 bit synchronization and frame synchronization. With bit synchronization, the timing sampling is generated on each bit at the receiver. Therefore, an agreement on the bit duration in between the transmit- ter and receiver must exist to avoid errors. These errors can originate more bits than the transmitted ones if the bit-sampling duration is shorter at the receiver (see Figure 2.17(a)). On the other hand, if the sampling duration of the bit is longer at the receiver compared with the bit-transmitting duration, the receiver will miss some bits and the data transmitted will be incomplete (see Figure 2.17(b)). Other problems arise when a long trail of 1s or Os is transmitted. This difficulty can impair the receiver's synchronization. To avoid this last issue, scramblers can be used, which add extra information to the bits in a random manner. DC components are also eliminated by the use of scramblers. Bit synchronization distortion can arise whenever the sampling drifts in time with respect to the correct sample period. The synchronization will present timing jitter, and errors will occur at the receiver. Error propagation can appear whenever scrambling is being used and one bit in error is present. This error will derive more bits in error, and the only solution is to fill the scrambler with full correct bits. Frame synchronization refers to the timing per frame in the transmission. Whenever a transmission starts, the receiver must know at which point a signal is present (or has Timing and Synchronization 110111 111100111111 110111 Fig. 2.17 (a) Oversampling and (b) undersampling at the receiver of a digital signal. arrived). No identification of the signal frame will occur in the errors that are because of the interpretation of noise as random bits. Therefore, the frame synchronization starts with identifying the beginning of the frame. Another important aspect is to recognize subdivi- sions or frames within the transmitted signal. To do that, specific bit patterns are used as prefixes. The repetition of this pref