How broadband works: Difference between revisions

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==Frequency==

Normal telephone service is an analogue of sound waves. It carries sound using electrical signals that work in the same way as they do though air. In terms of frequency this means that telephone calls use very low frequency signals on the telephone line.

Normal telephone service is an analogue of sound waves. It carries sound using electrical signals that work in the same way as they do though air. In terms of frequency this means that telephone calls use very low frequency signals on the telephone line. This is called baseband.

The line itself can carry signals at a much higher frequency. The limiting factor is the quality and length of the cable used. Normal telephone lines are a copper pair (two insulated copper conductors twisted around each other) that can go for several miles from the local telephone exchange to your premises.

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==ADSL1==

ADSL has been defined for use alongside normal phone service as well as alongside ISDN. The different frequency for ISDN means it is a different standard. In the UK we use ADSL over POTS (Plain Ordinary Telephone Service) following ITU G.922.1 Annex A which allows in theory up to 12Mbit/s downstream and 1.3Mbit/s upstream. However BT offer only 8.128Mbit/s downstream and 832kbit/s upstream maximum using ADSL1.

In the early 2000s, ADSL has been defined for use alongside normal phone service as well as alongside ISDN. The different frequency for ISDN means it is a different standard. In the UK we use ADSL over POTS (Plain Ordinary Telephone Service) following ITU G.922.1 Annex A which allows in theory up to 12Mbit/s downstream and 1.3Mbit/s upstream. However BT offer only 8.128Mbit/s downstream and 832kbit/s upstream maximum using ADSL1.

It is important to realise that the above is a technical statement about the ADSL1 technology. An actual service will achieve a sync speed (which includes various overheads) depending on the line length and quality and other factors, and may even change over time. To assess the likely speed of your service, please use the availability checker.

==ADSL2+==

A new standard for ADSL called ADSL2+ ~~provides~~ extended bandwidth. This follows ITU G.992.5 and provides up to 24Mbit/s downstream and 1Mbit/s upstream.

In the late 2000s, A new standard for ADSL called ADSL2+ provided extended bandwidth. This follows ITU G.992.5 and provides up to 24Mbit/s downstream and 1Mbit/s upstream.

A variation of the ADSL2+ specification called Annex M allows up to 24Mbit/s downstream and up to 3.5Mbit/s upstream. In the UK we cannot achieve the full 3.5Mbit/s upstream as there is a frequency plan that must be followed on all phone lines to avoid interference. Therefore, in the UK, Annex M allows around 2Mbit/s uplink.

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==VDSL==

In the 2010s, Openreach started to offer VDSL

Fibre To The Cabinet (FTTC) operates using VDSL from a street cabinet rather than ADSL all the way from the exchange. VDSL uses different frequencies and powers to ADSL but is otherwise very similar technology. VDSL can provide speeds over 100Mbit/s on very short lines. The speed available drops off quickly with distance - but this is not usually an issue as cabinets are usually close to premises. There are cases where FTTC can be slower than ADSL all of the way from the exchange.

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==Fibre==

In ~~some~~ ~~cases~~ fibre optic cable (glass) can be used to provide a service to a customer premises. Fibre is usually a single frequency using a laser, so not actually broadband at all~~. Whilst speeds to change with distance on fibre services, they are so high that services are sold with specific speeds which do not vary~~. Various services are available, see each service description for the speeds that are offered.

In the 2020s, Openreach and many Altnets are deploying fibre optic cable (glass) can be used to provide a service to a customer premises, generally using PON (Passive Optical Network) technology, Fibre is usually a single frequency using a laser, so not actually broadband at all. Various services are available, see each service description for the speeds that are offered, typically between 80Mbit/s and 1000Mbit/s.

The Openreach FTTP rollout is progressing, with 25 million premises (80%+ of the UK) expected by December 2026

[https://media.ccc.de/v/emf2024-305-the-last-miles-of-the-internet James Harrison provided an excellent talk PON/FTTP at EMF 2024]

Most services sold by other companies as fibre are not, in fact, fibre optic services to your premises. All Internet access services, even ADSL from the exchange, make use of fibre optic cable for part of the service, but what matters for speed and reliability is the weakest link, which is normally the bit to your home.

Some companies have previously branded coax based services as Fibre, which can muddy the water a little.

==Upgrade to 21CN==

Existing lines on 20CN will be upgraded to 21CN and so allow ADSL2+. This is part of an ongoing programme. The upgrade may be some months after ADSL2+ is available on the exchange, and so customers can pay a small fee to upgrade sooner if the exchange is ready. Upgrades as part of our upgrade programme are free of charge and we email you several weeks in advance with details. When we upgrade we initially keep your line on ADSL1 on the new 21CN kit, and then change to ADSL2+ a few days later.

==Beyond the exchange==