interview question for network administrator 6

Describe Domain Name System. There are two types of client/server programs. First is used directly by the user, such as e-mail, Second supports other application programs. The Domain Name System (DNS) is a supporting program that is used by other programs such as to find the IP address of an e-mail recipient.

Describe Hierarchical Namespace. In a hierarchical name space, each name is combination of several parts as: Type of the organization. Name of an organization. Departments in the organization. A central authority can assign the part of the name that defines the nature and name of the organization. The other things of the name the organization provides itself. The organization can add suffixes (or prefixes) to the name to define its host. The company need not worry about the prefix if it is same as that of another organization because, even if part of an address is the same, the whole address is different. Example: Suppose central authority has given names ABC.com and XYZ.com to two separate organization but these organization can add prefix as career so the name becomes career.XYZ.com and career.ABC.com. The names are unique without the need for assignment by a central authority. The central authority controls only part of the name, not the whole.

Define Fully Qualified Domain Name and Partially Qualified Domain Name. In hierarchical name space, names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127. Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string (empty string). In this tree, all the labels have different name, which guarantee the uniqueness of the domain names. A full domain name is a sequence of labels separated by dots (.). The domain names are always read from the node up to the root. Fully Qualified Domain Name If a label is terminated by a null string or empty string, it is called a fully qualified domain name (FQDN). Partially Qualified Domain Name If a label is not terminated by a null string, it is called a partially qualified domain name (PQDN). A PQDN starts from a node, but it does not end with the root. What is transmission impairment? Signals need transmission media for travel from one point to another. Transmission media are not perfect because it provides resistance. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. There are three causes of impairment are attenuation, distortion, and noise.

Describe attenuation, distortion, and noise in brief. Attenuation: When a signal travels through a medium, it loses some of its energy due to resistance of the medium. This loss of energy is called Attenuation. This is the reason why a wire carrying electric signals gets warm. Some of the electrical energy in the signal is converted to heat. To overcome for this problem, amplifiers are used to amplify the signal. Unit of the decibel is used to find out if a signal has lost or gained strength. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. If decibel is negative then a signal is attenuated and if positive signal is amplified. Formula: Where PI and P2 are the variable and the powers of a signal at points 1 and 2, respectively. Distortion When the signal travels through the medium from one point to another it may chance to change the form or shape of signal. It is called distortion. Distortion can occur in a composite signal, made up of different frequencies. Each signal component has its own propagation speed through a travelling medium and, therefore, its own delay in reaching at the final destination. Means signal components at the receiver have phases different from what they had at the sender. Noise The third cause of impairment is Noise. Following types of noise are possible. -Thermal noise. -Induced noise. -Crosstalk noise.  -Impulse noise. These noise may corrupt the signal. Thermal noise is produced due to the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and other electronic appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on the other. Impulse noise comes from power lines, lightning etc.

Describe composite single. Composite Signals are the combination of more than one sine waves. Simple sine waves can be used to carry electric energy from one place to another or we can use a single sine wave to send an alarm to a security center when a thief opens a door or window in the house. A single frequency sine wave is not useful in data communications we need to send a composite signal. A composite signal can be periodic or non-periodic. The figure of composite periodic signal is given below.

Describe MIME Electronic mail has a simple structure. It can send messages only in NVT 7-bit ASCII format. It cannot be used for those languages that are not supported by 7-bit ASCII characters so languages such as French, German, Hebrew, Russian, Chinese etc cannot use this format. Also, it cannot be used to send binary files or video or audio data. We need a protocol that allows non-ASCII data to be sent through e-mail. So Multipurpose Internet Mail Extensions (MIME) is the answer. MIME transforms non-ASCII data at the sender site to NVT (Network Virtual Terminal) ASCII data and delivers them to the client MTA to be sent through the Internet. The message at the receiving side is transformed back to the original data. Main task of MIME is to transform non-ASCII data to ASCII data and vice versa. MIME header Multipurpose Internet Mail Extensions defines five types of header.

SMTP is push protocol Justify your answer. SMTP is a push protocol; it pushes the message from the client to the server. For sending email SMTP is used two times, between the sender and the sender's mail server and between the two mail servers. To send mail, a system must have the client MTA (Message transfer agents), and to receive mail, a system must have a server MTA. SMTP uses commands and responses to transfer messages between an MTA client and an MTA server. Commands are sent from the client to the server and Responses are sent from the server to the client. SMTP is not used in pulling the message.

