value between 1–7. LGV Nord Nine-bits contain the network identification code and checksum. The exactly meaning of the remaining 18-bits depends on the particular network code and is known as the "parametrisation". For the first deployment on the LGV Nord, 8-bits are assigned for the speed information, 6-bits for the current block length and 4-bits for the gradient of the current block: Encoding The decoded signal takes the form of a 27-bit digital word, with each bit corresponding to one of the 27 frequencies modulated onto the carrier frequency in the track circuits. The presence of a particular frequency indicates a "1" bit and lack of corresponds to a "0". The resulting word contains several fields, in the following order: Speed Codes containing three pieces of information: the current maximum safe speed in the block, the target speed at the end of the block, and the target speed at the end of the next block. Each of these can take on six different values; in the case of a high speed line these are (in km/h) 320, 300, 270, 230, 170, 80 and 0, roughly corresponding to a typical braking and deceleration profile. Gradient information, averaged over the length of the block. This allows the train's signalling computers to account for this in speed calculations. Block Length, which can vary quite a bit, and is also important in speed calculations. For example, on a flat stretch of high speed track, a block can be a full 1500 metres long, while in the terminal areas of the Channel Tunnel, blocks are one-tenth as long. Network Code, a number which determines the interpretation of the speed codes which should be taken by the train's computer. For example, on high speed lines where the maximum allowable speed is 300 km/h (186 mph), a different network code is used from that in the Channel Tunnel, where the speed limit is 160 km/h (100 mph). Eurostar trains need this information since they operate both on high speed tracks and in the tunnel. Error-Checking code, allowing the integrity of the entire 27-bit word to be checked if the information has been misread. This allows error correcting as well as error detection in most cases. The code takes the form of a 6-bit cyclic redundancy check (CRC). These 27 bits of information are used as an input to the train's signalling computer, the onboard part of the