Computer Network unit-1 complete Notes.pptx

Computer Network Presentation

1. Computer Network (503) Unit – I Basic Concepts: Presented By : Lucky Berwal

2. UNIT Ist Basic Concepts: Components of data communication, distributed processing, standards and organizations. Line configuration, topology, Transmission mode, and categories of networks. OSI and TCP/IP Models: Layers and their functions, comparison of models. Digital Transmission: Interfaces and Modems: DTE-DCE Interface, Modems, Cable modems.

3. Introduction to computer network A Network is a set of devices(nodes) connected by media, A node can be a computer, printer or any other devices capable of sending and/or receiving data generated by other nodes on the network. Definition of computer network :- A COMPUTER NETWORK is a group of computer systems and other computing hardware devices that are linked together through communication channels to facilitate communication and resources sharing among a wide range of users. Networks are commonly categorized based on their characteristics.

4. Layered Architecture:- Computer Network are generally organized as a series of layers or levels, each one built upon the one below, Every layer needs a mechanism for identify sender and receiver. Node: It can be any network device(router, printer, camera). Host: It represents computer stations(interface). Hub: Network device used to increase the reach ability.

5. Definition Computer Network , two or more  computers  that are connected with one another for the purpose of communicating data electronically. Besides physically connecting computer and communication devices, a  network  system serves the important function of establishing a  cohesive   architecture  that allows a variety of equipment types to transfer information in a near-seamless fashion. 

6. Data Communication   Data Communication  is defined as exchange of data between two devices via some form of transmission media such as a cable, wire or it can be air or vacuum also. For occurrence of data communication, communicating devices must be a part of communication system made up of a combination of hardware or software devices and programs.

7. Criteria for a Data Communication Network Performance: Performance is the defined as the rate of transferring error free data. It is measured by the Response Time. Response Time is the elasped time between the end of an inquiry and the beginning of a response. Consistency: Consistency is the predictability of response time and accuracy of data. Reliability: Reliability is the measure of how often a network is useable. MTBF (Mean Time between Failures) is a measure of the average time a component is expected to operate between failures. Normally provided by the manufacturer. A network failure can be: hardware, data carrying medium and Network Operating System.

8. Criteria for a Data Communication Network   Recovery: Recovery is the Network's ability to return to a prescribed level of operation after a network failure. This level is where the amount of lost data is nonexistent or at a minimum. Recovery is based on having Back-up Files. Security: Security is the protection of Hardware, Software and Data from unauthorized access. Restricted physical access to computers, password protection, limiting user privileges and data encryption are common security methods. Anti-Virus monitoring programs to defend against computer viruses are a security measure.

9. Components of Data Communication The different components of Data communication are shown in the following figure. 1 . Message: The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.

10. 2. Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on. 3. Receiver: The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. 4. Transmission medium: The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.  5. Protocol: A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese.

11. Transmission Mode/ types of data communication: A given transmission on a communications channel between two machines can occur in several different ways. The transmission is characterized by: the direction of the exchanges the transmission mode: the number of bits sent simultaneously synchronization between the transmitter and receiver. Types of Transmission mode Simplex Half Duplex Full Duplex

12. 1.Simplex A simplex connection is a connection in which the data flows in only one direction, from the transmitter to the receiver. This type of connection is useful if the data do not need to flow in both directions (for example, from your computer to the printer or from the mouse to your computer...). 2.Half Duplex A half-duplex connection (sometimes called an  alternating connection  or  semi-duplex ) is a connection in which the data flows in one direction or the other, but not both at the same time. With this type of connection, each end of the connection transmits in turn. This type of connection makes it possible to have bidirectional communications using the full capacity of the line. 3.Full Duplex A full-duplex connection is a connection in which the data flow in both directions simultaneously. Each end of the line can thus transmit and receive at the same time, which means that the bandwidth is divided in two for each direction of data transmission if the same transmission medium is used for both directions of transmission.

