COMPUTER NETWORK UNIT 1&2

1. What Is Meant by Data communication?

Data communication is the exchange of data (in the form of 0’s and 1’s ) between two devices

via some form of transmission medium (wire or wireless).

2.Define data communications systems fundamental characteristics?

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.

3. Timeliness: The system must deliver data in a timely manner . Data delivered late are useless, in the case of video and audio, timely delivery means delivering data a they are produced , in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.

3. What are components are used data communications?

1. Message: The message is the information (data) to be communicated. It can consist of text, number, pictures , sound or video-or any combination of these.

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 compute, workstation , telephone handset, television, and so on.

4. Medium: The transmission medium is the physical path by which a message travels from sender to receiver. It could be a twisted –pair wire, coaxial cable, fiber optic cable, or radio waves (terrestrial or satellite microwave).

5. Protocol: a protocol is a set of rules that governs data communication . 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.

5. Define various types is transmission Modes.

Simplex

In Simplex mode, the communication is unidirectional , as on one-way street, Only one of two devices on a link can transmit; the other can only receive

simplex

Fig 1.2 Simplex

Keyboards and traditional monitors are both examples of simplex devices. The keyboard can only introduce input, the monitor can only accept output.

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 (see fig 1.3)

half-duplex2

Fig 1.3 Half-duplex

The half duplex mode is like a one-lane road with two-directional traffic. While cars tare traveling 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 . Walkie-talkies and CB (citizens band), radios are both half-duple systems.

Full duplex:

In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously (See fig 1.4)

full-duplex1

6. What are the general categories of network?

1. LAN

2. MAN

3. WAN

7. Define distributed processing ?

Distributed Processing:

Most networks use distributed processing, in which a task is divided among multiple computers . Instead of a single large machine being responsible for all aspects of a process , separate computers (usually a personal computers or workstation ) handle a subset.

8. Define network criteria?

Performance: Performance can be measure in many ways, including transit time and response time. Transit time is the amount of time required for a massage to travel from one deice to another. Response time is clasped time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software.

Reliability:

In addition to accuracy of delivery, network reliability is measure by the frequency of failure , the time it takes a link to recover from a failure , and the network ‘s robustness in a catastrophic.

Security:

Network security issues include protecting data from unauthorized access.

9. Define topology?

Topology refers to the way a network is laid out, either physically or logically. Two or more devices connect to a link two or more links from a topology.

10. Define protocol?

A protocol is a set of rules that governs data communications.

11. What are the key elements of protocol?

  • Syntax: Syntax refers to the structure or format of the data, meaning the order in which they are presented. For example, 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: 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: Timing refers to two characteristics: when data should be sent and how fast they can be sent. Fox 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 data will be largely lost.

12. Why are standards needed?

Standards are essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunication technology and processes.

13. Define some important characteristics of a protocol?

1) Direct/Indirect

2) Monolithic/structured

3) Symmetric /asymmetric

4) Standard /non standard

14. Define protocol Transmission services?

A protocol may provide a variety of additional services to the entities that use it. We mention here three common examples:

  • Priority: certain messages , such as control message , may need to get through to the destination entity with minimum delay. An example would be a close-connection request. Thus , priority could be assigned on a message basis. Additionally, priority could be assigned on a connection basis.

  • Quality of service: Certain classes of data may require a minimum throughput or a maximum delay threshold.

  • Security: Security mechanisms, restricting access, may be invoked.

15. Define parallel Transmission?

The mechanism for parallel transmission is a conceptually simple one :Use n wires to send n bits at one time. That way each has its own wire, and all n bits of one group can be transmitted with each clock tick from one device to another. Figure 4.25 show how parallel transmission works for n=8 . typically, the eight wires are bundled in a cable with a connector at each end.

The advantages of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission . But there is a significant disadvantage : cost.

16.Define serial transmission?

In serial transmission one bit follows another, so we need one communication channel rather than n to transmit data between tow communicating device. The advantages of serial over parallel transmission is that with only one communication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n. Serial transmission occurs in one of two ways: asynchronous or synchronous.

17. Define Asynchronous Transmission?

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

Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized : their durations are the same.

18. Define synchronous transmission?

In synchronous transmission, we send bit one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the file.

19. Advantage of star Topology?

Robustness if one link fails, only that link is affected. All other links remain active.

20. Disadvantages of star Topology?

A star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).

