A cable modem termination system (CMTS) performs the converse operation
for multiple subscribers at the cable operator??™s head-end. Typically, a few hundred users
can share a downstream channel and one or more upstream channels. The downstream
channel occupies the space of a single television transmission channel in the
cable operator??™s channel lineup; it can provide up to 38.8 Mbps (for 6 MHz channels) or
55.6 Mbps (for 8 MHz channels). In the DOCSIS 1.0 and 1.1 specification, the upstream
channels can be up to 3.2 MHz wide and can deliver up to 10 Mbps per-channel (typically
limited to 5 Mbps for DOCSIS 1.0). In the DOCSIS 2.0 specification, upstream
channels can deliver up to 30 Mbps over channels as wide as 6.4 MHz. A Media Access
Control (MAC) layer coordinates shared access to the upstream bandwidth. In the
DOCSIS 3.0 specification, the MAC layer has been enhanced to support multiple physical
channels in each direction simultaneously.
Although sometimes referred to as a ???last-mile??? (or ???first-mile???) access technology, a
DOCSIS network can operate over much greater distances. In fact, the DOCSIS specifications
are designed to operate up to a maximum optical-electrical distance of ~100 miles.
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