The purpose of this test is to measure bit error ratio (BER) of downstream (forward path) broadband telecommunications QAM signals. This procedure will address mainly pre-forward error correction BER results for 64- and 256-QAM.

It is important to rely upon both the pre-FEC and post-FEC BER results. In North America, most 64- and 256-QAM signals incorporate ITU-T J.83 Annex B FEC encoding. It is therefore important to reference the post-FEC result, but extremely dangerous to rely solely upon it. BER suffers from what is commonly known as a "cliff effect" in which the post-FEC BER remains error free up to a point and then begins to rapidly decline. Referring to the pre-FEC BER result can yield insight as to how much margin is available prior to reaching the cliff.