Clocking determines how individual routing nodes or entire networks sample transmitted data. As streams of information are received by a device in a network, a clock source specifies when to sample the data. In asynchronous networks, the clock source is derived locally, and synchronous networks use a central, external clock source. Interface clocking indicates whether the device uses asynchronous or synchronous clocking.
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Note: Because truly synchronous networks are difficult to design and maintain, most synchronous networks are really plesiochronous networks. In a plesiochronous network, different timing regions are controlled by local clocks that are synchronized (with very narrow constraints). Such networks approach synchronicity and are generally known as synchronous networks. |
Most networks are designed to operate as asynchronous networks. Each device generates its own clock signal, or devices use clocks from more than one clock source. The clocks within the network are not synchronized to a single clock source. By default, devices generate their own clock signals to send and receive traffic.
The system clock allows the device to sample (or detect) and transmit data being received and transmitted through its interfaces. Clocking enables the device to detect and transmit the 0s and 1s that make up digital traffic through the interface. Failure to detect the bits within a data flow results in dropped traffic.
Short-term fluctuations in the clock signal are known as clock jitter. Long-term variations in the signal are known as clock wander.
Asynchronous clocking can either derive the clock signal from the data stream or transmit the clocking signal explicitly.
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