The Need to Control the Speed
Some types of Ethernet NICs and router interfaces can run at
various Ethernet-supported speeds. For those NICs and interfaces, you can either
configure the PC or router to use one speed, or the device can autonegotiate the
speed.
Serial links can run at different speeds, but the speed must be
preconfigured. A component of a serial interface called the CSU/DSU (channel service unit/data
service unit) controls the speed. The CSU/DSU functions, which include several
other things besides setting the speed, can be done with an external device or
as a function of the serial interface card on the router. If you use a CSU/DSU
that sits outside the router, you must configure the speed on that external
device; if the CSU/DSU is built into the router, you must configure the speed of
the WAN link on the router.
When you order a router, you can order a serial interface
either with or without the CSU/DSU on the serial interface. If you order a
serial interface with a CSU/DSU, it's called an internal CSU/DSU,
and if you order a router's serial interface without a CSU/DSU, you have to use
an external CSU/DSU. Figure 14-4 shows an example of both an
internal and an external CSU/DSU.
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