transfer using TCM modulation. Other additional defined data transfer rates are 24.0 kbit/s and 19.2 kbit/s as well as all the permitted V.32 and V.32bis rates. Additionally, V.34 modems employ shell mapping as shaping code to reduce the transmit power. V.34 (10/96) is an updated ITU-T recommendation for a modem, building on the V.34 standard but allowing up to 33.6 kbit/s bidirectional data transfer. Other additional defined data transfer rates are 31.2 kbit/s, as well as all the permitted V.34 rates. Modems implementing this standard were often marketed under the moniker V.34+. V.34 (02/98) commonly rendered as V.34bis, is a further update to V.34 which corrected some errata in the original 1996 document. Ad hoc standards In order to gain first-mover advantage, many modem companies introduced models based on upcoming V-series standards before they reached final ratification. In other cases, companies introduced non-standard systems but gave them ITU-like names. V.32terbo, or V.32ter for short, was a 19.2 kbit/s standard introduced by AT&T Paradyne. It was based on V.32bis and did little other than increase the data rate. V.32ter is compatible with V.32bis at speeds of 14.4 kbit/s and lower, but it is not compatible with V.34 at 19.2. V.FC, short for V.Fast Class and sometimes referred to as V.FAST, was developed by Hayes and Rockwell to introduce a 28.8 kbit/s standard while the V.34 efforts dragged on. V.FC was not compatible with V.34, although most V.34 modems could support V.FC, notably, those using Rockwell chipsets. K56flex was developed by Rockwell and Lucent while the V.90 standards were underway. Compatibility problems between the two companies' implementations were a concern, and the standard quickly disappeared. X2 was USRobotics' answer to K56flex, and likewise disappeared rapidly with the introduction of V.90. Wideband modems Applies to V.35–V.39 V.35 is an ITU-T standard located on layer 1 on the OSI model. Max speed is 2 Mbit/s and synchronous only. Withdrawn ITU-T recommendation for 48 kbit/s data transmission over wideband circuits. The physical and electrical characteristics of this interface are now specified in ITU-T recommendation V.11. V.36 is an ITU-T standard located on layer 2 on the OSI model called Modems For Synchronous Data Transmission Using 60–108 kHz Group Band Circuits. 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.