CSU
Channel Service Unit
A device connecting a terminal to a digital line, often paired with a DSU.
A Channel Service Unit terminates a digital carrier circuit such as a T1 or T3 at the customer premises, conditioning the signal and supporting line diagnostics and loopback testing. It is almost always paired with a Data Service Unit as a combined CSU/DSU that sits between the provider line and a router. On the exam it represents legacy WAN demarcation equipment rather than a security control in its own right.
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Looking CSU up is step one. Getting tested on it is step two.
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