HYBRID FIBRE COAXIAL CABLE (HFC)

Compiled by Yvonne Chan.


This is a declaration that aside from referenced sources, this webpage is my own work.
If there is any information that is not referenced please email: ypchan@uts.edu.au


Warning: This is a website written by a student of 48480 Communications Networks. Reference to other sources may be required to confirm this information is correct before further use.


Index

 

1.     What are HFCs?

The abbreviation HFC is used to describe a type of network which uses the cheaper, coaxial cable as well as the higher capacity, fibre optic cable to provide video, voice telephony, data and other interactive services. Some sources believe that HFC stands for Hybrid Fibre Cable while some others call it Hybrid Fibre Coax or Coaxial.


2.     Applications of HFCs

Currently in Australia, HFC technology is mainly used for Cable Television (CaTV) and Cable Internet but is also used in telephony.

CaTV is currently not too popular in Australia as it has a limited budget and often repeats content. Cable Internet, however, has been quickly adopted due to the need for bandwidth at an affordable price. This was achieved with major rollouts in Australia’s capital cities by two competing companies, Telstra and Optus. Optus currently also supports telephone use on HFCs, using a different frequency to CaTV and Cable Internet.

HFCs can be used in all moving digital applications although must of the technology is not yet available. Digital, as opposed to the current analogue television signals maybe an application for the future. Video On Demand (VOD) is a technology where the customer requests a film which is pay-per-view. This is an attempt to tap into the video market and this is currently under trial in Orlando.


3.     Evolution of Technology

3.1.    Cable Television (CaTV)

3.1.1. Topology

HFC finds its origins in Cable Television (CaTV) networks. The original cable network solved the problem of poor TV reception by using shielded coaxial cables to provide strong and equal in strength TV signals to the home.

The traditional CaTV network is made up of four main parts: the headend, the trunk, the feeder (also known as the distribution network) and the drop cable.

The headend is the “regional origination point for broadcast signals in the CaTV system” (Elfeitori 1997). It gathers signals from satellites, distant TV broadcast channels and local channels. These signals are modulate into 8-MHz TV channels using a technique called frequency division multiplexing, where channels are allocated certain frequencies. These channels are broadcast to the cable network through the trunk.

Approximately covering only 12 percent of the plant feet of the network, the trunk is traditionally made up of high quality coaxial cable with amplifiers required at approximately every 600 metres. Lower quality coaxial cables are used in the feeder and drop sections of the network. The trunk is designed to carry signals from the headend to the distribution network. These are often large distances, sometimes well over 15 kilometres (Donaldson 2001). To cover these large distances, there are typically 20-40 amplifiers in each trunk, limiting the bandwidth of the coaxial cable. Each amplifier provides undesirable characteristics to the signals, namely noise and nonlinear distortions to the signal (Donaldson 2001), creating an unreliable result.

The feeder or distribution portion of the network is a node which feeds the drop cables leading to individual premises. It is no longer than 1.5 kilometres but takes up approximately 38% of the total footage in a cable system. This length limitation is because Radio Frequency (RF) energy is tapped off to each premise. In comparison to the trunk, the feeder must support relatively high power levels to be able to maintain enough RF energy for the required subscribers. The feeder must be able to support high levels of activity where subscribers are connected and disconnected regularly. Donaldson, states that “it must be worker-friendly, physically robust, and easy to reconfigure.”

The drop is the flexible coaxial cable that connects the home to the feeder. Often being less than 45 metres, it can still be effective at a maximum length of 120 metres. It is this portion of the network which is physically connected and disconnected from the home when subscribers choose to opt-in or opt-out of the network.

Amplifiers are an important part of the traditional cable network. They are located in the trunk and feeder portions of the network. Amplifiers are used to amplify and restore signal strength to compensate for its deterioration in the transmission cable and signal-splitting devices. The more times a cable and signal is split and the longer the cable, the more amplifiers are needed. However, too many amplifiers in a network can cause distortion to the signals, providing its own noise and non-linear distortions.

