In order to allow bidirectional communications over the coax portion of the plant,
and to maintain compatibility with the legacy downstream services (analog and digital
video), the upstream communications operate at the low-frequency end of the spectrum
(5??“42 MHz in North America, 5??“65 MHz in Europe), whereas the downstream
transmissions operate in the normal video transmission band (typically 54??“870 MHz
in North America, 87.5??“862 MHz in Europe).
Because cable networks were originally designed as one-way transmission media, the
spectrum allocation for downstream (to the customer) and upstream (to the network)
communication in cable networks is very asymmetric. The upstream radio frequency
(RF) spectrum is fairly narrow compared to the amount of downstream spectrum. That,
coupled with the highly robust modulation required for upstream transmissions, leaves
a large asymmetry in capacity between the two channels. That capacity asymmetry has,
fortunately, matched fairly well with the demand asymmetry historically seen in websurfing
traffic for residential broadband users and the predominantly downstream entertainment
video services.
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