Data communication and Networking exams short notes, Lecture notes of Data Communication Systems and Computer Networks

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What is Computer Network?

Computer network is interconnectivity of two or more computer system for purpose of sharing data. A computer network is a communication system much like a telephone system, any connected device can use the network to send and receive information. In essence a computer network consists of two or more computers connected to each other so that they can share resources. Networking arose from the need to share resources in a timely fashion.  Data communications Data communications refers to the transmission of this digital data between two or more computers and a computer network or data network is a telecommunications network that allows computers to exchange data. The physical connection between networked computing devices is established using either cable media or wireless media. The best-known computer network is the Internet.

 Protocol and Standards In Networking

In computer networks, communication occurs between entities in different systems. An entity is anything capable of sending or receiving information. However, two entities cannot simply send bit streams to each other and expect to be understood. For communication to occur, the entities must agree on a protocol. A protocol is a set of rules that govern data communications. It defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are syntax, semantics, and timing.  Syntax. The term syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, a simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself.  Semantics. The word semantics refers to the meaning of each section of bits. How is a particular pattern to be interpreted, and what action is to be taken based on that interpretation? For example, does an address identify the route to be taken or the final destination of the message?  Timing. The term timing refers to two characteristics: when data should be sent and how fast they can be sent. For example, if a sender produces data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost.

Network Topologies The term topology in computer networking 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 links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring.  Bus Topology: The Bus topology consists of a single cable that runs to every work-station. The bus topology is also known as linear bus. In other words, all the nodes (computers and servers) are connected to the single cable (called bus), by the help of interface connectors. This central cable is the back bone of the network and every workstation communicates with the other device through this bus.  Ring Topology: The ring topology connects computers on a single circle of cable. There are no terminated ends. A ring topology connects one host to the next and the last host to the first. The signal travels around the loop in one direction and pass through each computer. Unlike the passive bus topology, each computer acts like a repeater to boost the signal and send it on to the next computer. Because the signal passes through each computer, the failure of one computer can impact the entire network  Star Topology: In the star topology, computers are connected by cable segments to centralized component, called a hub or switch. Signals are transmitted from the sending computer through the hub or switch to all computers on the network. This topology originated in the early days of computing with computers connected to a centralized mainframe computer. It is now a common topology in microcomputer networking. 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.  Mesh Topology: 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. In a mesh topology, Node1 must be connected to n1 nodes, node2 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. In other words, we can say that in a mesh topology, we need n (n  1) / 2.  Hybrid Topology: Before starting about Hybrid topology, we saw that a network topology is a connection of various links and nodes, communicating with each other for transfer of data. We also saw various advantages and disadvantages of Star, Bus, Ring, Mesh. Hybrid, as the name suggests, is mixture of two different things. Similarly, in this type of topology we integrate two or more different topologies to form a resultant topology which has good points (as well as weaknesses) of all the constituent basic topologies rather than having characteristics of one specific topology. This combination of topologies is done according to the requirements of the organization

 Types of Transmission

Data Flow in Communication means how data transfers between devices. Communication means

the transfer or exchange of data between two different devices. In Communication process whenever data transfers it depends on the condition or depends on devices that through which type data will transfer.  Simplex Transmission In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction. Examples are Radio and Television broadcasts. They go from the TV station to your home television.  Half Duplex Transmission 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 Transmission 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 time. 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.

3. Asynchronous Transmission In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1) at the end of each byte. There may be a gap between each byte.

 Definition of Data Transmission Media

Transmission media is a pathway that carries the information from sender to receiver. We use different types of cables or waves to transmit data. Data is transmitted normally through electrical or electromagnetic signals. An electrical signal is in the form of current. An electromagnetic signal is series of electromagnetic energy pulses at various frequencies. These signals can be transmitted through copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different properties like bandwidth, delay, cost and ease of installation and maintenance. Transmission media is also called Communication channel. Transmission media is broadly classified into two groups. Wired or Guided Media or Bound Transmission Media and Wireless or Unguided Media or Unbound Transmission Media. (See the diagram below).

Transmission Channel Parameters Some parameters are required in description of transmission channel. Some of the parameter is discussed below. Bandwidth: The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers. For example, if a composite signal contains frequencies between 2000 and 6000, its bandwidth is 6000 - 2000, or 4000. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. The bandwidth determines the channel capacity. Noisy Channel: In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a noisy channel: Bit Rate: Most digital signals are non-periodic, and thus period and frequency are not appropriate characteristics. Bit rate is used to describe digital signals. The bit rate is the number of bits sent in 1s, expressed in bits per second (bps). Bit Length: The bit length is the distance one bit occupies on the transmission medium. Bit length=propagation speed x bit duration.

1. Guided Transmission Media Wired or Guided Media or Bound Transmission Media: Bound transmission media are the cables that are tangible or have physical existence and are limited by the physical geography. Popular bound transmission media in use are twisted pair cable, co-axial cable and fiber optical cable. Each of them has its own characteristics like transmission speed, effect of noise, physical appearance, cost etc.
2. Unguided Transmission Media (Wireless Transmission Medium) In unguided Transmission Media data signals flows through the air. Wireless or Unguided Media or Unbound Transmission Media: Unbound transmission media are the ways of transmitting data without using any cables. These media are not bounded by physical geography. This type of transmission is called Wireless communication. Nowadays wireless communication is becoming popular. Wireless LANs are being installed in office and college campuses. This transmission uses Microwave, Radio wave, Infrared are some of popular unbound transmission media.  Transmission Media Problems and Impairment: Attenuation Distortion: Attenuation results in loss of energy. When a signal travels through a medium, it loses some of its energy in overcoming the resistance of the medium. The electrical energy in the signal may convert to heat. To compensate for this loss, amplifiers are used to amplify the signal. Crosstalk Crosstalk: is when one line induces a signal into another line. In voice communications, we often hear this as another conversation going on in the background. In digital communication, this can cause severe disruption of the data transfer. Cross talk can be caused by overlapping of bands in a multiplexed system or by poor shielding of cables running close to one another. There are no specific communications standards applied to the measurement of crosstalk.