13. Type of computer network/ Categories of computer network Data Communication and Computer Network Generally, networks are distinguished based on their geographical span. A network can be as small as distance between your mobile phone and its Bluetooth headphone and as large as the internet itself, covering the whole geographical world. Personal Area Network Local Area Network Metropolitan Area Network Wide Area Network

14. Personal Area Network (PAN): A Personal Area Network (PAN) is smallest network which is very personal to a user. This may include Bluetooth enabled devices or infra-red enabled devices. PAN has connectivity range up to 10 meters. PAN may include wireless computer keyboard and mouse, Bluetooth enabled headphones, wireless printers, and TV remotes.

15. 2.Local Area Network (LAN): LANs are the most frequently discussed networks, one of the most common, one of the most original and one of the simplest types of networks.  LANs  connect groups of computers and low-voltage devices together across short distances (within a building or between a group of two or three buildings in close proximity to each other) to share information and resources.

16. 3.Metropolitan Area Network(MAN): These types of networks are larger than LANs but smaller than WANs – and incorporate elements from both types of networks. MANs span an entire geographic area (typically a town or city, but sometimes a campus). Ownership and maintenance is handled by either a single person or company (a local council, a large company, etc.).

17. 4.Wide Area Network(WAN): Slightly more complex than a LAN, a  WAN  connects computers together across longer physical distances. This allows computers and low-voltage devices to be remotely connected to each other over one large network to communicate even when they’re miles apart. The Internet is the most basic example of a WAN, connecting all computers together around the world. Because of a WAN’s vast reach, it is typically owned and maintained by multiple administrators or the public.

18. Data Processing Collection , manipulation, and processing collected data for the required use is known as data processing. It is a technique normally performed by a computer; the process includes retrieving, transforming, or classification of information.

19. Methods of Data Processing Let us now discuss the different methods of data processing. Single user programming Multiple programming Real-time processing On-line processing Time sharing processing Centralized processing Distributed processing

20. Centralized, decentralized and distributed processing Centralized Processing: In centralized processing, one or more terminals are connected to a single processor. Note that terminal is the combination of mouse, keyboard, and screen. In-library there is one processor attached to different terminals and library users can search any book from the terminal (mouse, keyboard, and screen). In centralized processing all the terminals are controlled by a single processor (CPU) and any command can be fulfilled by a single processor and this type of network is called  centralized network . Decentralized Processing: In decentralized processing, there are different CPU connected on the network and each processor can do its job independent of each other. For example, in a Net cafe, all computers can perform their own tasks. This type of network is called  decentralized network . Distributed processing: Another type of processing also exists named distributed processing. In this type of processing different CPU are connected to the network and are controlled by single CPU. For example in air reservation system there exists different terminals and processing is done from many locations and all the computers are controlled by the single main processor. This type of network is called  distributed network .

21. Distributed processing Distributed processing is a setup in which multiple individual central processing units (CPU) work on the same programs, functions or systems to provide more capability for a computer or other device.  distributed processing refers to  local-area networks (LANs)  designed so that a single program can run simultaneously at various sites.  Another form of distributed processing involves  distributed databases . This is  databases  in which the  data  is  stored  across two or more computer systems. The  database system  keeps track of where the data is so that the distributed nature of the database is not apparent to  users .

22. Line configuration Line configuration refers to the way two or more communication devices attached to a link . Line configuration is also referred to as connection. A Link is the physical communication pathway that transfers data from one device to another. For communication to occur, two devices must be connected in same way to the same link at the same time. There are two possible line configurations.        1. Point-to-Point.        2. Multipoint.

