Basic CS-Networks [TCS Placement]: Sample Questions 97 - 98 of 133

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Question 97

Networks
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Describe in Detail

Essay▾

What is protocol Data Unit?

Explanation

  • A protocol data unit is information delivered as a unit among peer entities of networks containing control information, address information, or data.
  • PDU represents a unit of data specified in the protocol of a given layer, which consists of protocol control information and user data.
  • PDU is related to the initial four layers of the OSI model. In Layer 1, PDU is a bit, in Layer 2 it is a frame, in Layer 3 it is a packet and in Layer 4 it is a segment. In Layer 5 and above, PDU is referred to as data.
  • Contains four fields:
    • Destination service access point (DSAP)
    • Source service access point (SSAP)
    • Control field
    • Information field
  • DSAP, SSAP are addresses used by the LLC to identify the protocol stacks on the receiving and sending machines that are generating and using the data.
  • The control field specifies whether the PDU frame is an information frame (I - frame) , a supervisory frame (S - frame) , or an unnumbered frame (U - frame) .
Given the Image is Define the Protocol Data Unit (PDU)

Question 98

Networks
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Describe in Detail

Essay▾

What is stop-and -wait protocol?

Explanation

  • In Stop and wait protocol, sender sends one frame, waits until it receives confirmation from the receiver (okay to go ahead) , and then sends the next frame.
  • In this method of flow control, the sender sends a single frame to receiver & waits for an acknowledgment.
  • The next frame is sent only when acknowledgment of previous frame is received- process continues as long as the sender has data to send.
  • To end transmission sender transmits end of transmission (EOT) frame.
  • The main advantage of stop & wait protocol is accuracy and order- there is no chance of frame being lost or arriving out of order.
  • The method is however inefficient- transmission process is slow as a single frame travels from source to destination and single acknowledgment travels from destination to source- one by one in serial order. Each frame sent and received uses the entire time needed to traverse the link.
Given the Image is Define the Stop and Wait Protocol

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