Practical TCP/IP and Ethernet Networking- P17: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 62 Practical TCP IP and Ethernet Networking Physical medium independent PHY sub layer The PHY layer specifies the 4B 5B coding of the data data scrambling and the non return to zero - inverted NRZI data coding together with the clocking data and clock extraction processes. The 4B 5B technique selectively codes each group of four bits into a five-bit cell symbol. For example the binary pattern 0110 is coded into the five-bit pattern 01110. In turn this symbol is encoded using non return to zero - inverted NRZ-I where a 1 is represented by a transition at the beginning of the cell and a 0 by no transition at the beginning. This allows the carriage of 100 Mbps data by transmitting at 125 MHz and gives a consequent reduction in component cost of some 80 . With a five-bit pattern there are 32 possible combinations. Obviously there are only 16 of these that need to be used for the four bits of data and of these each is chosen so that there are no more than three consecutive zeros in each symbol. This ensures there will be sufficient signal transitions to maintain clock synchronization. The remaining 16 symbols are used for control purposes. This selective coding is shown in Table . 4-bit Data Pattern 5-bit Symbol 0000 11110 0001 01001 0010 10100 0011 10101 0100 01010 0101 01011 0110 OHIO 0111 01111 1000 10010 1001 10011 1010 10110 1011 10111 1100 11010 1101 non 1110 11100 1111 11101 Table 4B 5B data coding This coding scheme is not self clocking so each of the receivers maintains a separate data receive clock which is kept in synchronization with the transmitting node by the clock transitions in the data stream. Hence the coding cannot allow more than three consecutive zeros in any symbol. 100Base-TX and -FX physical media dependent PMD sub layer This uses the ANSI PMD layer and operates on two pairs of category 5 twisted pair. It uses stream cipher scrambling for data security and MLT-3 bit encoding. The multilevel threshold-3 MLT-3 bit .
