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What instances are modems needed?

M

MrEgo

Senior member
I'm currently enrolled in the CCNA academy program, and we're starting to learn about WAN technologies. Now.. I know that modems are installed for DSL users, cable users, dial-up users, etc. I know that modems convert analog to digital and vice-versa, but I guess what I'm trying to ask is what the is "rule" for needing a modem?

One thing that's confusing me is that DSL stands for DIGITAL subscriber line. So.. the signal between my computer and the ISP is digital, right? So what's the point of the modem? From my understanding, the modem will change the low-frequency signal into a much higher frequency signal, and the ISP's end will change the high freq to a low. Do modems have other functions than just converting analog and digital signals?

I took a look at MichNet's (Michigan's biggest WAN network, I'm assuming) WAN scheme, and most of the routers are connected by serial lines.. but no modems are needed in between, or at least the diagram didn't list any. Is this because fiber lines or T1/T3 do not require mod-demod of data? I feel like I should know this stuff by now 🙁
 
no worries, at least your willing to learn!

You need a modem at every point you connect to your isp. This converts their analog signal into a digital signal your computers can understand. While DSL stands for Digital subscriber line, a modem is still needed to convert the digital signals from you into a voltage signal of a suitable frequency range which can be applied to the phone line.

Serial lines require a device like a modem (no mod/demod), but called a CSU/DSU. CSU (Channel Service Unit) recieves / transmits signals to and from the WAN line, provides a barrier for electrical interference, and echo loopback signals from the ISP for testing purposes. DSU (Data Service Unit) manages line control, converts input/output between rs-232, V.x, etc frames from the LAN, timing errors, signal regeneration, etc.

CSU/DSU's can be external boxes between the demarc and your router, or they can be internal to your Serial WIC card on the router.
 
A cable or DSL "modem" isn't a modem by the original definition... modulate/demodulate. You sound like you understand the concept, you're just getting confused by terminology.
 
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