• Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
    Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.

Multiple Signals on One Cable

A

Atrye

Limp Gawd
Joined
Sep 11, 2004
Messages
222
I've been reading up on some network stuff, and I came across a definition that has made me curious. The text in my book says:

"...baseband signaling means that there is just one signal on the cable at any time, as
opposed to a broadband system (such as TV coax), which can have multiple signals
on the cable."

I understand what it's saying. My question is how does the broadband system allow more than one signal at a time? Any smart people have the answer?
 
Broadband phone, aka DSL, used different frequencies of sound. The frequencies above human hearing is split off for use with DSL, and those within human hearing is used for voice.
 
So. On the data level, can more than one computer talk on the line at once?
 
Good. lol

That makes sense.

So, the core of my question is: How do multiple signals get carried at once?

e.g. How does my cable modem know which signal is mine?
 
If you've ever operated a Ham radio or CB the answer is easy. Your modem is set to "listen" to a particular frequency or frequency range.

I got my dad to understand the concept by drawing to parallel lines and then I drew multiple differeing sine waves between the two lines.

But as for DSL, DSL is a digital signal that fills in the gaps between the peaks and valleys of the analog voice(sine wave) signal.
 
So... On my street, some number of people are connected to the main line via individual cable modems. Are each of these modems preconfigured to listen to a certain range, and then my ISP knows to transmit my data via that range automatically?

...that would explain something I've wondered for a while: Why can't people just tap into a cable line and steal free service?

Am I getting this or is it more dynamic than that?
 
This is called multiplexing and has been used in telecom for decades. This specific type of multiplexing is called frequency division multiplexing.

Cable modems are not preset for a specific frequency. The frequencies are decided upon during negotiation of the link. AFAIK. I could be wrong. If you want all the technical details of how that works, do a search for DOCSIS and you'll be quite overwhelmed quite quickly. :D I believe the cable modem system also uses some sort of time division multiplexing to allow multiple cable modems on the same frequency on the same line.

The reason people can't just tap a line and get free internet service is because internet communication is necessarily bidirectional. IE, for that connection to be of any use to you, you would have to send requests to the ISP. The ISP would see those requests(obviously) and could block you for unauthorized access. That's in contrast to stealing cable tv, which they have no practical method of detecting(assuming you weren't careless about it) because you aren't sending anything back up the line.
 
I'd suggest stopping by at howstuffworks.com and go to the Computer section. Do a search on Broadband and a search on Cable Internet. Those may answer your questions more thouroughly.
 
Technically, broadband means that the system has a "large" amount of bandwidth (frequency bandwidth, not data rate). The opposite of baseband communications is bandpass communications. The opposite of broadband communcations is narrowband communications. Marketers liked the term "broadband" and started advertising high data rates as broadband. It just turned out that you needed more bandwidth to push a faster data rate anyways.

Roughly, most signals start off as baseband signals (voltages that correspond to 1's and 0's), are then modulated and upconverted (mixed with a carrier) to a bandpass signal, transmitted through a medium, received by the far end, downconverted to a baseband signal again, and then sampled and demodulated to give you back your 1's and 0's. Wherever your signal is upconverted to needs to be different than the other users of the medium so that downconversion and demodulation will work without any garbled bits.

That's the very simplistic explanation of how multiple users can share any communications medium (cables, air, etc). It's also only the first part of how it all works. Do a search for the OSI model to get an idea of all the different layers of communications that has to take place to get data from one place to another.

Also, the answer of why you can't just steal cable internet is in a layer higher up than the physical layer (signaling). Most cable services register the MAC address of your cable modem just as in the way you can set your wireless access point to only communicate with certain MAC addresses (I won't get into the security of that here). If the MAC checks out, the DHCP server will give you an IP address and away you go.
 
You must log in or register to reply here.
Back
Top