One additional feature of HDLC and PPP not mentioned so far is that they are both synchronous. Synchronous simply means that there is an imposed time ordering at the link's sending and receiving ends. Essentially, the sides agree to a certain speed, but it is expensive to build devices that truly can operate at exactly the same speed. So, the devices operate at close to the same speed and listen to the speed of the other device on the other side of the link. One side makes small adjustments in its rate to match the other side.
Synchronization occurs by having one CSU (the slave) adjust its clock to match the clock rate of the other CSU (the master). The process works almost like the scenes in spy novels in which the spies synchronize their watches; in this case, the watches or clocks are synchronized automatically several times per second.
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