23. 1.Point-to-Point: A Point to Point Line Configuration Provide dedicated link between two devices use actual length of wire or cable to connect the two end including microwave & satellite link. Point to point network topology is considered to be one of the easiest and most conventional network topologies. It is also the simplest to establish and understand. To visualize, one can consider point to point network topology as two phones connected end to end for a two way communication

24. 2.Multipoint Configuration: Multipoint Configuration also known as  Multidrop line configuration one or more than two  specific devices share a single link capacity of the channel is shared. More than two devices share the Link that is the capacity of the channel is shared now. With shared capacity, there can be two possibilities in a Multipoint Line Configuration: Spatial Sharing : If several devices can share the link simultaneously, its called Spatially shared line configuration. Temporal (Time) Sharing: All devices are given a time spam in which they can utilize that single link. At that time other device will not send their data on link.

25. Topologies Network topologies refers to the way in which a network is laid out physically. The topology of network is the geometric representation of the relationship for all the links and linking devices (usually called nodes) to one another. Type of topologies: Bus Topology Star Topology Ring Topology Mesh Topology Tree Topology Hybrid Topology

26. Type of topologies Bus Topology: Bus topology use the single communication line called backbone which is shared by all the nodes or computers to communicate. It is used for smaller networks mainly and is one of the most simplest and reliable ways to communicate. (Cable end) Node

27. Bus Topology: Advantages: Suitable for smaller networks. Easy to implement. Even if one node breaks down, network does not get hampered. Disadvantages: Cable fail then whole network will be fail. Within a given cable length, limited number of nodes can be added. Data transfer rate gets slow down if more nodes are added. Only one data packet can be transferred at a particular moment.

28. 2. Star Topology In star topology, every node is connected to central hub or switch. The switch is the server and the peripherals are the clients. Data from the source is first delivered to the hub and then transferred to the transferred to the other nodes. HUB

29. 2. Star Topology Advantages: Easy to install and implement. Give better performance as message does not pass through various nodes unlike bus topology. Faulty nodes can be easily removed without affecting the other nodes in the loop. Disadvantages: If the central hub fails then the whole network is break. Data transfer rate depends on capacity of the central hub. Cost of installation is high.

30. 3.Ring Topology Ring topology is a networks topology in which the nodes or the computers are connected in a closed loop. It is called as circular topology. A signal is passed along the ring in one direction, until it reaches its destination. Ring topology NODE Server(sender)

31. 3.Ring Topology Advantages: Ring topology is easy to install. Transmitting network is not affected by high traffic. A signal travels around the circle in a clockwise direction. Disadvantages: If one node is break then the whole network gets down. It becomes difficult to add/remove nodes.

32. 4.Mesh Topology In mash topology all devices are cross-connected to communicate each other. There are two type of mesh topology: Full mesh topology:- Each and every nodes or devices are connected to other. Partial mesh topology:- In this topology some of the systems are connected same function as mesh topology but some devices are only connected to two or three devices

33. 4.Mesh Topology Advantages: When one node fails, others continue to work without disruption. We can send data from one node to many nodes Each connection can carry its own data load. Disadvantages: It is quite expensive since a bulk wiring is required. Installation and configuration can be a very difficult task.

34. 5.Tree Topology Tree topology is the combination of bus and star topology. In this data flow from top to bottom i.e from the central hub to secondary and then to the devices or from bottom to top i.e. devices to secondary hub and then to the central hub.

35. 5.Tree Topology Advantages: It is most suitable for large networks where ring and star topologies are not efficient. It divides the network in sub-parts, so it becomes more manageable. Disadvantages: If the central hub gets fails the entire system fails. The cost is high because of cabling.

36. 6.Hybrid Topology A network structure whose design contains more than one topology is said to be hybrid topology. Hybrid topology inherits merits and demerits of all the incorporating topologies.

37. 6.Hybrid Topology Advantages: Hybrid network can be designed according to the requirements of the organization. Special care can be given to nodes where traffic is high as well as where chances of fault are high. Disadvantages: One of the biggest drawback of hybrid topology is its design. Its not easy to design this type of architecture and its a tough job for designers. As hybrid architecture are usually larger in scale, they require a lot of cables, cooling systems.