3.1.2. Advent of CaTV

In the sixties, television signals were transmitted by air to antennas on television sets. This meant television reception was poor, especially in suburban areas where the middle class began moving in. The traditional CaTV network provided the much needed television reception into the suburban areas. A good quality antenna tower received television signals from the air and placed them onto the traditional CaTV network where it was broadcast to the community. This was the beginning of the traditional CaTV networks.

The traditional CaTV networks had many advantages. It did not require upgrades of televisions as its signals were replicas of the ones broadcast by air. It could also provide more channels, with good quality signals to every home because signals in the air are more readily lost than ones transmitted through cable.

The quick uptake of CaTV in North America meant that the traditional network quickly became saturated and hard to manage. Quality of the CaTV became very poor as more and more people shared the network. This was because signals transmitted through coaxial cables decline in quality as they are split between more users and also because of the need for more amplifiers lead to too many amplifiers, ultimately leading to signal distortion. CaTV providers started looking for an alternative to remedy this situation and Optical Fibres looked like the answer.

3.2    Upgrade to HFC

In the 1980s, a breakthrough in the CaTV network came when fibre optic cables were used to replace trunks. Optical fibres, sometimes abbreviated to simply fibres, have higher bandwidth and lower losses than coaxial cables, meaning that fewer amplifiers (4 to 6) were needed. However, they are relatively more expensive initially. In saying that, the 20-40 amplifiers within coaxial cable trunks meant that they had higher maintenance costs and over time, optical fibres were relatively less expensive.

The traditional CaTV network provided one-way analog signals to the subscriber. It was not capable of carrying two-way signals. Physically, coaxial cables are capable of transmitting in both directions but the unidirectional amplifiers that were required to strengthen signals were unable to transmit in both directions. Use of optical fibres in the traditional CaTV network meant that there was a higher quality in services and by upgrading the existing amplifiers within the feeder part of the network, two-way communication was plausible.

Service Providers found that multiple optical fibres could be wired from a single headend without loss of quality. The CaTV network, quickly evolved from a trunk and branch topology to a ring and star topology.

This technology was adopted very quickly as it merely replaced trunks with fibre optic cables and there was no need to change the already existing coaxial networks to the homes. The ever- increasing need for bandwidth amongst Internet users quickly created a market for this high bandwidth, yet affordable technology. This new generation cable network is now used for both Internet and Cable Television.


4.     Advantages and Disadvantages

4.1.    Advantages

4.2.    Disadvantages


5.     Capabilities and Limits

HFC network speeds are limited by the number of users using the network at the same time. Even though a single 7MHz channel can have theoretical speeds of 30Mbps, much of this speed is shared between all the users on the neighbourhood node using a cable modem at the same time. Each node is capable of supporting services to 500-2000 homes.

Each 7MHz band can carry one analogue video channel, five digital video channels, one 30Mbps data stream shared by cable modems or 384 digitised voice conversations (BYTE.com). Generally, in Australia, 5MHz to 65MHz are used for upstream channels and 85MHz to 700MHz (for Optus Cable) or 750MHz (for Telstra Cable) are used for downstream. See HFC Frequencies for more details.

Because original networks did not anticipate the need for bandwidth in both directions, the upstream paths in the HFC network is slower than downstream. Currently, this asymmetric structure reflects typical usage of the general Internet user but this may have to be changed in the future when interactive multimedia becomes more common. Cable providers are generally reluctant to provide too much upstream bandwidth due to the idea that users could exploit this by hosting servers using their Cable Internet accounts.


6.     Competitors

Asynchronous Digital Subscriber Line (ADSL) is often discussed as the main competitor to HFC technology. This is another widely available broadband technology, providing fast speeds and large bandwidth.

