Broadband Services(CABLE)


Accessing the Internet through a cable network is a popular option used by teleworkers to access their enterprise network. The cable system uses a coaxial cable that carries radio frequency (RF) signals across the network. Coaxial cable is the primary medium used to build cable TV systems.
Cable television first began in Pennsylvania in 1948. John Walson, the owner of an appliance store in a small mountain town, needed to solve poor over-the-air reception problems experienced by customers trying to receive TV signals from Philadelphia through the mountains. Walson erected an antenna on a utility pole on a local mountaintop that enabled him to demonstrate the televisions in his store with strong broadcasts coming from the three Philadelphia stations. He connected the antenna to his appliance store via a cable and modified signal boosters. He then connected several of his customers who were located along the cable path. This was the first community antenna television (CATV) system in the United States.
Walson's company grew over the years, and he is recognized as the founder of the cable television industry. He was also the first cable operator to use microwave to import distant television stations, the first to use coaxial cable to improve picture quality, and the first to distribute pay television programming.
Most cable operators use satellite dishes to gather TV signals. Early systems were one-way, with cascading amplifiers placed in series along the network to compensate for signal loss. These systems used taps to couple video signals from the main trunks to subscriber homes via drop cables.
Modern cable systems provide two-way communication between subscribers and the cable operator. Cable operators now offer customers advanced telecommunications services, including high-speed Internet access, digital cable television, and residential telephone service. Cable operators typically deploy hybrid fiber-coaxial (HFC) networks to enable high-speed transmission of data to cable modems located in a SOHO.

The electromagnetic spectrum encompasses a broad range of frequencies.
Frequency is the rate at which current (or voltage) cycles occur, computed as the number of "waves" per second. Wavelength is the speed of propagation of the electromagnetic signal divided by its frequency in cycles per second.
Radio waves, generally called RF, constitute a portion of the electromagnetic spectrum between approximately 1 kilohertz (kHz) through 1 terahertz. When users tune a radio or TV set to find different radio stations or TV channels, they are tuning to different electromagnetic frequencies across that RF spectrum. The same principle applies to the cable system.
The cable TV industry uses a portion of the RF electromagnetic spectrum. Within the cable, different frequencies carry TV channels and data. At the subscriber end, equipment such as TVs, VCRs, and high-definition TV set-top boxes tune to certain frequencies that allow the user to view the channel or, using a cable modem, to receive high-speed Internet access.
A cable network is capable of transmitting signals on the cable in either direction at the same time. The following frequency scope is used:
Downstream - The direction of an RF signal transmission (TV channels and data) from the source (headend) to the destination (subscribers). Transmission from source to destination is called the forward path. Downstream frequencies are in the range of 50 to 860 megahertz (MHz).
Upstream - The direction of the RF signal transmission from subscribers to the headend, or the return or reverse path. Upstream frequencies are in the range of 5 to 42 MHz.

The Data-over-Cable Service Interface Specification (DOCSIS) is an international standard developed by CableLabs, a non-profit research and development consortium for cable-related technologies. CableLabs tests and certifies cable equipment vendor devices, such as cable modems and cable modem termination systems, and grants DOCSIS-certified or qualified status.
DOCSIS defines the communications and operation support interface requirements for a data-over-cable system, and permits the addition of high-speed data transfer to an existing CATV system. Cable operators employ DOCSIS to provide Internet access over their existing hybrid fiber-coaxial (HFC) infrastructure.
DOCSIS specifies the OSI Layer 1 and Layer 2 requirements:
Physical layer - For data signals that the cable operator can use, DOCSIS specifies the channel widths (bandwidths of each channel) as 200 kHz, 400 kHz, 800 kHz, 1.6 MHz, 3.2 MHz, and 6.4 MHz. DOCSIS also specifies modulation techniques (the way to use the RF signal to convey digital data).
MAC layer - Defines a deterministic access method, time-division multiple access (TDMA) or synchronous code division multiple access method (S-CDMA).
To understand the MAC layer requirements for DOCSIS, an explanation of how various communication technologies divide channel access is helpful. TDMA divides access by time. Frequency-division multiple access (FDMA) divides access by frequency. Code division multiple access (CDMA) employs spread-spectrum technology and a special coding scheme in which each transmitter is assigned a specific code.
An analogy that illustrates these concepts starts with a room representing a channel. The room is full of people needing to speak to one another-in other words, needing channel access. One solution is for the people to take turns speaking (time division). Another is for each person to speak at different pitches (frequency division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. In radio CDMA used by many North American cell phone networks, each group of users has a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other. S-CDMA is a proprietary version of CDMA developed by Terayon Corporation for data transmission across coaxial cable networks. S-CDMA scatters digital data up and down a wide frequency band and allows multiple subscribers connected to the network to transmit and receive concurrently. S-CDMA is secure and extremely resistant to noise.
Plans for frequency allocation bands differ between North American and European cable systems. Euro-DOCSIS is adapted for use in Europe. The main differences between DOCSIS and Euro-DOCSIS relate to channel bandwidths. TV technical standards vary across the world, which affects the way DOCSIS variants develop. International TV standards include NTSC in North American and parts of Japan; PAL in most of Europe, Asia, Africa, Australia, Brazil, and Argentina; and SECAM in France and some Eastern European countries.
More information is available at these websites:
About DOCSIS: http://www.cablemodem.com/specifications
About Euro-DOCSIS: http://www.euro-docsis.com
Delivering services over a cable network requires different radio frequencies. Downstream frequencies are in the 50 to 860 MHz range, and the upstream frequencies are in the 5 to 42 MHz range.
Two types of equipment are required to send digital modem signals upstream and downstream on a cable system:
Cable modem termination system (CMTS) at the headend of the cable operator
Cable modem (CM) on the subscriber end
A headend CMTS communicates with CMs located in subscriber homes. The headend is actually a router with databases for providing Internet services to cable subscribers. The architecture is relatively simple, using a mixed optical-coaxial network in which optical fiber replaces the lower bandwidth coaxial.
A web of fiber trunk cables connects the headend to the nodes where optical-to-RF signal conversion takes place. The fiber carries the same broadband content for Internet connections, telephone service, and streaming video as the coaxial cable carries. Coaxial feeder cables originate from the node that carries RF signals to the subscribers.
In a modern HFC network, typically 500 to 2,000 active data subscribers are connected to a cable network segment, all sharing the upstream and downstream bandwidth. The actual bandwidth for Internet service over a CATV line can be up to 27 Mb/s on the download path to the subscriber and about 2.5 Mb/s of bandwidth on the upload path. Based on the cable network architecture, cable operator provisioning practices, and traffic load, an individual subscriber can typically get an access speed of between 256 kb/s and 6 Mb/s.
When high usage causes congestion, the cable operator can add additional bandwidth for data services by allocating an additional TV channel for high-speed data. This addition may effectively double the downstream bandwidth that is available to subscribers. Another option is to reduce the number of subscribers served by each network segment. To reduce the number of subscribers, the cable operator further subdivides the network by laying the fiber-optic connections closer and deeper into the neighborhoods.

1 comments:

dave88 on November 14, 2008 at 9:59 PM said...

If you want a free, objective way to check the reception in your area BEFORE you lock yourself with a specific carrier, you should really check out "Got Reception?" (http://www.gotreception.com).

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