(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

• 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.