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 .
Uni-directional distribution systems or broadcast systems are an extreme version of asymmetric communications systems .Quite often ,bandwidth limitations ,differences in transmission power ,or cost factors prevent a communication system from being symmetrical.Symmetrical communication systems offer the same transmission capabilities in both communication directions ,i.e the channel characteristics form A to B are as same as from B to A .
Examples of symmetric communications services are the plain old telephone service (POTS) or GSM ,if end-to-end communication is considered .In this case ,it does not matter if one mobile station calls the other or the other way round,bandwidth and delay are the same in both services .
This symmetry is necessary for a telephone service ,but many other applications do not require the same characteristics for both directions of information transfer.Consider a typical client /server environment.Typically ,the clients needs more data from server than the server needs from the client .Today's prominent example is world wide web .Millions of users download the data using there browsers form the web servers .A user only returns information to server from time to time .Single requests for new page with typical size of several hundreds bytes result in responsese of upto some 10 kbytes on average.
A television with a set-top box represents more extreme scenario .While high resolution video stream requires several mbits/s, a typical user returns some bytes from time to time to switch between channels or return some information for television shopping
Finally ,todays pagers and radio work completely one-way.These devices can only reveive information backto e.g,the radio station .Typically the telephone system is used for this purpose.
A special case of asymetrical communication systems are unidirectional broadcast systems where typically a high bandwidth data stream exists from one sender to many receivers.The problem arising from this is that the sender can only optimize transmitted data for the whole group of the recivers and not fro individual users .