21. Advantage of mesh Topology?

A mesh offers several advantages over other network topologies. First , 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. Second, a mesh topology is robust. If one link becomes useable, it does not incapacitate the entire system . Another advantage is privacy or security.

22. Disadvantages or mesh Topology

The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconnection are difficult, second, the sheer bulk of the wiring can be greater than the available space (in walls, ceiling, or floors) can accommodate.

23. Advantages of Bus Topology?

Advantages of a bus topology include case of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths

24. Disadvantage of Bus topology/

Disadvantage include difficult reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation.

25. Define Ring Topology?

Ring: in a ring topology, each device has a dedicated point-to-point connection only with 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. Each device in the ring incorporates a repeater.

26. Define LAN?

A LAN can be as simple as two PCs and a printer in someone’s home office .LAN size is limited to a few kilometers .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.

The most common LAN topologies are bus, ring and star. Traditionally, LANs have data rates in a the 4 to 16 megabits per second (Mbps)range. Today’s, however speeds are increasing and can reach 100Mbps with gigabit system in development.

27. Define MAN?

It may be a single network such as a cable television network , or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN-to-LAN as well as device-to-device. A MAN may be wholly owned and operated by a private company, or it may be a service provided by a public company , such as a local telephone company. Many telephone companies provide a popular MAN service called Switched multi-megabit Data Services (SMDS)

28.Define WAN?

Wide Area Network (WAN) : A wide area network (WAN) provides long-distance transmission of data, voice, image , and video information over large geographic areas that may comprise a country, a continent , or even the whole world.

29. Explain function of 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 ). Figure 2.6 shows the relationship of the data like layer to the network and physical layers.

30. Define classes of Transmission media?

transmission_media


31. Define Guided media?

Guided media, which are those that provide a conduit from one device to another include twisted –pair cable, coaxial cable and fiber optic cable.

32. Explain UTP Vs STP?

The most common twisted –pair cable used in communications is referred to as unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its use called shielded twisted pair (STP). STP cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors. Although metal casing improves the quality of cable by preventing the penetration of noise or cross talk, it is bulkier and more expensive. Figure 7.4 shows the difference between UTP and STP. Our discussion focuses primarily on UTP because STP is seldom used outside of IBM.

33. Define Application of Co-axial case?

The use of coaxial cable started in analog telephone networks where a single coaxial network could carry 10,000 voice signals. Later it was used in digital telephone networks where a single coaxial could carry digital data up to 600Mbps. However , coaxial cable in telephone has largely been replaced today with fiber –optic cable.

34. Define multimedia step-Index fiber?

In multimode step-index fiber, the density of th core remains constant from the center to the edges. A beam of light moves through this constant density in a straight line until it reaches the interface of the core and cladding. At the interface, there is an abrupt change to a lower density that alters the angel of the beam’s motion. The term step index refers to the suddenness of this change.

35. Define multimode graded –Index fiber ?

Multimode graded-index fiber, decreases , this distortion of the signal through the cable. The word index here refers to the index of refraction . The index of refraction is related to density. A graded –index fiber, therefore, is one with varying densities. Density is highest at the center of the core and decreases gradually to its lowest at the edge.

36. What is DTE, DCE?

Terminals and computer, are generally referred to as data terminal equipment (DTE). A DTE makes use of the transmission system through the mediation of data circuit-terminating equipment (DCE)

37. Interface between DCE & DTE. Important Characters?

  • Mechanical
  • Electrical
  • Functional
  • Procedural

38. Define bandwidth?

Every line has a upper limit and lower limit on the frequencies of the signals it can carry. This limited range is called the bandwidth.

39. The best known example of a circuit-switching work is the __________________

Public telephone network.

40. Give the basic difference between broadcast radio and microwave.

The principal difference between broadcast radio and microwave is that former is omni directional and the latter I directional . Thus broadcast radio does not require dish shaped antennas need not be rigidly mounted to a precise alignment.

41. What are the four classes of Servicer Primitives?

q Request -An entity wants the service to do some work

q Indication -An entity is to be informed about an event

q Request -An entity wants the respond to an event

q Confirm – The response to an earlier request has come back.

42. What are the phases of data communication?

Connection oriented service: like e.g., the telephone

  1. Establish a connection (dial and get an answer)
  2. Communicate over the connection (speak)
  3. Delete the connection (hang up)

Connection less service:

A complete address must be put on each message. E.g., like on letter (snail-mails). There is normally no guarantee that message arrive in the same order they are sent.

