DSLAM? ADSL? How does it all work

[firewall]

I'm having trouble understanding how the DSLAM, exchange and your phone line all come together.

OK, so your phone line goes from your house to a Telkom box on the street called a SDC (Street Distribution Cabinet). Sometimes they another box next to it called the DSLAM (Digital Subscriber Line Access Multiplexer. And then it all goes from there to the exchange which is the brown windowless telkom building somewhere in your suburb. As far as I understand, the exchange is the thing which handles your area code e.g. 021 911 xxxx
Am I correct so far?

Now, to get a stable DSL connection you must be around 4.5km from your exchange (or at least the cable from the exchange to you must not exceed 4.5km).

1. Do you need to be within 4.5km from your friendly neighbourhood white DSLAM box, or 4.5km from your exchange, the building?

2. When they run out of ADSL ports, and need to add more to the exchange, are they talking about the exchange building, or the DSLAM box nearby?

3. What happens in the DSLAM box? As I understand it, your data portion of the line is at this point changed to ATM or some other kind of medium/protocol/something?

4. Does your voice portion of the line also terminate at the SDC/DSLAM with the data, or does that carry on all the way to the exchange building?

 

[Gnome]

http://computer.howstuffworks.com/dsl.htm

DSLAM sits in the exchange, it's a building. Some areas the exchange is too far so they have a mini-DSLAM next to the road. It looks like the transformer boxes but with a Telkom sign on it. My area is on a mini-DSLAM that is connected VIA fibre to the actual exchange. I'll maybe try get a pic when I drive by again. The line also ends at the DSLAM, so our areas phones go over fibre.

So it's either a Telkom building, or a huge Telkom box next to the road.

 

[firewall]

So this is a mini-DSLAM: http://img484.imageshack.us/img484/731/31082006147mediumml4.jpg ?

Do you need to within 4.5km from the closest mini-DSLAM?
And when they add ports, do they add it to the mini-DSLAM or to the exchange?

 

[Sting]

So this is a mini-DSLAM: http://img484.imageshack.us/img484/731/31082006147mediumml4.jpg ?

Do you need to within 4.5km from the closest mini-DSLAM?
And when they add ports, do they add it to the mini-DSLAM or to the exchange?

1. Yes.
2. Both.

 

[ambo]

So this is a mini-DSLAM: http://img484.imageshack.us/img484/731/31082006147mediumml4.jpg ?

The box on the left might contain a DSLAM. The blue and green one is just a SDC which contains wires and connectors - no electronics in there.

Where do you get that 'magical' 4.5km number from? The further you are from the DSLAM the less you maximum possible sync rate will be. Most of the people with good 10Mbps connections are less than 2km from the DSLAM. The quality of the copper connection has far more impact on your theoretic speeds though.

 

[Gnome]

So this is a mini-DSLAM: http://img484.imageshack.us/img484/731/31082006147mediumml4.jpg ?

The blue/gray box is the right size for the mini-DSLAM but the one close to us has a Telkom logo on it. The small blue box (in that pic) with the Telkom logo on it, is just the termination point. All the wires from various properties are wired together in that box. It isn't a DSLAM, it's just wires.

EDIT: Ambo beat me to it

Do you need to within 4.5km from the closest mini-DSLAM?
And when they add ports, do they add it to the mini-DSLAM or to the exchange?

The same rules that apply to the DLSAM apply to the mini-DSLAM, the only difference is that the mini-DLSAM only has a portion of the ports and can usually be housed on the street whereas the proper larger DSLAM usually a lot larger and is therefore housed in a structure.

If you ARE on a mini-DSLAM then they will add ports there, if you are on a normal DSLAM they'll add ports at the normal DSLAM. So far as I know the mini-DSLAM comes equipped with as many ports as it can have out of the box. But honestly this is speculation on my part. I know we are on one because I've asked the techie and he also told me where it is (I live very close to my DSLAM which happens to be a mini-DSLAM over fibre).

Telkom only run mini-DSLAMs in affluent areas btw. so very few places have it (well that is how I understand it), I don't fall into that category (on 384 still) but most people around us are on 4mb, in fact I haven't talked to a neighbour who doesn't have ADSL and isn't on 4mb ADSL (or however much it is now).

You can also usually tell when you phone 0800 DSL DSL, they can't do a line test on my line (because it is over fibre) and they usually send a technician out instead of trying things like port resets and various tests. Only once have they done a port reset for me (out of 10 call outs since I moved here) and even then they still sent a technician out which I hear is counter to what other ppl on MyBB say.

 

[firewall]

The unfriendly telkom lady told me about the 4.5km. She said they need to do line tests if you are close to or further away from your exchange than 4.5km
Also, the wikipedia page on DSL has a graph that shows that things get pretty grim after 4.5km when you use the standard 13.81 dB/km cable loss into account.

 

[Sting]

The blue/gray box is the right size for the mini-DSLAM but the one close to us has a Telkom logo on it. The small blue box (in that pic) with the Telkom logo on it, is just the termination point. All the wires from various properties are wired together in that box. It isn't a DSLAM, it's just wires.

