Telecommunication
The other meaning of the term "channel" in telecommunications is seen in the phrase communications channel, which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. For example, one radio station can broadcast radio waves into free space at frequencies in the neighborhood of 94.5 MHz (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighborhood of 96.1 MHz. Each radio station would transmit radio waves over a frequency bandwidth of about 180 kHz (kilohertz), centered at frequencies such as the above, which are called the "carrier frequencies". Each station in this example is separated from its adjacent stations by 200 kHz, and the difference between 200 kHz and 180 kHz (20 kHz) is an engineering allowance for the imperfections in the communication system.
Personal radio service
FRS (規範頻率), 14 channels, 12.5 kHz spacing:Since 3 February 2004, the Infocomm Development Authority of Singapore (IDA) has allocated the 446.0–446.1 MHz frequency band for low-powered walkie-talkies on a non-interference, non-protected and shared-use basis. As these walkie-talkies are low-powered devices which do not potentially cause interference to other licensed radio services, it need not be licensed for use in Singapore. However, the device must be type approved by IDA for local sale.
Citizens Band radio is a family of services available in different countries and with different operating rules, generally using channels in the 27 MHz part of the radio spectrum. 26–27 MHz occupies the "boundary area" between HF (3–30 MHz) and VHF (30–300 MHz). This means that CB signals provide local coverage similar to low-band VHF during times of low sunspot activity. However, during the peak of the sunspot cycle, CB frequencies exhibit skywave propagation just like the lower parts of HF do, making communication hundreds or even thousands of miles (km) away possible. While some operators seek out long distance "DX" contacts on CB frequencies and on frequencies above channel 40 and below channel 1 (a practice referred to as "freebanding" or "outbanding"), interference from distant stations will often make local communication extremely difficult or impossible during band openings. CB was, and still is, designed for short-distance (local) communications needs. US FCC law prohibits communicating with any station more than 250 km (155.3 miles) on CB frequencies.(150 mile rule deleted by FCC September -2017) Like many rules regarding the HF CB services, the distance prohibition is largely ignored and unenforced. Often as a result of channel overcrowding and interference, many HF CB users have turned to purchasing "export" or "10-meter" radios that operate in the legal CB band but also provide access to frequencies above and below the CB band. Other CB users purchase amplifiers to increase their output power and "punch through" interference caused by distant stations (or by local stations running amplifiers). This has created a tragedy of the commons situation in and near the HF CB spectrum (25–28 MHz). As more users purchase amplifiers and operate on out-of-band frequencies, more interference is produced, forcing others to acquire even more powerful equipment to punch through even more interference, and/or to acquire transceivers capable of accessing more frequencies so that a clear frequency may be found. Many CB users have moved to other personal radio services to avoid these issues.