Important Short Questions and Answers:Computer Networks - Physical Layer

1. What is mean by data communication?

Data communication is the exchange of data (in the form of 1s and 0s) between two devices via some form of transmission medium (such as a wire cable).

2. What are the three criteria necessary for an effective and efficient network?

The most important criteria are performance, reliability and security.

Performance of the network depends on number of users, type of transmission medium, and thecapabilities of the connected h/w and the efficiency of the s/w.

Reliability is measured by frequency of failure, the time it takes a link to recover from thefailure and the network’s robustness in a catastrophe.

Security issues include protecting data from unauthorized access and viruses.

3. What are the three fundamental characteristics determine the effectiveness of the data communication system?

The effectiveness of the data communication system depends on three fundamental characteristics:

Delivery: The system must deliver data to the correct destination.

Accuracy: The system must deliver data accurately.

Timeliness: The system must deliver data in a timely manner.

4. What are the advantages of distributed processing?

An advantage of distributed processing includes security/encapsulation, distributed databases, faster problem solving, security through redundancy and collaborative processing.

5. Why are protocols needed?

In networks, communication occurs between the entities in different systems. Two entities cannot just send bit streams to each other and expect to be understood. For communication, the entities must agree on a protocol. A protocol is a set of rules that govern data communication.

6. Why are standards needed?

Co-ordination across the nodes of a network is necessary for an efficient communication. If there are no standards, difficulties arise. A standard provides a model or basis for development to which everyone has agreed.

7. For n devices in a network, what is the number of cable links required for a mesh and ring topology?

Mesh topology – n (n-1)/2.

Ring topology – n

8. What is the difference between a passive and an active hub?

An active hub contains a repeater that regenerates the received bit patterns before sending them out. A passive hub provides a simple physical connection between the attached devices.

9. Distinguish between peer-to-peer relationship and a primary-secondary relationship. Peer-to-peer relationship:

All the devices share the link equally.

Primary-secondary relationship:

One device controls traffic and the others must transmit through it

10. Assume 6 devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device?

Number of cables=n (n-1)/2=6(6-1)/2=15

Number of ports per device=n-1=6-1=5

11. Group the OSI layers by function.

The seven layers of the OSI model belonging to three subgroups. Physical, data link and network layers are the network support layers; they deal with the physical aspects of moving data from one device to another. Session, presentation and application layers are the user support layers; they allow interoperability among unrelated software systems. The transport layer ensures end-to-end reliable data transmission.

12. What are header and trailers and how do they get added and removed?

Each layer in the sending machine adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it. This information is added in the form of headers or trailers. Headers are added to the message at the layers 6,5,4,3, and 2. A trailer is added at layer2. At the receiving machine, the headers or trailers attached to the data unit at the corresponding sending layers are removed, and actions appropriate to that layer are taken.

13. Discuss the mode for propagating light along optical channels.

There are two modes for propagating light along optical channels, multimode and single mode.

Multimode: Multiple beams from a light source move through the core in different paths.

Single mode: Fiber with extremely small diameter that limits beams to a few angles, resulting inan almost horizontal beam.

14. How are the guided media differing from unguided transmission media? Guided transmission media

1. Guided indicate, medium is contained have any within physical boundary

2. Transmission takes place through wire.

Unguided transmission media

1. Unguided medium does not Physical boundary

2. It is a wireless transmission.

15. What are the disadvantages of optical fiber as a transmission medium?

The disadvantages of optical fiber are

·Very expensive.

·Installation and maintenance is difficult.

·Fragility.

16. What are the criteria used to evaluate transmission medium?

The criteria used to evaluate transmission medium are

·Throughput

·Propagation speed

·Propagation time

·Wavelength

17. Give the relationship between propagation speed and propagation time?

Propagation time = distance / propagation speed

The time required for a signal or a bit to travel from one point to another is called Propagation time. Propagation speed is the distance, a signal or a bit travel through a mediumin one second.

18. Explain cross talk and what is needed to reduce it?

Effect of one wire on another is called as cross talk. One wire will be the sending antenna and the other wire will be the receiving antenna. We can use the shielded twisted pair cable or coaxial cable for transmission, which contains metal foil to reduce cross talk.

19. Compare datagram networks and virtual circuit networks.

20. What is TCP/IP?

TCP/IP is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality: however, the modules are not necessarily interdependent.

21. State the role of DSL.

DSL Technology supports high speed digital communication over the existing local loops.

22. What is the role of DSL Modem?

DSL Technology is one of the most promising for supporting high-speed digital communication over the existing local loops.

23. What are the features provided by layering?

Two nice features are:

o    It decomposes the problem of building a network into more manageable components.

o    It provides a more modular design.

Glossary:

ADSL Lite: A splitterless ADSL. This technology allows an ASDL Lite modem to be pluggeddirectly into a telephone jack and connected to the computer. The splitting is done at the telephone company.

Bandwidth: The difference between the highest and the lowest frequencies of a compositesignal. It also measures the information-carrying capacity of a line or a network.

Bayone-Neill-Concelman (BNC) connector: A common coaxial cable connector.

Bit Stuffing: In a bit-oriented protocol, the process of adding an extra bit in the data section of aframe to prevent a sequence of bits from looking likes a flag.

Bit-Oriented Protocol: A protocol in which the data frame is interpreted as a sequence of bits.Cable Modem: A technology in which the TV cable provides Internet access.

Cable Modem Transmission System (CMTS): A device installed inside the distribution hubthat receives data from the Internet and passes them to the combiner.

Cable TV Network: A system using coaxial or fiber optic cable that brings multiple channels of

video programs into homes.

Circuit Switching’s: switching technology that establishes an electrical connection betweenstations using a dedicated path.

Cladding: Glass or plastic surrounding the core of an optical fiber; the optical density of thecladding must be less than that of the core.

Coaxial Cable: A transmission medium consisting of a conducting core, insulating material, anda second conducting sheath.

Community Antenna TV (CATV): A cable network service that broadcasts video signals tolocations with poor or no reception.

Constellation Diagram: A graphical representation of the phase and amplitude of different bitcombinations in digital-to-analog modulation.

Datagram Network: A packet-switched network in which packets are independent from eachother.

Digital Subscriber Line Access Multiplexer (DSLAM): A telephone company site device thatfunctions like an ADSL modem.

Error Control: The handling of errors in data transmission.

Flow Control: A technique to control the rate of flow of frames (packets or messages).

Guided Media: Transmission media with a physical boundary.

Hop-To-Hop Delivery: Transmission of frames from one node to the next.

Internet service provider (ISP): Usually, a company that provides Internet services.

Jitter: A phenomenon in real-time traffic caused by gaps between consecutive packets at thereceiver.

Local Area Network (LAN) A network connecting devices inside a single building or insidebuildings close to each other.

Node-To-Node Delivery: Transfer of a data unit from one node to the next.

Optical Fiber: A thin thread of glass or other transparent material to carry light beams.

Packet Switching: Data transmission using a packet-switched network.

Peer-To-Peer Process: A process on a sending and a receiving machine that communicates at agiven layer.

Physical Layer: The first layer of the Internet model, responsible for the mechanical andelectrical specifications of the medium.

Switch: A device connecting multiple communication lines together.

Topology: The structure of a network including physical arrangement of devices.

Wide Area Network (WAN): A network that uses a technology that can span a largegeographical distance.