Questions about congested ADSL exchanges

[undesign]

I've been battling with Telkom for the last 7 weeks about my 4Mb ADSL line non-performance and I thought I'd ask for some more advice about the issue.

It seems, from reading on this forum, and talking to different people that a "congested exchange" is the probably the issue. My questions are -

1) Is it a normal characteristic of a congested exchange to (for the majority of the time) consistently achieve download speeds slower than the upload speeds?

2) What exactly is the "exchange"? Is it the mini-DSLAM down the road?

Thanks!

 

[MickeyD]

Q2: What exactly is the "exchange"? Is it the mini-DSLAM down the road?

No, that is not an exchange. Guys like ponder can correct me here... but I'll give it a go for you!

A simple ADSL network = Customer -> DP -> SDC -> MDF -> DSLAM -> Switch (this is all copper based).
[DP = Distribution Point, SDC = Street Distribution Cabinet, MDF = Main Distribution Frame]

Due to copper length distances the telco could decide to shorten the length of the "local loop" between the Switch and the customer.

They have various options at their disposal:
1. Build a new switch that includes a small brick building.
2. Install a containerised exchange.
3. Install a remote exchange in a cabinet.
4. Install a mini-DSLAM.
5. Install a MSAN (the very latest addition to their toolbox).

Option 1 is obvious. It will include new optic fibre backhaul in the nearest local fibre ring. While this will always be the best option it is the most expensive and also requires the purchase of a property (land). Brand new telephone numbers will be allocated to all customers connecting to this new exchange.

Option 2 has been phased out and existing units are being replaced by options 3 and (soon) option 5.

Option 3 was popular as it was quicker to install and occupied very little physical space. All equipment would fit into a single metal cabinet mounted on a concrete plinth on a pavement. In most cases this remote unit was connected by optic fibre back to the nearest "big" exchange. Brand new telephone numbers will be allocated to all customers connecting to this new remote exchange.

Option 4 is very popular to increase broadband capacity. Instead of installing new exchanges or remote exchanges, a mini-DSLAM would be installed in a metal cabinet on a pavement or in a shopping centre, etc. and it would be connected to a "parent" exchange nearby. Each ADSL port on the mini-DSLAM will require a dedicated port on the DSLAM at the "parent" exchange. Numbering is done at the "parent" exchange and not at the remote unit. So, while it is a relatively quick fix, it is not ideal.

Option 5 is the latest and greatest currently being piloted in South Africa by Telkom. It is an "all-in-one" MSAN unit that is connected via optic fibre to the nearest optic fibre run. The MSAN does everything and does not need to be connected to the nearest "parent" or "big" exchange. Telkom will use these to provide 20Mbps services. Pilot projects are underway in Joburg, Pretoria, Cape Town and Durban.

 

[undesign]

Thanks for the detailed info MD...and for confusing me.

So the DSLAM sits at the exchange, and from what I gather I'm connected to it via a mini-DSLAM, one like this -

http://img484.imageshack.us/img484/731/31082006147mediumml4.jpg

On the assumption that the place where I live (Durbanville) only has one exchange, if that is congested then the whole of Durbanville must be experiencing the same ****ty speeds like I do. Since that is emperically not the case, then it means the mini-DSLAM is in other words congested?

Back then to question one, whether it is the exchange or the mini-DSLAM which is congested, would that cause consistently achieving speeds of less than 10% of sync speed?

 

[MickeyD]

Your pic - the cabinet on the left contains, amongst others, a mini-DSLAM. The green and blue cabinet is a SDC.

Possible issues could include:
1. insufficient backhaul capacity between the mini-DSLAM and the "parent" exchange.
2. a cable problem between the mini-DSLAM and the "parent" exchange.
3. a cable problem between your premises and the mini-DSLAM.
4. wiring issues with the mini-DSLAM "box"
5. faulty ADSL port at either the DSLAM in the "box" or at the "parent" exchange.
6. distance from mini-DSLAM to your premises.

 

[Tinuva]

I have news for you, even the mini-DSLAMs won't have cable problems to the BRAS, in fact they usually use fibre, and fibre either works or not at all. Running at capacity yes (a.k.a. congested) they can, but it won't be specifically a cable problem.

But back to the OP, yes if the uplink from the DSLAM to the BRAS is congested or running at 90-100% capacity yes you will experience slow downloads and high latencies.

 

[undesign]

Thanks guys. I'm trying to eliminate the possible causes as Telkom has steadfastly refused to send a technician out to me. The only "detail" they have given me is a "general network fault".

I'm starting to think it is NOT a congestion problem as their are no pattern to the low speeds (happens throughout the day/night), pings are generally ok (less than 50) and speeds are either 100% or less than 10%.

 

[undesign]

At long last managed to get hold of someone in the area that actually knows what is going on. My mini-DSLAM (J19) is congested, so much so that it regularly "flatlines" hence the almost non-existent speed.

They're "waiting for Pretoria" to upgrade the capacity. ETA unknown.

Apparently it is impossible to connect me to another mini-DSLAM as the whole complex is wired towards this one.