#TechExchange: Communications Cable and the Electromagnetic Spectrum

The electromagnetic spectrum encompasses a broad range of frequencies. Remember the rise of all mobile networks and satellite services depend on the correct allocation of Radio Frequencies.

By Cisco Eng. Shingie Lev MuringiĀ 

Frequency is the rate at which current, or voltage, cycles occur. Frequency is computed as the number of waves per second. Wavelength is the distance from the peak of one wave to the peak of the next wave. Wavelength is calculated as the speed of propagation of the electromagnetic signal divided by its frequency in cycles per second.

Radio waves, often called RF, constitute a portion of the electromagnetic spectrum between approximately 1 kHz and 1 THz, as indicated in the figure. When users tune a radio or television to find different radio stations or television 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, Blu-ray players, DVRs, and HDTV set-top boxes tune to certain frequencies that allow the user to view the channel or use 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 scopes are used:

  • Downstream – The direction of an RF signal transmission, such as TV channels and data, from the source, or headend, to the destination, or subscribers. Transmission from source to destination is called the forward path. Downstream frequencies are in the range of 50 to 860 MHz.
  • Upstream – The direction of the RF signal transmission from subscribers to the headend. Upstream frequencies are in the range of 5 to 42 MHz.

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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 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, or 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 technique, which is how 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, each person needs 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.

 

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