The advantage of ADSL, especially in Australia, is that it runs on the existing twisted-pair copper wiring used by telephones. Most, if not all homes, have a telephone while this is not so with the CaTV network where most lines have to be installed. Similar speeds can be achieved with both ADSL and HFC although HFC speeds decline as more users are added to the system while ADSL is not affected by other users due to a dedicated connection from subscriber to Service Provider. ADSL can run on the same phone line as a telephone conversation at the same time as they are used at different frequencies.

However, ADSL does have disadvantages. Firstly, it is not available on some telephone lines where older telephone technology can block the use of digital services like ADSL. Similar to HFCs, ADSL also provides asymmetrical service where downstream data is slower than upstream data. However, newer xDSL technologies such as HDSL and SDSL can provide symmetrical service although not available in Australia.

Dial-up can also be a competitor for low usage Internet users. Dial-up is cheaper than HFC services but it is slower as it is a narrowband connection. Users who cannot get access to HFC services can use a Dial-up service as it runs on a phone line, similar to ADSL. Dial-up is also usually available even if ADSL cannot be used as Dial-up uses a simple phone call. This means that if a telephone can be used, Dial-up is available.


7.     How Does it Work?

7.1    Topology

HFCs use what is called a “ring and star” topology. It consists of several sections: the headend, the fibre trunk, the fibre nodes (FN), the feeder and the drop cable.

The headend, feeder and drop cable act in the same way as in the traditional CaTV network as the headend and drop cable consist of coaxial cables. However, this time, the headend distributes signals to a few optical fibre trunks rather than the coaxial trunks.

The fibre trunk of the network carries signals from the headend to the feeder, bypassing a fibre node (FN). The purpose of the FN is to convert the optical signals from the optical fibre to electrical signals for further transmission through the rest of the network which is made up of coaxial cables. The placing of the FN may vary with the FTTx technologies such as Fibre To The Neighbourhood (FTTN) where fibre is extended to the Main Distribution Frame (MDF) room from the service provider exchange or Fibre To The Curb (FTTC) where fibre is extended right to the curb of the home. The FN is always located where it is necessary to convert optical signals to electrical signals (and vice versa), i.e. where the optical fibre and coaxial cable meet in the network.

The amplifiers in a HFC network slightly differ from traditional CaTV networks. Because two way communication is required, a way must be found to enable amplifiers to amplify both the incoming and outgoing signals. Since amplifiers only operate in one direction, the amplifier unit first splits the signals travelling in opposite directions using diplexer circuits before amplifying these signals and returning them to the coaxial cable (Donaldson 2001). In a HFC network, amplifiers are mostly located in the feeder section of the network with 4 to 6 within the fibre section as fibre can distribute signals for up to 30km, requiring much less amplifying than with a coaxial trunk which requires 20-40 amplifiers.

7.2    HFC Frequencies

HFCs operate between 5MHz to 750MHz. The 5MHz to 65MHz of the spectrum is reserved for upstream communications while downstream data is situated in the 85 to 750 MHz band.


Image from BYTE.com

The downstream section of the network contains channels which are 7MHz each in Australia. In CaTV, each channel is transmitted down one of these 7MHz bands. Each downstream band can carry one analog video channel, one 30Mbps data stream shared by cable modems or 384 digitised voice conversations (BYTE.com).

Upstream bands located in the 5MHz to 65MHz band are much noisier and require more robust modulation schemes which are not as efficient as the downstream channels. This part of the spectrum has to serve as the return path for interactive TV, voice telephony and cable modems. This myriad of activity, coupled with noise from unterminated connectors, bad wiring, household appliances, shortwave radio and even Air Force radar (BYTE.com) means that upstream data must be transmitted using a slower but more reliable technology for adequate communication to take place.


Hence, slower speeds in HFC networks and asymmetric communication speeds. There are some modems which offer symmetric speeds but this is often done by limiting the downstream bandwidth. Some experimentation has been done with moving the upstream communications to the gigahertz part of the frequency spectrum, but this was found to be too expensive for most cable providers.