43. Give some example for Networks.

Some WAN networks

SDH/SONET

ATM

Some LAN networks.

Ethernet

Pronet (token ring)

FDDI (token ring on fiber)

ATM

44. Define cell switching?

Cell switching is a special case of packet switch where the packets are very small and have fixed size throughout the network. Cell switching relies on a connection set up phase.

45.Define RS-232

RS-232 defines serial, asynchronous communication.

1. Serial- bits are encoded and transmitted one at a time (as opposed to parallel transmission)

2. Asynchronous – characters can be at any time and bits are not individually synchronized.

46. What are the four types of redundancy check used in data communication?

(i) Vertical redundancy Check (VRC) or parity check

(ii) Longitudinal redundancy Check (LRC)

(iii) Cyclical redundancy Check (CRC)

(iv) Check Sum.

47. Define interleaving.

Taking a specific amount of data from each device in a regular order.

48. Define Modem?

A device consisting of a modulator and demodulator converts a digital signal in to an analog signal (Modulation and demodulate the analog signal into digital signals.

49. What are the four methods that convert a digital signal to an analog signal?

(i) Amplitude Shift Keying (ASK)

(ii) Phase Shift Keying (PSK)

(iii) Frequency Shift Keying (FSK)

(iv) Quadrature Amplitude Modulation (QAM)

50. Define Ethernet?

The standard developed by IEEE Project 802 for local area networks. It covers the MAC for networks using the CSMA/CD access method and provides a formal definition for the Ethernet.

51. What is meant by 1 Bass 5?

IEEE 802.3 standard for low-data-rate Ethernet using twisted –pair cable and daisy –chained connection.

52. Define Networks.

Networks is a set of devices connected by media links A node can be a Computer, Printer, or any other device capable of sending and /or receiving data generated by other node on the network. The links Connecting the devices are called Communication channels.

53. List the Uses of Computer Networks. (April 2004 AU)

· Resource Sharing

· High reliability

· Saving money

· Provider Powerful Communication Medium

· Electronic mailing

· Video Conferencing

54. Differentiate Connection Oriented and Connection Less Services.

Connection Oriented Service

Connection less Service

1)It guarantees that data transmitted from a sender to a receiver.

1) It does not make any guarantees about eventual delivery

2) All the Packets belonging to a message are sent in the same path

2) All the Packets belonging to a message are not sent in the same path

55. What are the different types of layers in OSI model?

· Application Layer

· Presentation layer

· Session layer

· Transport layer

· Network layer

· Data link layer

· Physical layer

56.Give the signal-to-noise ratio for quantizing noise.

The signal-to-noise ratio for quantizing noise can be expressed as

SNR = 20 log 2n + 1.76 dB

= 6.02n + 1.76 dB

Thus each additional bit used for quantizing increases SNR by about 6dB, which is a factor of 4.

57. What are the advantages and disadvantages of Micro wave transmission ?

Advantages

Þ Higher frequency than radio and television

Þ An established medium used by the long-distance telephone companies

Þ Single direction transmission

Disadvantages

Þ Must have a clear path for transmission

Þ More expensive than copper wire and glass fiber

Þ High maintenance costs

58.Define PSTN.

Network, which refers to the international telephone system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI.

59.Mention some of the Network Hardware.(NOV 2004 AU)

  • Topolgy (Ring, Bus, Mesh, Tree, Star)
  • Transmission Technology (Broadcasting, Unicasting, Multicasting)
  • Internetworking
  • Wireless Application

60.Differentiate distributed system and computer network.(Nov 2004 AU)

Distributed System.

A distributed system consists of a collection of autonomous computers, connected through a network and distribution middleware, which enables computers to coordinate their activities and to share the resources of the system, so that users perceive the system as a single, integrated computing facility.

Computer Network

A computer network is a set of computers that are connected and able to exchange messages

61.List out the Centralised System Characteristics

  • One component with non-autonomous
  • parts
  • Component shared by users all the time
  • All resources accessible
  • Software runs in a single process
  • Single Point of control
  • Single Point of failure

62.List out the Distributed System.

  • Multiple autonomous components
  • Components are not shared by all users
  • Resources may not be accessible
  • Software runs in concurrent processes on
  • different processors
  • Multiple Points of control
  • Multiple Points of failure
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