Received an interesting question from end-user.... for us IT guys...

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kwaggawerner

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So, most of us, as we move up in the ranks, from IT Technician, to something 'more experienced', be it Administrator, Consultant, Expert, Associate, Professional, the list goes on... for some of us, we never forget the things we learned from being a technician... however... a friend ask mine wanted ADSL, and I offered to 'install' aka plug the pots filter and rj11 in the router - as telkom puts it... and he asked me something I never thought about.... "How does a pots filter work?" - We all know that DSL is digital, but we as humans, are analog, (as are faxes)... so... here is my question to hopefully someone in the Electronic/s Engineering industry... How does a POTS filter, on-the-fly convert analog to digital, and how does the DSLAM or exchanges further up the service provider, know how to split it all... (tagging of data?)
 
ADSL uses a high frequency signal
Normal voice and fax uses a low frequency signal. A POTS filter works by cutting out the low frequency signal (like a crossover in a amp), preventing the ADSL equipment from getting confused by the mixed signal.
 
Isnt the human brain digital? I thought it uses binary electrical impulses to process and store?
 
Sapphiron said:
ADSL uses a high frequency signal
Normal voice and fax uses a low frequency signal. A POTS filter works by cutting out the low frequency signal (like a crossover in a amp), preventing the ADSL equipment from getting confused by the mixed signal.
Click to expand...

The Filter actually just filters out high frequency ADSL signals to protect your Telephone handset from damage since it was not designed for the high frequencies in mind. ADSL though took the low frequency analogue signal into account at design time and thus you do not need a filter for your router.

Unless there is some filter on the market that can filter out the analogue signal that I don't know about?
Maybe a ADSL SPLITTER works like that but I don't think they are (readily) available in SA.

At the Exchange they just split the 2 frequencies to the 2 systems. Although I think they may have hybrid systems now days that can do the split internally on the dslam maybe?
 
rurapente said:
Isnt the human brain digital? I thought it uses binary electrical impulses to process and store?
Click to expand...

You ears are very much analogue, and can't physically hear the ADSL signal (outside our hearing range). The human ear can hear signals up to about 20kHz, whereas ADSL runs on frequencies higher than 25kHz. So sorry, but I don't think you can browse MyBB directly via the ADSL line. ;-)

Most telephone equipment won't be influenced by the ADSL signals and most ADSL equipment won't be influenced by the telephone signal. The POTS filters I've opened before was simply LPsF (low pass filters), meaning that it only filters out the ADSL signal and not the POTS signal. Personally I don't use them - none of our telephones has any noise on so I can't see the reason for adding another lossy element into the circuit.

*Edit - I think the splitters simply passes the ADSL signal straight through while LPFing the POTS signal. I haven't seen any HPFs in the POTS filter environment yet. Easy enough to build though - I think the POTS filters are a 2nd or 3rd order filter. :-)
 
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agreed, i meant our brain, not our hearing. I thought thats what they were referring to. :)
 
its filters the frequencies between the Analog signal 0-4000hz and the DSL signal 4000hz+. Because both are running on the line if you dont have a filter on your DSL line would sync and work fine untill the a analog connection is made by you picking up the phone that will drop your DSL connection. <--- This is a basic understanding but there are loads of info in alot of documentation

And it filters the higher frequencies not the analog frequencies. That is why you place the filter normaly only on your "phone's" line.

edit** frequency ranges taken from official Cisco Documentation (ICND1 Official Exam Certification guide ed2 by Wendell Odom page 520 chapter 16 WAN Concepts)
 
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eclipsedx said:
LMFAO sorry man but could i perhaps point you to some books about DSL technologies?
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May I maybe point you to the telkom self install documentation and the documentation from billion and netgear routers about the correct placement of the pots filters?

The info was given in relation to the filters available from Telkom shops and those shipped with the routers. Not some third party specialist filters that can filter out the analogue signal.
 
eclipsedx said:
its filters the frequencies between the Analog signal 0-4000hz and the DSL signal 4000hz+. Because both are running on the line if you dont have a filter on your DSL line would sync and work fine untill the a analog connection is made by you picking up the phone that will drop your DSL connection. <--- This is a basic understanding but there are loads of info in alot of documentation

And it filters the higher frequencies not the analog frequencies. That is why you place the filter normaly only on your "phone's" line.

edit** frequency ranges taken from official Cisco Documentation (ICND1 Official Exam Certification guide ed2 by Wendell Odom page 520 chapter 16 WAN Concepts)
Click to expand...

Cisco doesn't write specifications. For the telco world that is done mostly by i.e. ETSI and ITU (at least, we in SA base most on those two). You're right in POTS utilizing only up to 4kHz (please use the capital "H" for Hz if you want to discuss something technical). ADSL doesn't, however, use 4kHz+: the band between 4kHz and 25kHz is a guard band and is not used in a system where POTS are running on the same copper.

"And it filters the higher frequencies not the analog frequencies."
False: everything is filtered. The frequencies are block and the low frequencies are passed. This is what a LPF does. Same as what Johan said, and what you obviously meant.

@rurapente: maybe we could try pluggin the ADSL line into the brain somewhere? Maybe the brain will learn how to decode the data, given enough time. :-)
 
It's a simple low pass filter that attenuates frequencies higher than the cutoff frequency.
 
kwaggawerner said:
We all know that DSL is digital, but we as humans, are analog, (as are faxes)... so... here is my question to hopefully someone in the Electronic/s Engineering industry... How does a POTS filter, on-the-fly convert analog to digital, and how does the DSLAM or exchanges further up the service provider, know how to split it all... (tagging of data?)
Click to expand...

It has nothing to do with ADSL being digital and the pots filter does no conversion from analogue to digital.

The pots filter is merely a low-pass filter that lets lower frequencies through and attenuates the higher frequencies. http://en.wikipedia.org/wiki/Low-pass_filter
http://www.epanorama.net/documents/telecom/adsl_filter.html
http://www.accesscomms.com.au/Reference/ADSL_filter.htm
http://www.gaoresearch.com/resources/whitepapers/other/g992.php
http://www2.electronicproducts.com/An_XDSL_primer-article-JANPUL1-JAN2001.aspx
http://services.exeter.ac.uk/cmit/modules/the_internet/webct/ch-introduction.html#id662517
http://www.paradigm.com.tw/Support_FAQ-ADSL.html#08 (Uses incorrect terminology, splitter instead of filter)
 
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