8.     HFCs in Australia

Unlike in countries such as the USA, Canada and many European countries, Australia did not have an existing traditional CaTV infrastructure. The first CaTV networks to be installed in Australia were HFC networks. Two companies, Telstra and Optus, raced to build two of these modern HFC networks, often serving the same areas. This means that we have two modern high-performance HFC networks in some areas of our major cities (Whittle 2001).

Because of this, there have been studies suggesting that the market in Australia is not large enough for two CaTV providers. In fact, it was even suggested that a monopoly cable system would be unprofitable over the next 10 years (APRO 1996) due to the cost of laying down cables for this completely new network in Australia.

The Optus network started its rollout of HFC network in Blacktown in Sydney and East Burwood in Melbourne in February 1995. It is now also available in Brisbane. It provides premium TV, local telephony, fully two-way high-speed transmission and other digital and interactive services (ACCC 2001). Currently, most, if not all, of the Optus HFC network is connected using overhead cables on power poles. This is more susceptible to breakage from vehicles and storms but is cheaper than buried cable. It is reported that Sydney Electricity charge Optus $9.00 per pole per annum rent, giving it an income of about $3m per year (APRO 1996). By the end of 2000, Brisbane, Melbourne and Sydney contained 21,000km coax cable (0.625” coaxial) and 5,500km fibre cable (single mode optical fibre from 24 to 144 fibres per sheath) (BIS 2001). Optus stopped this rollout in 1997, spending approximately $3bn on its HFC network.

Telstra started its HFC rollout in 1994. By December 1997, however, the network had passed 2.5 million homes, 1.5 million short of the planned 4 million. Network rollout stopped in 1999 after a $4bn investment in HFC networks. Telstra currently provides CaTV and Cable Internet to users in Sydney, Melbourne, Brisbane and the Gold Coast via both above and underground cables. Telstra is currently investing in another form of broadband technology, Asynchronous Digital Subscriber Line (ADSL).

In 1998/1999, both Telstra and Optus have decided not to expand their networks. This was largely due to the idea that there were HFC network duplications in some areas of Australia. (Below: HFC network rollout in Australia, Courtesy of ACCC.)


9.     Companies using HFC technology

Currently, Telstra and Optus are the main suppliers of HFC technology in the capital cities of Australia. There are also a few regional operators: Neighbourhood Cable and Austar (Windytide), who are building their networks in regional Australia, leaving Telstra and Optus to compete in the State Capitals. West Coast Radio (iiNet) is the only cable provider to Perth (Ellenbrook). Below is a table from the ACCC outlining the HFC operators in Australia and their Coverage.

10.     Future Trends of HFCs

HFCs were once thought to be the way of the future but this idea is being questioned as there are increasing technological advances. Asynchronous Digital Subscriber Line (ADSL) is an alternative which Service Providers and consumers are considering, placing it in competition with HFCs. ADSL gives access to the Internet via the already existing telephone lines at a different frequency to telephone communication. This means that both the telephone and computer can receive signals at the same time with little or no interference. Because not every home in Australia has CaTV, it is more convenient to use existing telephone lines for ADSL than it is to install a coaxial cable from the street to the home for HFCs. This allows ADSL to provide cheaper services and attract more users.

As optical fibres become cheaper, HFCs may take on a different form. Because applications require more and more bandwidth, optical fibres are an ideal solution.
Technologies such as Fibre To The Curb (FTTC) and Fibre To The Home (FTTH) will push optical fibre further into the HFC network. These technologies replace existing coaxial cables to the curb or home with optical fibres. These types of HFCs are often called FTTx as the “x” can be replaced with many other parts of the network such as Fibre To The Node (FTTN), Fibre To The Building (FTTB) and even Fibre To The User (FTTU). As more of the network is made up of optical fibre, a higher network capacity can be reached. However, this is limited by the current cost of optical fibres. Currently FTTN is being used.

