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Digital Vs Analog Modem (Communication Problem)

Posted on 1998-11-16
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Last Modified: 2013-12-27
My Factory is located in a small valley surrounded by small mountains. The normal telephoneline could not function so we got Radio Towers Installed on both the sides (one on our factory site and on on the Telephone Exchange site). As per the information provided by the Telephone department, the signal received at the Factory side is digital. For the voice communication there is no problem. But we can not use email at all. Fax also, we can send only one page and we can receive about 70% of the incoming fax.

I would like this problem resolved. One person has suggested that the problem is because of the incompatibility resulted from Analog vs Digital signal. He suggested that if we use the digital modem, the problem would be solved.He says that the fax machines use in-built analog modems.

I would like to have the solution as early as possible. Any advice ?

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Question by:deveshb
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20 Comments
 

Author Comment

by:deveshb
ID: 1145479
Edited text of question
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Expert Comment

by:joopv
ID: 1145480
The data corruption can take place at saveral points.  You don't give much information so lots of things can be wrong.

The radio link :
Ask the provider if they did bit error rate tests over the radio link.  And what the results were...

The radio link modem interface point.
Your PABX is connected to the radio link at some point.  Is this connection ok - type of cabling, interface settings (clock source) etc.

Give us some more information please.  Like what kind of connection do you have (T1 /T2 / E1 (isdn-30), what kind of PABX do you have, how many outgoing lines do you have.

Can you fax internally within your factory ?  Does this work ok ?

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Expert Comment

by:Otta
ID: 1145481
If you take the FAX to your home,
and use the normal "land-lines" to send/receive,
does it work correctly?

What manufacturer and model of modems are you using for E-mail?
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Expert Comment

by:rosefire
ID: 1145482
ISDN is digital and an ISDN modem can be used to connect two analog devices (ordinary phones, faxes, modems, or whatever).  With the right equipment it seems highly unlikely that the digital nature of your phone lines is the problem.  I hook my modem to an ISDN box that has two analog (analog is what a normal telephone uses) ports that are connected to the digital ISDN line.

It sound as if the data rate of your digital signals are too slow for your connection rate or you have noise somewhere in the setup so you get transmission errors that are not a problem to audible sound.  The people who set up/sold you the radio towers should fix the problem.  Analog to digital lines is not a problem with the right equipment.

Keith
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Expert Comment

by:joopv
ID: 1145483
please give us some feedback.
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Author Comment

by:deveshb
ID: 1145484
1)
Thank you all for putting valuable comments. I am sorry as I am responding a little late as I was collecting Information. We don't have any PABX system.There are 6 lines which are directly connected to the TRX box, which is connected to the Tower.

The picture could be like this

 Telephone Exchange                Factory Site
      Site
   
      __                               __
     (  )                             (  )
     /  \ ____                        /  \_____
    /----\    |                      /----\    |
   /------\   |                     /------\   |
              |                                |
              |                                |
          ____|__                          ____|__
         |       |                        |       |
         |  TRX  |                        |  TRX  |
         |_______|                        |_______|
           ||||||                           ||||||
        ( 6 lines)                        ( 6 lines)
       To Tel xchange                      To Factory


The people who installed are not able to provide much information. Following details are printed on the TRX (Transmitter/Receiver) box :

          NEC-digital Radio C6/C12
          TRP400C6/C12-1A
          sr no 640 wt. 18 Kilos (with option)
          NEC Corportion
          Mar 1993 (C8594)
          Made In Japan

2) We tried to fax within the factory using two line (please note that there is no PABX, we need to dial the whole number, similar to any other outside number). The problem remains same i.e only 70 % of the fax could be sent. I think , as there is no PABX, the Faxing route include the Factory to Telephone exch via TRX and Tower and then coming back to the factory.


Please help.
           
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Expert Comment

by:rosefire
ID: 1145485
Without more specifics, I have no more ideas.  Sorry
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Expert Comment

by:joopv
ID: 1145486
> as there is no PABX, the Faxing route include the Factory to
>Telephone exch via TRX and Tower and then coming back to
>the factory.

I think you are quite right here.

There must be some kind of modem between your lines and the digital radio's.

So the signal goes through these points :

(analog) phone, fax or  pc modem.
|
some kind of analog/digital vv converter
|
digital radio
|
radio link (RF cable, antenna, radio attenuation because if distance to other point, hills, trees etc etc, antenna, RF cable)
|
digital radio
|
again some kind of digital/analog vv converter)
|
(analog) phone line to tel. exchange

Now you will have to find out where the data corruption takes place.  It could be each and every point in the above diagram.

