Error Control and Data Compression
Applies to V.40–V.49
- V.41 is a code-independent error control system. Defines the ITU-T standard CRC-16 polynomial as x16 + x12 + x5 + 1
- V.42 is an error correction protocol. Its function is to allow the receiver to immediately request re-transmission of any lost data packets. However, it does not guarantee how quickly the error-free data will be delivered to the receiving end. V.42 is generally included in dialup modems. Also defines the ITU-T standard CRC-32 polynomial as x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1, and includes the LAPM framing protocol.
- V.42bis, also an adaptive data compression standard, is based on the Lempel Ziv dynamic dictionary approach, and may go to "transparent mode," in which data is transmitted uncompressed. The specific algorithm is "BTLZ" (British Telecom Lempel Ziv), which was developed by Alan Clark (then with BT, now with Telchemy).
- V.44 is based on LZJH (Lempel-Ziv-Jeff-Heath) adaptive data compression developed by Hughes Electronics for its DirecPC satellite Internet, and incorporated into the v.92 dial-up modem standard. V.44 offers somewhat better compression performance for certain types of data than the V.42bis standard, on average allowing 15% greater throughput. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3×6 = 320 kbit/s using pure text. In reality, the overall data rate rarely exceeds 3:1 compression (~150 kbit/s). It should be noted that the compression tends to get better and worse over time due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). At some points the modem will be sending compressed files at 53 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.
Read more about this topic: List Of ITU-T V-Series Recommendations
Famous quotes containing the words error, control, data and/or compression:
“Knowledge, like matter, [my father] would affirm, was divisible in infinitum;Mthat the grains and scruples were as much a part of it, as the gravitation of the whole world.—In a word, he would say, error was error,—no matter where it fell,—whether in a fraction,—or a pound,—’twas alike fatal to truth.”
—Laurence Sterne (1713–1768)
“Above and beyond paying attention to feelings before and after a separation, never threaten your child with leaving or loss of love in an effort to control her behavior. Children believe their parents’ assertions that “I will send you away,” “I won’t love you any more,” “I’ll go away,” and are terrified with good reason. Fear is a very poor way of disciplining a child, and it can cause severe lifelong anxiety.”
—Alicia F. Lieberman (20th century)
“This city is neither a jungle nor the moon.... In long shot: a cosmic smudge, a conglomerate of bleeding energies. Close up, it is a fairly legible printed circuit, a transistorized labyrinth of beastly tracks, a data bank for asthmatic voice-prints.”
—Susan Sontag (b. 1933)
“Do they [the publishers of Murphy] not understand that if the book is slightly obscure it is because it is a compression and that to compress it further can only make it more obscure?”
—Samuel Beckett (1906–1989)