38. Standards and Organizations A standards and organization, sometimes referred to as a standards body, is an organization with authority to endorse official standards for given applications.

39. Examples of standards organizations include: ANSI  (American National Standards Institute) is the primary organization for fostering the development of technology standards in the United States.  The  IEEE  (Institute of Electrical and Electronics Engineers) fosters the development of standards that often become national and international standards. The  BSI  (British Standards Institution) is a service organization that produces standards across a wide variety of industry sectors. The  IETF  (Internet Engineering Task Force) is the body that defines standard Internet operating  protocol s such as  TCP/IP .  OASIS  (Organization for the Advancement of Structured Information Standards) exists to promote product-independent standards for information formats such as  XML  and  HTML .

40. Internetwork An internetwork is defined as two or more computer network LANs or WAN or computer network segments are connected using devices, and they are configured by a local addressing scheme. This process is known as  internetworking . An interconnection between public, private, commercial, industrial, or government computer networks can also be defined as  internetworking . An internetworking uses the  internet protocol . The reference model used for internetworking is  Open System Interconnection (OSI) .

41. Types of Internetwork: Internet:   Internet is a worldwide, publicly accessible computer network of interconnected computer networks (internetwork) that transmit data using the standard  Internet Protocol (IP) . Largest Internetwork in the world is Internet. The terms World Wide Web (WWW) and Internet are not the same. The Internet is a collection of interconnected computer networks, linked by copper wires, fiber-optic cables, wireless connections, etc. The World Wide Web is one of the services accessible via the Internet, along with various others including email, file sharing, remote administration, video streaming, online gaming etc. World Wide Web (WWW) is a collection of interconnected documents and other resources, linked together by hyperlinks and URLs

42. Types of Internetwork: Extranet:  An extranet is a communication network based on the internet protocol such as  Transmission Control protocol  and  internet protocol . It is used for information sharing. The access to the extranet is restricted to only those users who have login credentials. An extranet is the lowest level of internetworking. It can be categorized as  MAN ,  WAN  or other computer networks. An extranet cannot have a single  LAN , at least it must have one connection to the external network. Intranet:  An intranet is a private network based on the internet protocol such as  Transmission Control protocol  and  internet protocol . An intranet belongs to an organization which is only accessible by the  organization's employee  or members. The main aim of the intranet is to share the information and resources among the organization employees. An intranet provides the facility to work in groups and for teleconferences.

43. Computer network model OSI and TCP/IP Models

44. History of OSI Model Here are essential landmarks from the history of OSI model: In the late 1970s, the ISO conducted a program to develop general standards and methods of networking. In 1973, an Experimental Packet Switched System in the UK identified the requirement for defining the higher-level protocols. In the year 1983, OSI model was initially intended to be a detailed specification of actual interfaces. In 1984, the OSI architecture was formally adopted by ISO as an international standard

45. OSI Model Open System Interconnect is an open standard for all communication systems. OSI model is established by International Standard Organization (ISO). An open system is a model that allow any two different systems(devices) to communication regardless of their underlying architecture. The purpose of the OSI model is to open communication between different systems without requiring changes to the logic of the underlying hardware and software. It is a model of understanding and designing a network architecture which is flexible.

46. Organization of the layers   The 7 layers can be grouped into 3 subgroups   Network Support Layers : Layers 1,2,3 - Physical, Data link and Network are the network support layers. They deal with the physical aspects of moving data from one device to another such as electrical specifications, physical addressing, transport timing and reliability.   Transport Layer: Layer 4 , transport layer, ensures end-to-end reliable data transmission on a single link.    User Support Layers: Layers 5,6,7 – Session, presentation and application are the user support layers. They allow interoperability among unrelated software systems.

