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.
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