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Half Duplex, In half duplex mode each station can both transmit and receive but not at the same time. When one device is sending the other can only receive and vice versa. The half duplex mode is like a one lane road with traffic allowed in both directions When cars. are traveling in one direction cars going the other way must wait. In a half duplex transmission the entire capacity of a channel is taken over by whichever of the. two devices is transmitting at the time,Full Duplex. In full duplex mode both stations can transmit and receive simultaneously. The full duplex mode is like a two way street with traffic flowing in both directions at the same. One common example of full duplex communication is the telephone network When two. people are communicating by a telephone line both can talk and listen at the same time. 2 What is network topology Explain the different network topologies. The term physical topology refers to the way in which a network is laid out physically I O or more. devices connect to a link two or more links form a topology. There are four basic topologies possible mesh star bus and ring. Mesh Topology, Mesh In a mesh topology every device has a dedicated point to point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. Node 1 must be connected to n 1nodes node 2 must be connected to n 1 nodes and finally. node n must be connected to n 1 nodes We need n n 1 physical links. Type text Page 2,Advantages, The use of dedicated links guarantees that each connection can carry its own data load thus. eliminating the traffic problems that can occur when links must be shared by multiple devices. A mesh topology is robust If one link becomes unusable it does not incapacitate the entire. There is the advantage of privacy or security When every message travels along a dedicated. line only the intended recipient sees it, Point to point links make fault identification and fault isolation easy.
Disadvantages, Amount of cabling and the number of I O ports required. The sheer bulk of the wiring can be greater than the available space in walls ceilings or floors. can accommodate, The hardware required to connect each link I O ports and cable can be prohibitively expensive. Star Topology, In a star topology each device has a dedicated point to point link only to a central controller. usually called a hub, The devices are not directly linked to one another Unlike a mesh topology a star topology does. not allow direct traffic between devices, The controller acts as an exchange If one device wants to send data to another it sends the.
data to the controller which then relays the data to the other connected device. Type text Page 3,Advantages, A star topology is less expensive than a mesh topology. In a star each device needs only one link and one I O port to connect it to any number. of others This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed and additions moves and deletions involve only one. connection between that device and the hub, Other advantages include robustness If one link fails only that link is affected All other links. remain active,Disadvantages, The dependency of the whole topology bon one single point the hub If the hub goes down the. whole system is dead, Although a star requires far less cable than a mesh each node must be linked to a central hub. Bus Topology, The preceding examples all describe point to point connections A bus topology on the other hand is.
multipoint One long cable acts as a backbone to link all the devices in a network. Type text Page 4,Advantages,Bus topology includes ease of installation. A bus uses less cabling than mesh or star topologies. Disadvantages,Difficult reconnection and fault isolation. A fault or break in the bus cable stops all transmission even between devices on the same side. of the problem,Ring topology, Ring Topology In a ring topology each device has a dedicated point to point connection with only the. two devices on either side of it A signal is passed along the ring in one direction from device to device. until it reaches its destination,Advantages,A ring is relatively easy to install and. reconfigure Fault isolation is simplified,Disadvantages.
Unidirectional traffic can be a disadvantage, In a simple ring a break in the ring such as a disabled station can disable the entire network. Type text Page 5,Hybrid Topology, A network can be hybrid For example we can have a main star topology with each branch connecting. several stations in a bus topology, 3 What are the different types of networks Explain in detail. Today when we speak of networks we are generally referring to two primary categories. Local area networks LAN and wide area networks WAN. The category into which a network falls is determined by its size. A LAN normally covers an area less than 2 mi a WAN can be worldwide Networks of a size in. between are normally referred to as metropolitan area networks and span tens of miles. A local area network LAN is usually privately owned and links the devices in a single office building or. campus LANs are designed to allow resources to be shared between personal computers or. workstations The resources to be shared can include hardware e g a printer software e g an. application program or data A common example of a LAN found in many business environments links. a workgroup of task related computers for example engineering workstations or accounting PCs. A wide area network WAN provides long distance transmission of data image audio and video. information over large geographic areas that may comprise a country a continent or even the whole. A metropolitan area network MAN is a network with a size between a LAN and a WAN It normally. covers the area inside a town or a city It is designed for customers who need a high speed connectivity. normally to the Internet and have endpoints spread over a city or part of city A good example of a MAN. is the part of the telephone company network that can provide a high speed DSL line to the customer. Type text Page 6, 4 Explain the OSI reference model with neat diagram important question. An open system is a set of protocols that allows any two different systems to communicate regardless of. their underlying architecture, 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,Physical Layer, The physical layer coordinates the functions required to carry a bit stream over a physical medium It. deals with the mechanical and electrical specifications of the interface and transmission medium. The physical layer is also concerned with the following. 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,Type text Page 7, 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 I s 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. 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, 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 every device is connected to every other. device a star topology devices are connected through a central device a ring topology each. device is connected to the next forming a ring a bus topology every device is on a common. link or a hybrid topology this is a combination of two or more topologies. 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. Data Link Layer, The data link layer transforms the physical layer a raw transmission facility to a reliable link It makes. the physical layer appear error free to the upper layer network layer. Other responsibilities of the data link layer include the following. 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 If. Type text Page 8, the frame is intended for a system outside the sender s network the receiver address is the. address of the device that connects the network to the next one. 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, 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. Network Layer, The network layer is responsible for the source to destination delivery of a packet possibly across. multiple networks links Whereas the data link layer oversees the delivery of the packet between two. systems on the same network links the network layer ensures that each packet gets from its point of. origin to its final destination, Other responsibilities of the network layer include the following. Logical addressing The physical addressing implemented by the data link layer handles the. addressing problem locally If a packet passes the network boundary we need another. addressing system to help distinguish the source and destination systems 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 We discuss logical addresses later in this chapter. Routing When independent networks or links are connected to create internetworks network. of networks or a large network the connecting devices called routers or switches route or. switch the packets to their final destination One of the functions of the network layer is to. provide this mechanism,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 Whereas the network layer oversees source to destination. delivery of individual packets it does not recognize any relationship between those packets. Other responsibilities of the transport layer include the following. Service point addressing Computers often run several programs at the same time For this. reason source to destination delivery means delivery not only from one computer to the next. Type text Page 9, but also from a specific process running program on one computer to a specific process. running program on the other The transport layer header must therefore include a type of. address called a service point address or port address The network layer gets each packet to. the correct computer the transport layer gets the entire message to the correct process on that. 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 connection oriented. A connectionless transport layer treats each segment as an independent packet and delivers it. to the transport layer at the destination machine A connection oriented 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. Flow control Like the data link layer the transport layer is responsible for flow control. Type text Page 1 DATA COMMUNICATION AND NETWORKS IMPORTANT QUESTIONS WITH ANSWERS 1 Explain the types of transmission modes Communication between two devices can be simplex half duplex or full duplex

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