Frequently asked questions / answer
Q. 1. What is data communication?
Ans :- Data communication is the exchange of the data between the two devices via some of transmission media.
Q. 2. What are the different types of data transmission?
Ans :- 1. Digital data Transmission
(2). Analog data Transmission
(1) Digital A digital data is discrete discontinuous, voltage pulses. Each pulse is a single element.
(2)Analog It is continuous a set of specific point of data. And the possible point between the two specific end.
Q. 3. What is Asynchronous and synchoronoous Transmission?
Ans :- Asynchronous refers to the series of event that donot take place one after the other. Synchoronoous Transmission refer to the event which take place one after other.
Q. 4. What is baud rate?
Ans :- The signalling rate of a line is measured in terms of baud rate. It is the switching speed or number of Transmission that take place per second.
Q. 5. What is frequency? What is its unit?
Ans :- Number of period in one second. Unit of frequency is hertz (Hz).
Q. 6. Explain why each specific set of Traffic description and QoS parameters were selected for each of the ATM services categories?
Ans :- The ATM forum has defined 5 service categories. The 1st 2 categories apply to Connections that are Real time in sense of stringent delay and timing requirements :
1. Constant bit rate (CBR) : This service category is intended for traffic with rigorous timing requirements, such as voice, circuit emulation, and video, that require constant cell transmission rate. The traffic descriptor used is Peak Cell Rate (PCR). This is chosen because this service category has a constant - bit rate.
The QoS parameter used are
Cell Transfer Delay (CTD), Cell Delay Variation (CDV)
These QoS parameters are used because this service category has stringent delay requirement.
2. Real Time Variable bit rate (rt-VBR) : This service category is intended for variable bit traffic, like video with rigorous timing requirements.
Traffic descriptors used are : PCR, SCR (Sustainable Cell Rate), and MBS (Maximum Burst Size).
There are used since the bit rate is variable and thus these should be a peak cell rate and sustainable cell rate defined for the connection. Also a maximum burst size (maximum number of cells transmitted at peak cell rate) is specified above which size, transmission cant's occur.
The QoS is specified by CLR, CTD, CDV because of the rigorous time requirements.
Non - Real Time Service Categories :
3. Non - Real Variable Bit Rate (nrt-VBR) : This category addresses bursty sources such as data transfer, that do not have strict timing requirements.
The traffic descriptors used are PCR, SCR and MBS, because of the use in bursty traffic mode. The QoS parameters used is CLR (Cell Loss ratio) and no delays related parameters are used since no delay requirements are there
4. Available Bit Rate (ABR) : This category is used for sources that can dynamically adapt the rate at which they transmit cells in response to feedback from the network.
The traffic parameters used are PCR and MCR (Minimum Cell Rate) since the rate may vary and thus a peak cell rate need to be specified.
The QoS parameter CLR is low for such adaptive connections.
5. Unspecified Bit Rate (UBR) : This service category does not provide any QoS guarantee. Since no timing requirements is there and no specified bit rate is there.
Also, traffic descriptors PCR, may or may not be specified.
Q. 7. What are the advantages of using UDP over TCP?
Ans :- Advantages of UDP over TCP :
1. UDP Protocol is connectionless as compared to TCP (which is connection oriented, connection must be established before transmission.
2. UDP has very low overhead as compared to TCP, since no acknowledgements are sent. Thus, lesser congestion problems are there.
3. The transmission speed in UDP is very high and thus is useful for multimedia applications in contrast to TCP.
4. There is no need for error and flow control is UDP as compared to TCP which lays emphasise on these.
5. UDP can be used for applications where speed matters rather than integrity of data whereas TCP can't be used for such applications.
Example - Video streaming.
Q. 8. Some people say that flow control congestion control are equivalent. Is it true? Are the objectives of the two same? Discuss.
Ans:- Flow control and congestion control are 2 distinct control mechanisms with different objectives.
1. The objectives of congestion control is to see that the subnet is able to carry the offered traffic. It involves controlling the behaviour of all hosts, all the routers, so as to control the amount of data in the subnet.
2. The objectives of flow control on the other hand, is to control the traffic between a specific sender does not continually send data faster that the receiver can absorb it. Flow control involves some feedback from the receiver to the sender.
Q. 9. What do you mean by Quality of service? What does it signify? Discuss some QoS parameters?
Ans :- Quality of service means the ability to guarantee certain level of performance to a data flow in the network.
The QoS a flow is characterised by 4 primary parameters :
1. Reliability, 2. Delay, 3. Jitter, 4. Bandwidth
These parameters are explained as follows :
Reliability
Reliability is a characteristic that a flow needs. Lack of reliability means losing a packet or acknowledgement, which entails retransmission. However, the sensitivity of applications programs to reliability is not the same. For example, it is more important that electronic mail, file transfer, and Internet access have to reliable transmission than telephony or audio conferencing.
Delay
Source - To - destination delay is another flow characteristics. Again applications can tolerate delay in different degrees. In this case, telephony or audio conferencing.
Jitter
Jitter is the variation in delay for packets belonging to the same flow. Real time audio and video can not tolerate high jitter. For example, a real time video broadcast is useless if there is a 2 - ms delay for the first and second packets and a 60 - ms delay for the third and fourth. On the other hand, it does not matter if packet carrying information in a file have different delays. The transport Layer at the destination waits until all packets arrive before delivery to the application Layer.
Bandwidth
Different applications need different bandwidths. In video conferencing we need to send millions of bits per second to refresh a color screen while the total number of bits in an email may not reach even a million.
Q. 10. What is the use of ICMP packets? Discuss their advantages and disadvantages?
Ans :- Advantages of ICMP packets :
1. It helps to detect error in transmission.
2. It helps to detect congestion in Network.
3. It helps to detect whether a direction is reachable or not.
4. It helps to update the routing information using router solicitation and advertisements.
Disadvantages of ICMP packets :
1. The ICMP packets are an overhead since they utilise some part of the bandwidth thus reducing the available bandwidth for data transmission.
2. The ICMP packets may cause further congestion in an already congested network when they try to inform the sender about the congestion in the network.
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