47. OSI Model Layers and their functions Application Layer: This layer is responsible for providing interface to the application user. This layer encompasses protocols which directly interact with the user. Presentation Layer: This layer defines how data in the native format of remote host should be presented in the native format of host. Session Layer: This layer maintains sessions between remote hosts. For example, once user/password authentication is done, the remote host maintains this session for a while and does not ask for authentication again in that time span. Transport Layer: This layer is responsible for end-to-end delivery between hosts. Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network. Data Link Layer: This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer. Physical Layer: This layer defines the hardware, cabling, wiring, power output, pulse rate etc. NOTE: For sender application layer is first and for receiver physical layer is first.

48. OSI Model Layers Application layer Message format, file transfer Presentation Layer Encryption and decryption Session Layer Authentication, Permission Transport Layer End-to-End connection, flow control Network Layer Path determination, routing Data link Layer Erro r detection Physical Layer Media, signal, binary transmission Host layers Media layers (Hardware Layers) 1 7 6 5 4 3 2 Network Support Layers User Support Layer Ensures end-to-end reliable data transmission

49. Sender Receiver S/W Layer S/W Layer H/W Layer H/W Layer

50. Structure of a data when it goes to all layers

51. OSI Model Layers and their functions

52. Physical Layer: The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

53. The other responsibilities of physical layer Physical characteristics of interfaces and media - The physical layer defines the characteristics of the interface between the devices and the transmission medium. Representation of bits - To transmit the stream of bits, it must be encoded to signals. The physical layer defines the type of encoding. Data Rate or Transmission rate - The number of bits sent each second – is also defined by the physical layer. Synchronization of bits - The sender and receiver must be synchronized at the bit level. Their clocks must be synchronized. Line Configuration - In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices. Physical Topology - The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh, bus, star or ring topology. Transmission Mode - The physical layer also defines the direction of transmission between two devices: simplex, half-duplex or full-duplex.

54. Data Link Layer It is responsible for transmitting frames from one node to next node.

55. The other responsibilities of this layer are Framing - Divides the stream of bits received into data units called frames. Physical addressing – If frames are to be distributed to different systems on the n/w , data link layer adds a header to the frame to define the sender and receiver. Flow control - If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender ,the Data link layer imposes a flow ctrl mechanism. Error control - Used for detecting and retransmitting damaged or lost frames and to prevent duplication of frames. This is achieved through a trailer added at the end of the frame. Access control -Used to determine which device has control over the link at any given time.

56. NETWORK LAYER This layer is responsible for the delivery of packets from source to destination. It is mainly required, when it is necessary to send information from one network to another.

57. The other responsibilities of this layer are Hope to hope transmission Logical addressing - If a packet passes the n/w boundary, we need another addressing system for source and destination called logical address. Routing – The devices which connects various networks called routers are responsible for delivering packets to final destination. Fragmentation Congestion control

58. TRANSPORT LAYER It is responsible for Process to Process delivery. It also ensures whether the message arrives in order or not.

59. The other responsibilities of this layer are Port addressing - The header in this must therefore include a address called port address. This layer gets the entire message to the correct process on that computer. Segmentation and reassembly - The message is divided into segments and each segment is assigned a sequence number. These numbers are arranged correctly on the arrival side by this layer. Connection control - This can either be connectionless or connection-oriented. The connectionless treats each segment as a individual packet and delivers to the destination. The connection-oriented makes connection on the destination side before the delivery. After the delivery the termination will be terminated. Flow and error control - Similar to data link layer, but process to process take place.

60. SESSION LAYER This layer establishes, manages and terminates connections between applications.  

61. The other responsibilities of this layer are Dialog control - This session allows two systems to enter into a dialog either in half duplex or full duplex. Synchronization -This allows to add checkpoints into a stream of data.

62. PRESENTATION LAYER It is concerned with the syntax and semantics of information exchanged between two systems.

63. The other responsibilities of this layer are Translation – Different computers use different encoding system, this layer is responsible for interoperability between these different encoding methods. It will change the message into some common format. Encryption and decryption -It means that sender transforms the original information to another form and sends the resulting message over the n/w. and vice versa. Compression and expansion -Compression reduces the number of bits contained in the information particularly in text, audio and video.

