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CS1302- Computer Networks 2-Mark Question & Answer
CS 1302 - Computer Networks
1. Which Technologies of this age had led to the emergence of computer network? The technologies are Computer technology and Communication technology with
the support of VLSI Technology.
2. What are the two broad classifications under which Networks can be divided? All computer networks fit in one of the two dimensions namely,
Transmission Technology, this focuses on the basic underlying physical network, for e.g. whether the nodes share a communication media or each pair of node has a separate dedicated link. Scale, it focuses on the scale of network how large is your network.
3. Mention different categories of computer networks (on the basis of scale) and distinguish one from the other.
Local Area Network (LAN): It is privately owned communication systems that cover a small area, say a building or a complex of buildings. Length is about 10 meters to few kilometers and operates at a high speed like 10 MBPS to 1000 MBPS. It has very low error rate (1:1011). Metropolitan Area Network (MAN): It is public or privately owned communication system that typically covers a complete city. Speed is about 10 MBPS and follows DQDB (Distributed Queue Double Bus) standard. Its reliability is moderate. Wide Area Network (WAN): It covers a large geographical area and usually owned by a state. Data transfer rate is low (few KBPS to 10 MBPS) and error rate is much higher.
4. What are the two types of Transmission technologies, basis on which computer networks can be categorized?
Broadly there are two types of transmission technology: Broadcast networks: a single communication channel that is shared by all the machines on the network Point-to-point networks: This network consists of many connections between individual pairs of machines. To go from the source to destination a message (or packet) may have to visit one or more intermediate machines
5. What is Internet? Internet is a collection of networks or network of networks. Various networks such as LAN and WAN connected through suitable hardware and software to work in a seamless manner. It allows various applications such as e-mail; file transfer, remote log-in, World Wide Web, Multimedia, etc run across the internet.
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CS1302- Computer Networks 2-Mark Question & Answer
6. How do you account for higher reliability and scalability of computer network? Computer network can have a large number of computers, which can share
software, database and other resources. In the event of failure of one computer, its workload can be taken over by other computers. So, it provides higher reliability than centralized computing system.
Requirement of software, hardware, database etc. increases gradually. In centralized computing system, if one computer is not able to serve the purpose, we have to replace it by new one. Replacement of new computer requires lot of investment and effort, which can be avoided in computer network system. If there is need for more, one can buy another powerful computer, add it to computer network and use it. The various resources like computers, peripherals, etc. can be added in a scalable manner.
7. Mention important benefits of computer network. Important benefits of computer networks are:
Resource sharing Powerful communication medium Higher reliability Higher flexibility Lower cost Incremental expansion
8. What are the main categories based on which applications of computer network can be categorized?
The main areas under which the applications for computer network can be categorized are as follows:
Scientific and Technical Computing – Client Server Model, Distributed Processing – Parallel Processing, Communication Media
Commercial – Advertisement, Telemarketing, Teleconferencing – Worldwide Financial Services
Network for the People (this is the most widely used application nowadays)
– Telemedicine, Distance Education, Access to Remote Information, Person-to-Person Communication, Interactive Entertainment
9. Why it is necessary to have layering in a network? A computer network is a very complex system. It becomes very difficult to
implement as a single entity. The layered approach divides a very complex task into small pieces each of which is independent of others and it allow a structured approach in implementing a network. The basic idea of a layered architecture is to divide the design into small pieces. Each layer adds to the services provided by the lower layers in such a manner that the highest layer is provided a full set of services to manage communications and run the applications.
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CS1302- Computer Networks 2-Mark Question & Answer
10. What are the key benefits of layered network? Main benefits of layered network are given below:
Complex systems can be broken down into understandable subsystems. Any facility implemented in one layer can be made visible to all other layers. Services offered at a particular level may share the services of lower level. Each layer may be analyzed and tested independently. Layers can be simplified, extended or deleted at any time. Increase the interoperability and compatibility of various components build by different vendors.
11. What do you mean by OSI? The Open System Interconnection (OSI) reference model describes how
information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Standardization Organization (ISO) in 1984, and it is now considered the primary architectural model for inter-computer communications.
12. What are the seven layers of ISO’s OSI model? The seven layers are:
Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer
13. What are the key functions of data link layer? Data link layer transfers data in a structured and reliable manner so that the
service provided by the physical layer is utilized by data link layer. Main function of data link layer is framing and media access control.
14. What do you mean by Protocol? In the context of data networking, a protocol is a formal set of rules and
conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers.
15. On what parameters the quality of transmission depends in case of guided transmission media?
It is mainly decided by the frequency of transmission and the characteristics of the transmission media.
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CS1302- Computer Networks 2-Mark Question & Answer
16. Why wires are twisted in case of twisted pair of transmission medium? It minimizes electromagnetic interferences between the pairs of wires, which are
bundled together, so that the cross talk is minimum.
17. Give a popular example where co-axial cables are used for broadband signaling. Use of co-axial cable for broadband signaling is cable TV (CATV) application.
18. What devices are used as source and detector in case of single mode of fiber?LASER is used as source and photodiode is used as detector in case of single
mode of fiber.
19. In what way multi-mode and single-mode fibers differ? The core diameter of single-mode fiber is much smaller than that of multi-mode
fiber. For example, For multi-mode fiber:
- Core diameter lies in the range of 50-200μm - Cladding diameter lies in the range of 125-400μm - Repeater spacing is 2Km.
For single-mode fiber: - Core diameter lies in the range of 8-12μm - Cladding diameter 125μm - Repeater spacing is 20Km.
20. Why does single-mode fibres are used for large distance communications rather than multi-mode fibres?
In a multi-mode fiber, the quality of signal-encoded light deteriorates more rapidly than single-mode fiber, because of interference of many light rays. As a consequence, single-mode fiber allows longer distances without repeater. For multi-mode fiber, the typical maximum length of the cable without a repeater is 2km, whereas for single-mode fiber it is 20km.
21. What is crosstalk? How is it minimized in case of twisted-pair of wire? Crosstalk refers to the picking up of electromagnetic signals from other adjacent wires by electromagnetic induction. When a pair of wires is twisted together, the electromagnetic signals generated by the two wires cancel each other as these are of opposite polarity. This helps to reduce the susceptibility of interference to the adjacent wires.
22. What are the factors responsible for attenuation in case of terrestrial microwave communication?
Attenuation due to distance is 10 log (4πd/λ)2. Factors responsible for attenuation are given below:
Distance – Attenuation is more if distance increases.Wavelength – Attenuation is less if wavelength is longer. (i.e high frequency components are attenuated more than the low frequency component)Rainfall – Attenuation is less if there is no rain.
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CS1302- Computer Networks 2-Mark Question & Answer
23. What parameters decide the spacing of repeaters in case of terrestrial microwave communication?
Parameters are the height of the antenna ‘h’ and adjustment factor ‘k’ based on the relation d = 7.14√kh, where d is the distance in Km between two the two antennas.
24. Why two separate frequencies are used for uplink and downlink transmission in case of satellite communication?
Two separate frequencies are used so that one cannot interfere with the other and full duplex communication is possible. And other reason is that the Power required to transmit a signal is proportional to the frequency of the signal. And more power requirement more would be the weight of the system. As there are constraints on the load that can be carried with the satellite, mainly down linking frequency is lower than the up linking one.
25. Why uplink frequencies are higher than downlink frequencies in case of satellite communication?
The satellite gets power from solar cell. So, the transmitter is not being of higher power. On the other hand the ground station can have much higher power. As we want less attenuation and better signal-to-noise ratio, lower frequency is more suitable for downlink and higher frequency is commonly used for uplink.
