New services are
constantly being introduced. Subscribers constantly demand greater bandwidth,
higher data rates in both directions, and extremely low latency. In addition,
there are constant requirements to increase the reliability of all services. As
a result, some CATV service providers are converting their traditional hybrid
fiber coax (HFC) networks to fiber to the home (FTTH.) Here are some of the
reasons why and how it can be accomplished.
For many years,
cable operators have been building and maintaining HFC (Hybrid Fiber Coax)
networks. HFC is a broadband network that enables triple-play services - CATV,
IPTV, Internet, telephony, etc. HFC began when AT&T developed the
distributed feedback (DFB) laser, a high power laser that was capable of high
frequency conversion of signals with minimal distortion. This permitted CATV
operators to convert analog frequency modulation (FM) CATV signals to optical
systems, allowing them to extend their CATV networks well beyond the limits of
traditional coaxial cables.
About the same
time, a network was developed that allowed Ethernet signals to be converted to
FM and sent as data in place of a channel on a CATV system. Since Ethernet was
the basis of the Internet, this allowed CATV systems to become providers of
Internet service, offering much higher speeds than the telephone lines in
current use and always-on service. CATV systems quickly became the dominant
provider of Internet service.
became standardized in 1997 as DOCSIS – Data Over Cable Service Interface
Specification. As the Internet grew, so did the DOCSIS standard. In order to keep
up with user requirements, CableLabs, the research arm of the CATV industry has
constantly upgraded the DOCSIS standard.
The DOCSIS 1.0
standard was introduced in 1997, which enabled what was then an incredible
40/10 Mbps. In the next 10 years, DOCSIS 2.0 and DOCSIS 3.0 were introduced,
which significantly increased bandwidth and date rates in both directions.
These two DOCSIS standards are still in use. DOCSIS 3.0 allowed a maximum of 1
Gbps downstream and 200 Mbps upstream. The DOCSIS standard upgrade, from DOCSIS
1.0 to DOCSIS 3.0 did not require major changes to the HFC network.
changes were related to the electronics, the equipment in the head end called
the CMTS (Cable Modem Termination System,) minor corrections to the coaxial
part of the network (shortening of coax lines and increasing the number of
optical nodes) and upgrades on the cable modem at the customer premises.
Note the biggest
problem with DOCSIS HFC networks; they are asymmetrical – download speeds are
generally much higher than upload speeds, which can be a problem for some user
applications like video distribution and gaming.
DOCSIS 3.1 and future 4.0 standards require major changes in the HFC network
due to frequency range expansion. In addition to replacing CMTS, major changes
are required in the coaxial part of the HFC network, such as replacing passive
equipment (RF splitters) and active equipment (broadband amplifiers and optical
nodes). These are very big changes that many operators are not prepared for.
DOCSIS 3.1 and 4.0 bring benefits but with big changes. Because of this, many
cable operators are starting to build FTTH.
passive optical network (PON) used in most FTTH networks has been present for
more than 15 years. The main problems with PONs are the complexity of designing
the network, especially with large numbers of subscribers and installation of
large numbers of cables to subscribers. FTTH, however, enables very easy
migration to new higher speed PON standards (e.g. from GPON to XGS-PON).
However, once the network is built, upgrades are simply a matter of changing
electronics at the head end and subscriber. By design, one can even have GPON
and XGS-GPON operate simultaneously on the same network since they operate at
PONs means that subscribers are provided with reliable services with high
bandwidth, high data rates in both directions and low latency. If we compare
PON and HFC, all the advantages are on the side of the PON network.
have cable operators waited so long to convert to PONs? The reasons are
generally economics. The main reason is the existing HFC network, in which they
have invested a lot of money and time. Second, the cost of optical equipment
was high. And finally, the cost of rebuilding or building a new cabling network.
New FTTH network
Building a completely new FTTH network is one of the most common
ways of migrating from HFC to FTTH. Rather than converting the current HFC over
to FTTH which would disrupt service for many users, overbuilding the cable
plant with new fiber is generally a better solution. A FTTH PON solution
requires completely different network, an optical line terminal (OLT) in
Headend, building an optical network (ODN – Optical Distribution Network) and
migration of subscribers from the old to the new network (replacement of CPE
equipment and home network connection (optical network terminal, ONT). Cable
operators usually use GPON (OLT with GPON service boards) or parallel GPON and
Installation of the appropriate OLT with the appropriate service
boards is required. The OLT can be of different capacities, depending on the
number of subscribers to be connected. The number of subscribers on a PON port
depends on several factors, such as the PON standard, distance, and provision
for future expansion of subscribers in a geographic area.
The design of the cable plant will probably follow the design of
the current cable plant, overbuilding on aerial cables and pulling additional
cables into current underground ducts. The construction of the FTTH cable plant
can be realized in the traditional way (fusion splicing of optical fibers) or
using pre-terminated solutions (optical cables with connectors). The use of
pre-terminated solutions significantly reduces realization time.
Operators can build a completely new ODN or sometimes use parts
of an existing HFC network. In order to save time and money, most cable
operators want to use what is available from the existing HFC network. If there are dark
fibers in existing optical cables, the construction of the feeder segment of
the new network may be simplified. This reduces costs, shortens the time of
realization, and the quality is the same as long as the fibers are in good
there are not enough dark fibers in the existing optical cables, cable
operators have the options of installation of new feeder cables, reconfiguring
the network to a remote OLT solution and/or replacement of old optical nodes
with new optical nodes with WDM modules.
Generally, FTTH construction reuses the same poles, pipes,
manholes, handholes, and optical cables. This significantly reduces the time
and cost of a new network. This is very important since there may be delays and problems
obtaining new building permits.
The coaxial components of the HFC network (optical nodes,
broadband amplifiers, different boxes and cabinets, and coaxial cables - RG11,
RG6, etc.) must be replaced for conversion to a PON. However, on that part of
the network, cable operators can use poles, conduits, handholes and manholes
for the FTTH network.
a fiber optic distribution and drop segment is necessary. Special boxes and
cabinets are required for housing optical splitters. If possible, boxes for
splitters, as well as cables, are installed on the same poles or existing
manholes or handholes.
indoor applications, such as multi-dwelling buildings, distribution and drop
cables should use existing pipes in the walls or special metal or plastic
channels. It is important that the network is built according to all the rules
and that the aesthetics of the building are not damaged.
construction is completed, the
service provider will have two parallel
networks - the old HFC network and the new FTTH network.The migration of subscribers from HFC to FTTH
can then be gradual and planned in
the FTTH technician connects the subscriber (migrating from HFC to FTTH), the
drop cable is installed and the coaxial cable (most often RG11) may be reoved. Most
drop cables are available with factory-installed connectors (plug & play
solution), which enables quick connection. One end of the drop cable is
connected to the last optical splitter on the network, while the other end of
the drop cable ends at the ONT. Cable modem and coaxial network may then
uninstalled. This is the procedure for each and every subscriber. Only when all
subscribers migrate from HFC to FTTH, then the HFC network can be completely
Vladimir Grozdanovic is a graduate electrical engineer for
telecommunications with more than 10 years of experience in access
networks (HFC and FTTH) in large cable operators in Serbia (SBB and
Fiber Optic Association Fiber To The Home Handbook:
For Planners, Managers, Designers, Installers And
Operators Of FTTH - Fiber To The Home - Networks The
Fiber Optic Association Fiber To The Home
in paperback or as an eBook on the Amazon
direct from Amazon.com,
local booksellers and other distributors. Also available in Spanish.