$4.$ Mobile phone network operators need to know where their subscribers$$ mobile phones $($hence their users$)$ are located. Explain why this is bad for users. Now give reasons why this is good for users.

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$5.$ Is an oil pipeline a simplex system, a half$-$duplex system, a full$-$duplex system, or none of the above? What about a river or a walkie$-$talkie$-$style communication?

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$6.$ Ten signals, each requiring $4000$ Hz$,$ are multiplexed onto a single channel using FDM$.$ What is the minimum bandwidth required for the multiplexed channel? Assume that the guard bands are $400$ Hz wide.