26. Why do you need encoding of data before sending over a medium? Suitable encoding of data is required in order to transmit signal with minimum
attenuation and optimize the use of transmission media in terms of data rate and error rate.
27. What are the four possible encoding techniques? Give examples. The four possible encoding techniques are
Digital Data to Digital Signal; Example - Transmitter Analog Data to Digital Signal; Example - Codec (Coder-Decoder) Digital Data to Analog Signal; Example - Modem Analog Data to Digital Signal; Example - Telephone
28. Between RZ and NRZ encoding techniques, which requires higher bandwidth and why?
RZ encoding requires more bandwidth, as it requires two signal changes to encode one bit.
29. How does Manchester encoding differ from differential Manchester encoding?
In the Manchester encoding, a low-to-high transition represents a 1, and a high-to-low transition represents a 0. There is a transition at the middle of each bit period, which serves the purpose of synchronization and encoding of data. In Differential Manchester, the encoding of a 0 is represented by the presence of a transition at the beginning of a bit period, and a 1 is
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CS1302- Computer Networks 2-Mark Question & Answer
represented by the absence of a transition at the beginning of a bit period. In this case, the midbit transition is only used for synchronization.
30. How Manchester encoding helps in achieving better synchronization? In Manchester encoding, there is a transition in the middle of each bit period and
the receiver can synchronize on that transition. Hence better synchronization is achieved.
31. Why B8ZS coding is preferred over Manchester encoding for long distance communication?
The B8ZS encoding is preferred over Manchester encoding, because B8ZS encoding requires lesser bandwidth than Manchester encoding.
32. Why is it necessary to limit the band of a signal before performing sampling? It is necessary to limit the bandwidth of a signal before sampling so that the basic
requirement of sampling theorem, i.e. the sampling rate should twice or more than twice the maximum frequency component of the signal, is satisfied. This is known as Nyquist rate. If it is violated, original signal cannot be recovered from the sampled signal.
33. Distinguish between PAM and PCM signals? In order to convert Analog data to Digital signal, initially sampling is done on the
analog data by using Sample & Hold (S/H) circuit. The output of the S/H circuit is known as PAM (Pulse Amplitude Modulated) signal. The PAM signal is then converted to PCM (Pulse Code Modulated) data by using a Analog-to-Digital (A/D) converter circuit. This digital data (PCM) is passed through an encoder to generate PCM signal.
34. What is quantization error? How can it be reduced? To convert analog signal to digital signal, the analog signal is first sampled and
each of these analog samples must be assigned a binary code. In other words, each sample is approximated by being quantized into some binary codes. As the quantized values are only approximations, it is impossible to recover the original signal exactly. Error due to this quantization is known as quantization error. Quantization error can be minimized by using non-linear encoding.
35. Explain how and in what situation DPCM performs better than PCM DPCM performs better when the input is slowly changing, as in case of a voice
signal.
36. Distinguish between data rate and baud rate?
Data rate is the rate, in bits per second (bps), at which the data can be communicated. Baud rate is the rate of transmitting the signal elements including the bits required for synchronization.
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CS1302- Computer Networks 2-Mark Question & Answer
37. What is bit-stuffing? Why is it used?
Bit-stuffing: In case of synchronous transmission, flag bits (8-bit length sequence 01111110) are attached at the beginning and end of each frame. These flag bits are used for synchronization. It may so happen that the flag bit sequence may appear somewhere inside the frame and this will cause a problem for synchronization. To avoid this problem a process called bit-stuffing is used, in which extra bit is stuffed if flag bit like sequence appears inside the frame. For example, in HDLC the transmitter introduces a 0 after each occurrence of five 1’s in the data sequence by. At the receiving end these extra 0’s are removed.
38. Why do you need error detection?
As the signal is transmitted through a media, the signal gets corrupted because of noise and distortion. In other words, the media is not reliable. To achieve a reliable communication through this unreliable media, there is need for detecting the error in the signal so that suitable mechanism can be devised to take corrective actions.
39. Explain different types of Errors?
The errors can be divided into two types: Single-bit error and Burst error.
Single-bit Error - The term single-bit error means that only one bit of given data unit (such as a byte, character, or data unit) is changed from 1 to 0 or from 0 to 1.
Burst Error - The term burst error means that two or more bits in the data unit have changed from 0 to 1 or vice-versa. Note that burst error doesn’t necessary means that error occurs in consecutive bits.
40. Explain the use of parity check for error detection?
In the Parity Check error detection scheme, a parity bit is added to the end of a block of data. The value of the bit is selected so that the character has an even number of 1s (even parity) or an odd number of 1s (odd parity). For odd parity check, the receiver examines the received character and if the total number of 1s is odd, then it assumes that no error has occurred. If any one bit (or any odd number of bits) is erroneously inverted during transmission, then the receiver will detect an error.
41. What are the different types of errors detected by parity check?
If one bit (or odd number of bits) gets inverted during transmission, then parity check will detect an error. In other words, only odd numbers of errors are detected by parity check. But, if two (or even number) of bits get inverted, and then the error remains undetected.
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42. What are the key functions of error control techniques?
There are basically two types of errors, namely, (a) Damaged Frame (b) Lost Frame. The key functions for error control techniques are as follows:
Error detection Sending of positive acknowledgement (ACK) by the receiver for no error Sending of negative acknowledgement (NAK) by the receiver for error Setting of timer for lost frame Numbering of frames
43. Why is flow control needed?
In case of data communication between a sender and a receiver, it may so happen that the rate at which data is transmitted by a fast sender is not acceptable by a slow receiver. IN such a situation, there is a need of flow control so that a fast transmitter does not overwhelm a slow receiver.
44. Mention key advantages and disadvantages of stop-and-wait ARQ technique?
Advantages of stop-and-wait ARQ are:
Simple to implement Frame numbering is modulo-2, i.e. only 1 bit is required.
The main disadvantage of stop-and-wait ARQ is that when the propagation delay is long, it is extremely inefficient.
45. Consider the use of 10 K-bit size frames on a 10 Mbps satellite channel with 270 ms delay. What is the link utilization for stop-and-wait ARQ technique assuming P = 10-3?
Link utilization = (1-P) / (1+2a) Where a = (Propagation Time) / (Transmission Time) Propagation time = 270 msec Transmission time = (frame length) / (data rate)
= (10 K-bit) / (10 Mbps) = 1 msec Hence, a = 270/1 = 270 Link utilization = 0.999/(1+2*270) ≈0.0018 =0.18%
46. What is the channel utilization for the go-back-N protocol with window size of 7 for the problem 3? Channel utilization for go-back-N = N(1 – P) / (1 + 2a)(1-P+NP)
P = probability of single frame error ≈ 10-3
Channel utilization ≈ 0.01285 = 1.285%
47. In what way selective-repeat is better than go-back-N ARQ technique?
In selective-repeat scheme only the frame in error is retransmitted rather than transmitting all the subsequent frames. Hence it is more efficient than go-back-N ARQ technique.
48. In what situation Stop-and-Wait protocol works efficiently?
In case of Stop-and-Wait protocol, the transmitter after sending a frame waits for the acknowledgement from the receiver before sending the next frame. This protocol works efficiently for long frames, where propagation time is small compared to the transmission time of the frame.
49. How the inefficiency of Stop-and-Wait protocol is overcome in sliding window protocol?
The Stop-and-Wait protocol is inefficient when large numbers of small packets are send by the transmitter since the transmitter has to wait for the acknowledgement of each individual packet before sending the next one. This problem can be overcome by sliding window protocol. In sliding window protocol multiple frames (up to a fixed number of frames) are send before receiving an acknowledgement from the receiver.
