That portion of the ionosphere existing between approximately 160 and 400 km above the surface of the Earth, consisting of layers of increased free-electron density caused by the ionizing effect of solar radiation. Note 1: The F region reflects normal-incident frequencies at or below the critical frequency (approximately 10 MHz) and partially absorbs waves of higher frequency. Note 2: The F₁ layer exists from about 160 to 250 km above the surface of the Earth and only during daylight hours. Though fairly regular in its characteristics, it is not observable everywhere or on all days. The principal reflecting layer during the summer for paths of 2,000 to 3,500 km is the F₁ layer. The F₁ layer has approximately 5 × 10⁵ e/cm³ (free electrons per cubic centimeter) at noontime and minimum sunspot activity, and increases to roughly 2 × 10⁶ e/cm³ during maximum sunspot activity. The density falls off to below 10⁴ e/cm³ at night. Note 3: The F₁ layer merges into the F₂ layer at night. Note 4: The F₂ layer exists from about 250 to 400 km above the surface of the Earth. The F₂ layer is the principal reflecting layer for HF communications during both day and night. The horizon-limited distance for one-hop F₂ propagation is usually around 4,000 km. The F₂ layer has about 10⁶ e/cm³. However, variations are usually large, irregular, and particularly pronounced during magnetic storms.

In laser diodes, a phenomenon in which a high field intensity of stimulated optical radiation causes degradation of the facets, i.e., those forming the cavity mirrors, decreasing reflectivity and resulting in a decrease of the internal quantum efficiency and an increase in the threshold current.

A fixed, mobile, or transportable structure, including (a) all installed electrical and electronic wiring, cabling, and equipment and (b) all supporting structures, such as utility, ground network, and electrical supporting structures. 2. A network-provided service to users or the network operating administration. 3. A transmission pathway and associated equipment. 4. In a protocol applicable to a data unit, such as a block or frame, an additional item of information or a constraint encoded within the protocol to provide the required control. 5. A real property entity consisting of one or more of the following: a building, a structure, a utility system, pavement, and underlying land. [JP1]

A group of interconnected equipment and transmission media uniquely identified and dedicated to a specific type of service. [T1.238-1994/97]

A two-digit code (FC) used in the exchange-carrier-to-interexchange carrier (EC-to-IC) facility/service selective signaling protocol and the IC-to-EC facility/service selective signaling protocol that identifies the type of facility requested. [T1.104-1991]

The electrically interconnected system of conductors and conductive elements that (a) provides multiple current paths to the earth electrode subsystem, and (b) consists of the earth electrode subsystem, the lightning protection subsystem, and the fault protection subsystem.

The particular group of facilities to route the call. [T1.667-1999]

The specific member of a trunk group or a multi-line hunt group. [T1.667]

A form of telegraphy for the transmission of fixed images, with or without half-tones, with a view to their reproduction in a permanent form. [47CFR] In this definition the term telegraphy has the same general meaning as defined in the Convention. [NTIA] [RR] 2. The process by which fixed graphic images, such as printed text and pictures, are scanned, and the information converted into electrical signals that may be transmitted over a telecommunications system and used to create a copy of the original, or an image so produced. Note 1: Wirephoto and telephoto are facsimile via wire circuits. Radiophoto is facsimile via radio. Note 2: Technology now exists that permits the transmission and reception of facsimile data to or from a computer without requiring hard copy at either end. Note 3: Current facsimile systems are designated and defined as follows:

• Group 1 Facsimile: The mode of black and white facsimile operation, defined in ITU-T Recommendation T.2, that uses double sideband modulation without any special measures to compress the bandwidth. Note 1: A 216 × 279-mm document, i.e., an 8½ × 11-inch document, may be transmitted in approximately 6 minutes via a telephone-type circuit. Additional modes in this group may be designed to operate at a lower resolution suitable for the transmission of 216 × 279-mm documents in 3 to 6 minutes. Note 2: The CCITT frequencies used are 1300 Hz for white and 2300 Hz for black. The North American standard is 1500 Hz for white and either 2300 or 2400 Hz for black.
  

• Group 2 Facsimile: The mode of black and white facsimile operation, defined in ITU-T Recommendation T.3, that accomplishes bandwidth compression by using encoding and vestigial sideband, but excludes processing of the document signal to reduce redundancy. Note: A 216 × 279-mm document, i.e., an 8½ × 11-inch document, may be transmitted in approximately 3 minutes using a 2100-Hz AM/PM/VSB, over a telephone-type circuit.
  

• Group 3 Facsimile: The mode of black and white facsimile operation, defined in ITU-T Recommendation T.4, that incorporates means for reducing the redundant information in the signal by using a one-dimensional run-length coding scheme prior to the modulation process. Note 1: A 216 × 279-mm document, i.e., an 8½ × 11-inch document, may be transmitted in approximately 1 minute or less over a telephone-type circuit with twice the Group 2 horizontal resolution. The vertical resolution may also be doubled. Note 2: Group 3 Facsimile machines have integral digital modems. Note 3: An optional two-dimensional bandwidth compression scheme is also defined within the Group 3 Facsimile Recommendation. Note 4: When any CCITT or CCIR Recommendation is modified by the ITU-T,
  the modified document is designated as an ITU-T Recommendation.
  

• Group 3C Facsimile: The Group 3 digital mode of facsimile operation defined in ITU-T Recommendation T.30. Note: Group 3C is also referred to as Group 3 Option C or as Group 3-64 kb/s.
  

• Group 4 Facsimile: The mode of black and white facsimile operation defined in ITU-T Recommendation T.563 and CCITT Recommendation T.6. Note 1: Group 4 Facsimile uses bandwidth compression techniques to transmit, essentially without errors, a 216 × 279-mm document, i.e., an 8½ × 11-inch document, at a nominal resolution of 8 lines/mm in less than 1 minute over a public data network voice-grade circuit. Note 2: When any CCITT or CCIR Recommendation is modified by the ITU-T, the modified document is designated as an ITU-T Recommendation.
  

• Type I Facsimile: The mode of digital black and white facsimile operation defined in MIL-STD-188-161 used for transmission of bi-level information (e.g., text and simple graphics). Note: Type I facsimile is interoperable with the black-and-white facsimile mode of STANAG 5000 and is designed for operation over noisy communications links such as tactical channels.
  

• Type II Facsimile: The mode of gray-scale facsimile operation defined in MIL-STD-188-161 used for transmission of multi-level information (e.g., photographs). Note: Type II facsimile is interoperable with the black-and-white facsimile mode of Type I or STANAG 5000 equipment and is designed for operation over noisy communications links such as tactical channels.