Testing this is not easy and is dependent on the availability of complex equipment and knowledge of the used equipment.

What you could do :
- bit-error rate tests between the digital radio's.
- maybe the radio's can be put in a local- and remote loopback mode.  Do also bit error rate tests in these modes.
- frequency range  and signal-to-noise tests between the analog points.
- checking the right configuration of the equipment.  Is the clocking and cabling configured right between the digital radio's and the analog/digital converters
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Expert Comment

by:joopv
ID: 1145487
deveshb,

Any progress on this issue ?

Did the installer of the radio link also deliver the interface boxes to the phone lines ?
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Author Comment

by:deveshb
ID: 1145488
Hello joopv
The installer have not been able to give any feedback on biterror test. I will update you as soon as I get some feedback.

Yes they have provided interface boxes(as mentioned above as TRX boxes) to the phone lines.
Thanks.
0
 

Expert Comment

by:bruneel
ID: 1145489
You have a serious problem on your connection and it is located between your phone connection point and the phone compagny. The loop-back you made by phoning yourself via fax failed is the prove. On a audio connection you can have problem but you don't hear them. Fax and data communication is very strikly in what they want to see, missing bits are not allowed. There is a tolerance for corrections but the window is very small. When you will do a bit error rate test you'll find out a lot of errors.
What you need is a quality check for your line. This requires high-tech measurement equipment like those of Wandel&Goltermann. More info for measuring techniques and line qualifications can be found on there side: www.wg.com 
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Expert Comment

by:joopv
ID: 1145490
We should try to find out what bit rate these digital radio's support. For a connection that also should support fax and modem signals it should be at least 56kbit/sec, or better 64kbit/sec.  This is PER PHONE LINE, so the combined bitrate to support 6 lines should be at least 336 kbit/sec.

I already searched the web about these NEC digital radio's but could not find anything.

deveshb,

- can you find anything about the bit rate that the digital radio's are specified for ?

- Is the installed radio link actually specified to transport modem and fax signals ?  Could it be that it is only specified to transport voice signals ?

- The cabling and configuration of the connection between the digital radio's and the TRX boxes is very error-prone.  This is (very probably) not a normal serial connection but a synchronous clocked connection.  It is quite likely that something is installed wrong concerning the clocking source, or that the wrong cable wiring is used.

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Author Comment

by:deveshb
ID: 1145491
Thanks for the inputs. I will get further information with the technicians.
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Expert Comment

by:joopv
ID: 1145492
Daveshb,
Any news yet ?  Did you got more information ?

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Expert Comment

by:joopv
ID: 1145493
Daveshb,
Any news yet ?  Did you got more information ?
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Author Comment

by:deveshb
ID: 1145494
We could not perform the bit-error test as the company which installed the equipment is Government one and the support is not good. The vendor company's technicians are showing ignorance for the requirement of such tests but at the same time they are not able to give any answer also.
I am trying to contact some private companies which do such installations. Once I get some feedback, I will come back to you all.
Thanks anyway for your patience and help
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Accepted Solution

by:
joopv earned 100 total points
ID: 1145495
since this Q seems to have fallen asleep i will try to wake it up again.

In my opinion, the digital transmission channel is corrupt because of :

- The cabling and configuration of the connection between the digital radio's and the TRX boxes is very error-prone.  This is (very probably) not a normal serial connection but a synchronous clocked connection.  It is quite likely that something is installed wrong concerning the clocking source, or that the wrong cable wiring is used.

- The installed radio link is not specified to transport modem and fax signals.  Probably some compression scheme is used to bundle all 6 lines over 1 rf channel.

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Expert Comment

by:serialband
ID: 1145496
This might not have anything to do with anything.  but...

Maybe you could get a voltmeter and measure the voltage coming in at the line.  If the fax can connect and send a single page, somehow you must be getting an analog signal at your end.   Possibly the voltage level can't be maintained, or   Or as joopv suggested, the digital connection is corrupt.

From what I've been told by phone company techs, digital lines are usually at a much higher voltage (>100V), while analog is around ?20V?40V? (At least in California).  If your line is digital, it would have fried your fax or modem(and computer if its internal), unless you have a very robust piece of hardware.  If your line is purely digital, you can't use any analog fax/modems at all.
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Author Comment

by:deveshb
ID: 1145497
Hello JoopV
Though I have not received the final solution yet, I do count the efforts put in by you. In fact we could not complete our part as the vendor's technitians are not able to follow up the suggestions.

As a result I do accept the answer. But could you please give me your email address so that if required we will contact you to get further advice.
Thanks
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Expert Comment

by:joopv
ID: 1145498
my email address is pe1dna@amsat.org

hope to hear from you !


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