A technique that provides real-time data on high-frequency ionospheric-dependent radio propagation, using a basic system consisting of a synchronized transmitter and receiver. Note: The time delay between transmission and reception is translated into effective ionospheric layer altitude. Vertical incident sounding uses a collocated transmitter and receiver and involves directing a range of frequencies vertically to the ionosphere and measuring the values of the reflected returned signals to determine the effective ionosphere layer altitude. This technique is also used to determine the critical frequency. Oblique sounders use a transmitter at one end of a given propagation path, and a synchronized receiver, usually with an oscilloscope-type display (ionogram), at the other end. The transmitter emits a stepped- or swept-frequency signal which is displayed or measured at the receiver. The measurement converts time delay to effective altitude of the ionospheric layer. The ionogram display shows the effective altitude of the ionospheric layer as a function of frequency.

Ongoing disturbances of the ionosphere that scatter incident electromagnetic waves. Note: Ionospheric turbulence results in irregularities in the composition of the ionosphere that change with time. This causes changes in reflection properties. These, in turn, cause changes in skip distance, fading, local intensification, and distortion of the incident waves. [From Weik ’89]

1. Intelligent Peripheral, 2. Internet Protocol.

A device’s or resource’s numerical address as expressed in the format specified in the Internet Protocol. Note 1: In the current addressing format, IP version 4 (IPv4), an IP address is a 32-bit sequence divided into four groups of decimal numbers separated by periods (“dots”), commonly referred to as “dotted decimals.” The IP address of a device is made up of two parts: the number of the network to which it is connected, and a sequence representing the specific device within that network. An IP address may be used on private intranets, as well as The Internet. Note 2: Due to inefficiencies that have arisen in address assignment, available IPv4 addresses are nearly exhausted. A newer version of IP addressing (IP version 6, consisting of a 128-bit numerical sequence) is currently being developed. Synonyms Internet address, IP number.

The fundamental unit of information passed across any network utilizing Internet protocol. Note: An IP datagram contains source and destination addresses along with data and a number of fields that define such things as the length of the datagram, the header checksum, and flags that indicate whether the datagram can be (or has been) fragmented.

Nonpermanent access, by means of a switched network (e.g., the public switched telephone network), to any network utilizing The Internet Protocol, especially The Internet. Note: Two well-known protocols for dialup access are PPP (point-to-point) and SLIP (serial line internet protocol). [After 2382-pt.35]

A technique that allows packets to be simultaneously transmitted over the Mbone (multicast backbone on The Internet) to a selected set of destinations. Note: Standard Internet traffic requires a separate set of packets for each destination. IP multicast allows for one set of packets to be sent to multiple destinations.

Synonym IP address.

A technique used by hackers to access computer systems by modifying packet headers to make them appear to have originated from a trusted port. 2. The practice of falsifying an e-mail header to make it appear as though it originated from a different address.