Comment Re:Hams (Score 2, Informative) 456
thc69 wrote Er...if BPL is up in said emergency, and ham fails, then why not just use the BPL connection?
BPL may be down in the disaster area but it may be up in another area, preventing that area from receiving the transmission.
Let's put this in computer terms:
You've got an 802.11g wireless access point in your house. It is using a specific set of frequencies to maintain your connection between the AP and your computer. Let's say that you are using channel 9 in the US, meaning that your AP is communicating on a small slice of spectrum surrounding 2.452GHz (2452MHz).
Now let's put a microwave oven in your house. Your microwave oven is set to generate energy at 2.450GHz (2450MHz). There is a magnetron inside the microwave designed to generate energy at that frequency. It most likely doesn't generate just 2450MHz waves but generates energy on frequencies that generate a narrow bell curve centered on 2450MHz.
Now let's say that your microwave oven is a little out of spec. The shielding is a little leaky and the magnetron is generating energy at a little higher frequency. Now your microwave is putting out energy at 2451MHz +/- 1MHz. That means that the generated energy stretches from 2450MHz to 2452MHz.
Your wireless access point actually uses 22MHz of spectrum, meaning that the signal stretches from 2441MHz to 2463MHz. But look! Your microwave is "transmitting" from 2450MHz to 2452MHz, right on top of the bandwidth used by your AP. Not only that, part of that energy is right on your center or carrier frequency, which is part of what makes this all work.
Now it's time to talk about noise. Imagine that you're trying to hold a conversation across a table in a crowded room. If you're in a quiet restaurant, you should be able to hear each other across the table easily. If you're in a diner with music playing, it because a little more difficult to hear because you've got the constant level of background music. If you're in a bar, it's a little more difficult to hear since the background noise is even louder. If you're in a dance club, it's going to be even more difficult. As the background noise becomes louder, you have to increase your speech volume to overcome the background noise.
The signal to noise ratio is the amount of received signal compared to the amount of noise in the environment. In the conversation scenario, it's how much louder your voice is than the other noise in the room. As the background noise increases, you have to increase your volume (power) to be heard over the noise.
The same thing happens with your wireless access point. There is a certain amount of radio noise in the environment. As long as the AP signal is a certain amount stronger than the noise, when measured at your computer's antenna, the computer can communicate with the AP. If the signal isn't stronger than the noise, the computer can't talk to the AP.
Now you decide to turn on the microwave. It starts generating additional energy (noise, as far as your computer and AP are concerned) right on the frequencies used to communicate. If the new noise level is higher than the signal level, you're offline. (I know of one installation where I can reproduce this exact scenario on command. Turn off the microwave and one computer can't talk until the microwave is turned off.)
The same thing happens with ham radio. If there is an emergency and BPL goes offline, I can receive fairly well because the noise level just dropped significantly. When I try to transmit (most likely at a reduced power level, since I'm running on emergency power), my signal has to be received by someone else. If that person is right next to a BPL source that is leaking lots of noise, he isn't going to be able to pick up my signal because the background noise masks the my signal.
thc69 wrote Damn the ham....
Let's look at potentially affected spectrum. Here are a couple of important links:
The potential BPL interference cuts a pretty broad swath, including the following services that may potentially be affected:
- Amateur radio
- Radiolocation services
- Fixed/mobile radio operations (taxis, police, shipping companies, etc.)
- Maritime mobile (telephone)
- Maritime mobile (ship-to-ship or ship-to-shore)
- Standard frequency and time reference signals (self-setting clocks)
- Space research
- Aeronautical mobile
- Broadcast spectrum (several chunks including TV channels 2-4)
- Radioastronomy
- ISM (Industrial, scientific, and medical bands)
That spectrum includes both government and non-government spectrum allocations. It's not just the hams who are going to be affected.
