point in the network. Receiving the bitstream intact and then extracting clocking information must be done before the payload can be ex- tracted. Even if the receiver is clocked externally, clock and data signals in the incoming stream must be separated and defined. Pay- load framing depends on timing relationships, and guess what, timing depends on clocking. Serial data streams just aren't real if they don't have a mechanism delineating framed data and clock signals. Understanding clock and data recovery as a stand-alone function is one of the keys to understanding digital communications networks. Figure 4-2 is a block diagram of a typical clock and data recovery function found in almost every network element. The functions in Figure 4-2 include receiving light or radio waves from the media, detecting and converting it to electrical voltage transitions. This requires a lens or optical interface in the case of light waves. The equivalent in radio waves would be an antenna. The purpose of either is to detect and focus the received energy into a signal containing the time variations between and during presence and absence of signal. The signal is then passed to a detector, which converts the signal into a series of transitions representing the serial bitstream originally sent by the transmitter. The raw bitstream is then fed to a clock extraction circuit and a buffer. The clock is extracted and used for two purposes: one is a clock signal representing the original clock used when the data was created or multiplexed, and the other drives a data recovery circuit. The data recovery circuit extracts and re-clocks the data to restore as precisely as possible the transitions between 1s and Os making up the data. At Lens & Clock Network Timing Detector Extraction Reference Buffer Recovery Incoming Data Common/Gnd FIGURE 4-2 Clock and Data Recovery Functional Block Diagram PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS this point, the data is simply just that. We don't know anything about framing or chan