50. What is piggybacking? What is its advantage?
In practice, the link between receiver and transmitter is full duplex and usually both transmitter and receiver stations send data to each over. So, instead of sending separate acknowledgement packets, a portion (few bits) of the data frames can be used for acknowledgement. This phenomenon is known as piggybacking.
The piggybacking helps in better channel utilization. Further, multi-frame acknowledgement can be done.
51. For a k-bit numbering scheme, what is the range of sequence numbers used in sliding window protocol?
For k-bit numbering scheme, the total number of frames, N, in the sliding window can be given as follows (using modulo-k).
N = 2k – 1 Hence the range of sequence numbers is: 0, 1, 2, and 3 … 2k – 1
52. What are the different types of stations Supported by HDLC?
HDLC specifies the following three types of stations for data link control:
Primary Station: It is used as the controlling station on the link. It has the responsibility of controlling all other stations on the link (usually secondary
stations). It is also responsible for the organization of data flow on the link. It also takes care of error recovery at the data link level.
Secondary Station: The secondary station is under the control of the primary station. It has no ability, or direct responsibility for controlling the link. It is only activated when requested by the primary station. It can only send response frames when requested by the primary station.
Combined Station: A combined station is a combination of a primary and secondary station. On the link, all combined stations are able to send and receive commands and responses without any permission from any other stations on the link.
53. What are the three different Configurations supported by HDLC?
The three configurations defined by HDLC:
Unbalanced Configuration: The unbalanced configuration in an HDLC link consists of a primary station and one or more secondary stations. The unbalanced occurs because one stations controls the other stations
Balanced Configuration: The balanced configuration in an HDLC link consists of two or more combined stations. Each of the stations has equal and complimentary responsibility compared to each other.
Symmetrical Configuration: It consists of two independent point to point, unbalanced station configurations. In this configuration, each station has a primary and secondary status. Each station is logically considered as two stations.
54. What are the three modes of operations of HDLC?
These three modes of operations are:
Normal Response Mode (NRM): The primary station initiates transfers to the secondary station. The secondary station can only transmit a response when, and only when, it is instructed to do so by the primary station
Asynchronous Response Mode (ARM): The primary station doesn't initiate transfers to the secondary station. In fact, the secondary station does not have to wait to receive explicit permission from the primary station to transfer any frames. The frames may be more than just acknowledgment frames.
Asynchronous Balanced Mode (ABM): This mode uses combined stations. There is no need for permission on the part of any station in this mode. This is because combined stations do not require any sort of instructions to perform any task on the link.
55. Name HDLC Non-Operational Modes.
HDLC also defines three non-operational modes. These three non-operational modes are:
Normal Disconnected Mode (NDM) Asynchronous Disconnected Mode (ADM) Initialization Mode (IM)
The two disconnected modes (NDM and ADM) differ from the operational modes in that the secondary station is logically disconnected from the link. The IM mode is different from the operations modes in that the secondary station's data link control program is in need of regeneration or it is in need of an exchange of parameters to be used in an operational mode.
56. What are the three basic steps involved in data communication through circuit switching?
The steps are: i) Circuit establishment (before data transfer)
ii) Circuit maintenance (When data transfer is going on) iii) Circuit disconnect (When data transfer is over)
57. Mention the key advantages and disadvantages of circuit switching technique. Advantages:
i) After path is established, data communication without delay. ii) Very suitable for continuous traffic. iii) It establishes a dedicated path. iv) No overhead after call setup. v) It is transparent and data passes in order.
Disadvantages: i) Provide initial delay for setting up the call.
ii) Inefficient for bursty traffic. iii) Data rate should be same because of fixed bandwidth. iv) When load increases, some calls may be blocked.
58. Why data communication through circuit switching is not efficient? In data communication, traffic between terminal and server are not continuous.
Sometimes more data may come or sometimes there is no data at all. Circuit switching is not efficient because of its fixed bandwidth 59. Compare the performance of space-division single-stage switch with multi-stage switch.
Space-division single-stage switch requires more number of crosspoints, nonblocking in nature but provides no redundant path. On the other hand multi-stage switches require lesser number of crosspoints, blocking in nature but provides redundant paths.
60. How the drawback of circuit switching is overcome in message switching? Message switching is based on store and forward technique. Instead of
establishing a dedicated path, the message is sent to the nearest directly connected node. Each node stores the message, checks for error and forwards it. It allows more devices to
share the network bandwidth and one message can be sent to several users. Destination host need not be on at the time of sending message.
61. What is the drawback of message switching? How is it overcome in packet switching?
In message switching, large storage space is required at each node to buffer the complete message blocks. On the other hand, in packet switching, messages are divided into subset of equal length, which are generated in the source node and reassembled to get back the initial complete message in destination node. Moreover, to transmit a message of large size, link is kept busy for a long time leading to increase in delay for other messages.
62. What are the key differences between datagram and virtual-circuit packet switching?
In datagram, the packets are routed independently and it might follow different routes to reach the destination in different order. In virtual-circuit packet switching, first a virtual connection is being established, and all the packets are sent serially through the same path. In this case, packets are received in order.
63. Distinguish between circuit switching and virtual-circuit packet switching. In circuit switching, a dedicated path is established. Data transmission is fast and
interactive. Nodes need not have storage facility. However, there is a call setup delay. In overload condition, it may block the call setup. It has fixed bandwidth from source to destination and no overhead after the call setup.
In virtual-circuit packet switching, there is no dedicated path. It requires storage facility and involves packet transmission delay. It can use different speed of transmission and encoding techniques at different segments of the route.
64. How packet size affects the transmission time in a packet switching network? Initially, transmission time decreases as packet size is reduced. But, as packet size
is reduced and the payload part of a packet becomes comparable to the control part, transmission time increases.
65. Describe Section, Line and path. A section is an optical link, connecting two neighbor devices: multiplexer to
multiplexer, multiplexer to regenerator, or regenerator to regenerator. A line is a portion of network between two multiplexers: STS to add/drop
multiplexer, two add/drop multiplexer, or two STS multiplexer. A Path is the end-to-end portion of the network between two STS multiplexer.
66. Define the functionality of Add/Drop multiplexer. A single-stage multiplexer/demultiplexer can multiplex various inputs into an
OC–N signal. It can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it without demultiplexing the entire signal. Instead of relying on timing and bit positions, add/drop multiplexer use header information such as addresses and pointers to identify individual streams. The add/drop
multiplexer provides interfaces between the different network signals and SONET signals.
67. Define the functionality of Wide band Digital Cross-Connect. A SONET cross-connect accepts various optical carrier rates, accesses the STS–1
signals, and switches at this level. It is ideally used at a SONET hub. One major difference between a cross-connect and an add/drop multiplexer is that a cross-connect may be used to interconnect a much larger number of STS–1s.
68. Describe various Overheads. Below is the description of the overheads in SONET:
Section Layer and Overhead: This layer is responsible for movement of a signal across a physical section. It handles framing, scrambling, and error control. Section overhead, which is added in this layer contains 9 bytes of the transport overhead accessed, generated, and processed by section-terminating equipment.
Line Layer and Overhead: This layer is responsible for the movement of a signal across a physical line. STS multiplexer and add/drop multiplexers provide line layer functions. Line overhead contains 18 bytes of overhead accessed, generated, and processed by line-terminating equipment.
69. Define Virtual Tributary. A virtual tributary is a partial payload that can be inserted into an STS1 and
combined with other partial payloads to fill out the frame. Instead of using 86 payload columns of an STS1 frame for data from one source, we can sub-divide the SPE and call each component as a VT.
