Unicity Distance redundancy clarification

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I have an example of an exam question here:

The open message source is a language with an alphabet represented by an 8-bit ASCII code stored in a byte (=8 bits). The relative redundancy of the language is 50%. Calculate the maximum Unicity Distance for an AES cipher operating in the ECB cryptographic mode of operation with an input block size of 128 bits.

I can't figure out how to get redundancy D to calculate Unicity Distance. I know that the answer is D=4. Any help?


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