Internet Access via Cable TV

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Internet Access via Cable TV
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Changes in the Cable Network

• The cable network was designed to deliver TV
  signals in one direction from the Head-End to the
  subscribers homes
• To provide TV services Cable Operators had to
  recreate a portion of the over-the-air radio
  frequency (RF) spectrum within a sealed coaxial
  cable line
• Operators had to upgrade the cable network so
  that signals could flow in both directions

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Changes in the Cable Network

• Cable Operators assign a spectrum of signal
  frequencies to the cable network
• One spectrum is used for the signals that move
  from the Head-End towards the cable subscriber
• Another spectrum of signal frequencies are used
  for the signals that move from the cable
  subscriber towards the Head-End

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Changes in the Cable Network

• By replacing existing one way amplifiers with two
  way amplifiers Cable Operators are able to
  separate the upstream and downstream signals and
  amplify each direction separately in the right
  frequency range

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Changes in the Cable Network
A Traditional Cable network
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Changes in the Cable Network
A Modern Cable network
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What is a Cable Modem?
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How Fast is a Cable Modem?

• Cable modem speeds vary widely
• Depends on the cable modem system
• Cable network architecture
• Traffic load.
• In the downstream direction (from the network to
  the computer), network speeds can be up to 27
  Mbps
• BUT, this is an aggregate amount of bandwidth
  that is shared by users.

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How Fast is a Cable Modem?

• Few computers will be capable of connecting at
  such high speeds or have exclusive access to the
  network
• A more realistic number is 1 to 3 Mbps.
• In the upstream direction (from computer to
  network), speeds can be up to 10 Mbps.
• However, most modem producers have selected a
  more optimum speed between 500 Kbps and 2.5 Mbps
• AND, many cable operators limit the upstream
  bandwidth to 128 or 384kbs

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How Fast is a Cable Modem?

• An asymmetric cable modem scheme is most common.
  The downstream channel has a much higher
  bandwidth allocation (faster data rate) than the
  upstream,
• primarily because Internet applications tend to
  be asymmetric in nature.
• Activities such as World Wide Web (http)
  navigating and newsgroups reading (nntp) send
  much more data down to the computer than to the
  network.

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How Fast is a Cable Modem?

• Mouse clicks (URL requests) and e-mail messages
  are not bandwidth intensive in the upstream
  direction.
• Image files and streaming media (audio and video)
  are very bandwidth intensive in the downstream
  direction.

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Real-world performance

• The theoretical performance of a Cable Modem is
  based upon all other devices being able to work
  at the same speed and performance as the Cable
  Modem
• However, in a similar way that the actual usable
  bandwidth on a 10Mbps Ethernet connection reduces
  to a 4Mbps, so too will the performance of a
  Cable Modem connection be reduced

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Real-world performance

• The Cable network itself will suffer the same
  problems of Internet performance as any other
  Internet Service Provider (ISP)
• Although performance to services on the cable
  network itself can be amazingly fast, access to
  'the outside world' will be slowed down by the
  performance of other connections on the way.

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Real-world performance

• As usage on your segment grows (as more customers
  are added) the bandwidth must be shared by more
  people
• Adding more cable network segments is very
  expensive for the cable operator
• If you connect to a remote Internet site that
  itself has a connection speed equivalent to a T1
  connection (1.5Mbps), then that is as fast as the
  data can be served to you, no matter how fast
  your receiving equipment is

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Who Makes Cable Modems?

• 3Com, Cisco Systems, Com21, General Instrument,
  Motorola, Nortel Networks, Phasecom, Samsung,
  Terayon, Toshiba, Zenith
• And many others

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Cable Modem Technology

• It MOdulates and DEModulates signals
• Much more complicated than their telephone
  counterparts
• Cable modems can be part modem, part tuner, part
  encryption/decryption device, part bridge, part
  router, part network interface card, part SNMP
  agent, and part Ethernet hub

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Cable Modem Technology

• Typically, a cable modem sends and receives data
  in two slightly different fashions
• In the downstream direction
• he digital data is modulated and then placed on a
  typical 6 MHz television channel, somewhere
  between 50 MHz and 750 MHz
• 64 QAM is the preferred downstream modulation
  technique, offering up to 27 Mbps per 6 MHz
  channel
• This signal can be placed in a 6 MHz channel
  adjacent to TV signals on either side without
  disturbing the cable television video signals.

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Cable Modem Technology

• The upstream channel is more tricky
• Typically, in a two-way activated cable network,
  the upstream (also known as the reverse path) is
  transmitted between 5 and 42 MHz
• This tends to be a noisy environment, with RF
  interference and impulse noise. Additionally,
  interference is easily introduced in the home,
  due to loose connectors or poor cabling
• Since cable networks are tree and branch
  networks, all this noise gets added together as
  the signals travel upstream, combining and
  increasing
• Due to this problem, most manufacturers use QPSK
  or a similar modulation scheme in the upstream
  direction, because QPSK is more robust scheme
  than higher order modulation techniques in a
  noisy environment
• The drawback is that QPSK is "slower" than QAM.

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Cable Modem Services

• The dominant service is high-speed Internet
  access
• This enables the typical array of Internet
  services to be delivered at speeds far faster
  than those offered by dial-up telephone modems
• Other services will include
• access to streaming audio and video servers,
  local content (community information and
  services)
• access to CD-ROM servers
• a wide variety of other service offerings. New
  service ideas are being developed daily.

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Cost of Cable Modem Service

• In North America, cable operators are packaging
  high-speed data services much like they do basic
  cable television service
• Typically charging 40 - 60 per month for an
  Internet service package
• Includes software, unlimited Internet access,
  specialized content and rental of a cable modem

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Cost of Cable Modem Service

• At the low end of this pricing scale, a very
  robust Internet service is available to consumers
  for about the cost of a dial-up account with a
  local Internet service provider and a second
  telephone line
• Even at 60 per month, cable is a far better
  value than ISDN.

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"Telco-Return" Modems

• Not really a cable technology
• Used more often with Direct Satellite video
  systems
• Satellite down link is used for fast downstream
  transmission
• A telephone modem handles upstream communication
  over the public telephone network.

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Support for Multiple PCs

• A cable modem can provide Intenet access to
  multiple PCs, if they are connected via a local
  area network (LAN)
• Cable modems typically have an Ethernet output,
  so they can connect to the LAN with a standard
  Ethernet hub or router
• Each PC must have an assigned IP address
• The cable ISP usually sells at a premium of
  5-10 a month per PC
• NAT (Network Address Translation) can allow
  multiple PCs to "hide" behind a single IP Address