The electrical characteristics of X.21 are defined by V.10/X.26 or V11/X.27. V.10 uses 4 to 6 Volts to represent binary value 0, and -4 to -6 Volts to represent binary value 1. It allows for connection distances of up to 1 km. Figure 2.27 illustrates how a DTE and a DCE may be connected using X.21.