I work doing tech support, and often walk people through setting up their DSL. Here we have PacBell ADSL, which has what is called a bridge as a gateway between the customer and the rest of the world. I’ve often seen and heard the DSL bridges like the Westell Wirespeed referred to as “modems”. I always assumed that this was just for convenience and simplicity for the users’ sake, however I’ve heard a couple arguements that the bridge is technically a modem, that it modulates the different signals between different protocols. Is the act of encapsulating packets just modulation when you get technical? I thought that you could only modulate analog signals, that digital was just on or off.
“Modem” means “modulator demodulator.”
An example of modulation is turning data into sound or a varying electrical current, as an analog modem does. But DSL “modems” do not turn the data into an analog signal, it’s a digital signal, transmitted at a very high frequency on the existing copper phone line.
This isn’t modulation. So a DSL gateway is not a modem. (Neither is a cable gateway, while we’re at it.)
This page has a definition of modulation that seems to match up pretty well with what’s involved in DSL.
An example of modulation is turning data into sound or a varying electrical current, as an analog modem does. But DSL “modems” do not turn the data into an analog signal, it’s a digital signal, transmitted at a very high frequency on the existing copper phone line.
I would certainly call what is transmitted between your DSL modem and the phone company as an analog signal. In order to achieve fast speed you do more than send the bits at a faster rate you send more than 1 bit at a time. So in stead of sending 1 1 1 0 0 1 1 0 1 1 1 1 you might send 11 10 01 10 11 11 or perhaps 111 001 101 111 or even 1110 0110 1111. One way of looking at it is for the sending one bit at a time you have 2 symbols say 0V=0 and 3V=1. Two bits at a time 0V=00 1V=01 2V=10 3V=11. Three bits at a time 0V=000 .42V=001 .85V=010 etc. You can keep doing this until the noise on the copper wire is to great to resolve the differences between symbols. What they do in practice is more complex than just sending different voltage levels. This is the same sort of encoding that your analog modem does. DSL does send stuff at higher frequency than an analog phone modem does but it is still sending an analog signal.
You have a DSL modem at your house the phone company has a DSL modem for you at their building. They probably have a bunch of DSL modems in this building for all the people with DSL in your neighborhood. All these modem will talk to a gateway at some point probably in the building maybe not. The modems and gateway might all be built into one box and probably are to make the whole system cheaper. If that is the case the gateway and modem are sort of the same.
Is the act of encapsulating packets just modulation when you get technical?
Not really modulation is basically encoding the bits in a manor that allows them to be sent and recovered over a channel. In DSL’s case the channel is the copper wire. Packets are done at a higher level. But that is the take of someone who worries about sending bits around.
I’d agree that modulation does not imply going from digital to analog and vice versa.
In addition, the OP’s title hints at another question: if it’s a modem, does this mean it’s not a bridge? Of course, the answer is that it can be both. My DSL modem can be either a modem+bridge, forwarding all traffic it sees on either side, or it can be a modem+router, forwarding only traffic it knows needs to go to the other side, depending on how it’s configured. (Bridge mode sucks, by the way, since it means any LAN traffic between your local machines can be seen by people on the other end of the DSL connection, which could be your neighbors if they are in bridge mode too).
Some DSL modems are literally just modems which attach to the PC using USB, and then require the PC to perform any routing or bridging functions. This setup sucks, in my opinion. Further, some DSL modems connect to the PC via ethernet, but still aren’t routers or bridges – the PC runs PPPOE software and talks to the modem using ethernet directly (as opposed to some higher layer protocol such as IP). This also sucks, because most PPPOE software I’ve seen is really lame.
Check out http://www.whatis.com for DSL and related terms. LINK