Serial communication

In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels. Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and difficulty of synchronization make parallel communication impractical. Serial computer buses have become more common even at shorter distances, as improved signal integrity and transmission speeds in newer serial technologies have begun to outweigh the parallel bus's advantage of simplicity (no need for serializer and deserializer, or SerDes) and to outstrip its disadvantages (clock skew, interconnect density). The migration from PCI to PCI Express (PCIe) is an example. Modern high speed serial interfaces such as PCIe send data several bits at a time using modulation/encoding techniques such as PAM4 which groups 2 bits at a time into a single symbol, and several symbols are still sent one at a time. This replaces PAM2 or non return to zero (NRZ) which only sends one bit at a time, or in other words one bit per symbol. The symbols are sent at a speed known as the symbol rate or the baud rate.