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There's more to broadband speeds than the headline, as anyone who has battled for bandwidth come 6 o'clock can attest.

This guide will explain how broadband speeds are affected by the lines in your area, the provider's policies and other users.

Actual vs advertised speeds

Before getting into the factors that make up the whole picture on broadband speeds, however, it's important for us to look at why we don't have the whole picture in the first place.

The answer lies in how speeds are advertised and the difference between those adverts and how speeds are actually measured.

How broadband speeds are measured

Broadband speeds are measured based on the number of megabits (basically, blocks of data) downloaded using the connection per second.

On our site we always write this as Mb. Elsewhere, however, you may also see it written as Mb/s, Mbps or even meg.

Speeds under 1Mb are measured in kilobytes (Kb) per second. 1Mb is 1024Kb and the typical dial-up connection provides speeds of up to 512Kb.

Speeds over 1024Mb are measured in Gigabytes (Gb) per second. 1Gb is 1024 megabytes.

A broadband speed test will measure both a download speed - which will affect the time it takes to see web pages and, um, download things - and an upload speed - which will affect the time that it takes you to upload information online - that the device is currently reaching.

In both cases, the speed test is a snapshot of speeds at a particular point in time.

Not only can a single speed test be unrepresentative of a connection's average connection capacity, it cannot show some aspects of traffic management which means that the end user will experience certain activities more slowly than others.

Most providers now slow peer-to-peer (P2P) software at peak times, for example.

How broadband speeds are advertised

As of 2007, the 'up to' broadband speeds that providers advertise are estimated download speeds, not the speeds you can actually expect when you sign up.

In Ofcom's broadband speeds voluntary code of practice (of all the ISPs listed on our site, only AOL aren't signed up) Ofcom say:

"...it is critical that all ISPs explain to consumers that actual throughput speeds are likely to be lower than the headline or advertised speeds..."

So, ISPs can advertise a headline 'up to' speed, which is a notional highest speed but they must also give you an estimated line speed based on the available information about your line.

It's that second estimated line speed that you should refer back to if you're trying to get your broadband up to speed.

The blame game

When slow speeds are frustrating it's nice to have someone to blame.

We've yet to hear of a broadband user who hasn't suffered from slow broadband speeds at one time or another so our broadband most-wanted list singles out the top reasons for slow broadband and explains why they can be a problem.

#1: Your Cables

The truth is that one of the most important factors when estimating broadband speeds - the most important for many users - remains the type of cable installed in the area.

There are four main types:

• FTTH (Fibre to the Home) - 50Mb +
• FTTC (Fibre to the Cabinet) - Up to 100Mb
• ADSL2+ - Up to 24Mb
• ADSLMax - Up to 8Mb

The fastest - FFTH - is currently only available in the UK as part of broadband trials.

Virgin Media and providers using BT's fibre network are offering FTTC services.

See our Broadband for Beginner's guide for more on this.

Because of the expense of installing new technology and because most of the decisions on what to install are being made by profit-making companies, urban areas with more people sharing an exchange are most likely to have the fastest cables installed.

#2: Postcode

The further away a property is from a telephone exchange the weaker broadband signal can become and the slower your actual connection.

This is due to a combination of attenuation - the signal getting weaker as it moves further away from its source -, the quality of the lines themselves and electromagnetic background 'noise' which interferes with the signal.

Fibre optic cable broadband is less prone to attenuation and noise problems which is why it's so much faster than other broadband services.

When you compare broadband by speed using one of our comparison tables we'll filter out services that aren't available at your exchange and also those packages offering speeds less than your line can handle - in other words leaving the fastest broadband packages available to you.

However, for the exact speed your line can receive you'll have to check with the provider for a more specific estimate.

#3: Provider

Intuitively, the best way to throw some broadband blame is at your service provider: after all, you are paying them.

As we noted above, there's a degree of truth to this.

Not only does the provider affect the cables available (see above) and the hardware and contention ratio (below) they can choose to affect connections directly.

Many so-called unlimited deals actually impose a fair use policy on their users.

People who breach the ISP's fair use policy will be 'throttled': it sounds nasty but really it means that broadband speeds are cut to prevent that user from slowing the service down for everyone else.

Most providers' fair use policies only slow the very heaviest downloaders.

In addition, during busy times, usually evenings during the week and afternoons and evenings at the weekends, some providers manage or shape their internet traffic.

In general, downloads, and in particular peer-to-peer (P2P) activity, tend to be slowed during these peak periods.

The big four ISPs - BT, TalkTalk, Virgin Media and Sky - now publish a standardised version of this information on these restrictions on their website.

#4 Hardware

Many deals that advertise higher speeds do so as much on basis of the hardware - usually a wireless router - that comes with the deal as the broadband itself.

Its important to remember newer - faster - wireless routers will be using the new 802.11n protocol - which means, while they are back compatible, if the wireless card in your computer or laptop was bought a few years ago and only supports 802.11b/g then you won't benefit from the connection speed boost of a wireless 'n' router.

For more on how to check your router see our faster broadband guide.

#5 Time of day

More people are online between 6pm and midnight - that means less bandwidth per person and slower speeds.

With more people at home during peak times there is also likely to be more noise on the line as a result of people using electromagnetic equipment in their homes which may also slow down your broadband speeds.

There's not much you can do about this it's a quirk of broadband technology which unlike water or electricity isn't just 'off' or 'on'.

All in all if broadband speed really matters to you and you're outside contract with your current provider, switching to one of the fastest broadband deals available is likely going to be the most hassle-free way of improving your broadband speeds.

#6 Contention ratio

Finally, the contention ratio describes the number of users sharing one unit of data capacity.

The lower the contention ratio the higher the quality of service. A 50:1 contention ratio would mean that up to 50 broadband customers are sharing the same bandwidth at any one time.

The quality and speed of your broadband connection is dependent on the number of users online at any given time. Business broadband services will often have much lower contention ratios to enable the ISP's to give business users a more consistent quality of service.

The higher the contention ratio the cheaper the broadband package is likely to be - and additionally, different ISPs have different contention ratios. The contention ratio is an important factor involved in the quality and speed of your chosen broadband package.

The average contention ratio for a home user package is 50:1 and 20:1 for a business package. That means that at any one time home users may be sharing their internet access with up to 49 other users all sharing the same bandwidth.

If you are using a 512kb connection, then, and each of the other users sharing the access are using it at the same time, your connection speed could be reduced to as little as 10kb!

Fortunately, it's unlikely that everyone uses the connection at the same time.

However, this does highlight the importance of choosing an ISP with a low contention ratio as the fewer people sharing the line, the more bandwidth you potentially have.

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