Let us assume that the broadcast sender is a radio station transmitting information about the radio conditions (block A ),the weather report (block B),the latest events in town and the menu to access these and other topics in addition to music.The sender can now assume ,knowing something about the importance of the data blocks,the block D is the most important to enable access to the other information.The second important block is A ,then B and finally C.A possible broadcast disk for this scenario could now look as follows.
DADBDADBADADDDDBDC............
It is now the receiver's task to cache the data blocks to minimize access delay as soon as a user needs a specific type of information.Again ,the receiver can only optimize caching if it knows something about the content of the data blocks.The receiver can store typical access patterns of a user to be able to guess which blocks the user will access with a high probability.Caching generally follows a cost-based strategy: what are costs for user? if if the data block has been requested but is currently not available in the cache.
A broadcast sender of data does not know when a receiver starts to,listen to the transmission .While for radio or television there is no problem,transmission of the other important information,such as traffic or weather conditions ,has to be repeated to give receiver a chance to receive this information after having listened for a certain amount of time.
The cyclical repetition of data blocks sent via broadcast systems is often called a broadcast disk according to the project in Acharya od data carousel,eg according to the DAB or DVB standatrds.The sender repeats the three data blocks A,B and C in a cycle.Using a flat disk , all blocks are repeated one after the another.Every blog is transmitted for an equal mount of time,the average waiting time for receiving a block is the same for A,B and C.Skewed disks favour one ofr more data blocks by repeating them once or several times.Thsi raises the probability of receiving a repeated block ,if the block was corrupted the first time.
Finally ,multi-disks distribute blocks that are repeated more often than others evenly over the cyclic pattern.This minimizes the delay if a user wants to access .
