Broadband Advisor

Broadband Download Speeds

Broadband advertised download speeds have been a high profile subject over recent months. The Chairman of Ofcom sent an open letter to the chairmen of the 6 top Broadband suppliers [pdf opens in new window] in December 2007 expressing his concern. Several magazines and the Gadget Show on Channel 5 have written about or campaigned on the consumers behalf.

The issue boils down to the advertised speed on many high speed broadband packages, that's "up to 8Mb/sec" and faster, not being the same as a customer recieves once signed up to the service. Often quite a bit less than advertised.

Theoretical maximum

The advertised speed seen on many broadband deals is actually the theoretical maximum. Many telephone exchanges have been upgraded with newer and faster equipment that gives a theoretical maximum speed of 8Mb/sec, known as ADSL+.

A download speed of 8Mb/sec is never actually possible because there is an overhead involved in making a connection and sending the data along the wire. The fastest possible speed is around 7.2Mb/sec. However this is only possible if the line is of top quality with no degradation of the signal. There are several factors that can affect the line quality including:-

  • distance from the exchange (the most likely cause)
  • quality of the wiring
  • your modem settings.

Real World download speeds vary considerably but are usually in the region of 2Mb/sec - 5Mb/sec for a product advertised as up to 8Mb/sec. Unfortunatly it is not possible to work out your download speed until you have signed a contract and started to receive a service from an ISP.

It's also worth noting as mentioned before that the quality of the line from your house to the exchange and also distance to the exchange affect the download speed. Therefore changing from one ISP who is a BT Wholesale customer to another who also resells BT Wholesale will make no difference to your connection speed.

Broadband over a cable connection

Broadband over a cable connection is not affected by the distance of the exchange from your home or office. Upload speeds are also generally higher. Virgin Media, the UK's cable provider, use fibre optic cables to transmit the data, sometimes using copper wires just for the last section into your home.

However download speeds over cable like ADSL can slow down from their maximum speed due to other factors like contention ratio which essentially means how many people are connecting at once and using the available bandwidth.

Other Reasons Your Download Speed may slow down.

Cable and ASDL service providers use several methods and techologies to share their available bandwidth amongst their customers and limit those who use the most. Your broadband connection could be subject to traffic shaping, download limits and bandwidth management with a resultant reduction in download speed.

Traffic Shaping
Software used by your ISP to control their network traffic.
Download Limits
Several broadband packages have a monthly limit for the amount of data downloaded.
Bandwidth Management
Similar to traffic shaping, banwidth management is carried out by an ISP to share out the available resources evenly. Some ISP's have fair use policies that throttle back a customers connection speed once a certain threshold has been reached in a day.