PCMech Forums - View Single Post - ADSL - Seeking Low Level Physical Layer Requirements

Web Gecko · 03-05-2004, 05:49 PM

Hey GLC

Think you have helped me out with other Qs before
( much appreciated ).

I think I have more understanding of the rim / fiber
sub exchange and how the AD process just doesn't work with
ADSL now. Since my initial post I have been on an extended web search. From what I can tell ADSL is a fully DIGITAL signal
before it gets to any sub or main exchange.

So basically you can't feed a digital signal through an A to D
converter and expect it to work ( to mux digital signals
onto fiber then demux DA etc etc.. The answer here was
a lot simpler than I suspected. Also with our PSTN ( Public Switched Telephone Network ) which is same as POTS ( we use a different dialling approach and most of our local loop is still analog) some of our phone lines are connected first to a rim /fiber sub exchange to the main exchange ( via the rim / fiber ) via an ANALOG switch to the DSLAM and onto the ISP ( fully digital from DSLAM to ISP ).

SO with the current system data coming from the main exchange switch from\to my line has to pass via fiber and
muxing. The ad and da approach is necessary to get standard analog voice grade signals over the fiber.

The way I think it works is taking a bunch of voice grade lines
( old pair gains cable sharing [multi pair per house line])
on copper going A to D and muxing at some point onto
fiber(s) then demuxing and D to A to the ANALOG switch in the exchange ( and vice versa back my way ) which is your basic current voice grade service. For ADSL they put a DSLAM
into the main exchange and it talks to the ANALOG switch
and aggregates ( muxes ) all the ADSL lines fully digital back to the ISP.

So the Minimux still has to deal with the ANALOG switch first before the signal gets to the DSLAM. I suspect that Minimux is a special multiplexer which muxes many inbound DSL digital lines BUT doesn't need to use any A to D conversion because the signal is already digital so this can be sent over fiber to the main exchange BUT the last D to A stage must remain in place
in order to get by the ANALOG switch in the main exchange
then to the DSLAM.

The real process will be more complicated than my simplistic explanation. BUT basically I'm pretty sure now the problem with pair gains phone lines running into old rim / fiber
sub exchanges IS the A to D conversion which for an already
DIGITAL ADSL SIGNAL running this via A to D converter CANNOT work !!!!

This extra A to D conversion also downgrades V.90 56K access
to around V.34 for dialup access and I think also stops ISDN from working as well.

The Minimux is a specialised multiplexing solution
( digital multiplexer ) to get round the rim / fiber to ANALOG
main exchange switch problem. I wish it could be said with
less words ( I don't have the necessary GIF !!! sorry if that sounds like a bad joke as it wasn't intended that way ;O)
A picture can paint a thousand words as they say ).

I have heard that your Telcos/data comms providers have been extending fiber out into the residential areas in the US but you have probably had this advanced infrastructure in place for far longer. We are just starting to roll out our cable and basic DSL infrstructure which started in the US a long time ago.

With fiber in your burbs you don't face the DSL exchange distance limitation ( around 18000 ft or around 4km on copper)
and you have access to faster forms of DSL like VDSL
( which gives bandwidths of over 50Mbps ). What bandwidths
are you getting there ? The fiber and the special street box I think houses what they call a DSL ( possibly VDSL ) gateway
and has a special DSLAM which you mentioned and this muxes
and demuxes yours DSL data channels both ways across fiber.

With fiber you should be getting quite fast bandwidth.
ADSL has two main transmission modes, CAP and DMT
but DSL has many different types depending on application,
quality of service required etc etc..

I found good info over at :

http://www.howstuffworks.com

Do a search on DSL and VDSL. Which explains ADSL well
and covers many different DSL modes etc..
Great site in general if you need the fundamentals on many
things.

So I think I have my main answers to this one.

Thanks again for your help.

Any other ideas/info you have are always welcome.

cheers

The Web Gecko

03-05-2004, 05:49 PM	#3
Web Gecko Member (8 bit) Join Date: Nov 2001 Location: Perth Western Australia Posts: 244	Hopefully I know a little more now Hey GLC Think you have helped me out with other Qs before ( much appreciated ). I think I have more understanding of the rim / fiber sub exchange and how the AD process just doesn't work with ADSL now. Since my initial post I have been on an extended web search. From what I can tell ADSL is a fully DIGITAL signal before it gets to any sub or main exchange. So basically you can't feed a digital signal through an A to D converter and expect it to work ( to mux digital signals onto fiber then demux DA etc etc.. The answer here was a lot simpler than I suspected. Also with our PSTN ( Public Switched Telephone Network ) which is same as POTS ( we use a different dialling approach and most of our local loop is still analog) some of our phone lines are connected first to a rim /fiber sub exchange to the main exchange ( via the rim / fiber ) via an ANALOG switch to the DSLAM and onto the ISP ( fully digital from DSLAM to ISP ). SO with the current system data coming from the main exchange switch from\to my line has to pass via fiber and muxing. The ad and da approach is necessary to get standard analog voice grade signals over the fiber. The way I think it works is taking a bunch of voice grade lines ( old pair gains cable sharing [multi pair per house line]) on copper going A to D and muxing at some point onto fiber(s) then demuxing and D to A to the ANALOG switch in the exchange ( and vice versa back my way ) which is your basic current voice grade service. For ADSL they put a DSLAM into the main exchange and it talks to the ANALOG switch and aggregates ( muxes ) all the ADSL lines fully digital back to the ISP. So the Minimux still has to deal with the ANALOG switch first before the signal gets to the DSLAM. I suspect that Minimux is a special multiplexer which muxes many inbound DSL digital lines BUT doesn't need to use any A to D conversion because the signal is already digital so this can be sent over fiber to the main exchange BUT the last D to A stage must remain in place in order to get by the ANALOG switch in the main exchange then to the DSLAM. The real process will be more complicated than my simplistic explanation. BUT basically I'm pretty sure now the problem with pair gains phone lines running into old rim / fiber sub exchanges IS the A to D conversion which for an already DIGITAL ADSL SIGNAL running this via A to D converter CANNOT work !!!! This extra A to D conversion also downgrades V.90 56K access to around V.34 for dialup access and I think also stops ISDN from working as well. The Minimux is a specialised multiplexing solution ( digital multiplexer ) to get round the rim / fiber to ANALOG main exchange switch problem. I wish it could be said with less words ( I don't have the necessary GIF !!! sorry if that sounds like a bad joke as it wasn't intended that way ;O) A picture can paint a thousand words as they say ). I have heard that your Telcos/data comms providers have been extending fiber out into the residential areas in the US but you have probably had this advanced infrastructure in place for far longer. We are just starting to roll out our cable and basic DSL infrstructure which started in the US a long time ago. With fiber in your burbs you don't face the DSL exchange distance limitation ( around 18000 ft or around 4km on copper) and you have access to faster forms of DSL like VDSL ( which gives bandwidths of over 50Mbps ). What bandwidths are you getting there ? The fiber and the special street box I think houses what they call a DSL ( possibly VDSL ) gateway and has a special DSLAM which you mentioned and this muxes and demuxes yours DSL data channels both ways across fiber. With fiber you should be getting quite fast bandwidth. ADSL has two main transmission modes, CAP and DMT but DSL has many different types depending on application, quality of service required etc etc.. I found good info over at : http://www.howstuffworks.com Do a search on DSL and VDSL. Which explains ADSL well and covers many different DSL modes etc.. Great site in general if you need the fundamentals on many things. So I think I have my main answers to this one. Thanks again for your help. Any other ideas/info you have are always welcome. cheers The Web Gecko