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Phone lines affected by terrorist attack?


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#1 of 8 OFFLINE   MickeS

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Posted September 13 2001 - 12:33 PM

The last few days when I've tried to call friends and family in Sweden, I haven't been able to get through all the time. The line is just dead. Then a little later I can call, then it's dead again. Does anyone know if the transatlantic telephone connections somehow were affected by this disaster, or if it's just regular overload from too many people calling at once? /Mike
/Mike

#2 of 8 OFFLINE   Rain

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Posted September 13 2001 - 12:38 PM

I've had problems calling Nova Scotia and Californa from here in BC. I imagine that it is due to busy lines. Even those not calling in or out of the affected areas are probably more likely to want to reach out to distant friends and family at this time.

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#3 of 8 OFFLINE   BrianW

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Posted September 13 2001 - 12:45 PM

Mike, it’s probably just an overload on the local switch. Just keep trying, and you should eventually get through. To anybody trying ot make phone calls into or out of the NYC area: Try to keep your calls short, out of consideration for others in the same situation. The shorter your call is, the sooner the next person will be able to make the next call. As an alternative, if you need to make multiple calls to friends and family just to give brief updates, call just one friend in another area of the country, give him your contact names and numbers, and have him make the rest of your calls on your behalf. As long as your friend’s calls don’t go through the affected phone switch in NYC, there should be no problem.
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#4 of 8 OFFLINE   Trace Downing

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Posted September 14 2001 - 09:15 AM

I'm an AT&T employee...

Let me try and explain without getting too technical or proprietary here. Posted Image

In any emergency (earthquakes, floods, hurricanes, etc) we shut down most (60-85%) incoming calls to the disaster area. Normal traffic usually flows freely between cities, but when a distater strikes, the most important calls have to come from the distaster area itself. So to keep from getting the trunks (lines) from overloading, we allow the max available trunks for outbound calling, most calls into a disaster area is blocked. It's easier for the affected to call everybody he/she knows to tell them they're all right than for everybody they know to call into a central area (like NYC), just to get a busy signal.

International calls are also subject to this form of manipulation. A disproportionate amount of int. trunks have been reserved for int. traffic out of NYC/DC. So what you got was our artificial blockage. Just keep trying.

The network will probably be this way until the traffic out of NYC falls to normal levels again.

If you have MCI, Sprint, etc...doesn't make a difference, they do the same thing. The phone network is all the same, seamless web, just us different companies own certain portions of it.

We had a switch in the WTC. It's been destroyed (of course) but luckily, I heard that all 50 or so fellow AT&T employees got out unharmed. That one switch was capable of handling 100,000 calls every 15 minutes. Don't worry, we have plenty more where that came from.

I am not speaking in any official position of AT&T PR...I'm just a tech. Posted Image

#5 of 8 OFFLINE   MickeS

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Posted September 14 2001 - 09:22 AM

Trace, thanks for the information. Turns out that my phone calls actually went through (it rang on the other side), but when they picked up they didn't hear anything. I didn't even know they went through, because it sounded completely dead (no signals at all) on my side. Why would that be, do you know? /Mike
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#6 of 8 OFFLINE   BrianW

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Posted September 14 2001 - 09:37 AM

This may be due to being able to establish a signal path, but no voice path. Modern phone systems (“modern” meaning build within the past couple of decades or so) often use “out-of-band” signaling, which means that the signals – ring, connect, disconnect, etc. – are exchanged on a pathway separate from the voice pathway. However, this being a trans-Atlantic connection, this really surprises me, since I thought that all calls through this pathway still used in-band signaling, meaning that the signals share the same pathway as the voice. Trace, that’s good news about your colleagues. I’m glad they’re safe.
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#7 of 8 OFFLINE   Trace Downing

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Posted September 14 2001 - 05:25 PM

quote:
Modern phone systems (“modern” meaning build within the past couple of decades or so) often use “out-of-band” signaling, which means that the signals – ring, connect, disconnect, etc. – are exchanged on a pathway separate from the voice pathway.[/quote]

Brian, Micke...

This is true for analog phone transmissions. Brian is most likely right. However, Micke was calling international or long distance (from here on out will be LD).

The in-band/out of-band signaling is handled by the LEC (local exchange carrier...BellSouth, Qwest, Verizon, etc). All analog calls in all Bell companies are converted to digital in the CO (Central Office...the exchange that gives you your NXX, or 3 middle digits in your phone number). Local digital phone service (cable/ISDN) is actually done with 8 bit signals and no D/A converters needed.

An LD network is 100% digital. There's just too much signal loss in analog transmissions that it's too expensive to have any analog circuits in an LD network. All calls in a trunk (24 channels that have been time division multiplexed) are on one carrier signal. Since there are no sine waves to deal with, the transmission is on a single frequency. The call is sampled 24 times a second, and multiplexed with 23 other calls. Each time slot is 8 bits, with a 9th bit for separation from the next channel. This bit is called a parity bit.

While it's certainly possible your parity bit was lost in the LD switch, this would've just held you for a second or two in the switch until it fixed itself (unless there's a problem with the Digital Interface or Time slot Interchange). What probably happened (come to think of it) is what Brian mentioned.

Micke, the problem you experienced occurs quite often and is usually due to the recieving LEC's problem. Since you got a ring signal and an offhook dial tone, I wouldn't say it was Qwest...it was most likely the LEC in the country your calling to. Brian sounds like he knows more about analog phone systems than I do. I had no idea that trans-Atlantic was in-band.

I surmise that you got a signal loss anyway, but Qwest did detect the connection, which is why your rings came in clear (They're the ones that give you the dial tone/rings/busy signals etc). Nothing is a fer-sure thing in this biz. Posted Image

Whew! Thanks for taking my mind off of Peter Jennings and this tragedy.

Sorry if you know all this. I just ramble on sometimes! Posted Image

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[Edited last by Trace Downing on September 14, 2001 at 08:49 PM]

#8 of 8 OFFLINE   BrianW

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Posted September 14 2001 - 08:45 PM

Trace, thanks for sharing your expertise. I appreciate the explanation.
-Brian
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