In this examination of cable modems, we will focus upon the asymmetric architecture of a Scientific Atlanta cable modem. The Scientific Atlanta cable modem we will examine is based upon an asymmetric design, using QAM in a 6MHz downstream channel to obtain an operating rate of 27 MHz.

In the opposite direction the modem uses QPSK modulation to provide an operating rate of 1.5 Mbps upstream. The modem supports downstream frequencies in the 54 to 750MHz spectrum and frequencies in the 14MHz to 26.5MHz range for upstream communications.

The Scientific Atlanta cable modem’s modulation method was proposed to the IEEE 802.14 Working Group and became the basis for use in both the IEEE standard and the DOCSI specification. Scientific Atlanta noted that QAM is non-proprietary and was previously selected as the European Telecommunications Standard.

In the firm’s proposal, two levels of modulation based upon 64 QAM and 256 QAM were defined to permit implementation flexibility. The standardization of QAM for downstream transmission results in a signaling rate of 5MHz using a carrier frequency between 151MHz and 749MHz spaced 6MHz apart to correspond to TV channel assignments.

The use of a 5MHz signaling rate and 64 QAM which enables six bits to be encoded in one signal change permits a transmission rate of 6 bits/symbol#5 MHz,or 30 Mbps. In comparison, the use of 256 QAM results in the packing of eight bits per signal change, resulting in a transmission rate of 8 bits/signal change#5 MHz,or 40 Mbps.

Through the use of forward error coding, the data rate throughput is slightly reduced from the modem’s operating rate to 35.504 Mbps for 256 QAM and 27.37 Mbps for 64 QAM. This reduction results from extra parity bits becoming injected into the data stream to provide the forward error detection and correction capability.

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