64. APPLICATION LAYER This layer enables the user to access the n/w. This allows the user to log on to remote user.

65. The other responsibilities of this layer are FTAM(file transfer, access ,mgmt) - Allows user to access files in a remote host. Mail services - Provides email forwarding and storage. Directory services - Provides database sources to access information about various sources and objects.

66. Layer Name Protocol Function Layer 7 Application SMTP, HTTP, FTP, POP3, SNMP To allow access to network resources. Layer 6 Presentation MPEG, ASCH, SSL, TLS To translate, encrypt and compress data. Layer 5 Session NetBIOS, SAP To establish, manage, and terminate the session Layer 4 Transport TCP, UDP The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine. Layer 3 Network IPV5, IPV6, ICMP, IPSEC, ARP, MPLS. To provide internetworking. To move packets from source to destination Layer 2 Data link RAPA, PPP, Frame Relay, ATM, Fiber Cable , etc. To organize bits into frames. To provide hop-to-hop delivery Layer 1 Physical RS232, 100BaseTX, ISDN, 11. To transmit bits over a medium. To provide mechanical and electrical specifications

67. TCP/IP Model Internet uses TCP/IP protocol suite, also known as Internet suite. This defines Internet Model which contains four layered architecture. OSI Model is general communication model but Internet Model is what the internet uses for all its communication. The internet is independent of its underlying network architecture so is its Model.

68. This model has the following layers:

69. OSI layers vs TCP/IP Layers /Internet/Internal

70. TCP/IP Model Layers and functions Application Layer: This layer defines the protocol which enables user to interact with the network. For example, FTP, HTTP etc. Transport Layer: This layer defines how data should flow between hosts. Major protocol at this layer is Transmission Control Protocol (TCP). This layer ensures data delivered between hosts is in-order and is responsible for end- to-end delivery. Internet Layer: Internet Protocol (IP) works on this layer. This layer facilitates host addressing and recognition. This layer defines routing. Link Layer: This layer provides mechanism of sending and receiving actual data. Unlike its OSI Model counterpart, this layer is independent of underlying network architecture and hardware.

71. How the web works

72. When you type a web address into your browser The browser goes to the DNS(Domain Name Servers) server, and finds the real address of the server that the website lives on (you find the address of the shop). The browser sends an HTTP request message to the server, asking it to send a copy of the website to the client (you go to the shop and order your goods). This message, and all other data sent between the client and the server, is sent across your internet connection using TCP/IP. If the server approves the client's request, which means "Of course you can look at that website! Here it is", and then starts sending the website's files to the browser as a series of small chunks called data packets (the shop gives you your goods, and you bring them back to your house). The browser assembles the small chunks into a complete web page and displays it to you (the goods arrive at your door — new shiny stuff, awesome!).

73. DNS(Domain name server) This is called an  IP address , and it represents a unique location on the web. However, it's not very easy to remember, is it? That's why Domain Name Servers were invented. These are special servers that match up a web address you type into your browser (like "mozilla.org") to the website's real (IP) address. Websites can be reached directly via their IP addresses. You can find the IP address of a website by typing its domain into a tool like

74. Comparison of models OSI(Open System Interconnection) TCP/IP(Transmission Control Protocol / Internet Protocol)  In OSI model the transport layer guarantees the delivery of packets. 2. In TCP/IP model the transport layer does not guarantees delivery of packets. Still the TCP/IP model is more reliable. OSI model has a separate Presentation layer and Session layer. 4. TCP/IP does not have a separate Presentation layer or Session layer. The OSI has seven layers TCP/IP has four layers. In OSI, the model was developed first and then the protocols in each layer were developed  In the TCP/IP suite, the protocols were developed first and then the model was developed. OSI model gives guidelines on how communication needs to be done TCP/IP protocols layout standards on which the Internet was developed. So, TCP/IP is a more practical model.