70. Describe the functionality of Pointers. SONET uses a concept called pointers to compensate for frequency and phase
variations. Pointers allow the transparent transport of synchronous payload envelopes (either STS or VT) across plesiochronous boundaries (i.e., between nodes with separate network clocks having almost the same timing). The use of pointers avoids the delays and loss of data associated with the use of large (125-microsecond frame) slip buffers for synchronization.
Pointers provide a simple means of dynamically and flexibly phase-aligning STS and VT payloads, thereby permitting ease of dropping, inserting, and cross-connecting these payloads in the network.
71. In what layers X.25 operates? X.25 operates in the network layer.
72. What are the key functions of X.25 protocol? Key functions of X.25 protocol are:
Call control packets are used for call set-up. Multiplexing of virtual circuits take place in packet layer. Both link layer and packet layer performs flow control and error control.
73. What limitation of X.25 is overcome in Frame Relay Protocol? In X.25, overhead on the user equipment and the networking equipment is very high
and it is also slower (can go up to 64 kbps only), which are overcome in Frame Control Protocol.
74. Explain the functionalities of DTE, DCE, PSE.
Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers. DCE devices are communications devices, such as modems and packet switches that provide the interface between DTE devices and a PSE, and are generally located in the carrier's facilities. PSEs are switches that compose the bulk of the carrier's network. They transfer data from one DTE device to another through the X.25 PSN
75. Describe the functionalities of Link Level.
The functions performed by the link level include:
Transfer of data in an efficient and timely fashion. Synchronization of the link to ensure that the receiver is in step with the
transmitter. Detection of transmission errors and recovery from such errors Identification and reporting of procedural errors to higher levels, for recovery.
76. What protocols can be used in Link Level? There are several protocols which can be used in the link level:
Link Access Protocol, Balanced (LAPB) is derived from HDLC and is the most commonly used. It enables to form a logical link connection besides all the other characteristics of HDLC.
Link Access Protocol (LAP) is an earlier version of LAPB and is seldom used today.
Link Access Procedure, D Channel (LAPD) is derived from LAPB and it is used for Integrated Services Digital Networks (ISDN) i.e. it enables data transmission between DTEs through D channel, especially between a DTE and an ISDN node.
Logical Link Control (LLC) is an IEEE 802 Local Area Network (LAN) protocol which enables X.25 packets to be transmitted through a LAN channel.
77. Explain the different level of operation of PLP.
The PLP operates in five distinct modes: call setup, data transfer, idle, call clearing, and restarting.
Call setup mode is used to establish SVCs between DTE devices. A PLP uses the X.121 addressing scheme to set up the virtual circuit. The call setup mode is executed on a per-virtual-circuit basis.
Data transfer mode is used for transferring data between two DTE devices across a virtual circuit. In this mode, PLP handles segmentation and reassembly, bit padding, and error and flow control.
Idle mode is used when a virtual circuit is established but data transfer is not occurring
Call clearing mode is used to end communication sessions between DTE devices and to terminate SVCs. This mode is executed on a per-virtual-circuit basis and is used only with SVCs.
Restarting mode is used to synchronize transmission between a DTE device and a locally connected DCE device. This mode is not executed on a per-virtual-circuit basis. It affects all the DTE device's established virtual circuits.
78. List out the advantages and drawbacks of bus topology. Advantages:
i) Easy to implement ii) It is very cost effective because only a single segment required iii) It is very flexible iv) Moderate reliability. v) Can add new station or delete any station easily (scalable)
Disadvantages: i) Required suitable medium access control technique. ii) Maximum cable length restriction imposed due to delay and signal unbalancing problem.
79. List out the advantages and drawbacks of ring topology. Advantages:
i) Data insertion, data reception and data removal can be provided by repeater ii) It can provide multicast addressing. iii) Point-to-point links to its adjacent nodes (moderate cost)
Disadvantages: i) The repeater introduces a delay ii) The topology fails if any link disconnects or a node fails. iii) Direct link not provided iv) It provides complex management
80. Why star topology is commonly preferred? It gives high reliability, more flexible and higher bandwidth. Since there is a
central control point, the control of network is easy and priority can be given to selected nodes.
81. Is there any relationship between transmission media and topology? Yes, medium should be selected based on the topology. For example, for bus
topology coaxial cable medium is suitable, and for ring/star topology twisted-pair or optical fiber can be used. 82. What are the goals in mind of IEEE 802 committee? IEEE 802 committee has few goals in mind, namely
– To promote compatibility – Implementation with minimum efforts – Accommodate diverse applications
83. List the functions performed by the physical layer of 802.3 standard? Functions of physical layer are:
i) Data encoding/decoding (To facilitate synchronization and efficient transfer of signal through the medium). ii ) Collision detection (It detects at the transmit side) iii) Carrier sensing (Channel access senses a carrier on the channel at both the transmit and receive sides) iv) Transmit/receive the packets (Frame transmitted to all stations connected to the channel) v) Topology and medium used (Mediums are co-axial cable, twisted pair and fiber optic cable)
84. Why do you require a limit on the minimum size of Ethernet frame? To detect collision, it is essential that a sender continue sending a frame and at the
same time receives another frame sent by another station. Considering maximum delay with five Ethernet segments in cascade, the size of frame has been found to be 64 bytes such that the above condition is satisfied.
85. What are the different types of cabling supported by Ethernet standard? Types of cabling are:
10 BASE 5 - Maximum cable length is 500 meters using 4” diameter coaxial cable.
10 BASE 2 - Maximum cable length is 185 meters using 0.25” diameter CATV cable.
10 BASE T - Maximum cable length is 100 meters using twisted-pair cable (CAT-3 UTP).
10 BASE FL - Maximum cable length is 2 Km using multimode fiber optic cable (125/62.5 micrometer).
86. What is the advantage of token passing protocol over CSMA/CD protocol? Advantage of token passing protocol over CSMA/CD protocol:
The CSMA/CD is not a deterministic protocol. A packet may be delivered after many (upto 15) collisions leading to long variable delay. An unfortunate packet may not get delivered at all. This feature makes CSMA/CD protocol unsuitable for real-time applications. On the other hand, token passing protocol is a deterministic approach, which allows a packet to be delivered within a known time frame. It also allows priority to be assigned to packets. These are the two key advantages of token passing protocol over CSMA/CD protocol.
87. What are the drawbacks of token ring topology? Token ring protocol cannot work if a link or a station fails. So, it is vulnerable to
link and station failure. 88. How the reliability of token ring topology can be improved?
Reliability of the ring network can be improved by implementing the ring topology using a wiring concentrator. This allows not only to detect fault, but also to isolate the faulty link/station with the help of a bypass relay. 89. What role the active token monitor performs?
Token ring is maintained with the help of active token monitor. Any one of the stations has the capability to act as active token monitor, but at a particular instant only one acts as active token monitor. It monitors various error situations such as multiple token, orphan packet, etc, and takes appropriate action to come out of the error situation.
90. In what way the MAC protocol of FDDI differs from that of token ring? In the frame format of FDDI protocol, preamble is eight bytes instead of one byte in token ring. Also token has one additional byte.
FDDI can have multiple frames simultaneously, which cannot be present in token ring. Here, the access method is timed token passing. Multiple frames can be transmitted after capturing a token.
First, the entire token is captured and then the data frames are introduced, whereas token ring follows token passing protocol and beginning of token is converted to the header of a frame.
In case of token ring token is released after receiving the acknowledgement (as the data frame returns after circulating the ring). On the other hand, in case of FDDI, token is released immediately after sending data frame, which is known as early token release.
