Symphony Symphony 8142, Installation And Setup Manual, Page: 12 / 68

Symphony Installation and Setup Guide
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Traditional coaxial cable systems typically operate with 330 MHz or 450 MHz of capacity,
whereas modern hybrid fiber/coax (HFC) systems are expanded to 750 MHz or more.
Logically, downstream video programming signals begin around 50 MHz, the equivalent of
channel 2 for over-the-air television signals. The 5 MHz - 42 MHz portion of the spectrum is
usually reserved for upstream communications from subscribers' homes.
Each standard television channel occupies 6 MHz of RF spectrum. Thus a traditional cable
system with 400 MHz of downstream bandwidth can carry the equivalent of 60 analog TV
channels and a modern HFC system with 700 MHz of downstream bandwidth has the
capacity for some 110 channels.
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.
An individual cable modem subscriber may experience access speeds from 500 kbps to 1.5
Mbps or more (depending on the network architecture and traffic load); blazing performance
compared to dial-up alternatives. However, when surfing the Web, performance can be
affected by Internet backbone congestion.
In addition to speed, cable modems offer another key benefit: constant connectivity. Because
cable modems use connectionless technology, much like in an office LAN, a subscriber's PC is
always online with the network. That means there's no need to dial-in to begin a session, so
users do not have to worry about receiving busy signals. Additionally, going online does not
tie up their telephone line, which (in the case of ) is very important in an emergency
reporting situation.
ADSL (Asynchronous Digital Subscriber Line) Primer
Asymmetric Digital Subscriber Line (ADSL) is a competing modem technology that converts
existing twisted-pair telephone lines into access paths for multimedia and high-speed data
communications. ADSL can transmit up to 6 Mbps to a subscriber, and as much as 832 kbps
or more in both directions.