It is capable of correcting any single burst of length formula_438. Interleaving is used to convert convolutional codes from random error correctors to burst error correctors. The basic idea behind the use of interleaved codes is to jumble symbols at the receiver. This leads to randomization of bursts of received errors which are closely located and we can then apply the analysis for random channel. Thus, the main function performed by the interleaver at transmitter is to alter the input symbol sequence. At the receiver, the deinterleaver will alter the received sequence to get back the original unaltered sequence at the transmitter. The figure below shows a 4 by 3 interleaver. The above interleaver is called as a block interleaver. Here, the input symbols are written sequentially in the rows and the output symbols are obtained by reading the columns sequentially. Thus, this is in the form of formula_468 array. Generally, formula_469 is length of the codeword. Capacity of block interleaver: For an formula_468 block interleaver and burst of length formula_439 the upper limit on number of errors is formula_472 This is obvious from the fact that we are reading the output column wise and the number of rows is formula_229. By the theorem above for error correction capacity up to formula_474 the maximum burst length allowed is formula_475 For burst length of formula_476, the decoder may fail. Efficiency of block interleaver (formula_477): It is found by taking ratio of burst length where decoder may fail to the interleaver memory. Thus, we can formulate formula_477 as   Drawbacks of block interleaver : As it is clear from the figure, the columns are read sequentially, the receiver can interpret single row only after it receives complete message and not before that. Also, the receiver requires a considerable amount of memory in order to store the received symbols and has to store the complete message. Thus, these factors give rise to two drawbacks, one is the latency and other is the storage (fairly large amount of memory). These drawbacks can be avoided by using the convolutional interleaver described below. Cross interleaver is a kind of multiplexer-demultiplexer system. In this system, delay lines are used to progressively increase length. Delay line is basically an electronic circuit used to delay the signal by certain time duration. Let formula_34 be the number of delay lines and formula_481 be the number of symbols introduced by each delay line.