 maintain accurate synchronization. Figure 3-1 illustrates combining technologies to achieve this. In this case, the reference is initially distributed via a satellite-based system and then transported at ground level using some form of network. Figure 3-1 Network Synchronization to a Common Reference Clock Synchronization can also be a fundamental requirement of many technologies used to transport data across a telecommunications network. In synchronous digital communications, networks require that frames arrive at the receiving node at the correct instant, SO that the allocated time slot is filled and processed correctly without dropping data. As data frames are exchanged between nodes connected to a synchronous network, it is important that clocks running at these nodes are closely synchronized. If not, eventually data would start to queue up at the slowest point and data would be lost or dropped. But, on the other hand, there are other transmission technologies that use asynchronous methods. 3.1.1 Asynchronous Networks In asynchronous networks, each node maintains its own clock, and there is no overriding control mechanism that coordinates the operation of the clocks in the network elements. Communications over an asynchronous link tend to be more byte- or packet-orientated rather than frame-orientated. These networks use methods (such as start and stop bits or a preamble) to delineate the transmission of a data segment. They also use mechanisms like flow control to stop the circuit being over- whelmed with data. As an example, older communications standards such as RS-232 use a stop and start bit to indicate the beginning and end of a byte of data. This method was widely used in the days of dial-up mo- dems and was frequently used for low-speed connections over copper wires. Some later communication technologies, such as many types of Ethernet, use a preamble that allows the receiving circuit to align its receiver to the frequency of the incoming bit pattern. As defined in IEEE 802.3-2018 standard, clause 3.