91. How FDDI offers higher reliability than token ring protocol? Token ring protocol is applicable in a single ring. Disadvantage of this protocol is
that, if one segment of wires fails or a node fails, the protocol cannot work. To increase reliability, dual counter ring topology used in FDDI protocol, where there are two rings, called primary ring and secondary ring. In case of failure of a node or a fiber link, the ring is restored the by wrapping up the primary ring to the secondary ring. Further improvement in reliability can achieve by using dual ring of trees and dual homing mechanism. It will provide multiple paths and if one path fails, another path will be available for passing token or data.
92. What are the functionalities of a Optical Bypass Switch? An optical bypass switch provides continuous dual-ring operation if a device on
the dual ring fails. This is used both to prevent ring segmentation and to eliminate failed stations from the ring. The optical bypass switch performs this function using optical mirrors that pass light from the ring directly to the DAS (dual-attachment station) device during normal operation. If a failure of the DAS device occurs, such as a power-off, the optical bypass switch will pass the light through itself by using internal mirrors and thereby will maintain the ring's integrity. When using the OB, you will notice a tremendous digression of your network as the packets are sent through the OB unit.
93. What are the functionalities provided by SMT standard? The Station Management (SMT) standard provides services that monitor and
control a FDDI station. SMT include facilities for connection management, node configuration, recovery from error condition, and encoding of SMT frames.
94. Describe various fields in frame format of FDDI? Let us have a look at the various fields:
SD: The first byte, after the preamble, of the field is the frame’s starting flag. As in Token ring these bits are replaced in physical layer by the control codes.
FC: it identifies the frame type i.e. token or a data frame. Address: the next 2 fields are destination and source addresses. Each address
consists of 2-6 bytes. Data: Each data frame carries up to 4500 bytes. FCS: FDDI uses the standard IEEE four-byte cyclic redundancy check. ED: this field consists of half a byte in data frame or a full byte in token frame.
This represents end of the Token. FS: FDDI FS field is similar to that of Token Ring. It is included only in
data/Command frame and consists of one and a half bytes.
95. Explain the basic difference between IEEE 802.3 and switched Ethernet, as far as implementation is concerned.
In Ethernet (IEEE 802.3) the topology, though physically is start but logically is BUS. i.e. the collision domain of all the nodes in a LAN is common. In this situation only one frame can send the frame, if more than one station sends the frame, there is a collision.
In Switched Ethernet, this collision domain is separated. Hub is replaced by a switch, a device that can recognize the destination address and can route the frame to the port to which the destination station is connected, the rest of the media is not involved in the transmission process. The switch can receive another frame from another station at the same time and can route this frame to its own final destination.
96. Explain the two techniques for implementing Ethernet switches. There are two techniques used in the implementation of Ethernet switches: store-and-forward and cut-through.
In the first case, the entire frame is captured at the incoming port, stored in the switch’s memory, and after an address lookup to determine the LAN destination port, forwarded to the appropriate port. The lookup table is automatically built up. On the other hand, a cut-through switch begins to transmit the frame to the destination port as soon as it decodes the destination address from the frame header.
Store-and-forward approach provides a greater level of error detection because damaged frames are not forwarded to the destination port. But, it introduces longer delay of about 1.2 msec for forwarding a frame and suffers from the chance of loosing data due to reliance on buffer memory. The cut-through switches, on the other hand, has reduced latency but has higher switch cost.
97. What are the different categories of Fast Ethernet? IEEE has designed two categories of Fast Ethernet: 100Base-X and 100Base-T4. 100Base-X uses two cables between hub and the station while 100Base-T4 uses four. 100-Base-X itself is divided into two: 100Base-TX and 100base-FX.
100 BASE-T4: This option is designed to avoid overwriting. It is used for half-duplex communication using four wire-pairs of the existing category 3 UTP cable, which is already available for telephone services in homes/offices. Two of four
pairs are bi-directional; other two are unidirectional. This means that there are 3 pairs to be used for carrying data, in each direction (2 bi-directional and 1 uni-directional). Because 100Mbps data cannot be handled by voice-grade UTP, this specification splits the 100 Mbps flow into three 33.66Mbps flow.
100 BASE TX: This option uses two category 5 UTP or two shielded (STP) cable to connect a station to hub. One pair is used to carry frames from the hub to the station and other to carry frames from station to hub. Encoding is 4B/5B to handle 100 Mbps; signaling is NRZ-I. The distance between station and hub should be less than 100 meters.
100 BASE FX: This option uses two Fiber optic cables, one carry frames from station to hub and other from hub to station. The encoding is 4B/5B and signaling in NRZ-I. the distance between station and hub should be less than 2000 meters.
98. What are the Objectives of The Gigabit Ethernet Alliance? The objectives of the alliance are:
Supporting extension of existing Ethernet and Fast Ethernet technology in response to demand for higher network bandwidth.
Developing technical proposals for the inclusion in the standard Establishment of inter-operability test procedures and processes
99. Explain GMII (Gigabit Media Independent Interface) in brief. The GMII is the interface between the MAC layer and the Physical layer. It allows any physical layer to be used with the MAC layer. It is an extension of the MII (Media Independent Interface) used in Fast Ethernet. It uses the same management interface as MII. It supports 10, 100 and 1000 Mbps data rates. It provides separate 8-bit wide receive and transmit data paths, so it can support both full-duplex as well as half-duplex operation.
The GMII provides 2 media status signals: one indicates presence of the carrier, and the other indicates absence of collision. With the GMII, it is possible to connect various media types such as shielded and unshielded twisted pair, and single-mode and multi mode optical fiber, while using the same MAC controller. It has three sub-layers namely: PCS (Physical Coding Sublayer), PMA (Physical Medium Attachment) and PMD (Physical Medium Dependent)
100. What are the reasons for wireless LANs not popular, if we look at recent past and make them popular now? Until recently wireless version of LANs were not popular because of the following reasons:
High cost: Previously the equipments cost more. Low data rate: Initially, the data rate supported by the WLAN is too less, so it
supports only a few applications. Occupational safety concerns Licensing requirements
101. State some advantages of Wireless LANs. Some of the advantages of wireless LANs are mentioned below:
Mobility: An increasing number of LAN users are becoming mobile. These mobile users require that they are connected to the network regardless of where they are because they want simultaneous access to the network.
Installation speed and simplicity: Wireless LANs are very easy to install. There is no requirement for wiring every workstation and every room.
Installation flexibility: If a company moves to a new location, the wireless system is much easier to move than ripping up all of the cables that a wired system would have snaked throughout the building. This also provides portability.
Reduced cost of ownership: While the initial cost of wireless LAN can be higher than the cost of wired LAN hardware, but long term cost benefits are greater in dynamic environment requiring frequent moves and changes.
Scalability: Wireless LAN can be configured in a variety of topologies to meet the users need and can be easily scaled to cover a large area with thousands of users roaming within it.
103. State few disadvantages of wireless LANs. Some of the limitations and challenges are mentioned below:
Lower reliability due to susceptibility of radio transmission to noise and interference.
Fluctuation of the strength of the received signal through multiple paths causing fading.
Vulnerable to eavesdropping leading to security problem. Limited data rate because of the use of spread spectrum transmission techniques
enforced to ISM band users. 104. Explain in brief the Frequency Hopping Spread Spectrum (FHSS) technique.
The idea behind spread spectrum is to spread the signal over a wider frequency band, so as to make jamming and interception more difficult and to minimize the effect of interference from other devices In FH it is done by transmitting the signal over a random sequence of frequencies; that is, first transmitting at one frequency, then second, then a third and so on. The random sequence of frequencies is generated with the help of a pseudorandom number generator.