75. data Transmission of digital data The data transmission mode decides how data is transmitted between two computers. The binary data in the form of 1s and 0s can be sent in two different modes: Type of Data transmission Data transmission Asynchronous Synchronous Serial Parallel

76. Parallel Transmission All the bits of data are transmitted simultaneously on separate communication lines. In order to transmit n bits, n wires or lines are used. Thus each bit has its own line. Parallel transmission is used for short distance communication. Advantages: Faster form of transmission, i.e. able to send multiple bits simultaneously. Disadvantage: It is costly method of data transmission as it required n lines to transmit n bits at the same time.

77. 2. Serial Transmission In serial transmission the various bits of data are transmitted serially one after the other. It required only one communication line rather than n lines to transmit data from sender to receiver. Advantage: Use of single communication line reduces the transmission line cost by the factor of n as compared to parallel transmission. Disadvantage : This method is slower as compared to parallel transmission as bits are transmitted serially one after the other

78. Asynchronous Transmission:   It sends only one character at a time where a character is either a letter of the alphabet or number or control character, it sends one byte of data at a time. Asynchronous transmission between two devices is made possible using start bit and stop bit. Start bit usually 0 is added to the beginning of each byte(alerts the receiver to the arrival of new group of bit). Stop bit usually 1 is added to the end of each(to let the receiver know that byte is finished).

79. II. Synchronous Transmission It does not use start and stop bit. They can contain multiple bytes. There is no gap between the various bytes in the data steam. Since the various bytes are placed on the link without any gap, it is the responsibility of receiver to separate the bit stream into bytes.

80. Modem A modem is a hardware device that allows the computer to connect to the internet over the existing telephone line. A modem is not integrated with the motherboard rather than it is installed on the PCI slot found on the motherboard. It stands for Modulator/Demodulator. It converts the digital data into an analog signal over the telephone lines. Based on the differences in speed and transmission rate, a modem can be classified in the following categories: Standard PC modem or Dial-up modem Cellular Modem Cable modem

81. Cellular Modem A device that adds 3G or 4G (LTE) cellular connectivity to laptops, desktop computers and various tablets. Typically available as an external USB module, for desktop computers, the modem may also be available on a PCI or PCI Express ( PCIe ) card that plugs into an empty slot on the motherboard. For older laptops, modems may be available for ExpressCard or PC Card/ CardBus slots. Known By Many Names A cellular modem via USB is a "USB modem." On a card, it can be an "air card," "data card" or "broadband card." Generic terms are "broadband adapter," "broadband modem," "cellular adapter," "3G or 4G modem." Alternatives: Smartphones and Routers While a cellular modem plugs directly into the computer, smartphones and stand-alone broadband routers convert cellular signals into a Wi-Fi hotspot to connect to nearby devices. See  cellular hotspot  and  mobile broadband router .

82. Cable modems A cable  modem  router is a device that not only acts as an  access point  for  cable Internet  access, but also doubles as a network router. The network router built into the  cable modem  allows more than one computer to share  Internet access  by directing traffic to two or more computers in a single location, such as a home or office building. A  cable modem  is a hardware device that allows your computer to communicate with an  Internet service provider  over a landline connection. It converts an analog signal to a digital signal for the purpose of granting access to  broadband  Internet. There are two basic types of cable modem routers: wired and wireless. A wired model will require all computers that need Internet access to be physically connected to the device via  Ethernet  cabling. A wireless model can communicate to all computers using radio waves, negating the need for cables.

83. DTE-DCE Interface DTE(Data Terminal Equipment): It is an end equipment that convert user information into signals or reconvert the received signals. DTE is the device where communication lines end. DCE (Data Circuit Terminal): It is the equipment that sits between DTE & data transmission. DCE is usually a modem/ Routers. Provide path for communication.

84. DTE-DCE Interface, Modems