dold@mitisft.Convergent.COM (Clarence Dold) (07/13/90)
After conversing with "hegarty@Berkeley.EDU" about the modem problem he posted
last week, we decided I should post some news to news:
For best operation, your modem Carrier should be at -13 dBm when it reaches
the Central Office. In the old days, you ordered an RJ45 data block on a
"Data Line". In addition to ensuring that the line would be of good enough
quality to support datacom, there was also a resistor in the RJ45 box that
modems could read. The value of this resistor was determined after the Telco
installer measured the line loss between you and the C.O. The modem would then
produce carrier sufficient to reach the C.O. at -13 dBm.
Most modems now send carrier at -9 dBm, which usually works. If you are having
trouble on your modem line, you should check for noise, and loss to the C.O.
Loss can be measured from the C.O. to you with a Digital Voltmeter. Loss from
you to the C.O. is assumed to be the same.
Using a High Impedance (almost all DVM qualify), measure your carrier. You
can send carrier on most "Hayes" modems with ATDT2;<RETURN> ATAA.
To check for noise, dial into the C.O., but don't complete the dialing, the
line should be noticeably quiet, no pops, clicks, or buzzing (ATDT9,2)
Check the C.O. loss by dialing your prefix-0020, or -0049, in my case, I dial
9, 435-0020. This connects to a 1 milliwatt source at the C.O. Measure this
carrier. Loss on a data line is guaranteed to be no more than 19 dBm. On a
voice line it is not guaranteed, but it will be fixed if it is greater than 21.
The combination of these two should be -13 dBm. Your modem might be internally
adjustable.
You can check the carrier of the remote modem by measuring the answer tone.
On a long distance call, it should be -29 dBm or stronger. Most modems will
work fine to -40 dBm, some even have jumpers to go to -45.
You might be able to interrupt an existing connection with "+++", then ATC0
to quiet your modem. Then you can measure the carrier on this call, if some
are better, or worse, than most.
The following chart gives DBM equivalences, as measured on a 600 ohm line
using a Fluke DVM. They agree with the measurements made with a Phone Line
Analyzer, and with theoretical calculation of milliwatts.
The theoretical calculation is (mVolts RMS) = square root of ( 600 x mWatts )
dBm is calculated as = log (mV / 775) * 20.
10
USE THIS CHART TO MEASURE
LINE LEVELS WITH A DVM
dBm RMS mVolts mWatts
+ 3 1095 2000
0 775 1000
- 3 547 500
- 6 387 250
- 9 273 125
- 12 193 62.5
- 15 137 30
- 20 77.5
- 25 43.6
- 30 24.5
- 35 13.8
--
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Clarence A Dold - dold@tsmiti.Convergent.COM (408) 435-5293
...pyramid!ctnews!tsmiti!dold FAX (408) 435-3105
P.O.Box 6685, San Jose, CA 95150-6685 MS#10-007rpw3@rigden.wpd.sgi.com (Rob Warnock) (07/17/90)
In article <1532@mitisft.Convergent.COM> dold@mitisft.Convergent.COM (Clarence Dold) writes: +--------------- | The following chart gives DBM equivalences, as measured on a 600 ohm line... | dBm RMS mVolts mWatts | 0 775 1000 +--------------- 1.000 [missing decimal point in original] But notice that most phone lines are closer to *900* ohms, not 600. The 600 ohm reference is for use inside the CO plant, and most lines have (or at least, used to have) 900:600 transformers on them. Most "DAA" (Part 68) modules also convert from the 900 ohm line impedance to 600 ohms internally to the modem, so if you measure the A.C. voltage on the inside of that module you'll get the numbers indicated. If you just stick a meter across the line, you'll see more like: dBm RMS mVolts mWatts 0 949 1.000 etc. -Rob ----- Rob Warnock, MS-9U/510 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311