Originally posted by mental ambiguity:
- Related to the butt sets, when I've been looking at them online, I see some that aren't supposed to interfere with DSL. If I remember reading right, it has something to do with voltages. Could someone explain further? What happens if it were to connect a butt set to a line with DSL on it? Does it just drop the DSL connection? Is there potential for equipment damage? What about connecting it to a data circuit, such as a 2 wire T-1?
It affects the operation of a data circuit because by placing the set on to the line you're affecting the impedance of the line itself. These sets were designed for testing basic phone lines (known as a POTS line for 'plain old telephone service'). When you connect your set on to the line with it on-hook you're bridging on to the line. The DC resistance is high because a capacitor inside blocks DC from passing while the device is on-hook. AC will still pass through the capacitor and activate the ringing circuit of the telephone if it's at the right voltage (90V at 20 Hz)... The AC going through the capacitor is fine because phones operate on -48V DC for what's known as 'talk battery'. When somebody picks up the phone, contacts inside the phone (or butt-set) connect the circuitry inside to the line and you can talk on it, make a call, whatever...the voltage at that point will then drop to about -10V or whatever needs to be there to provide approximately 21 mA of current. Once the line is back on-hook, the voltage will go back to -48V since again the DC will be blocked by the capacitor.
That's a rather long explanation but as an engineer I imagined you'd appreciate it.
Anyway...that's all good and fine for voice, but with data there is no on-hook or off-hook...you have data streaming continuously and by nature it's AC and therefore will pass through that capacitor in the butt-set's ringing circuit just fine. If you put a butt-set on prior to the DSL filter, like Sam said, it will hurt the speeds and maybe knock it down a bit but DSL is a very 'hearty' service (for lack of a better word) and will overcome a lot of changes and survive. A T1, however, is a bit different and is a lot more sensitive to the impedance depending upon what type of delivery method is being used to bring it to the site. Usually there is 100 ohms of impedance on a T1 line and it is not designed to operate with things bridged on to the line...
