Tiêu đề Unit IV.pdf ✅

Unit IV: Computers and Communication Computer Network: A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. Software modules in one system are used to communicate with one or more software modules in the distance System. Such interfaces across a distance are termed as “peer-to-peer” interfaces; and the local interfaces are termed as “service” interfaces. The modules on each end are organized as a sequence of functions called “layers”. The set of modules organized as layers is also commonly called a “protocol stack”. Over the years, some layered models have been standardized. The ISO Open Systems Interconnection (ISO/OSI) layered model has seven layers and was developed by a set of committees under the auspices of International Standards Organization (ISO). Classification of Computer Networks: 1) Based on Transmission Mode: Transmission mode defines the direction of signal flow between two linked devices. There are three types of transmission modes. A) Simplex: In simplex mode, the communication is unidirectional. Among the stations only one can transmit and the other can only receive. B) Half-Duplex: In half-Duplex mode, the communication is bidirectional. In this both station can send and receive but not at the same time. C) Full-Duplex: In Full-Duplex mode, both stations can transmit and receive simultaneously. 2. Based on Time in Transmission Type: A) Synchronous Transmission: In synchronous Transmission both the sender and the receiver use the same time cycle for the transmission. We send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits. Bit stream is delivered with a fixed delay and given error rate. Each bit reaches the destination with the same time delay after leaving the source.
B) Asynchronous Transmission: In Asynchronous Transmission we send one start bit at the beginning and one stop bit at the end of each byte. There may be a gap between each byte. Bit stream is divided into packets. Packets are received with varying delays, so packets can arrive out of order. Some packets are not received correctly. 3. Based on Authentication: A) Peer to Peer Connection: In peer-to-peer networks, there are no dedicated servers. All the computers are equal and, therefore, are termed as peers. Normally, each computer functions as both a client and a server. No one can control the other computers. B) Server Based Connection: Most networks have a dedicated server. A dedicated server is a computer on a network which functions as a server, and cannot be used as a client or a workstation. A dedicated server is optimized to service requests from network clients. A server can control the clients for its services. 4) Based on Size: A) Local Area Networks (LAN): LAN is a small high speed network. In LAN few numbers of systems are interconnected with networking device to create network. As the distance increases between the nodes or system it speed decreases. So it is limed to few meters only. Networks which cover close geographical area. LAN used to link the devices in a single office, building or campus. It provides high speeds over short distance. Systems are connecting directly to Network. The LAN is owned by private people. Example: A computer lab in a school
B) Wide Area Network (WAN): WAN is collection of network (or LAN). This network speed is less than the LAN network speed.WAN network connects systems indirectly. WAN spread over the world may be spread over more than one city country or continent. Systems in this network are connected indirectly. Generally WAN network are slower speed than LAN’s. The WAN network are owned or operated by network providers. If it is owned by a single owner then it is called Enterprise network. Often these types have combination of more than one topology. Best example is the internet. C) MAN (Metropolitan Area Network): Metropolitan area network is an extension of local area network to spread over the city. It may be a single network or a network in which more than one local area network can share their resources. Example: Organization with different branches located in the city. Other types: A) WLAN (Wireless LAN): A LAN that uses high frequency radio waves for communication. Provides short range connectivity with high speed data transmission. B) PAN (Personal Area Network): Network organized by the individual user for its personal use. C) SAN (Storage Area Network): Connects servers to data storage devices via fiber-optic cables. Example: Used for daily backup of organization or a mirror copy 5. Based on Reliability: Reliability is maintained by authentication. A) Connection-oriented: This type of communication establishes a session connection before data can be sent. This method is often called a "reliable" network service. It can guarantee that data will arrive in the same order. B) Connection less: This type of communication does not require a session connection between sender and receiver for data transfer. The sender simply starts sending packets to the destination. A connectionless network provides minimal services.
Topology: Topology refers to physical layout including computers, cables, and other resources; it determines how components communicate with each other. Today’s network designs are based on three topologies:  Bus consists of series of computers connected along a single cable segment  Star connects computers via central connection point or hub  Ring connects computers to form a loop All computers, regardless of topology, communicate by addressing data to one or more computers and transmitting it across cable as electronic signals. Data is broken into packets and sent as electronic signals that travel on the cable. Only the computer to which the data is addressed accepts it. Physical Topology: The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring A) Mesh: A mesh topology is the one where every node is connected to every other node in the network. A mesh topology can be a full mesh topology or a partially connected mesh topology. In a full mesh topology, every computer in the network has a connection to each of the other computers in that network. The number of connections in this network can be calculated using the following formula (n is the number of computers in the network): n(n-1)/2 In a partially connected mesh topology, at least two of the computers in the network have connections to multiple other computers in that network. It is an inexpensive way to implement redundancy in a network. In the event that one of the primary computers or connections in the network fails, the rest of the network continues to operate normally. Advantages of a mesh topology:  Can handle high amounts of traffic, because multiple devices can transmit data simultaneously.  A failure of one device does not cause a break in the network or transmission of data.  Adding additional devices does not disrupt data transmission between other devices. Disadvantages of a mesh topology:  The cost to implement is higher than other network topologies, making it a less desirable option.  Building and maintaining the topology is difficult and time consuming.  The chance of redundant connections is high, which adds to the high costs and potential for reduced efficiency. B) Star: A star network, star topology is one of the most common network setups. In this configuration, every node connects to a central network device, like a hub, switch, or computer. The central network device acts as a server and the peripheral devices act as clients. Depending on the type of network card used in each computer of the star topology, a coaxial cable or a RJ-45 network cable is used to connect computers together. Advantages of star topology:  Centralized management of the network, through the use of the central computer, hub, or switch.  Easy to add another computer to the network.  If one computer on the network fails, the rest of the network continues to function normally.