The one on the right is a SDC and the other one is a UMC, which could contain a mini-DSLAM.

 

[Gnome]

Yeah 4.5km is fairly far, still might be possible to get 384. I'm 1km from the exchange and even with one of the wires cut and the other one barely touching and completely rusted and burnt (as in I the phone was dead when I picked it up, I saw the wires when the techie came over) I could still synch and dl on 384.

384 is very reliable

 

[firewall]

I'm moving to an estate which they expect to house about 6000 people and the houses are all around 1.5m. So what is this story telkom tells me about DSL not being available over fibre?

 

[Gnome]

The one on the right is a SDC and the other one is a UMC, which could contain a mini-DSLAM.

That is what I said? I don't know the official names.

 

[firewall]

My exchange is 3.9km straight line distance. If you travel by road, its a bit more between 4.5 and 6km depending in which road you take. If I can find a mini-DSLAM box in my estate, I guess my chances of being able to get ADSL is significantly better?

 

[Gnome]

I'm moving to an estate which they expect to house about 6000 people and the houses are all around 1.5m. So what is this story telkom tells me about DSL not being available over fibre?

That is true, the ADSL signal must be terminated BEFORE the fibre. Like in our case the wires all terminate at the DSLAM then go over fibre from there, in your case there probably isn't a DSLAM and Telkom don't deem it financially viable to roll out ADSL there or perhaps the houses are linked with fibre in which case it isn't possible either.

 

[firewall]

There definitely is ADSL in the area. Some of the members on MyBB have responded that they have ADSL, but I haven't been able to get their ADSL Router stats from them to determine how far away they might be from their DSLAM. I'll go drive around there tomorrow and see if I can spot one of them white DSLAM boxes. Would Telkom be able to provide me with the location of the DSLAM where my line will terminate if I ask them?

 

[Sting]

I'm moving to an estate which they expect to house about 6000 people and the houses are all around 1.5m. So what is this story telkom tells me about DSL not being available over fibre?

My exchange is 3.9km straight line distance. If you travel by road, its a bit more between 4.5 and 6km depending in which road you take. If I can find a mini-DSLAM box in my estate, I guess my chances of being able to get ADSL is significantly better?

Ummmm big flashing lights go off when you mention Estates. Your best bet is to find out if anyone else in the estate already has ADSL service.

It is no longer just a Telkom issue; many estate refuse to allow Telkom into the complex to provide cable infrastructure.

 

[Gnome]

Not sure if they'd tell you. The customers involvement in the network ends at their premises. The only way to go is to apply for ADSL. Or are you basing your buying decision on if there is ADSL available?

Honestly you can't know for sure if ADSL is available even if you see such a box because the ports could all be occupied or the wires run a strange route and are too far away or a part of your suburb uses some kind of different wiring, etc. etc. Only Telkom know what the deal is over there.

 

[firewall]

many estate refuse to allow Telkom into the complex to provide cable infrastructure.

I sorted that: phoned the Telkom network planner for the area and he said that they will be ordering the material for lines to the complex and it should be done and installed in 4 - 6 weeks.

 

[Gnome]

Ummmm big flashing lights go off when you mention Estates. Your best bet is to find out if anyone else in the estate already has ADSL service.

It is no longer just a Telkom issue; many estate refuse to allow Telkom into the complex to provide cable infrastructure.

x2 on that. I think many estates plan on the future and wire the places well themselves (many times exceeding wire quality used by Telkom) but Telkom and just about every other telco on earths policy is that they must own the infrastructure up until the home. But then the Estate may ask why, and in the end Telkom just decides to not bother.

I understand both sides, Telkom must own the infrastructure in order to maintain it (else there may be legal and ownership issues) and I understand the Estate may have invested in proper infrastructure. Catch 22 really.

 

[firewall]

In this case, the developer has installed drawboxes to every house, and I'm guessing wires which go from each drawbox to some central point outside the complex. The houses already have 2 telephone points inside connected to the drawbox IIRC. The complex is inside the estate, and the estate is publicly accessible.

 

[Gnome]

In this case, the developer has installed drawboxes to every house, and I'm guessing wires which go from each drawbox to some central point outside the complex. The complex is inside the estate, and the estate is publicly accessible.

Yes but in order to provide the service the wires must meet a certain standard. Furthermore if anything is damaged (supposedly) by a Telkom employee who is held responsible (legally). Lastly Telkom don't want ANYONE to mess with wiring (even on their premises), forget DB boxes, but if they don't own it they have no legal leg to stand on so to speak but if you open a Telkom DB you are committing a crime. Just legal issues really.

It's not unique to Telkom, they are better than some of the overseas Telco's. I've heard some horror stories of European Telco's they have very strict rules. You cannot at any point touch the wiring running right up to your home until AFTER their wiring terminal. It is a legal requirement. They own everything up to that point.

Anyway just mentioning it, you have to call and ask, no-one can tell you for sure.