Currently, HFCs are asymmetrical, meaning that upstream data is slower than downstream. Optical Solutions (2001) states that future HFC networks will need to support more upstream traffic due to more interactive applications such as video conferences and peer to peer connections and therefore there will be a need for symmetrical services. New ideas will be needed to solve this issue as there have been no feasible solutions so far.

 


Key learning points:

 

Review Questions:

  1. What are some of the components of a HFC network?

    (a) Headend, coaxial trunk, feeder
    (b) Headend, feeder, drop cable
    (c) Headend, coaxial trunk, drop cable
    (d) Coaxial trunk, feeder, drop cable

  2. What are HFCs used for?

    (a) Telephone
    (b) Internet
    (c) Television
    (d) All of the above

  3. Which technology competes with HFC?

    (a) ADSL
    (b) FTTx
    (c) CaTV
    (d) None of the Above

  4. What is NOT an advantage of HFC?

    (a) Cheap to Maintain
    (b) Smaller and thinner cables
    (c) Constant Connectivity
    (d) Asymmetrical

  5. How much of the network does the drop take up?

    (a) 10%
    (b) 25%
    (c) 50%
    (d) None of the Above

  6. What does VOD stand for?

    (a) Video On Domain
    (b) Video On Demand
    (c) Very Optimal Domain
    (d) Very Optimal Demand

  7. What is the definition of asymmetrical in reference to networking?

    (a) Data is not identical on both sides
    (b) Fibre gives analogue and digital information
    (c) Analogue
    (d) Upstream and Downstream bandwidth not identical.

  8. What frequencies do HFCs operate at?

    (a) 5MHz - 750MHz
    (b) 5MHz - 65MHz
    (c) 85MHz - 750MHz
    (d) 65MHz - 750MHz

  9. Which capital city does NOT have Telstra or Optus HFC networks?

    (a) Brisbane
    (b) Melbourne
    (c) Sydney
    (d) Perth

  10. What is the main difference between HFC and traditional CaTV networks?

    (a) Optical fibres replace Coaxial Trunks
    (b) Optical fibres replace all coaxial cables
    (c) There are no amplifiers in HFCs
    (d) FTTH is introduced

  11. Why were HFCs such a breakthrough?

    (a) They provided strong and consistent television signals to the home
    (b) They provided two-way communication
    (c) They provided interactive multimedia
    (d) None of the above

  12. Dial-up can be seen as a competitor to HFC Internet because...

    (a) Dial-up has more bandwidth
    (b) Dial-up operates on a larger frequency band
    (c) Dial-up has constant connectivity
    (d) Dial-up is cheaper for low usage

  13. A purpose of the fibre node is to...

    (a) convert electrical signals to optical signals
    (b) connect the user to the feeder line
    (c) amplify signals
    (d) All of the above

  14. What are future uses of HFCs?

    (a) VOD
    (b) xDSL
    (c) FTTN
    (d) MHz

  15. Which country has a traditional CaTV system which needed to be upgraded?

    (a) USA
    (b) Canada
    (c) Most European countries
    (d) All of the above

  16. What is NOT an FTTx technology?

    (a) FTTP
    (b) FTTB
    (c) FTTN
    (d) FTTU

  17. Which portion of the HFC network is physically connected or disconnected from a home?

    (a) Fibre Trunk
    (b) Fibre Node
    (c) Feeder
    (d) Drop Cable

  18. What is stopping the symmetrical development of HFC technology?

    (a) Cable companies believe user can exploit upstream bandwidth
    (b) Too costly to move frequency for upstream data
    (c) No current feasible solutions
    (d) All of the Above

  19. What does ADSL stand for??

    (a) Asymmetrical Digital Subscriber Line
    (b) Asynchronous Digital Subscriber Line
    (c) Asymmetrical Data Subscriber Line
    (d) Asynchrounous Data Subscriber Line

  20. What technologies use a telephone line?

    (a) ADSL, dial-up
    (b) HFC, ADSL
    (c) HFC, dial-up
    (d) None of the above

Answers: 1.b, 2.d, 3.a, 4.d, 5.c, 6.b, 7.d, 8.a, 9.d, 10.a, 11.b, 12.d, 13.a, 14.a, 15.d, 16.a, 17.d, 18.d, 19.b, 20.a.



