ELEC3506

Topics

DHCP and WANsLecture 9 PDF10 min

Cable and HFC

How cable TV networks grew into a data access network, what hybrid fibre-coaxial changed, why coaxial beats twisted pair on interference, and how subscribers share the upstream band.

By the end of this page you should be able to

  • Describe the traditional cable network and the roles of the head end and amplifiers
  • Explain what makes an HFC network different from a traditional cable network
  • Say why the lecture considers coaxial cable better than DSL's twisted pair for data
  • State the upstream bandwidth and channel figures and explain how subscribers share them

The idea

Cable networks did not start as data networks. They existed to deliver TV to people whose antennas could not pick up a good signal. The same coaxial plant that already ran past each home turned out to be a second route into the house, alongside the telephone line.

How it works

Traditional cable network

Cable networks were created to give TV access to subscribers who had no reception because of natural obstructions such as mountains. Later they became popular with people who just wanted a better signal. They also gave access to remote broadcasting stations through microwave connections.

Cable TV began distributing broadcast video signals to locations with poor or no reception in the late 1940s. Community antenna TV (CATV) received the signals from the TV stations and distributed them via coaxial cables. The head end performs the signal processing, and amplifiers strengthen the signal at the user end.

How it works

Hybrid fibre-coaxial (HFC)

HFC is the second generation of cable networks, combining fibre-optic and coaxial cable.

  • From the cable TV office to a box called the fibre node, the medium is optical fibre.
  • From the fibre node through the neighbourhood and into the house, it is still coaxial cable.

How it works

Cable TV for data transfer

DSL uses the existing unshielded twisted-pair cable, which is very susceptible to interference. The coaxial cable used in a cable network alleviates this problem, which is why cable is a good vehicle for data.

How it works

Sharing

Both the upstream and downstream bands are shared by the subscribers. The lecture gives specific upstream figures:

  • The upstream data bandwidth is 37 MHz.
  • Only six 6 MHz channels are available in the upstream direction.
  • Subscribers share these channels in time to send data upstream.

When there are few users, cable gives a much higher data rate than xDSL for the same distance and bandwidth. Because the channels are time-shared, rates drop as the number of users grows. The lecture closes the slide by pointing to FTTB and FTTN as alternatives that use a DSLAM. See FTTx.

DSLCable (HFC)
Last-mile mediumUnshielded twisted pair (the telephone local loop)Coaxial cable from the fibre node
InterferenceVery susceptibleAlleviated by coaxial cable
The lecture's comparison is on interference and sharing, not on a list of speeds.

In the exam

  • Traditional cable is CATV: coaxial cable, a head end for signal processing, amplifiers at the user end.
  • HFC adds fibre from the cable office to the fibre node and keeps coaxial after it.
  • Numbers to memorise: 37 MHz upstream, six 6 MHz channels.
  • Sharing is by time, and rates fall as users increase.

Check yourself

  • Cable began as CATV, delivering TV over coaxial cable with a head end and amplifiers.
  • HFC is fibre to the fibre node and coaxial from there to the house.
  • Coaxial reduces the interference that hurts DSL’s twisted pair.
  • Upstream is 37 MHz as six 6 MHz channels, time-shared among subscribers.

Check yourself

  1. What distinguishes an HFC network from a traditional cable network?
  2. Why does the lecture say cable is better placed than DSL for data?
  3. The upstream band in the lecture is 37 MHz with six 6-MHz channels. What is the effect on a subscriber as more users join?