Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 1 Solution Overview
Solution Components
Diplex Filters—For coax cabling, diplex filters must be installed in the RF path between the cable
modem cards in the CMTS and cablemodems and/or STBs. A diplexer has three ports: low, high,
and common. The downstream cable attaches to the high port because high frequency signals flow
in the downstream direction from the CMTS to cable modems and STBs.. The upstream cable
attaches to the low port because low frequency signals flow in the upstream direction from the
cable modems to the CMTS. The common port attaches to a splitter attached to one or more MTAs
and/or STBs.
In two-way data cable networks, the diplexer takes the upstream and downstream and combines
them on one cable for the MTA. Downstream output signals from the CMTS run through the
upconverter, then enter the high filter port of the diplex filter. The signal exits the common port of
the filter and is distributed to the MTAs. The upstream signal from the MTAs enters the low port of
the diplex filter and flows to the upstream receive ports of the CMTS’s cable modem cards.
• Fiber-Optic Cable—Transports signals between the fiber node and the headend location. In most
systems, at least two fiber links connect each fiber node to the headend; one for upstream traffic and
one for downstream traffic
• Multimedia Terminal Adapter—The MTA converts analog voice signals into digitized voice
samples, which are then packetized into IP packets.
– For data connections, the MTA interacts with the core IP network to provide data connectivity
at the customer premises.
– For voice connections, the MTA interfaces with the Cisco BTS 10200 Call Agent and other
elements of the network to establish on-net or off-net voice calls.
• Fiber Links—In most systems, a minimum of two fiber links connect each fiber node to the hub,
one for the upstream traffic and one for the downstream traffic.
Upstream Traffic—In the upstream direction, the MTA encapsulates these IP packets into DOCSIS
frames for transmission uplink to the hub/headend (Cable Modem Termination System, CMTS) over
the Hybrid Fiber Coax (HFC) distribution network. The fiber node converts the electrical signal it
receives from the coax tree and branch network to an optical signal.
The upstream frequency between 5 and 42 MHz can be divided into channels of varying width
(between 0.20 to 3.20 MHz). Frequency ranges and channel widths are chosen based on known
typical ingress patterns. Assuming a typical cable plant, it might select upstream data channels of
1.6 MHz in the unused frequency ranges of the upstream frequency range. Depending on specific
HFC plant characteristics, it might select either QPSK (Quadrature Phase Shift Keying) or QAM16
as the upstream modulation technique. When a 1.6 MHz channel is selected, 1280 kilo-symbols/sec
are supported. Hence, QPSK can theoretically support up to 2.56 Mbps and 16 QAM can
theoretically support up to 5.12 Mbps.
Downstream Traffic—In the downstream direction, the CMTS converts the IP packets received
from the remote end into DOCSIS frames for delivery on the downlink between the CMTS and the
MTA. The MTA then removes the IP packets from DOCSIS frames, retrieves the digitized samples
from IP packets and converts them into voice to be sent to the analog end points. The fiber node
converts the optical signal to an electrical signal that it sends out on the coax tree and branch
network to MTAs.
The downstream frequency above 50 MHz is divided into 6 MHz channels, most of which are used
to support the CATV company’s video service. One or more of these channels is designated to
support downstream data transmission. Digital data is modulated onto the 6 MHz channel using
either 64QAM (Quadrature Amplitude Modulation) or 256 QAM. With 64 QAM, each 6 MHz
channel can support up to 27 Mbps of data. With 256 QAM, each 6 MHz channel can support up to
38 Mbps of data.