Describe the POP3 protocol. Post Office Protocol, version 3 is used for accessing the message. POP3 is very simple protocol and has limited functionality. POP has different version but currently version 3 is used. A POP3 server listens on well-known port 110. POP3 supports two types of mode. Delete Mode: In this mode, the mail is deleted from the mailbox after each retrieval. Keep Mode. The mail remains in the mailbox after retrieval.

Describe IMAP4 (Internet Mail Access Protocol) in detail. This protocol is also used in Internet for accessing the mail. IMAP4 has more feature than POP3 and is more powerful. Main difference in POP3 and IMAP4 are as follows.

What is Anonymous FTP? Some sites provide the functionality that user can access the file with limited access. To access these files, a user does not need to have an account or password. For using FTP, the user enters "anonymous" as a user ID and guest as password. Usually, you can enter anything as a password. What is User Agent? User agent is a software that provides the service to the user to make the process of sending and receiving a message easier. The following are User Agent services. Describe Classful Addressing IPv4 addressing uses the concept of classes. In classful addressing, the address space is divided into five classes:A, B, C, D, and E. We can find the class of an address when the given address is in binary notation or dotted-decimal notation.  Example  Find the error, if any, in the following IPv4 addresses. a. 200.56.045.78  b. 22.34.7.8.20  c. 75.45.279.14  d. 11100010.23.24.66 Solution a. There must be no leading zero (045).  b. There can be no more than four numbers in an IPv4 address.  c. Each number needs to be less than or equal to 255 (279 is outside this range).  d. A mixture of binary notation and dotted-decimal notation is not allowed. Describe how many types are wireless transmission? In general we can divide five types of wireless transmission 1. The Electromagnetic Transmission  2. Radio Transmission  3. Microwave Transmission  4. Infrared Transmission  5. Light Transmission 1. The Electromagnetic Transmission When electrons move, they create electromagnetic waves that can propagate through space and in a vacuum. The electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. 2. Radio Transmission Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication. These waves can travel in all directions. 3. Microwave Transmission Microwaves travel in a straight line, so if the towers are too far apart, the earth will get in the way. Thus, repeaters are needed periodically. The higher the towers are, the farther apart they can be. Unlike radio waves at lower frequencies, microwaves do not pass through buildings well. 4. Infrared Transmission It is used for short-range communication. The remote controls used for televisions, VCRs, and stereos, use infrared communication. They are relatively directional and can not pass through solid objects. 5. Light Transmission These types of wave can easily effected by wind and temperature changes. They normally work well on sunny days. Explain Random Excess Method. In Random Excess Method, any station can send the data at any time without the permission of other station. At any time, if a station has information to send to other station, it uses a specific method defined by the protocol to make a decision on whether or not to send. There is no scheduled time for a station to transmit the data that is why it is called Random Excess Protocol. Random protocol has the following types.  Explain Channelization in networks. Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations. There are three basic approaches to achieve channelization. Describe FDMA and TDMA in short. In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. Different frequency bands are used by different stations. In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in is assigned time slot. Explain CDMA in wireless networks. CDMA differs from FDMA and TDMA because only one channel occupies the entire bandwidth of the link and also there is no timesharing. In CDMA a code is available for every station wanting to send the data. In this protocol the assigned codes have two properties. 1. If we multiply each code by another, we get zero.  2. If we multiply each code by itself, we get the number of stations. So suppose there are three stations and code and data are (c1, d1), (c2, d2), (c3, d3) respectively. The data that go on the channel are the sum of all these terms. Any station wanting to receive data from one of the other two, multiplies the data on the channel by the code of the sender. For example, suppose stations 1 and 2 are talking to each other. Station 2 wants to hear what station I is saying. It multiplies the data on the channel by cl the code of station 1. Because (cl . cl) is 3, but (c2 . cI) and (c3 . cI) are all Zero, station 2 divides the result by 3 to get the data from station 1. Describe the functionality of data link layer. Data Link Layer is the second layer from the bottom in OSI model. This layer has two main functions - Data link control  - Media access control. Data link control provides the functionality of include framing, flow and error control that provide smooth and reliable transmission of frames between nodes. Media Access Control (MAC) handles the sharing of link. Explain Stop-and-Wait Automatic Repeat Request for noisy channel. In this protocol the sending device keeps a copy of the last frame transmitted until it receives an acknowledgment for that frame. A data frames uses a sequence number; an ACK frame uses an acknowledgment number. The sender has a control variable, which we call Sn (sender, next frame to send), that holds the sequence number for the next frame to be sent (0 or 1).