is called bit interleaved. Bit interleaving the simplest form of multiplexing, was used by the first commercially available multiplexers. It is simple because it only requires the multiplexer to be able to detect the boundaries of the incoming input data. In addition, if the sequence of the databits is known at the receiver end, the de-multiplexing process can be easily done for any type of input data. However, because this means sending all databits for an asynchronous char- acter, including the stop, start, and parity bits, the adopted multiplexing technique quickly moved to byte interleaved. In fact, when choosing the size of the timeslots, there is a trade- off between efficiency and delay. With very small timeslots (such as in bit interleaving), the multiplexer should be very fast to switch between different input channels. On the other Multiplexing hand, if timeslots are longer than a bit length, then data from each input should be stored while the other input channels are served. In the case of very long timeslots, a significant delay will be introduced. Notice that data interleaving can occur inside each input channel as well. This is usually achieved in the application layer before the data is sent to the lower layers to be multiplexed with other input channels. This is entirely different from the interleaving during the multi- plexing process. Because most of the transmission errors occur as bursts, the aim of higher- layer interleaving is to spread the erroneous data to improve the perceived quality of the received data. Frame Alignment Needless to mention, the de-multiplexer needs to identify which timeslot is allocated to which input channel. Therefore, a predetermined binary sequence is periodically repeated during the multiplexed data. A frame is then defined as a set of successive timeslots that can be identified by reference to a frame alignment signal. The frame alignment signals commonly occupy a finite number of timeslots (usually one or two) during each frame. These timeslots are referred to 