across that one link. As a CCNA, you will be required to understand and configure a variety of
protocols used on point-to-point links, including Link Access Procedure Balanced (LAPB),
High-Level Data Link Control (HDLC), and Point-to-Point Protocol (PPP). Each of these WAN
protocols has the following functions in common:
link.
asynchronous functions as well.)
protocols defines framing so that receiving stations know where the beginning of the frame is,
what address is in the header, and the point at which the packet begins. By doing so, the router
receiving data can distinguish between idle frames and data frames. Synchronous links, rather
than asynchronous links, are typically used between routers.
to build devices that can truly operate at exactly the same speed, the devices adjust their rates
to match a clock source. The process works almost like the scenes in spy novels, when the spies
synchronize their watches; in this case, the watches or clocks are synchronized automatically
multiple times per minute. Unlike asynchronous links, in which no bits are sent during idle
times, synchronous data links define idle frames. These frames do nothing more than provide
plenty of signal transitions so that clocks can be adjusted on the receiving end, consequently
maintaining synchronization.
used WAN terminology is useful. Table 8-2 lists the terms.
practically speaking, a device will try to use the same speed
as another on the other end of a serial link. However, by
examining transitions between voltage states on the link, the
device can notice slight variations in the speed on each end so
that it can adjust its speed.