Recall that AAL3/4 has a CRC-10 checksum at the end of each cell, while AAL5 has a single CRC-32 checksum at the end of the PDU. If a PDU is carried in 12 AAL3/4 cells, then AAL3/4 devotes nearly four times as many bits to error detection as AAL5.
(a) Suppose errors are known to come in bursts, where each burst is small enough to be confined to a single cell. Find the probability that AAL3/4 fails to detect an error, given that it is known that exactly two cells are affected. Do the same for three cells. Under these conditions, is AAL3/4 more or less reliable than AAL5? Assume that an N-bit CRC fails to detect an error with probability 1/2 N (which is strictly true only when all errors are equally likely).
(b) Can you think of any error distribution in which AAL3/4 would be more likely than AAL5 to detect an error? Do you think such circumstances are likely?