Session Layer Specific responsibilities of the session layer include the following: – Dialog control(determine transmission mode). The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode. – Synchronization. The session layer allows a process to add checkpoints, or synChronization points, to a stream of data. – For example, if a system is sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. In this case, if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. Pages previous to 501 need not be resent. – Authentication and Authorization Data flow  Transport Layer The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host. The transport layer, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.  Transport Layer Cont. Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission. Connection control. The transport layer can be either connectionless or connectionoriented. – A connectionless(UDP) transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. – A connectionoriented(TCP) transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.  Transport Layer Cont. Flow control. Like the data link layer, the transport layer is responsible for flow control. However, flow control at this layer is performed end to end rather than across a single link. Error control. Like the data link layer, the transport layer is responsible for error control. However, error control at this layer is performed process-to process rather than across a single link. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage, loss, or duplication). Error correction is usually achieved through retransmission.  Network Layer The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). Other responsibilities of the network layer include the following: – Logical addressing: The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver. – Routing. When independent networks or links are connected to create intemetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination.  Data Link Layer Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames. Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.  Data Link Layer Cont. Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time. Figure 2.7 Hop-to-hop delivery 2.30  Figure 2.7 Hop-fa-hop delivery To send data from A to F, three partial deliveries are made. – First, the data link layer at A sends a frame to the data link layer at B (a router). – Second, the data link layer at B sends a new frame to the data link layer at E. – Finally, the data link layer at E sends a new frame to the data link layer at F. Note that the frames that are exchanged between the three nodes have different values in the headers. – The frame from A to B has B as the destination address and A as the source address. – The frame from B to E has E as the destination address and B as the source address. The frame from E to F has F as the destination address and E as the source address. – The values of the trailers can also be different if error checking includes the header of the frame.  Physical Layer Physical characteristics of interfaces and medium. The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium. Representation of bits. The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals-- electrical or optical. The physical layer defines the type of encoding (how Os and Is are changed to signals). Data rate. The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts. Synchronization of bits. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.  Physical Layer Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices. Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology, a star topology, a ring topology, a bus topology, or a hybrid topology Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. – In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication. – In the half-duplex mode, two devices can send and receive, but not at the same time. – In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time TCP/IP PROTOCOL SUITE  2-4 TCP/IP PROTOCOL SUITE The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. Topics discussed in this section: Physical and Data Link Layers Network Layer Transport Layer Application Layer 2.39 Figure 2.16 TCP/IP and OSI model 2.41  2-5 ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific. Topics discussed in this section: Physical Addresses Logical Addresses Port Addresses Specific Addresses 2.42 Figure 2.17 Addresses in TCP/IP 2.43 Figure 2.18 Relationship of layers and addresses in TCP/IP 2.44 Section 4 Section 4 Cont. Thanks! Any questions?