Why do I not get the promised 8Mbps? [Article]


King of de Jungle
Mar 17, 2005
I found this pretty interesting in figuring out what is going on with your line...

From this site.... http://www.talktalkmembers.com/content/view/99/68/

Something we hear often is "Why am I not getting 8Mbps" ? Well the offer is up to 8Mbps and we would love to be able to say to people, you will get 2.4Mbps and your neighbour will get 4.7Mbps dfor the following reasons but unfortunately its not simple. We would love to give an accurate speed before signing up but unfortunately its a chicken and egg situation and we only know the speed you will be getting once you are connected. The TalkTalk website actually does quote an estimated speed when you order.

Two main factors affect your speed. Line attenuation and noise, both of these are explained below.

Based on your telephone line length in the "copper loop database" all ISPs will be able to do a quick calculation of the expected attenuation (line loss) and therefore say you will get approximately xMbps.

This is when the problems can start, we only know the true attenuation when we connect your line to our equipment and therefore your speed can be different as factors other than line length affect your attenuation. For example

* The quality of the cable
* The true length of the cable which can differ substantially from what we have been told
* The number of joins in the cable
* The age of the cable
* The dryness of the cable
* The quality of connections in the cable
* Is any part of the cable Aluminium
* The last few centimetres of cable coming into the house, these are usually heavily battered by Hoovers and dusting. Prone to damp from condensation. Covered in paint or damaged through not being fitted to the wall correctly

In particular the following things affect the attenuation and this is why you are asked to test in the master socket if you have one

* the number of internal wired extensions you have
* the number of items connected to those sockets
* the number of microfilters you have
* the quality of your internal cabling
* are you using a telephone extension lead (urgh).

Assuming you have good attenuation then the next issue is electrical noise. Again many factors affect this.

* the quality of the cable used in and around the house
* all the same things that affect attenuation
* extension leads
* central heating pumps
* dimmer switches
* Christmas tree lights
* faulty street lights near the premises or the distribution point to the premises
* fridges
* stair lifts
* electric lawn mowers
* whirlpool baths
* railway lines, tram lines or tube lines
* faulty central heating thermostats
* faulty immersion heater thermostats
* fish tank pumps
* touch sensitive switches
* power supply units for TV's, laptops, PCs
* earth leakage faults at the customers premises
* security PIR systems detecting movement
* security lights going on and off
* telecoms cable being routed too close to main cable

The list just goes on and on and depending on the amount of electrical noise these generate this will in turn affect your sync speed.

The final thing that will affect your sync speed is "cross talk". When your signal is at its weakest it is more susceptible to noise. The signal is at its weakest at the end of its journey so stuff you transmit will be at its weakest in the exchange and stuff you receive is at its weakest at your house.

If your line travelling 6km comes in to the exchange close to another line that travels 300m then it is possible that the signal from the 300m lines will "bleed" on to the weaker 6km signal. Alternatively the transmission of a signal down a 6km line may bleed over the weaker signal required to transmit on the 300m line but this is less likely. This is known as exchange cross talk. As the upstream is slower then exchange cross talk doesn't normally have as much affect as the other type of cross talk.

Distribution cross talk is exactly the same as exchange cross talk but happens at the other end of the cable. The PCP box (the green box on street corners) is a major culprit.

As more and more people get broadband then cross talk slows down the internet connection. You may find that you used to be able to get max speed on your broadband but its slowing down in the evenings and will sync slower if you reboot your router. This is a classic sign of cross talk or local noise. If you live in an area where no one has broadband but you then you will get little, as people sign up it will degrade over time.

The copper network was never mean to take anything other than voice and is being pushed to its limit with ADSL and now ADSL2+.

This is why speed is an inexact science. Hope this clarifies some points.


Executive Member
Jul 5, 2009
Very interesting article!

The effects of noise can vay from CRC errors, which are inconvenient but temporary, or a lower synch speed which is disappointing but not fatal, to failue to synchronise or dropped connection which is hugely frustrating...

I think that's also a reason for users to be on ADSL2+, since the G.Dmt.BisPlus modulation is better at dynamically correcting errors. See: http://en.wikipedia.org/wiki/G.DMT

I was on ADSL2+ (at 4Mbps) for 2 weeks after they upgraded my exchange, but when I was bumbped up to 7Mbps Telkom reverted me back to G.Dmt. This means I now get more CRC errors (on G.Dmt.BisPlus I got none), although my overall quaility is fine.
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