电信专业英英词典F-4（在线收听）

　　Frame

　　In time division multiplexing systems, Frame is a sequence of time slots each containing a sample from one of the channels carried by the system. The frame is repeated at regular intervals, (normally the sampling rate used in analog-to-digital conversion processes for signals being multiplexed) and each channel usually occupies the same sequence position in successive frames.

　　Frame Relay

　　Frame relay is a network interface protocol defined in CCITT Recommendation 1.122 "Framework for additional packet mode bearer services," as a packet mode service. In effect it combines the statistical multiplexing and port sharing of X.25 packet switching with the high speed and low delay of time division multiplexing and circuit switching. Unlike X.25, frame relay implements no layer 3 protocols and only the so-called core layer 2 functions. It is a high-speed switching technology that achieves ten times the packet throughput of existing X.25 networks by eliminating two-thirds of the X.25 protocol complexity The basic units of information transferred are variable length frames, using only two bytes for header information. Delay for frame relay is lower than for X.25, but it is variable and larger than that experienced in circuit switched networks. This means that currently frame relay is not suitable for voice and video applications where excessive and variable delays are unacceptable.

　　Frequency

　　Frequency is acoustic waves and electrical signals might be made up of only a single tone, like a single note on a piano. In this case the signal waveform is made up of repeating identical "cycles" and is said to be of a single frequency, equal to the number of cycles that occur in one second of time. In communications, frequency was traditionally expressed in cycles per second, but is now expressed in hertz (Hz), still equal to one cycle per second. Thus, one thousand cycles per second is equal to one thousand hertz, or a kilohertz (kHz).

　　Frequency Division Multiplexing (FDM)

　　FDM divides the frequency bandwidth (spectrum) of a broadband transmission circuit into many subbands, each capable of supporting a single, full time communications channel on a non- interfering basis with other multiplexed channels. FDM multiplexing is generally suitable for use with analog carrier transmission systems.

　　FSS

　　fully separated subsidiary

　　FTAM

　　file transfer access & management

　　Full Duplex

　　Full duplex is a transmission path capable of transmitting signals in both directions simultaneously.

　　FX

　　foreign exchange