105. Explain multi-path interference and a solution to it in brief.
Interference caused by signals bouncing off of walls and other barriers and arriving at the receiver at different times is called multipath interference. Multipath interference affects IR, RF, and MW systems. FHSS inherently solves the multipath problem by simply hopping to other frequencies. Other systems use anti-multipath algorithms to avoid this interference. A subset of multipath is Rayleigh fading. This occurs when the difference in path length is arriving from different directions and is a multiple of half the wavelength.
106. Explain Exposed station problem in brief.
Consider a situation where B is transmitting to A, and C sense the medium and detects the ongoing transmission between B and A. C falsely conclude that it can not transmit to D, when the fact is that such transmission would cause on problem. A transmission could cause a problem only when the destination is in zone between B and C. This problem is referred as Exposed station Problem. In this scenario as B is exposed
to C, that’s why C cannot transmit to D. So this problem is known as Exposed station problem (i.e. problem caused due to exposing of a station).
The problem here is that before transmission, a station really wants to know that whether or not there is any activity around the receiver. CSMA merely tells whether or not there is any activity around the station sensing the carrier.
107. Why do you need internetworking? As stations connected to different LANs and WANs want to communicate with each other, it is necessary to provide this facility. Internetworking creates a single virtual network over which all stations in different network can communicate seamlessly and transparently.
108. Why a repeater is called level-1 relay? A repeater operates in the physical layer. Data received on one of its ports is
relayed on the remaining port bit-by-bit without looking into the contents. That is why repeater is called a level-1 relay.
109. What is bridge? How it operates in the internetworking scenario? A bridge operates in the Data link layer. It looks into various fields of a frame to
take various actions. For example, it looks at the destination address field so that it can forward the frame to a port where destination stations is connected. It also looks at the FCS field to check error in the received frame, it any. A bridge helps to create a network having different collision domains.
110. Why spanning tree topology is necessary for routing using a bridge? If there exit more than one path between two LANs through different bridges,
there is a possibility of continuous looping of a frame between the LANs. To avoid the loop problem, spanning tree topology is used. It is essentially an overlay of tree topology on the physical graph topology, providing only one path between any two LANs.
111. What is discovery frame? In the source routing protocol, a host can discover a route by sending a discovery
frame, which spreads through the entire network using all possible paths to the destination. Each frame gradually gathers addresses as it goes. The destination responds to each frame and the source host chooses an appropriate route from these responses.
112. What limitation of transparent bridge protocol is overcome by the source routing protocol?
Transparent bridge protocol uses spanning tree algorithm, where a unique path is used for communication between two stations. As a consequence, it does not make use of other paths leading to lesser utilization of network resources. This problem is overcome in source routing algorithm.
113. What limitations of a bridge are overcome by a router? A router overcomes the following limitations of a bridge:
Linking of two dissimilar networks Routing data selectively and efficiently Enforcement of security Vulnerability to broadcast storm
114. Why do you need ARP protocol? Two machines on a network can communicate only if they know each other’s
physical address. So, IP address is not enough to deliver a packet to the destination node. It is necessary to know its physical (LAN) address. The ARP protocol allows a host to find out the physical address of a destination host on the same physical network, given only the IP address of the destination host.
115. What is the purpose of dotted decimal representation? Give dotted decimal representation of the IP address 11011101 10001111 11111101 00001111.
To represent the 32-bit IP address in short and easy to read form, Internet addresses are represented in decimal form with decimal points separating the bytes. This is known as dotted decimal notation. For the given IP address the dotted decimal representation is 221.143.253.15.
116. How is masking is related to subnetting? Masking is a process that extracts the physical network address part from the 32-
bit IP address. When subnetting is done, the masking is performed to get the subnetwork address rather than the network address.
117. What is the function of NAT? The Network Address Translation (NAT) approach is a quick interim solution to
this problem of acute shortage of IP addresses for individual hosts in IPv4. NAT allows a large set of IP addresses to be used in an internal (private) network and a handful of addresses to be used for the global internet.
118. What is the function of the ICMP? The ICMP has been designed as companion protocol to compensate two
important deficiencies of the IP protocol, namely error-control mechanism and the lack of mechanism for host and management queries.
119. What is the relationship between TCP/IP and Internet? Internet is a network of different types of network. TCP/IP is a set of rules and
procedures that govern the exchange of messages between hosts linked to different networks. TCP/IP creates an environment as if all hosts are connected to a single logical network.
120. Distinguish between TCP and UDP? Both TCP and UDP belong to transport layer. The UDP is simpler with much less
overhead. UDP provides unreliable connectionless service. On the other hand, TCP provides connection oriented reliable service with the help of suitable flow control and error control protocols. As a consequence, TCP has much more overhead.
121. What is the main function of UDP protocol? UDP protocol provides user programs the ability to communicate using unreliable connectionless packet delivery service with minimum overhead.
122. What is Data Collision?The event that destroys the data when two transmitters send data packets at the
same time on a common shared channel.
123. Define Active and Passive hub? Active Hub: Receives frame from an incoming link and propagates it into all
outgoing links. Passive Hub: Receives frame from an incoming link and regenerates it and
propagates it into all outgoing links.
124. Differentiate between Baseband and Broadband Transmissions?
SL.No Baseband Transmission Broadband Transmission1 Whole capacity of the medium is occupied by
the signalCapacity of the medium is divided into channels
2 Multiplexing is not possible Frequency division multiplexing(FDM) is used
3 Digital signaling used Analog signaling is used
125. Difference between Hub and switch?Sl.No Hub Switch1 Does not recognize destination
addressRecognizes destination address
2 Receives frame from an incoming lines, either regenerates or simply propagates to all outgoing lines
Receives frame from an incoming line and routes the frame to appropriate outgoing line.
3 All stations receive the frame Only destination station receives the frame
4 Physically STAR Topology but logically BUS Topology
Both are STAR Topology
5
126. What routing is important in a computer network?
In a packet switched network, there are number of nodes and different stations are communicating through these nodes. A packet is introduced in the network, which has to be delivered at a destination station. The path to be followed by the packet is decided by the routing algorithm. Routing tries to find out the least-cost or the optimized path between the source and the destination stations. If routing is not done properly, congestion may take place.
127. What are the primary conditions that affect routing? The primary conditions that affect routing are
• Failure (Link / Node failure) • Network congestion
128. Distinguish between virtual circuit and datagram type of routing? In case of virtual circuit, a session is established between source and destination.
At the beginning of the session, route is decided for all the packets to be sent for that session. In datagram type of routing, each packet is independently routed.
129. List out the advantages and disadvantages of fixed routing. The advantages of fixed routing are as follows.
• The routes are always fixed and hence the routing overhead is minimum. • The routing is dependent on network topology, i.e., static in nature. • Routing is same for datagram and virtual circuit type of services.
The major disadvantages are:
• Lack of flexibility. • The system is not robust. In case of link failure or node failure, the system cannot recover. • Congestion may occur on a particular route.
130. What is flooding? Why flooding technique is not commonly used for routing? Flooding is one type of non-adaptive routing technique where no network information
is used. In case of flooding as each node receives a packet, it is re-transmitted or forwarded to all the links connected to the node (except the link through which the packet has arrived).
Flooding is not commonly used for routing for the following reasons:
• Flooding leads to unbounded number of packets • May lead to congestion in the network • A number of copies of the same packet is delivered at the destination node
131. In what situation flooding is most appropriate? How the drawbacks of flooding can be minimized?
Flooding is most appropriate in some critical operations, like military network, because of its robustness. In flooding routing technique the packet delivery is guaranteed if a path exists.
