ADSL - Seeking Low Level Physical Layer Requirements

[Web Gecko]

Howdy

I am currently in the unenviable situation of having virtually no access to broadband. Satellite is the only broadband access availabale here but as this is a rental, satellite installation is not really practical ( may also be moving house soon ). Cable is also not available from either of our Telcos and there is no access via cable TV Cos.

My current dialup ( more like sub dialup at a max of 28.8Kbps or
31.2Kbps with compression this can be OK but this is rare as
data rates are mostly 24Kbps and WAY lower ) is being limitted
because my phone line is connected to our main exchange
via a rim / fiber optic sub exchange ( old technology ).
At the sub exchange a whole bunch of analog voice grade lines
( old pair gains cable sharing lines with 6 copper pairs per )
are going AD and muxed across fiber(s) then demuxed and DA
to the main exchange.

This AD DA conversion over fiber effectively defeats
ADSL as well ( and the main exchange is ADSL enabled !!!
so close but yet so far !!! ). This is also the reason why I can't get the full 56K digital bandwidth which has been allocated to me by my ISP ( and for which I am paying and yet the best I get is mostly 24Kbps or less with compression ).
An extra A to D conversion between your 56K V.90 modem and your ISP means that your 56K access will be downgraded
to V.34 at least i.e. around 33.6 Kbps !!!!

ISDN will also NOT work over the old rim / fiber sub exchanges
possibly due to the same reason. I am in Australia and compared with the US and other places we are way behind
as regards rollout of broadband technologies. We have only 2
major Telcos ( Telstra and Optus ) and many other smaller network service providers who mostly lease data comms
( and cell phone ) infrastructure off the main Telcos.

Any ideas are welcome here but what I am looking for is input from people with a detailed knowledge/understanding of the low level physical layer requirements for ADSL over copper.
All I know right now is that the rim / fiber sub exchange connection defeats ADSL because of the AD and DA process over fiber. I have some ideas as to why that might be but need either verification or extra info as to exactly why
ADSL is defeated by these conversions.

Our main Telco Telstra has started trialling a new technology
in the states/cities/suburbs on our east coast called Minimux which when installed in rim / fiber sub exchanges will
allow ADSL to finally work. Unfortunately it could still take over a year for Telstra to get the Minimuxes installed for old exchanges in Western Australia.

I have been told that ADSL will not travel over fiber but I have heard that in the states VDSL is being used ( much higher bandwidths like 54Mbps and over ) where your local loop in many places is already digital ( still a rarity in Australia ) by extending fiber out to your residential areas and using a
special VDSL gateway which adjusts the VDSL signals for transmission over fiber.

The issue is clearly NOT that DSL can't be sent over fiber as special electronics can compensate where differences in amplitude, frequency, phase etc or other timing/signal changes are required to take an analog signal and then
convert this to digital and then modulate a LED or laser light source on and off ( now we have an digital signal ) for transmission over fiber which either then is transmitted digitally to the ISP or at some point may require another DA step etc..
Also multiple data channels ( voice and data for upstream and downstream ) with both formats of ADSL may need to be split and multiplexed over fiber connections.

The two main ADSL data transmission modes are :

- CAP or carrierless amplitude/phase system where the
bandwidth of the ADSL copper line is split into :
0-4 Khz Voice
25-160Khz for upstream data
240Khz to 1.5Mhz for downstream data

The guard bands are the unallocated frequency ranges
which keep the various voice and data bands separate
to retain signal integrity
i.e. as in guard bands used for normal voice grade usage.

DMT - divides the full ADSL copper line bandwidth into
247 separate channels each being 4Khz wide
with a guardband between each channel.
This transmission format is much more complicated
as some lower channels are used for bidirectional
up/down stream info and the system has to monitor
all channels and will only use those channels for
transmission which test as having the best line
conditions ( lowest error rates ).

Much more complex transmission mode than CAP
but allows greater efficiency and higher data rates
in some circumstances.

So the DSLAM at the main exchange has a lot of work
to aggregate many lines ( extra channels with DMT )
back to the ISP.

My Main Questions are :

1. It has been indicated to me that the ADSL carrier
or perhaps the transmission mode becomes a
problem when we get to the AD DA conversion over
fiber.

Q : It seems to me that an ADSL transceiver ( ATU )
or ADSL modem is connected to the phone line by
way of USB or Ethernet. These are digital connections
which indicates that the ADSL data portion of the
signal is already DIGITAL. Being DSL I guess I should
have sussed this before BUT my knowldege of
broadband technologies ( especially at the low
detail level ) is not that great.

BUT the other standard phone accessories on the
same line are still analog and connected via low
pass filters ( In Australia most of our local loop is
still ANALOG as opposed to the US situation )
which could indicate that we are using the ADSL CAP
transmission mode i.e. voice is carried over the low
part of the signal bandwidth.

The low pass filters allow the phone and other line
accessories to work filtering out the high frequency
ADSL digital data so Internet and phone/fax etc can
work at the same time.

It seems to me that the problem IS the AD DA
conversion over fiber and the problem is that
the ADSL signal is not all baseband ( analog ) or
all digital it is a MIXED signal using baseband for
voice and digital for the computer/Internet data !

The current rim /fiber exchanges are probably
working on an all ANALOG/BASEBAND basis
which means taking a bunch of ANALOG lines
and going AD and muxing etc over fiber.

I am just trying to get at the real low level
problem and it seems to be that part of the ADSL data
is already digital and the voice is still baseband or analog
so the current rim / fiber deal could never work.

2. Does anyone know how the new Minimux technology works at a low level ? It seems possible that if the AD DA
conversion over fiber is the problem as ADSL is mostly
digital already then Minimux must be a totally copper based
digital multiplexing solution which involves NO extra AD DA
conversions ??? Can anyone verify this ?

This could also suggest that the voice band in ADSL
( CAP or DMT ) is handled as a digital signal as well.

You can't run a digital signal through a standard
A to D converter and expect any meaningfull or usefull output !!! So the issue is down to the AD conversion
probably.

I have run these questions by support staff in ISPS and even they are unsure about exactly why rim / fiber defeats
ADSL. I think I have the answer BUT need confirmation
from others who have a greater understanding of DSL that I
( initially thought it may be down to carrier/multiplex or timing issues going from copper to fiber and back ).

Any ideas are welcome on this one. BUT I really need confirmation or not of my ideas from all you DSL afficionados
out there.

Thanks again

cheers

The Web Gecko

?8O)}

It's better to burn out than it is to rust !!!!! NY

[glc]

I don't know what a "minimux" is, but they are extending DSL here by installing "remote terminals" - which connect to your premises with standard copper and are full of DSLAM cards. They are connected back to the exchange with fiber. This may be the same thing for all I know. I'm on one of these.

[Web Gecko]

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

[glc]

I'm getting 768/128 on mine - faster is possible but that's all they are selling off the RT's here to conserve bandwidth (max out the DSLAM slots). Even with a RT, they have to take you off the pair gain, you must have straight copper from the premises to the RT. All the RT does is allow them to offer DSL past the distance limit. The RT is just a glorified Slick (SLC - subscriber line concentrator) with DSLAM cards instead of all the A/D converters.

[Deimos11]

The RT is also fiber fed, a SLC is copper fed using T1 lines. The RT is basically a small Central Office without the switch.