References:

[1] Australian Competition and Consumer Commission, 2001,
Telecommunication Infrastructres in Australia 2001,
http://www.accc.gov.au/pubs/Publications/Utilities/Telecommunications/bis_shrapnel/ch6_bis_shrapnel.pdf">http://www.accc.gov.au/pubs/Publications/Utilities/Telecommunications/bis_shrapnel/ch6_bis_shrapnel.pdf
[2] Bisdikian, C.; Maruyama, K.; Seidman, D.I.; Serpanos, D.N.
IEEE Communications Magazine, Volume 34 Issue:11, Nov. 1996
Pages 128-135

[3] Halfhill, T., Byte.com, 1996
Break the Bandwidth Barrier
http://www.byte.com/art/9609/sec6art1.htm

[4] Cable & Satellite International, 2002,
Solutions for expanding cable return paths
Last Accessed 15 August 2002
http://www.cable-satellite.com/NEW%20feature_month.htm

[5] Donaldson, G., 2001,
Cable television broadband network architechtures
IEEE Communications MAgazine, Volume:39 Issue:6, June 2001
Page(s): 122-126

[6] Elfeitori, A., Leung, V., 1997
Personal Communications Services Over HFC CaTV Networks
Electrical and Computer Engineering 1997 Engineering Innovation: Voyage of Discover. IEEE 1997 Canadian Converence On,
Volume: 2, 1997, Page(s) 462-465.

[7] ETC-Telecomm
Dual Host Digital Terminal Systems - Overview
San Diego, California, 2000
Last Accessed 14 August 2002
http://etc-telecomm.com/publications/hfc2000_pub2.html

[8] Foxtel, 2001
Last Accessed 15 August 2002
http://www.foxtel.com.au/my/htlp/technology.jsp

[9] Partners of the Project SU2104 CONCORDIA (CONCORD 2),
CONCORD 2 - An Overview of Evolving Telecom Network Capabilities to the year
2000.
Last Accessed 14 August 2002
http://www.ejeisa.com/nectar/concord2/01/doc0005.htm



Glossary of Terms:

ADSL - Asymmetric Digital Subscriber Line
Modems attached to twisted pair copper wiring that transmit from 1.5 Mbps to 9 Mbps downstream (to the subscriber) and from 16 kbps to 800 kbps upstream, depending on line distance.

Asymmetrical - Where upstream and downstream data bandwidth is different.

FTTC - Fiber To The Curb - Network where an optical fiber runs from the telephone switch to a curbside distribution point close to the subscriber where it is converted to copper pair.

FTTH - Fiber To The Home - Network where an optical fiber runs from the telephone switch to the subscriber's premises.

HDSL - High bit-rate Digital Subscriber Line - Modems on either end of one or more twisted wire pair that deliver T1 speeds. At present, this requires two lines.

HFC - Hybrid Fiber-Coax

xDSL - A set of technologies which use the telephone line for connection to the Internet.

 

Acronyms:

CaTV: Cable Television
FN: Fibre Node
FTTB: Fibre To The Building
FTTC: Fibre To The Curb
FTTH: Fibre To The Home
FTTN: Fibre To The User
FTTU: Fibre To The User
FTTx: Fibre To The x
HDSL: High Bit-rate Digital Subscriber Line
HFC: Hybrid Fibre Coax or Coaxial. Also called Hybrid Fibre Cable.
MDF: Main Distribution Frame
RF: Radio Frequency
SDSL: Synchronous Digital Subscriber Line
VOD: Video On Demand
xDSL: x Digital Subscriber Line