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CS601 Current Final Term Papers Fall 2010 (11~26 Feb 2011)

Please Share Your Current Paper pattern, MCQs & subjective Questions to help each other. Thanks

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Replies to This Discussion

What is the formula to calculate the number of redundancy bits required to correct a bit error in a given number of data bits? [2]

What is R G rating of coaxial cable?

What are the advantages of thin ethernet?

 

What is the difference between a unicast, multicast, and broadcast address? [3]

 

T lines are designed for Digital data how they can be used for Analog Transmission ?

What are the three types of Guided Media?

 

Why do we need Inverse Multiplexing? [5]

Describe method of checksum briefly?

 

Explain Asynchronous Time Division Multiplexing in detail? Also discuss its advantages over synchronous TDM?

 

Ans:

Asynchronous time-division multiplexing (ATDM) is a method of sending information that resembles normal TDM, except that time slots are allocated as needed dynamically rather than pre-assigned to specific transmitters. ATDM is more intelligent and has better bandwidth efficiency than TDM.

 

Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

 

asynchronous time-division multiplexing comprising receive circuits (CRl/i) supplying cells received via input links, transmit circuits (CTl/j) transmitting retransmitted cells on output links, a buffer memory (MT) storing the received cells and delivering the cells to be retransmitted and a buffer memory addressing device (SMT) including a write address source (SAE) and a read address source (fsl/j).

The switching unit further comprises a write disabling circuit (pi) conditioned by a signal (adl) derived from the content of at least one received cell or a signal (tle) derived from the absence of any received cell and supplying a disabling signal (spi) and the address source includes a disabling device (pac, pal) influenced by the disabling signal (spi) so that no memory location is then occupied in the buffer memory (MT).

 

Advantages asynchronous TDM:

 

In asynchronous TDM, the timeslots are not fixed. They are assigned dynamically as needed.

In order to reduce the communications costs in time-sharing systems and multicomputer communication systems, multiplexing techniques have been introduced to increase channel utilization. A commonly used technique is Synchronous Time Division Multiplexing (STDM). In Synchronous Time Division Multiplexing, for example, consider the transmission of messages from terminals to computer, each terminal is assigned a fixed time duration. After one user's time duration has elapsed, the channel is switched to another user. With synchronous operation, buffering is limited to one character per user line, and addressing is usually not required. The STDM technique, however, has certain disadvantages. As shown in Figure 1, it is inefficient in capacity and cost to permanently assign a segment of bandwidth that is utilized only for a portion of the time. A more flexible system that efficiently uses the transmission facility on an "instantaneous time-shared" basis could be used instead. The objective would be to switch from one user to another user whenever the one user is idle, and to asynchronously time multiplex the data. With such an arrangement, each user would be granted access to the channel only when he has a message to transmit. This is known as an Asynchronous Time Division Multiplexing System (ATDM). A segment of a typical ATDM data stream is shown in Figure 2. The crucial attributes of such a multiplexing technique are:

1. An address is required for each transmitted message, and

2. Buffering is required to handle the random message arrivals.

 

Question#1
What are the Asynchronous protocols in data communication layer? ………….Marks (10)

 

Asynchronous protocols in data communication layer employed mainly in Modems. They treat each character in a Bit stream independently.

Disadvantage:

Inherent slowness is its disadvantage. It requires addition of start and stop bits

and extended spaces between frames, so these are mainly replaced with High speed synchronous mechanisms.

Advantages:

  1. Not Complex and Inexpensive to Implement.
  2. Transmission does not require timing coordination; Timing is done by using extra bits.

Different Asynchronous Protocols :

 A variety of Asynchronous protocols have been developed.

 

 

 

 


XMODEM :

 Control frames from the receiver ,

CAN: Aborts the transmission.

ACK: Acknowledgement.

NAK: Error or start of transmission.

YMODEM :

YMODEM is similar to X-MODEM with only the major differences:

  1. Two CANs to abort Transmission.
  2. ITU-T CRC 16 for Error Checking.
  3. Multiple fi les can be sent simultaneously.
  4. 1024-Byte data unit.

ZMODEM :

  1. Newer Protocol
  2. Combines features of XMODEM and YMODEM.

 

 

 



Question#2
What is Frequency division multiplexing ?.......Marks (5)

  1. In an analog technique , when BW of the link is greater than the combined BW of the signals to be TX
  2. Signals generated by each sending device modulate difference carrier frequencies.
  3. These modulated signals are then combined into a single Composite signal that can be transported by the link .
  4. Carrier frequencies are separated by BW to manipulate modulated signal.
  5. These BW ranges are channels through which the various signals travel.




Question#3
Find the binary equivalent ………….Marks (5)


Question#4
What is stop and wait ARQ in error control ?....Marks (3)
Stop-and-Wait is an extended form of flow control to include  retransmission of data in case of Lost or Damaged frames.

There are four main features added in it.

  1. Sending device keeps a copy of the last frame transmitted until it receives

the ACK for that frame .

  1. Both data and ACK frames are numbered 0 and 1 alternately.
  2. A data 0 frame is acknowledged by a ACK 1 frame indicating that the

receiver has received data 0 and is now expecting data 1 .

  1. Sending device keeps a copy of the last frame transmitted until it receives

the ACK for that frame.



Question#5
What is Interleaving ?........Marks (3)
if STDM considered as a very fast rotating switch. When this switch opens in front of a device, the device has the opportunity to send a specifies amount of data on to the path.