Encoding Techniques

The data encoding technique is divided into the following types, depending upon the type of

data conversion.

 Analog data to Analog signals − The modulaƟon techniques such as Amplitude ModulaƟon, Frequency Modulation and Phase Modulation of analog signals, fall under this category.  Analog data to Digital signals − This process can be termed as digiƟzaƟon, which is done by Pulse Code Modulation PCMPCM. Hence, it is nothing but digital modulation. As we have already discussed, sampling and quantization are the important factors in this. Delta Modulation gives a better output than PCM.  Digital data to Analog signals − The modulaƟon techniques such as Amplitude ShiŌ Keying ASKASK, Frequency Shift Keying FSKFSK, Phase Shift Keying PSKPSK, etc., fall under this category. These will be discussed in subsequent chapters.  Digital data to Digital signals − These are in this secƟon. There are several ways to map digital data to digital signals. Some of them are. 

Transmission Control Protocol (TCP): Transmission Control Protocol takes care of the

communication between your application software (i.e. your browser) and your network software. TCP is responsible for breaking data down into IP packets before they are sent, and for assembling the packets when they arrive. TCP is for communication between applications. If one application wants to communicate with another via TCP, it sends a communication request. This request must be sent to an exact address. After a "handshake" between the two applications, TCP will set up a "full-duplex" communication between the two applications. The "full-duplex" communication will occupy the communication line between the two computers until it is closed by one of the two applications.

Internet Protocol (IP): Internet Protocol is Connection-Less i.e, it does not occupy the

communication line between two computers. The Network Layer protocol for TCP/IP is the Internet Protocol (IP). It uses IP addresses and the subnet mask to determine whether the datagram is on the local or a remote network. If it is on the remote network, the datagram is forwarded to the default gateway which is a router that links to another network. IP keeps track of the number of transverses through each router that the datagram goes through to reach its destination. Each transvers is called a hop. If the hop count exceeds 255 hops, the datagram is removed and the destination considered unreachable.

 Special Purpose Protocol

The special purpose protocols are the set of protocols design to perform a single task on communication network system. Some of these protocols and their function are listed below: i. HTTP - Hyper Text Transfer Protocol: HTTP takes care of the communication between a web server and a web browser. HTTP is used for sending requests from a web client (a browser) to a web server, returning web content (web pages) from the server back to the client.

ii. HTTPS - Secure HTTP: HTTPS takes care of secure communication between a web server and a web browser. HTTPS typically handles credit card transactions and other sensitive data. iii. SSL - Secure Sockets Layer: The SSL protocol is used for encryption of data for secure data transmission. iv. MIME - Multi-purpose Internet Mail Extensions: The MIME protocol lets SMTP transmit multimedia files including voice, audio, and binary data across TCP/IP networks. v. IMAP - Internet Message Access Protocol: IMAP is used for storing and retrieving e-mails. vi. FTP - File Transfer Protocol: FTP takes care of transmission of files between computers. vii. NTP - Network Time Protocol: NTP is used to synchronize the time (the clock) between computers. viii. DHCP - Dynamic Host Configuration Protocol: DHCP is used for allocation of dynamic IP addresses to computers in a network. ix. SNMP - Simple Network Management Protocol: SNMP is used for administration of computer networks. x. LDAP - Lightweight Directory Access Protocol: LDAP is used for collecting information about users and e-mail addresses from the internet. xi. ICMP - Internet Control Message Protocol: ICMP takes care of error-handling in the network. xii. ARP - Address Resolution Protocol: ARP is used by IP to find the hardware address of a computer network card based on the IP address. xiii. RARP - Reverse Address Resolution Protocol: RARP is used by IP to find the IP address based on the hardware address of a computer network card.

The seven layers of OSI model are further categorized into Upper Layers, Network Independent Layer and Lower Layers. Physical Layer, Data link Layer and Network Layer are categorized as Lower Layers. Transport Layer as Network Independent Layer and Session Layer, Presentation Layer, Application Layer as Upper Layers.  Also, these layers are sequenced in fixed manner and this sequence cannot be changed. It means, any alterations in the positions of the layers are not accepted.

Architecture And Layers of OSI Reference Model A) The Physical Layer: The Physical Layer is the bottom most layer and is associated with electrical, mechanical and functional aspects of the transmission media for information and receiving over internet. B) The Data Link Layer: The Data Link Layer is second from bottom and comes under the lower layer category. It ensures that the data must be synchronized, error detection and control are enabled. C) The Network Layer: The Network Layer is third from bottom in OSI model and is responsible for establishing data communication channel between multiple networks or devices or hosts or nodes. D) The Transport Layer: Transport Layer is the middle most layer in OSI model and it acts as Network Independent Layer. It has no idea about the functioning of lower layer i.e. physical, data link and network layers. E) The Session Layer: Session Layer is the fifth layer of OSI model and it provides appropriate sessions between users and entities, where user interacts. This layer can be used on the basis of resources available and it can be skipped too if not required.  For example : Login Sessions in online banking. F) The Presentation Layer: This is the sixth layer of OSI model, and it provides appropriate representation of data through various data presentation techniques. G) The Application Layer: Application Layer is the topmost layer of the OSI model and has the responsibility for providing interface between various users and application.