Cable Modem Access
Networks
To deliver data services over a
cable network, one television channel (in the 50 - 750 MHz range) is
typically allocated for downstream traffic to homes and another channel (in
the 5 - 42 MHz band) is used to carry upstream signals.
A headend cable modem
termination system (CMTS) communicates through these channels with cable
modems located in subscriber homes to create a virtual local area network
(LAN) connection. Most cable modems are external devices that connect to a
personal computer (PC) through a standard 10Base-T Ethernet card or
Universal Serial Bus (USB) connection, although internal PCI modem cards are
also available.
The cable modem access network
operates at Layer 1 (physical) and Layer 2 (media access control/logical
link control) of the Open System Interconnect (OSI) Reference Model. Thus,
Layer 3 (network) protocols, such as IP traffic, can be seamlessly delivered
over the cable modem platform to end-users.
A single downstream 6 MHz
television channel may support up to 27 Mbps of downstream data throughput
from the cable headend using 64 QAM (quadrature amplitude modulation)
transmission technology. Speeds can be boosted to 36 Mbps using 256 QAM.
Upstream channels may deliver 500 Kbps to 10 Mbps from homes using 16QAM or
QPSK (quadrature phase shift key) modulation techniques, depending on the
amount of spectrum allocated for service. This upstream and downstream
bandwidth is shared by the active data subscribers connected to a given
cable network segment, typically 500 to 2,000 homes on a modern HFC network. |