The switch moves from device to device at a constant rate and in a fixed order. This process is called INTERLEAVING . Interleaving can be done by BITS, BYTES or by any other DATA UNIT



Question#6
What is DSU in terms of digital services?.........Marks (3)
DSU (Digital service unit) changes the rate of digital data created by the subscriber’s device to 56 Kbps and encodes it in the format used by service provider.it used in dialing process and is more expensive than MODEM. But it has

  1. better speed,
  2. better quality and
  3. less susceptibility to noise.

    Question#7
    Which architecture of Ethernet developed by ITU_T and
    ANSI?........... Marks (2)


FDDI (Fiber Distributed Data Interface)architecture of Ethernet developed by ITU_T and ANSI.

Question#8
What is a spike in noise term?.........Marks (2)

Spike is a signal with high energy in a very short period of time

that comes from power lines, lightening etc,


Question#9

What is even parity generator in VRC error detection mechanism?...........Marks (2)

Even parity generator counts the 1’s and appends the parity bit (1) at the end.

 

 

 

 

Compare line decipline methods ENQ/ACK and Poll/ Select?

Line Discipline is done in 2 ways:

ŠEnquiry / Acknowledgement (ENQ/ACK)

   (Used in Peer-to-Peer Communication)

when there  is a Dedicated Link b/w the two devices so that the only device that can receive data is the intended one.

It coordinate who may start a transmission and whether or not the  intended recipient is ready and enabled.

Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of equal rank,

 for example - a printer cannot initiate communication with a CPU.

initiating device establishes session in Half duplex and in full duplex.

In half-duplex:

 The initiator then sends its data while the responder waits

The respondent may take over the link when the initiator is finished or has

Requested a response

In full duplex:

 both the devices can TX  simultaneously once the link has been

 

 

 

 

 

ŠPoll / Select 

   (Primary-Secondary communication)

ŠIf the  primary wants to receive data, it asks the second-arise  if they have anything to send, This is called POLLING

ŠIf the primary wants to send data, it tells the target secondary to get ready to receive, This function is called SELECTING .

 

  1. The primary device controls the link and
  2. the secondary device follow sits instruction.
  3.  

Primary device determine which device is allowed to use the  channel.so it is always the initiator of the a session.

 

Multipoint systems must coordinate several nodes, not just two.

Whenever  a  multipoint link consists of  a primary device and multiple secondary  devices  using  a single  TX  line , all exchanges must be made through the primary device even when the ultimate destination is a secondary device.

 

Calculate the the capacity of T1 line which carries 8000 frames and 24 voice channels?

 

 

T-1 line is usually 193 bits divided into 24 slots of 8 bits each

+ 1 bit for synchronization.

 

 24*8+1=193

24 segments are interleaved in one frame .

 

Here,

given frames = 8000

                      =193 * 8000

 

 Capacity of the line = 1.544Mbps

 

Explain 10 Base 5 Thick Ethernet?5

 

IEEE divides the base band category into 5 standards:

10 Base 5 ,

10 Base 2,

10 Base-T,

1 Base 5,

100 Base-T

 

 

10 Base 5 Thick Ethernet:

The first of the physical standards defined in IEEE 802.3 model is called 10 Base 5 Thick net or Thick Ethernet .

 

It is given name because of  the size of the cable .

10 Base 5 is a bus topology LAN that uses base band signaling and has a max.

segment length of 500 meters.

Topology of 10 Base 5 :

The physical connectors and cables utilized by 10 base 5 include

coaxial cable,

Network Interface Card,

Transceivers and

Attachment Unit Interface (AUI) cables .

 

Repeaters and Bridges are used to overcome the size limitation of LAN.

to reduce collisions, the total  length of the bus should not exceed 2500

meters (5 segments)

Also the standard demands that each station be separated from each other 2.5 meters

200 stations per segment and 1000 stations in total.

 

 

 

 

 

 

Why addressing is required in Poll / Select method and not required in ENQ/ACK method?3

 

Addressing is required in Poll / Select method as it is a not point-to-point configuration, For the primary device in a multipoint topology to be able to identify and communicate with a specific secondary device, there must be some addressing, while ENQ/ACK method  is a point-to-point  method and for point-to-point configuration, there is no need for addressing.

 

 

 

 

 

What do you know about ITU-T Modems?3

 

ITU-T modem :

V-series: Today’s most popular modem standards

Bell modem compatible:

V.21/22/23/26/27/29

 

 

 

Following abbreviations stands for what?3

ARP…….…. (Address Resolution Protocol)

RARP……... (Reverse Address Resolution Protocol)

ICMP ……… (Internet Control Message Protocol)

 

 

 

 

 

Write names of Link Access Protocols developed by ITU-T?3

 

LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC

Write the names of different types of noise in the medium?3

 

 

Ans:

 

Thermal Noise

Induced Noise

Crosstalk

Impulse Noise

 

 

 

 

 

Write down some disadvantages of star topology.3

 

each node must be connected to a Hub , so Cabling is still much more

than some other Topologies

 

 

 

What are the two major classes of synchronous protocols at data link layer?2

 

  1. Character – Oriented Protocols

 

  1. Bit – Oriented Protocols

 

 

Whether Hamming code is the technique used for error detection or error correction?2

 

Define Multiplexing? What is its advantage?2

 

Ans:

 

Set of techniques that allows the simultaneous transmission of multiple signals

across a single data link

it allow multiple users to share total capacity of a Transmission Medium.

 

 

 

What is the purpose of dual ring?2

 

Unidirectional traffic movement is overcome by dual ring technology. 

 

 

Which modem was first  developed commercially in 1970?2

Bell modems

 

 

 

 

Write any two functions of physical layer?2

It defines characteristics of Interface between device and transmission Medium

It also defines the type of transmission medium

 

See the attached file please
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