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Where do lost megabits go?

2009-07-31 10:54:23

WHO, WHAT, WHY?

The Magazine answers...

Broadband users aren't getting the speeds they are paying for, says Ofcom. One

reason is because the signal degrades over distance when sent through copper

wires, so where do the missing megabits go?

To many people around the UK the results of Ofcom's broadband speed survey came

as no surprise when it was published this week.

The telecoms regulator says broadband customers are not getting the speeds they

are paying for. Nearly one fifth of those on an eight megabits per second

(Mbps) connection actually receive less than 2Mbps.

It also says the speed of broadband delivered through traditional copper wires

- rather than faster fibre-optic cables - is slower the further away you are

from your telephone exchange. So where do the missing megabits go?

They aren't lost in the way that you would lose water pressure through a leaky

water pipe. The extra megabits per second you are paying for and not receiving

are usually never given in the first place, say experts.

THE ANSWER

The Megabits you pay for are never given in the first place

Broadband connection is decided by the "sync speed"

This is decided before signal leaves the telephone exchange

In most cases the speed of your broadband ADSL connection is set from the

start, it doesn't get slower or faster. So if it's only 2Mbps then that's the

speed it was sent out from your local telephone exchange, even if you paid for

a faster connection.

Several factors decide this rate but the main one is "sync speed", says Richard

Shaw from SamKnows, a broadband measurement site and Ofcom's technical partner.

Broadband works best on a stable line and "sync speed" is the most stable speed

possible on your line. It is calculated between the exchange and the ADSL modem

in your home before the connection is fully established and working.

"You could think of it like a greeting between two people at the start of a

phone call before the main conversation starts," says Mr Shaw.

There are two factors that decide "sync speed". The first is line attenuation,

which is the natural loss of the signal due to the distance you are from an

exchange.

This is the most referred to factor in broadband quality because a signal sent

through copper wires degrades over distance. Quite simply the further you are

from an exchange, the longer the copper wire used and the worse the signal.

The second factor is signal to noise ratio (SNR). This is the quality of the

electrical signal being transmitted through the wiring and how it compares to

the electrical interference.

Such noise on a line blocks and reduces the amount of broadband signal that can

get through. The greater the signal that can get through the more stable the

line is, which also taking in the attenuation can lead to faster speeds.

"It's like a phone call with lots of noise in the background, you only might be

able to hear half of what the other party is saying, whereas with no noise you

can hear everything," says Mr Shaw.

Once these two calculations are done the fastest, the most stable speed at

which the signal can be sent from the exchange is decided. So, rather than

megabits being lost along the line - the "lost" megabits are never sent in the

first place.

Other much smaller factors that affect speed include using cheap hardware and

using phone extensions which introduce interference.

Broadly speaking, standard ADSL can work up to 5km from an exchange, says

Janusz Jezowicz, a director of BroadbandSpeedChecker. However, to get 8Mps you

would need to be located no further than 2km from an exchange.

Frustratingly, it is very difficult - if not impossible - to find out how far

you are from your closest exchange in terms of copper wire length, he adds.

This means it is hard to make an informed decision about what broadband package

it's worth buying. After all, who wants to fork out money for 8Mpbs if you can

only get broadband at 2Mbps.

"Distances apply to cable length and BT Wholesale does not publish how the

cables are laid out on our streets," says Mr Jezowicz.

"You can find out your distance from the exchange on various websites but it

will give you distance as the crow flies, not the actual cable length which in

most cases is much longer."