ial or parallel format (see Figure 2.3). In serial communication, the transmitter sends one bit at a time. In contrast, as the name implies, parallel data trans- mission involves several bits simultaneously. The serial transmission mode uses only one communication channel (for example, one pair of wires) to send a binary datastream (Figure 2.3(a)). In this case bits are sent into the channel sequentially according to a clock instance. In the case of parallel transmission (Figure 2.3(b)), a group of bits is transferred simultaneously from transmitter to receiver via a set of channels (one bus of information). Generally, parallel transmission is faster com- pared to the serial scheme but requires a greater number of transmission links. Parallel Data Transmission Basics Transmitter 10110101 10110111 10110100 10101010 Receiver System System Serial Transmission Fig. 2.4 Serial transmission of individual bytes in asynchronous mode. transmission is often used within or between co-sited equipment. It can also be used in longer-distance communications such as virtual concatenation in Next Generation SDH (see Chapter 11) or in inverse multiplexing in ATM. In addition, clock skew between different channels is not an issue in the case of serial communications while it can be problematic when data is transmitted in parallel channels. Finally, crosstalk is less of an issue in the case of serial communications because of the limited number of transmission links. However, serial transmission is more prone to dispersion. Serial-to-parallel conversion can be carried out at the transmitter when the channel is set to parallel transmission. In the opposite way, parallel-to-serial transmission conversion can occur if only one channel is used for the transmission. Serial transmission can be subdivided into asynchronous and synchronous data trans- fer. In asynchronous serial transmission (see Figure 2.4), no external clock is used for syn- chronization of the transmission process. In this case, timing information can be embedded in the 