Many other types of encodings have been defined to transmit information over an electrical cable. All physical layers are able to send and receive physical symbols that represent values 0 and 1. However, for various reasons that are outside the scope of this chapter, several physical layers exchange other physical symbols as well. For example, the Manchester encoding used in several physical layers can send four different symbols. The Manchesterencodingisadifferentialencodingschemeinwhichtimeisdividedintofixed-lengthperiods. Eachperiodis divided in two halves and two different voltage levels can be applied. To send a symbol, the sender must set one of these two voltage levels during each half period. To send a 1 (resp. 0), the sender must set a high (resp. low) voltage during the first half of the period and a low (resp. high) voltage during the second half. This encoding ensures that there will be a transition at the middle of each period and allows the receiver to synchronise its clock tothesender’sclock. Apartfromtheencodingsfor0and1,theManchesterencodingalsosupportstwoadditional symbols : InvH and InvB where the same voltage level is used for the two half periods. By definition, these two symbols cannot appear inside a frame which is only composed of 0 and 1. Some technologies use these special symbols as markers for the beginning or end of frames.