Data Communications Networking Devices - 4th Ed.pdf

4.9 BROADBAND MODEMS ________________________________________________________ 499Figure 4.77Constellation pattern for 256 QAM modulation one quadrant illustrated)Table 4.35Binary and Gray code equivalenceDecimal Binary Gray Code0 000 0001 001 0012 010 0113 011 0104 100 1105 101 1116 110 1017 111 100For both 64 and 256 QAM element coding the outputs are encoded using a phasechange based upon the inputs and previous inputs. Table 4.36 lists the proposed 64and 256 QAM element coding.In addition to proposing the use of 64 and 256 QAM for downstream operations,Scienti®c Atlanta recognized the need for error correction and detection due to thenoise levels on broadband systems. In the Scienti®c Atlanta proposal a ReedSolomon Parity code was proposed using interleaving to provide protection againstrandom as well as burst errors. Thus, the demodulated data rates of 30 Mbps for 64QAM and 40 Mbps for 256 QAM would result in actual corrected data rates of27.378 and 36.504 Mbps, respectively.DSL modemsAs mentioned earlier in this section, a DSL modem derives its name from the factthat it was developed to support the transmission of data via copper twisted-pairwire on the line linking a subscriber to a central of®ce. In actuality there is a family