View Full Version : What is bandwidth/where does it come from?

02-03-05, 04:45 PM
Maybe this is a stupid/n00b question, but it's something I've never really understood...what exactly IS bandwidth? Where does it come from? How come it is limited and how come DSL is slower than Cable and both are slower than DS3?

02-03-05, 04:57 PM
Bandwidth is how much data can be sent over a givin line.
Or how much they give you.

You can get DSL up to 3 megs now.
That is the same as most cable.

A DS3 can pump out more data.
Just like a T1 has 1.5meg up and down.
I think a DS3 can pump 45 megs. :eek: :angel:

02-03-05, 05:20 PM
Think of it this way - you have a collection of 0s and 1s. You need to transmit them by signifying them as on/off signals. The rate at which you can process those will determine your data rate.

This data rate is often termed bandwidth by PC people, almost mistakenly, because bandwidth actually means the frequencies available to transmit data. For example, a standard phone line has a bandwidth of 300Hz to 3KHz. So things get more confused when referring to broadband internet, as cable modems operate on a single channel in a broadband system (typically 50Mhz to 1GHz for coax/fiber systems).

So why would your data transfer rate be limited? The 0 and 1 on a DSL line is represented by a voltage and variation in frequency (lower frequencies upstream and higher downstream). How accurately the Layer 1 technology of DSL can successfully convey the 0s and 1s determines your data rate. This is why DSL has distance limitations. You hit the noise floor and everything goes to hell.

Cable, specifically digital cable, is a discrete signal on a channel (typically 45-55MHz upstream and 400-750MHz downstream depending on what "channel" your modem gets). It is digital signal with a much higher frequency allowinf for more ons and offs to be delivered. This give Cable the bandwidth edge as the technology improves to better utilize broadband.

A DS3 is a telco standard, 45Mbps which is encapsulated in an STS signal of 55Mbps. The SONET technology is a very set in stone, connection oriented bandwidth always available technology, ie if you pick up the phone, you are supposed to have dial tone How a DS3 became 45Mbps - 1 DS3 = 28 T1s, 1 T1 = 24 POTS lines. 672 lines at 64Kbps each = ~45Mps. Now there is overhead every step of the way making things very robust with set timeslot encapsulation of the data. SONET is not flexible.

So, if you want really low bandwidth, use Morse code on a telegram...

A little more on ethernet - Ethernet is a physical layer standard with many flavors; 10base2, 10baseT, 100baseT, 100baseFX, 1000baseT and so on. How it is used is determined by Layer 2 and layer 3 switching and routing methods stacked on top. Bridging/switching is a reflex protocol while routing is a learned intelligent protocol. You NIC has a permanent MAC address, layer 2 hardware, and your PC is given a flexible IP address, layer 3 software. How well pieces of equipment can shuffle packets of data will also effect your data rate. That is why a full 100Mpbs connect on FastE card is impossible. Layer sees the addition of TCP which works with IP to manage connections between points on the network. Further up the chain are the layers that get more involved, things like telnet and FTP. (If that cooks you, Google OSI model.)

Questions? :afro:

02-03-05, 05:38 PM

Ah...good memores :cool:

02-03-05, 06:17 PM
So higher voltages and frequencies make more bandwidth/faster speeds? That's what I don't understand, I guess. What exactly is it that Time Warner is doing when they upgrade our speed from 3mbps to 5mbps. Where is all that extra bandwidth/speed coming from? Just higher voltages and frequencies that the data is being trasmitted over?

02-03-05, 06:42 PM
The bandwidth upgrade you received can come from a few different things. Going to higher frequency channel (higher frequency means more times per second to convey a zero or one). More likely, it was an efficiency upgrade, making better use of available data rate in a channel.

BTW, too much voltage is not good either. In a discrete signal, you have to be able to decipher what is a one and what is a zero. If your levels are too high or too low, the relative differnce vs. the noise level makes deciphering the signal difficult. You also run into problems of reflection and channel bleed. And the frequency may be frequencies, if you know what I mean. A 8MHz chunk of bandwidth in the broadband network maybe dedicated to serving your area. That much is about 50Mbps...

02-03-05, 06:55 PM
Okay, so if I'm understanding this correctly...bandwidth is nothing more than a line on a certain frequency that allows the data to be send/received at a certain speed dictated by that same frequency and voltage?

02-03-05, 07:23 PM
Pretty much.

Look at it this way. Think water pipes. You can have a very large pipe that moves water slowly, or a very small pipe that move water quickly. Both can have the same flow rate. There is your bandwidth. In the case of DSL, you have a large variance in frequency, 240Khz to 1.5MHz. That's a big relative slice there, but because of the lower frequency, you are limited. Cable will run on a channel that is 6MHz wide (it's spacing). Narrower relative channel, but much higher frequecies.

That make sense?

Here is a good quote on how cable modems turn the CableTV signal into pure data. The most common demodulators have four functions. A quadrature amplitude modulation (QAM) demodulator takes a radio-frequency signal that has had information encoded in it by varying both the amplitude and phase of the wave, and turns it into a simple signal that can be processed by the analog-to-digital (A/D) converter. The A/D converter takes the signal, which varies in voltage, and turns it into a series of digital 1s and 0s. An error correction module then checks the received information against a known standard, so that problems in transmission can be found and fixed. In most cases, the network frames, or groups of data, are in MPEG format, so an MPEG synchronizer is used to make sure the data groups stay in line and in order. Digital cable works in a similar way, only they decode into video formats instead of ehternet. Analog cables are the "old-school" versions of radio frequnecies (Off The Air) funneled through the coaxial cable.

Yes, I have abackground in CATV and now work in Telecom and Data...

02-03-05, 09:16 PM
BrianG.... Electrical Engineer?? I am (well... 3rd year in college getting my degree)... and I acctually get everything your saying. :D Sweet.

02-03-05, 10:04 PM
Not quite. I was EE and then went to Math/Physics. Took a job between my junior and senior years, though I have about 120 hours.

Started out a cable brat, my dad followed work in CATV. I was drafting on mylar at age 12. Heh.

02-04-05, 03:40 AM
Okay, I think I got it now...thanks for clearing it up. :)