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Wednesday, July 10, 2013

Explain the concept of piggybacking?

- Piggybacking is a well known technique used in the transmission of data in the third layer of the OSI model i.e., the network layer. 
- It is employed in making a majority of the frames that are transmitted from receiver to the emitter. 
- It adds to the data frame, the confirmation that the sender sent on successful delivery of data frame. 
- This confirmation is called the ACK or acknowledge signal. 
- Practically, this ACK signal is piggybacked on the data frame rather than sending it individually by some other means. 

Principle behind Piggybacking
- The piggybacking technique should not be confused with the sliding window protocols that are also employed in the OSI model. 
- In piggybacking, an additional field for the ACK or the acknowledgement signal is incorporated in to the data frame itself. 
- There is only a difference of bit between the sliding window protocol and piggybacking.
- Whenever some data has to be sent from party to another, the data will be sent along with the field for ACK. 

The piggybacking data transfer is governed by the following three rules:
Ø  If both the data as well as the acknowledgement have to be sent by the party A, it has to include both the fields in the same frame.
Ø  If only the acknowledgement has to be sent by the party A, then it will have use a separate frame i.e., an ACK for that.
Ø  If only the data has to be by the party A, then the ACK field will be included within the data frame and thus transmitted along with it. This duplicate ACK frame is simply ignored by the receiving party B.

- The only advantage of using this technique is that it helps in improving efficiency. 
- The disadvantage is that is the service can be blocked or jammed by the receiving party if there is no data to be transmitted. 
- Enabling a receiver timeout by means of a counter the moment when the party receives the data frame can solve this problem to a great extent. 
- An ACK control frame will be sent by the receiver if the timeout occurs and still there is no data for transfer. 
- A counter called the emitter timeout is also set up by the sender which if ends without getting any confirmation from the receiver will make the sender assume that the data packet got lost in the way and therefore will have to re-transmitted.

- Piggybacking is also used in accessing the internet.
- It is used in establishment of a wireless internet connection by means of wireless internet access service of the subscriber without taking explicit permission from the subscriber. 
- However, according to the various jurisdiction laws around the world, this practice is under ethical and legal controversy. 
- In some places it is completely regulated or outlawed while at other places it is allowed.  
- A business customer who provides services related to hotspots, as of cafe and hotels, cannot be thought of using piggybacking technique via non – customers. - A number of such locations provide services for a fee. 

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