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What is piggybacking? Give an example of piggybacked frame. Sketch the appropriate HDLC frames for the following scenario involving primary station 'A' and two secondary stations B and C:

(I) Primary station 'A' wishes to establish a normal response mode link with secondary stations B and C

(II) Both the stations B and C send positive acknowledgements to A

(III) Station A sends a polling command to B and B sends 4 data frames. The third frame is lost during transmission.

(IV) Assuming selective repeat ARQ station A sends negative acknowledgment to station B.

(V) Station B resends the frame and A sends positive acknowledgement.

(VI) Station A now polls station C and station C responds with ready response. A sends three data frames to C and C sends positive acknowledgement to indicate the receipt of error free data frames.

Electronics > Sem 6 > Computer Communication Networks

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  • In two way communication, Whenever a data frame is received, the receiver waits and does not send the control frame (acknowledgement) back to the sender immediately.
    • The receiver waits until its network layer passes in the next data packet. The delayed acknowledgement is then attached to this outgoing data frame.
    • This technique of temporarily delaying the acknowledgement so that it can be hooked with next outgoing data frame is known as piggybacking.
    • The major advantage of piggybacking is better use of available channel bandwidth.
    • The disadvantages of piggybacking are:
  1. Additional complexity.

  2. If the data link layer waits too long before transmitting the acknowledgement, then retransmission of frame would take place.

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