The choice of upstream channel widths is also increased to include a 6.4 MHz
setting. The use of both TCM and 128-QAM is limited to the S-CDMA mode of operation,
and 128-QAM is only selectable when TCM is enabled, which effectively reduces
its spectral efficiency to that of 64-QAM.
The S-CDMA physical layer technology partitions the upstream channel into (up to)
128 subchannels kept distinct by a set of orthogonal spreading codes. That structure is
broken up in time into a series of equal-duration timeslots called frames. The CMTS then
schedules upstream transmissions by codes and frames. Because any frame may see several
CMs transmitting simultaneously (using different sets of codes), orthogonality of the
transmissions is maintained by precisely synchronizing the CM transmitters to within 1
percent of the modulation period. At the highest symbol rate (5.12 Msps) that results in
a synchronization accuracy of approximately 2 ns.
The DOCSIS MAC Layer Because the architecture of the cable system enforces a one-tomany
topology for downstream transmissions and a many-to-one topology for upstream
transmissions, the DOCSIS MAC layer protocol??™s primary task is to coordinate the
upstream transmissions from all of the attached CMs.
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