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Quote:

Originally Posted by tsk1979 (Post 885860)

T1 uses PCM right(8Khz)

Nope, there is no separate carrier like in AM or FM.

Once the analog voice is pulse code modulated, it becomes digital stream of 64kbps. Then this signal is time division multiplexed into T1 or E1.

T1 - 1544KHz digital stream - that is 24 channels of 64Kbps
E1 - 2048KHz digital stream - that is 32 channels of 64kbps

These signals can survive for 2.2Kms, therefore repeaters are provided every 2Kms. But modern cables have extended the range up to 3.5Kms. This is how modern offices with high telephony/internet needs are connected to the exchange.

Quote:

Originally Posted by tsk1979 (Post 885860)

Samu, isn't the carrier analog?
Just like ADSL etc.,

Don't confuse internet technology with telephony technology. Internet is packet switched, telephony is circuit switch. Yeah, VOIP is blurring the difference, but still it has to be maintain QOS or quality of service to be legible.

I was talking purely in the signal domain, no "higher layer involved"
I have never actually worked on all these T1 etc., only on ADSL primarily and some other related stuff and little fiber.
Mostly all devices and chips I worked on required a D2A at transmission end and a A2D at the receiving end.

Quote:

Originally Posted by tsk1979 (Post 885906)

Mostly all devices and chips I worked on required a D2A at transmission end and a A2D at the receiving end.

That raises a curiosity, does ethernet card have A2D/D2A chips ?

Quote:

Originally Posted by Samurai (Post 885809)

Digital signals can be fixed as new, but there is no such hope for analog. Once corrupted, it is spoiled, spilled milk.

I would like to take exception to that !

A digital signal, if corrupted , cannot be recovered ( thankfully this situation is reduced due to interleaving/error correction) , but if a digital 1 becomes a 0 or vice versa, the information is lost forever.
An analog signal degrades gracefully with noise. even to the point where the noise is greater than the signal. the reduction of signal fidelity with noise is a constant downward slope for analog signals , while for digital, it is a horizontal line which falls drastically once it reaches a threshold.

Quote:

Originally Posted by NetfreakBombay (Post 885916)

That raises a curiosity, does ethernet card have A2D/D2A chips ?

Nope. Ethernet or LAN is digital. Depending on cable the length can go upto 500metres.

Quote:

Originally Posted by greenhorn (Post 885924)

We are talking about transmission here. With digital you can "detect" this failure and ask sender to resend.

In terms of storage, yes this is an issue.

Quote:

Originally Posted by greenhorn (Post 885924)

Nope that doesn't correct, even if one bit has gone wrong or some others were have correction techniques for that ..... there are many methods of error correction in encoded digital data ......

Quote:

Originally Posted by Thad E Ginathom (Post 885677)

Isn't fibre-optic a purely digital medium carrying a purely digital sigal?

Light on = 1
Light off = 0

I've always made that assumption. It wouldn't surprise me to know it was completely wrong!

quoting condor's signature, "life is analog, digital is approximation".

what if you get a .3 lux on optical fibre assuming 1 lux is supposed to be 1, and zero lux zero? proves that the medium is analog.

Quote:

Originally Posted by NetfreakBombay (Post 885916)

That raises a curiosity, does ethernet card have A2D/D2A chips ?

No, that applies only to voice signals. Ethernet is purely data.

Quote:

Originally Posted by greenhorn (Post 885924)

I think you are missing the point completely. If you have digital communication background you would understand what I meant right away. When I said corrupted, I meant deteriorate over the cable length but still legible enough for regeneration by a repeater. That is why repeaters are provided every 2 Kms. If you put it every 5Kms, the repeater won't know whether it is a 1 or 0 and can't repeat.

Quote:

Originally Posted by greenhorn (Post 885924)

the reduction of signal fidelity with noise is a constant downward slope for analog signals , while for digital, it is a horizontal line which falls drastically once it reaches a threshold.

yes, and once a line is installed taking this risk into account, and repeaters installed at regular intervals, the system is good for a long time.

my father worked for DOT optical fibre network installations, and he said once they changed from copper to optical, the distance required between repeaters increased 50 kms or something like that (with copper it was ~2 kms....forgot :Frustrati).

plus, they did not have to face cable theft for copper :D.
this was back in 1990s.

Quote:

Originally Posted by Samurai (Post 885935)

That is why repeaters are provided every 2 Kms. If you put it every 5Kms, the repeater won't know whether it is a 1 or 0 and can't repeat.

Samurai, that was in copper days, now optical fibres are much more robust, and used almost everywhere except the last mile.

Quote:

Originally Posted by vivekiny2k (Post 885946)

my father worked for DOT optical fibre network installations, and he said once they changed from copper to optical, the distance required between repeaters increased 50 kms or something like that (with copper it was ~2 kms....forgot :Frustrati).

Your figure for copper is right. I didn't know the repeater distance for optical cable. Therefore didn't want to mention it. So, that is 50Kms.

Quote:

Samurai, that was in copper days, now optical fibres are much more robust, and used almost everywhere except the last mile.

I don't deal with telco-to-telco communications. I usually deal with last mile in digital communications.

Quote:

Originally Posted by NetfreakBombay (Post 885930)

We are talking about transmission here. With digital you can "detect" this failure and ask sender to resend.

In terms of storage, yes this is an issue.

If we are talking about SDH, SONET or WDM, NetworkElements usually dont ask the sender to resend it but actually plan in advance to get the best signal possible which is called path protection. For transmitting large ( MUXed) optical signals, NOC operators always provision for protection paths to make sure that best signals are picked up.

cheers