An explanation of why you don't get the ISP's Promised Speed.

Something we hear often is ''Why am I not getting 8Mbps?''  Although the ISPs offer up to 8Mbps, for example, and would love to be able to say to people, you will get 2.4Mbps and your neighbour will get 4.8Mbps for the following reasons but unfortunately it’s not that simple. They would love to give an accurate speed before signing up but unfortunately it’s a chicken-and-egg situation and they only know the speed you will be getting once you are connected.

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 data- base'' 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, they only know the true attenuation (reduction of signal) when they connect your line to their equipment and therefore your speed can be different as factors other than line length affect your attenuation. For example some of the factors in the external BT network:-

• 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
• if any part of the cable is Aluminium.

Within your premises the following things affect the quality of the signal:

• the number of internal wired extensions you have
• the number of items connected to those sockets
• the number of microfilters you have
• the last few centimetres of cable coming into the house - these can often be heavily battered by vacuum cleaners; prone to damp from condensation; covered in paint or damaged through not being fitted to the wall correctly.
• the quality of your internal cabling
• if you are using a telephone extension lead (urgh!)

Assuming you have good signal level 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
• faulty central heating thermostats
• faulty immersion heater thermostats
• fish tank pumps
• touch sensitive switches
• power supply units for TVs, laptops, PCs
• earth leakage faults at the customer’s premises
• security PIR systems detecting movement
• security lights going on and off
• telecoms cable being routed too close to main cables.

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

The final thing that will affect the 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 data you transmit will be at its weakest in the exchange and data 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 room 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 BT Cabinet (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 it’s slowing down in the evenings-and will syncronise slower if you re-boot your router. This is a classic sign of cross-talk or local noise. If you live in an area where only one or two have broadband, as other people sign up it will degrade over time. The copper network was never meant to take anything other than voice - it is now being pushed to its limit with ADSL and now ADSL2+.

This is why speed is an inexact science. We hope this clarifies some points.
For help in analyzing problems on the performance of your Broadband service call Broadband Doctor for a consultation.