Well, this question requires a two part answer. [
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Before I begin, I should clarify something though, which Telkom never seem to be able to explain to users, since the people working in the call centers just don't know how all of this works.
In almost all exchanges around the country, the exchange is connected to Telkom's backbone via fiber optic cables. This is not what is causing your line not to be compatible.
At the exchange, the DSLAM (DSL Acess Multiplexer) unit converts your traffic from analogue modulation to ethernet traffic, which then travels further on Telkom's network as EoA (ethernet over ATM) traffic. EoA traffic can traverse fiber just fine, so once your traffic has managed to reach the DSLAM, fiber optics are no problem. It is just that there must be an end to end copper connection between your modem and the DSLAM.
Now, the problem that's causing you not to be able to get service is not that the exchange is connected by fiber optics, but that there is no end to end copper cable running from your location to the exchange.
The reason for this is that line distribution point in your area is not an analogue mini-switch connected to the exchange via more analogue cable, but a digital switch connected to the exchange via fiber optic cables. These digital DPs have their advantages in that you need only one fiber optic cable from the DP to the exchange, as opposed to thick copper wire with many strands, which is normally required.
There is, however, a disadvantage. These digital DPs accept incoming analogue copper connections from the surrounding properties, but instead of switching the analogue transmission on to another copper wire running to the exchange, the call is compressed and digitized inside the DP and sent on to the exchange via fiber.
From the point of view of the exchange, this is a good thing, because the exchange no longer needs to perform the analogue to digital conversion before sending the call on to the backbone network, but from the point of view of an ADSL modem, this is a disaster.
That's because, as the analogue calls from properties served by the DP are converted to digital, the high frequencies used by ADSL connections are cut out, never reaching the exchange or the DSLAM unit connected to the exchange. This is discussed at length in the first part of the answer below.
Just as a point of interest, you can be assured that you will always have analogue copper wire at least to your local area DP. This is because no conventional telephones, voice band modems or even ADSL modems can be connected directly to fiber optic cables. In cases where your area is served by a digital DP, the analogue to digital conversion will always happen at the DP.
So, when Telkom tell you that you can not have ADSL because your exchange is fiber optic, what they actually mean is that you can not have ADSL because your DP is connected to the exchange via fiber optics.
This will also explain why the internet test will list your number as having ADSL support. The exchange in your area most likely does have the DSLAM unit required to support ADSL. Furthermore, it is possible that not all DPs in the area are digital, thus, those people lucky enough to be serviced by the old analogue mini-switch DPs will be able to get ADSL, but not those serviced by digital DPs connected via fiber optics.
Now, having gotten that clarification out of the way, we move on to the two part answer:
First off, it's impossible for ADSL to work over fiber optic cables since they use a completely different method of transmission to carry the signal. ADSL modems use very high frequency discreet multi-tone modulation on analogue cables. In other words, they send sound waves via analogue electrical pulses at frequencies way above that which would be needed by voice calls.
That's why you can use your line for voice calls and ADSL internet at the same time as long as you have a splitter or filter. The voice calls generally use frequencies between 300Hz and 3KHz. ADSL connection make use of frequencies between 20KHz and 1MHz. The splitter or filter devides these two frequency bands so that both kinds of devices can share the line without interference from the other device.
Fiber optic cables use light pulses created by low frequency lasers in small optical fiber "channels" inside the cable. The low frequency laser light can travel over huge distances and can also be extended by repeaters on the cable. The light pulses are used to directly signify digital data, as ones and zeros, as all digital transmission systems do.
Voice calls traveling over fiber optic networks are generally digitized by special filtering equipment at the exchange end and transmitted in digital form over the fiber optic cable. This is not possible for ADSL because of the very high sound frequencies it uses.
Generally, the algorithms used to compress voice call data will not handle any sound frequency above 3KHz as it is considered unneeded for voice calls. Voice band modems will also not use frequencies above 3KHz for compatibility reasons, so all of these will work just fine when the call is compressed, digitized and sent over fiber.
In the case of ADSL modulations, the equipment compressing and digitizing the call will definitely loose all of the modulation. Thus, your ADSL connection will dissapear at the point where it must be routed over a fiber optic connection.
Now, from the first part of my answer and the clarification above, it would be very obvious that you are unlucky enough to find yourself within the service area of one of these digital DPs and can not have ADSL, not just yet, anyway.
On to the second part of my answer:
The new digital DPs connected to exchanges via fiber optics are very popular with telephone operators, since they generally cost less and are easier to maintain than the older analogue mini-switch technology. For that reason, there has been a push to develop ways in which customers in the service area of these DPs can still receive ADSL service.
The solution is called a mini-DSLAM. This is a DSLAM unit which is much smaller than the DSLAM unit you might find at the local exchange. A normal DSLAM might have anything from 30 to 2000 ports, depending on the population density and service penetration in the area the DSLAM must service.
A mini-DSLAM will typically have no more than four or eight ports available. These mini-DSLAM units are not connected to exchanges, as with the traditional DSLAM, but are installed directly onto a digitally operated and connected distribution point.
Thus, instead of your DSLAM being at the exchange, it is at the DP. In this way, your ADSL signal can travel over the short distance of copper wire to the DP and the mini-DSLAM connected to it. Then, at the DP, it can be converted into EoA traffic by the mini-DSLAM and then sent on to the backbone network via the exchange, over fiber optic cable, without any problem.
With one of these in place, any DP connected to the exchange via fiber optics can still offer ADSL service directly to users within its coverage area.
Now, here is the problem you face:
Currently, you do not have direct copper connectivity to the exchange, where the nearest DSLAM is located. Thus, you can not have ADSL.
There is something you can install which will fix the problem, the mini-DSLAM, but this is not something that <b>you</b> can install, it can only be installed by Telkom.
Telkom have mentioned that they were testing mini-DSLAMs in the past, but the last mention of it from any Telkom representative was in late 2003.
The only way for you to be able to get ADSL in your area would be to convince Telkom to install one of these units in your local DP. This will not be easy though, since DSLAM equipment of any kind is supposedly quite expensive. For them to be interested in doing this for you, you will have to get your neighbours to sign on too.
Typically, a DP will service six to eight properties located around it. Sometimes, this can be higher, but is unlikely in sparsely populated areas.
Telkom will not consider it worth while to install a mini-DSLAM in your DP if you are in a sparsely populated area, or any area where interest in ADSL is low. For it to be profitable for them to install such a unit in your DP, you will have to get at least four neighbours to also demand ADSL service. If you can, you might be able to convince them to install one for your DP and your ADSL withdrawl symptoms should come to and end shortly.
That is, however, if they are willing, or able, to install it at all. Otherwise, I'm afraid you are stuck. [
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Willie Viljoen
Web Developer
Adaptive Web Development
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