7.1.1 Clocks in Digital Systems

Clock signals are containers that carry the physical quantity of frequency. Frequency is transferred across digital systems by the clock in a way that is different from the one in analog systems. Clocks are a new part of digital systems.

Digital systems are either synchronous or asynchronous. In the former case, all system elements utilize the same reference clock, while in asynchronous systems they do not. Current telecommunication networks, especially long-haul transmission networks, are basically synchronous. In each such network, the clock is synchronized and shared. It conveys the common frequency used to synchronize the network elements. In this configuration, the clock distribution area constitutes the whole network. Each asynchronous network, on the other hand, may have many clock distribution areas, each of which is usually formed between neighboring functional blocks. The physical separation between functional blocks varies from neighboring integrated circuits to several or several tens of kilometers.

Synchronous and asynchronous communication both require that the clock be transferred to the intended destination. The transfer process is a basic item for any system. In a synchronous system, the clock is distributed from the master node, which acts as a reference. This reference clock is regenerated at all other nodes and further transmitted to all systems in each node. Since all circuits in the network use the same clock (i.e., same frequency accuracy) this method makes it easy to multiplex/demultiplex and add/drop information using registers. If the clock frequency deviates from the...