interview question for network administrator 12

Explain stop and wait flow control. - Here transmitter sends one message and waits for acknowledgement(ACK) - After receiving ACK it transmits next message. - Its main advantage is it is very simple and easy to understand. - Its main disadvantage is its speed is very slow and time is wasted. - Here it takes twice as long to transmit data and receive ACK.

Explain Poll/select. - It is a line discipline used for centrally controlled networks. - Here primary station controls data and secondary station responds to instructions. - Primary polls to secondary to determine if it has data to transmit. - If secondary has data it responds to poll with a message and acts as a transmitter. - If primary selects secondary, it identifies secondary as the receiver and checks if it is ready to receive data.

Explain the function of line discipline. - Line discipline is used in the data link layer which coordinates hop to hop data delivery. - Here the hop can be a computer or a network controller. - It also coordinates the half duplex transmission on data communication network. - Acknowledgement discipline is used for simple networks. - Poll/select is used for centrally controlled data networks.

Give the difference between cable and voice modems. - Cable modem operates on higher frequencies up to 850MHz. - Cable modem uses more complex modulation and demodulation techniques than simple FSK modulation. - Cable modems connect subscribers to cable tv and voice modem connect subscriber to the telephone network. - Cable modem works on Bit rate of 30Mbps and voice modem works on 56000 bps. - Cable modem works on voice and video whereas voice modem work only on voice.

What are scramble and descramble circuits used for? - Scramble circuit detects the undesirable sequence of 0 and 1 and convert them into sequence more acceptable. - It randomizes the data to convert data back at the receiver. - For this different algorithms are used both at the transmitter and the receiver. - Descramble circuit generates the original sequence at the receiver. - Example: eight consecutive 0’s are converted to alternate 0’s and 1’s which is more acceptable to the clock recovery.

Explain adaptive equalizers. - These are located in the receiver section of the modem. - It automatically adjusts the gain and delay of the input received to compensate for the phase and amplitude impairments. - It determines the quality of the received signal. - It continuously vary its settings to achieve bandwidth characteristics for the circuit. - Because of their adjusting property they are called adaptive equalizers

Issues for data interfacing in physical layer. - Raw data is to be converted into binary form 0 and 1 to be sent to the next layer. - Different voltages are required for representing 0(0 to1.5 volts) and 1(3 to 5 volts). - How many nano seconds a bit should last. - How transmission may proceed from physical layer to data link layer or from communication channel to physical layer . - How many pins network connectors may use for serial communication.

Which physical property of information is changed for transmission? - Current or Voltage of signal is changed for transmitting it in the form of zeros and ones. - Signal broken into harmonics called frequency using Fourier transform. - Amplitude of signals due to Fourier transform may be reduced. - A filter is attached before the transmission medium which allows only required harmonics to pass through the medium. - Signal at the receiver end can be reconstructed from harmonics using inverse Fourier transform .

Explain resource sharing - Different resources are attached to a computer and if they can be used by other computers in the same network it is called resource sharing. - It is generally possible in local area networks confined to offices or institutes. - The host requires an operating system to access the resources of the server. - FTP called the file transfer protocol is used to provide the shared access between host and the server. - It is cheaper, faster and easier to maintain.

Which transmission medium radiates and how it can be avoided? - Twisted pair cable radiates energy during transmission. - Two wires present in the cable constitute an Antenna which radiates energy in all directions. - The wires are twisted like a DNA present in human blood. - Waves coming out from the two twisted wires are out of phase by 180 degree so they cancel out. - Leads to no radiations and also sheath could be provided outside the cable to provide insulation. Which category of twisted pair is mainly used. - There are up to 7 categories of twisted pair but Category 3 and 5 twisted pairs are mostly used. - Category 3 has 2 individually insulated wires twisted together . - 4 such twisted pairs are put together in a sheath to form the medium. - Category 5 is same as 3 with more number of twist per cm. More the number of twists the better the communication medium . - Leads to less cross talk and better signal transmission over longer distances. Which transmission medium works at 2 giga bits per second. - The fiber cable sends data at rate of 2 giga bites per second. - Cable consists of Hair like thin fiber which works on the principle of total internal reflection. - It transmits data with speed of light and works on bandwidth up to terahertz. - Transmitting medium is fiber of glass which is insulated on the outside. - Advantage:Attenuation of signal is minimum. Speed of transmission is very high. Can infrared be used for transmission - The infrared waves can be used for short range communication in networking (up to 45 km). - It is wireless communication so no physical medium is required to connect the transmitter and receiver . - Works on line of sight system i.e. the transmitting and receiving antennas should be in the sight of each other. - No government license is needed till now for this communication . - Example :used in television ,stereos. Compare satellite and cable medium for transmission. - Delay given by satellite during its transmission is 3 micro seconds per km. - Delay given by cable wire is 5 micro seconds per km. - Satellite is broadcast media placed in the geo stationary orbit around the earth. - Satellite is used in wireless communication system and cable is used in wired communication. - On security grounds cable is better than a satellite. Explain Store and forward Network. - Store and forward method is used in message switching. Example: SMS - The data sent by the sender is first stored at the server. - Data is later forwarded from the server to the receiver. - It may use time division multiple access for transmission i.e. the whole bandwidth is given to single user for smalltime slots. - No physical path is required for transmission of data from sender to the receiver.