ADSL EMC with LF operation?
Andre Kesteloot
andre.kesteloot@ieee.org
Wed, 04 Dec 2002 09:16:32 -0500
-------- Original Message --------
Subject: LF: Re: RE: Re: ADSL EMC with LF operation?
Date: Wed, 4 Dec 2002 13:45:02 +0100
From: "Stewart Nelson" <sn@scgroup.com>
Reply-To: rsgb_lf_group@blacksheep.org
To: <rsgb_lf_group@blacksheep.org>
References: <LOBBIDNOENMNHIBGDCKBAEICDEAA.g3wkl@btinternet.com>
Hi John and all,
A few comments on measuring DSL interference effects:
Here in Paris, I have the cheapest ADSL service from France Télécom.
Advertised speeds are 512 kbps downstream and 128 kbps upstream.
I am using a Cisco 827-4V, which is a combination modem, router, and
VOIP unit. A portion of the result of a "show dsl interface atm0"
command is shown below. There are verbose details about signal and
noise in each bin which are not posted. Your modem should have
similar information available, but the commands to show it will of
course be different.
The 608 kbps (DS) and 160 (US) values allow for protocol overhead.
You can see that this line could accommodate much higher speeds.
On my unit, the downstream Noise Margin updates every few seconds,
so one could easily test if transmitting LF causes it to degrade
(I don't have a transmitter here). If your real-time S/N is not
displayed, then just reboot the modem with the transmitter on and
see if the value (derived during training) changes.
The DMT bins are 4 kHz each, so you can see that 136 kHz lands
well between the US and DS bands; any trouble would be caused
by harmonics. IMO, interference to the upstream is very unlikely.
Moderate downstream interference will be corrected on the fly by
the Reed-Solomon code; check the error statistics. Uncorrectable
errors are retried at the DSL level, and when that fails, at the
TCP level. Before TCP gives up, the modem will renegotiate and
retrain with the DSLAM, resuming operation at a lower speed.
As a result, I doubt that you will see any TCP failures. But
I am quite curious if LF transmission causes any Reed-Solomon
errors, or if the displayed downstream Noise Margin is reduced.
73,
Stewart KK7KA
---------------------------------------------------------------------
ATU-R (DS) ATU-C (US)
Modem Status: Showtime (DMTDSL_SHOWTIME)
DSL Mode: ITU G.992.1 (G.DMT)
ITU STD NUM: 0x01 0x1
Vendor ID: 'ALCB' 'ALCB'
Vendor Specific: 0x0000 0x0000
Vendor Country: 0x00 0x0F
Capacity Used: 9% 23%
Noise Margin: 33.5 dB 31.0 dB
Output Power: 12.5 dBm 12.0 dBm
Attenuation: 32.0 dB 19.0 dB
Defect Status: None None
Last Fail Code: None
Selftest Result: 0x34
Subfunction: 0x02
Interrupts: 3197 (1 spurious)
Activations: 1
SW Version: 3.666
FW Version: 0x1A04
Interleave Fast Interleave Fast
Speed (kbps): 608 0 160 0
Reed-Solomon EC: 0 0 0 0
CRC Errors: 0 0 0 0
Header Errors: 0 0 0 0
Bit Errors: 0 0
BER Valid sec: 0 0
BER Invalid sec: 0 0
DMT Bits Per Bin
00: 0 0 0 0 0 0 0 0 0 2 2 3 4 4 4 4
10: 4 4 4 4 4 4 4 4 4 3 3 2 2 2 0 0
20: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 2
30: 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3
40: 3 3 4 3 3 3 0 3 3 3 3 3 3 3 3 3
50: 3 3 3 3 3 3 3 3 3 3 2 3 3 2 2 2
60: 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
70: 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
80: 2 2 2 2 2 2 2 2 2 2 2 2 2 0 0 0
90: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
A0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
B0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
C0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
D0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
E0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
F0: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
---------------------------------------------------------------------
> Thanks for the comments guys - as I would expect all seems OK with the
> "Engineer Installed". Does anyone have any experience of the "Wires
only",
> which to me seems more likely to be a problem?
>
> No-one has commented on any degredation of data on the ADSL
connection due
> to LF transmitting equipment in the shack - maybe the TCPIP protocol
to an
> extent hides any effect.
>
> Paul's comment below didn't seem to come through?
>
> 73 John, G3WKL