The drawbacks of flooding can be minimized by the following two ways:
While forwarding a packet each node should find whether the particular packet has been already transmitted. If so, the second transmission of the packet should be stopped. Hop-count information should be maintained at each node. A packet is not forwarded, if hop-count is more than the specified limit.
132. Why adaptive routing is preferred over fixed routing?
The major problem of fixed routing is that in case of link/node failure, the system cannot recover. This problem is taken care in adaptive routing. The popularity of adaptive routing is mainly due to the following reasons:
Adaptive routing improves performance of the network. It aids in avoiding congestion.
133. What kind of routing algorithm is used in ARPANET?
ARPANET uses network information supplied by adjacent nodes for routing rather than local information. Arpanet routing techniques have gone through three generations. Successive generations tried to improve the shortcomings of the previous ones. In the first generation (1969) and second generation (1979) techniques, the main metric was delay. Finally, in the third generation (1987) routing technique, the shortcomings are overcome by the following ways:
Instead of using best route at each node, average path (under heavy load) is considered. The cost function is revised and it is keyed to the network utilization rather than delay.
Finally, it can be concluded that ARPANET uses following routing technique:
Delay based metric when load is light. Capacity based metric when the load is heavy.
134. Define Autonomous Systems.
A routing domain generally is considered a portion of an internet under common administrative authority that is regulated by a particular set of administrative guidelines. Routing domains are also called autonomous systems.
135. Differentiate between Single path and Multi-path routing algorithms.
Single path algorithms are where only a single path (or rather single next hop) is stored in the routing table. Some sophisticated routing protocols support multiple paths to the same destination; these are known as multi-path algorithms. Unlike single-path algorithms, these multipath algorithms permit traffic multiplexing over multiple lines.
136. Differentiate between Link State and Distance Vector routing algorithms.
Link-state algorithms (also known as shortest path first algorithms) flood routing information to all nodes in the internetwork. Each router, however, sends only the portion of the routing table that describes the state of its own links. In link-state algorithms, each router builds a picture of the entire network in its routing tables. Distance vector algorithms (also known as Bellman-Ford algorithms) call for each router to send all or some portion of its routing table, but only to its neighbors. In essence, link-state algorithms send small updates everywhere, while distance vector algorithms send larger updates only to neighboring routers. Distance vector algorithms know only about their neighbors.
137. State few of the Routing metrics.
Routing algorithms have used many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used:
Path length: Path length is the sum of the costs associated with each link traversed. It is also defined as hop count, a metric that specifies the number of internetworking devices between source and destination. Delay: It is the length of time required to move a packet from source to destination through the internetwork. Bandwidth: It refers to the available traffic capacity of a link. Load: Load refers to the degree to which a network resource, such as a router, is busy. Reliability: In the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link. Some network links might go down more often than others. After a network fails, certain network links might be repaired more easily or more quickly than other links.
138. Describe Bellman-Ford Algorithm.
The algorithm is distributed because it involves a number of nodes (routers) within an Autonomous system. It consists of the following steps:
Each node calculates the distances between itself and all other nodes within the AS and stores this information as a table. Each node sends its table to all neighboring nodes.
When a node receives distance tables from its neighbors, it calculates the shortest routes to all other nodes and updates its own table to reflect any changes.
139. What are the disadvantages of Bellman Ford Algorithm? The main disadvantages of Bellman-Ford algorithm in this setting are:
1) Does not scale well 2) Changes in network topology are not reflected quickly since updates are spread node-by-node. 3) Counting to infinity
140. Describe various Timer used in RIP. RIP uses numerous timers to regulate its performance. These include a routing-update timer, a route-timeout timer, and a route-flush timer.
The routing-update timer clocks the interval between periodic routing updates, each router periodically transmits it entire routing table to all the other routers on the network. Generally, it is set to 30 seconds, with a small random amount of time added whenever the timer is reset. The route invalid timer (or route-timeout timer), which determines how much time must expire without a router having heard about a particular route before that route is considered invalid. This notification of invalid route must occur prior to expiration of the route flush timer. When the route flush timer expires, the route is removed from the routing table.
141. Explain the Count to Infinity problem. Count-to Infinity problem is a problem that bad news travel slowly through the
network and to advertise a bad news throughout the entire network will take a long time. This problem arises because routing update messages propagates slowly across the network. Choosing a small infinity limit (16) reduces it but doesn’t eliminate it. This problem causes inconsistencies in the routing tables of different routers. This problem is also called as slow convergence problem.
142. Name few solutions to Slow Convergence Problem. Some of the solutions to slow convergence problem are
Hold-downs Split horizons Poison reverse updates Triggered updates.
143. Explain Hold down Solution Hold-Down is one of the solutions to Slow Convergence problem. Hold-downs
prevent inappropriately reinstating a route that has gone bad when routers broadcast their regular update messages. Hold downs tell routers to hold on to any changes that might affect recently removed routes for a certain period of time, usually calculated just to be greater than the period of time necessary to update the entire network with a route
change. This prevents count-to-infinity problem. This time duration is known as Hold Down time. Typically hold down time is around 60 sec. So the idea is to wait long enough to ensure that all machines receive the bad news (link failure news) and not mistakenly accepts a message that is out dated
144. Explain Split Horizon Technique. It is never useful to send information about a route back in the direction from
which it came and thus split horizons is used to prevent updates that are redundant to the network. For this purpose Router records the interface over which it received a particular route and does not propagates its information about that route back to the same interface. This change is known as Split Horizon Technique.
145. Explain Poison reverse and triggered updates technique. Once a connection disappears, the router advertising the connection retains the entry for several update periods, and include an infinite cost in the broadcast. The updates are sent to remove downed route and place it in hold-down. This sending of update immediately is known as poison reverse.
To make Poison reverse more efficient, it must be combined with Triggered Updates. Triggered updates force a router to send an immediate broadcast when receiving bad news, instead of waiting for the next periodic broadcast. By sending an update immediately, a router minimizes the time it is vulnerable to believing in good news.
146. Explain steps involved in Link State Routing. In Link state routing, each router shares its knowledge of its neighborhood with
every other router in the internetwork. Following are few noteworthy points about the Link state routing:
Advertise about neighborhood: instead of sending its entire routing table, a router sends information about its neighborhood only. Flooding: Each router sends this information to every other router on the internetwork, not just to its neighbors. It does so by a process of flooding. In Flooding, a router sends its information to all its neighbors (through all of its output ports). Every router sends such messages to each of its neighbor, and every router that receives the packet sends copies to its neighbor. Finally, every router has a copy of same information. Active response: Each outer sends out information about the neighbor when there is a change.
147. Explain Routing Hierarchy in OSPF. Unlike RIP, OSPF can operate within a hierarchy. The largest entity within the
hierarchy is the autonomous system (AS), which is a collection of networks under a common administration that share a common routing strategy. An AS can be divided into a number of areas, which are groups of contiguous networks and attached hosts. Routers with multiple interfaces can participate in multiple areas. These routers, which are called Area Border Routers, maintain separate topological databases for each area.
Area partitioning creates two different types of OSPF routing, depending on whether the source and the destination are in the same or different areas. Intra-area
routing occurs when the source and destination are in the same area; interarea routing occurs when they are in different areas.
148. Explain various types of OSPF message formats. Type field in the header format identifies the OSPF packet type as one of the following:
Hello—Establishes and maintains neighbor relationships. Database description—Describes the contents of the topological database. These messages are exchanged when an adjacency is initialized. Link-state request—Requests pieces of the topological database from neighbor routers. These messages are exchanged after a router discovers (by examining
database-description packets) that parts of its topological database are outdated. Link-state update—Responds to a link-state request packet. These messages also are used for the regular dispersal of LSAs. Several LSAs can be included within a single link-state update packet. Link-state acknowledgment—Acknowledges link-state updates packets.
