for self-synchronizing code and error detection. J. M. Emile Baudot (1845–1903) worked out a five-bit code for telegraphs which was standardized internationally and is commonly called Baudot code. More than two voltage levels are used in advanced techniques such as FDDI and 100/1,000 Mbit/s Ethernet LANs, and others, to achieve high data rates. 1,000 Mbit/s Ethernet LAN cables use four wire pairs in full duplex (250 Mbit/s per pair in both directions simultaneously), and many bits per symbol to encode their data payloads. Digital television and OFDM example In digital television transmission the symbol rate calculation is: symbol rate in symbols per second = (Data rate in bits per second × 204) / (188 × bits per symbol) The 204 is the number of bytes in a packet including the 16 trailing Reed–Solomon error correction bytes. The 188 is the number of data bytes (187 bytes) plus the leading packet sync byte (0x47). The bits per symbol is the (modulation's power of 2) × (Forward Error Correction). So for example, in 64-QAM modulation 64 = 26 so the bits per symbol is 6. The Forward Error Correction (FEC) is usually expressed as a fraction; i.e., 1/2, 3/4, etc. In the case of 3/4 FEC, for every 3 bits of data, you are sending out 4 bits, one of which is for error correction. Example: given bit rate = 18096263 Modulation type = 64-QAM FEC = 3/4 then In digital terrestrial television (DVB-T, DVB-H and similar techniques) OFDM modulation is used; i.e., multi-carrier modulation. The above symbol rate should then be divided by the number of OFDM sub-carriers in view to achieve the OFDM symbol rate. See the OFDM system comparison table for further numerical details. Relationship to chip rate Some communication links (such as GPS transmissions, CDMA cell phones, and other spread spectrum links) have a symbol rate much higher than the data rate (they transmit many symbols called chips per data bit). Representing one bit by a chip sequence of many symbols overcomes co-channel interference from other transmitters sharing the same frequency channel, including radio jamming, and is common in military radio and cell phones. Despite the fact that using more bandwidth to carry the same bit rate gives low channel spectral efficiency in (bit/s)/Hz, it allows many simultaneous users, which results in high system spectral efficiency in (bit/s)/Hz per unit of area. In these systems, the symbol rate