149. For what purpose Dead Timer, Hellow Inter, Gateway Priority, designated router fields is used in OSPF Hellow Message.
Dead Timer gives time in seconds after which a non-responding neighbor is considered dead. Hellow Inter: It is the normal period, in seconds, between hello messages. Gway Prio: It is the interior priority of this router, and is used in selecting the backup designated router. Designated Router: IP address of the router, which is the designated router for the network as viewed by the sender.
150. Explain the various possible options for Link Type filed in Database Description message of OSPF.
Link Type: describes one link in network topology; it is repeated for each link. Different possible values for Link Type are as follows:
151. What is congestion? Why congestion occurs? In a packet switching network, packets are introduced in the nodes (i.e. offered
load), and the nodes in-turn forward the packets (i.e. throughput) into the network. When the “offered load” crosses certain limit, then there is a sharp fall in the throughput. This phenomenon is known as congestion.
In every node of a packet switching network, queues (or buffers) are maintained to receive and transmit packets (store/forward network). Due to busty nature of the network traffic there may be situations where there is overflow of the queues. As a result there will be re-transmission of several packets, which further increases the network traffic. This finally leads to congestion.
152. What are the two basic mechanisms of congestion control? The two basic mechanisms of congestion control are:
One is preventive, where precautions are taken so that congestion can not occur. Another is recovery from congestion, when congestion has already taken place
153. How congestion control is performed by leaky bucket algorithm? In leaky bucket algorithm, a buffering mechanism is introduced between the
host computer and the network in order to regulate the flow of traffic. Busty traffic are generated by the host computer and introduced in the network by leaky bucket mechanism in the following manner
• Packets are introduced in the network in one per tick • In case of buffer overflow packets are discarded
154. In what way token bucket algorithm is superior to leaky bucket algorithm? The leaky bucket algorithm controls the rate at which the packets are introduced
in the network, but it is very conservative in nature. Some flexibility is introduced in token bucket algorithm. In token bucket algorithm tokens are generated at each tick (up to certain limit). For an incoming packet to be transmitted it must capture a token and the transmission takes place at the same rate. Hence some of the busty packets are transmitted at the same rate if tokens are available and thus introduces some amount of flexibility in the system. This also improves the performance.
155. What is choke packet? How is it used for congestion control? Choke packet scheme is a close loop mechanism where each link is monitored to
examine how much utilization is taking place. If the utilization goes beyond a certain threshold limit, the link goes to a warning and a special packet, called choke packet is sent to the source. On receiving the choke packet, the source reduced the traffic in order to avoid congestion.
The congestion control in the choke packet scheme can be monitored in the following manner.
Each link is monitored to estimate the level of utilization. If the utilization crosses a certain threshold limit, the link goes to a warning state and a choke packet is send to the source. On receiving the choke packet, the source reduces the transmitting limit to a certain level (say, by 50%). If still warning state persists, more choke packets are sent further reducing the traffic. This continues until the link recovers from the warning state. If no further choke packet is received by the source within a time interval, the traffic is increased gradually so that the system doesn’t go to congestion state again.
156. What are the four services required for secured communication? The four services required for secured communication are: privacy, integrity,
authentication and nonrepudiation.
157. What is nonce? The nonce is a large random number that is used only once for the purpose of user
authentication.
158. Explain the operation of the Diffie-Hellman protocol with an example. Although the algorithm works on large numbers, it is illustrated with smaller numbers in this example.
Let N = 23 and G = 7. Sita chooses x = 5 and calculates R1 = 75Mod23 = 17 Sita sends 17 to Ram. Ram chooses y = 3 and calculates R2 = 73Mod23 = 21 Ram sends 21 to Sita Ram calculates K = 173Mod 23 = 14 Sita Calculates K = 215Mod23 = 14
159. Explain the function of Kerberos. Kerberos is a popular technique for key distribution. The Kerberos is an
authentication protocol and at the same time acts as a Key Distribution Center. It requires an authentication server and a ticket-granting server in addition to the real data server.
160. What is VPN? VPN allows private communication through public internet. It is essentially a
logical (virtual) network within a conventional network. It makes use of cryptography (IPSec in tunnel mode) to perform private communication through insecure and public internet.
161. What is the purpose of a Firewall? The purpose of the Firewall is to protect a private network from the threats of hackers coming from the Internet (a public network).
162. What are the commonly used Firewall types? Firewalls can be of the following three types: • Packet Filters
• Application-level Gateways • Circuit-level Gateways.
163. Explain the operation of the packet-filter firewall. A packet filters Firewall blocks or forwards packets based on the transport and
network layer addresses and protocols. It is typically set up as a list of rules based on matches of fields in the IP or TCP header.
164. Explain the operation of the Application Gateway Firewall. An Application Gateway blocks or forwards packets based on the information in the application layers.
165. What is NAT? How it improves network security? Network Address Translation (NAT) allows a private network to use a set of private addresses and a set of global Internet Addresses for external communication. It uses a translation table to route messages between the two networks and provides substantial security.
166. Define Device drivers.A device driver is a collection of operating system routines that initialize the
device, transmit frame on the link and field interrupts.
167. What is Internet Protocol (IP)?IP is an unreliable connection less datagram protocol that provides best effort
delivery. But effort means that IP provides no error checking or tracking.
168. Why do IP protocols reassembly at the destination host rather than at the routers.
The reassembly of the datagram is done only by the destination host because each fragment becomes an independent datagram. The fragmented datagram can travel through different routers and we can never control or guarantee which route a fragmented datagram may take, but all of the fragments belonging to the same datagram should finally arrive at the destination host. So it is logical to do the reassembly at the destination host.
169. What is HTTP?Hyper Text Transfer protocol is an application protocol used for retrieving web
pages from remote web servers in the internet.
170. The SNMP protocol runs over UDP rather than TCP. Why?The SNMP protocol runs over UDP rather than TCP to achieve robustness, to
maintain the effectiveness of SNMP exchanges and also to manage loss of connection.
171. Mention the methods used to reduce routing table size. Next hop routing Network specific routing Host specific routing Default routing
172. What do you meant by encryption and decryption?Encryption transforms a message (plaintext) into a form (cipher text)
unintelligible to an unauthorized person. On the other hand, decryption transforms an unintelligible (Cipher text) message into meaningful (plaintext) information by an authorized person.
173. What are the two approaches of encryption/decryption technique?There are basically two approaches as follows:
One key technique (or symmetric encryption) – In this case the same key is used for encryption and decryption. Public key (or asymmetric encryption) – In this case the transmitting end key is known (or public), whereas the receiving end key is secret.
174. For n number of users, how many keys are needed if we use private and public key cryptography schemes?
For n users n(n-1)/2 keys are required in private key cryptography and 2n keys are required in public key cryptography.
175. How triple DES enhances performance compared to the original DES?It was realized that the DES key length was too short to provide high security.
Triple DES is used to make DES more secure by effectively increasing the key length. Here two keys are used in three stages.
176. Explain how RSA Works.The steps of RSA are as follows:
1. Choose two large primes p and q (typically around 256 bits)2. Compute n = p x q and z = (p-1) x (q-1)3. Choose a number d which is relatively prime to z4. Find e such that e x d mod (p-1) x (q-1) = 1
For encryption: C = Pe (mod n)For decryption: P = Cd (mod n)
