Here are answers to questions which people often ask about broadband internet access:


What is ADSL? Go to top of page

ADSL (Asymmetric Digital Subscriber Line) is always-on broadband service delivered over a standard BT PSTN analogue line. ADSL uses different frequencies on the line which means you can make phone calls and use the Internet at the same time.

It is Asymmetric because the download speed is different from the upload speed.

BT supplies the service via its telephone exchanges but wholesales its service via Internet Service Providers (ISPs) who package the service in different ways or offer additional services to the end user.

What speeds does ADSL run at? Go to top of page

ADSL based on BT lines or other local loop telephone providers:

ADSL available depends on the BT exchange you are connected to and how far you are from it (length of the phone cable - not necessarily as the crow flies). BT exchanges generally provide a service UP TO 8Mbps download speed. The achievable speed depends on these and other factors. The exchange and router will try to find the best speed to achieve a reliable connection. This is referred to as "rate-adaptive". Upload speed is also "rate-adapted" and can be UP TO 448Kbps (Max products) or 832Kbps (Max Premium products).

Most BT exchanges have been updated to 21CN now and can provide a service UP TO 20Mbps but the achievable speed still depends on distance form the exchange and quality of lines and connections. if you were having problems on the old style exchange you won't necessarily get a better performance from an updated exchange.

It is possible to provide higher speeds download and upload by aggregating traffic across two or more ADSL connection using managed bonding.

ADSL circuits are shared services and are subject to a performance loss when many users are busy. There are various load-balancing

Cable services:

Cable TV providers - in most of the UK now only Virgin Media (who swallowed up NTL and Telewest) - offer broadband through the same cable which is installed to run the TV. This is usually a pretty stable service with speeds available up to 50Mbps. You don't have to have a BT phone line for this service and in fact they offer a phone service bundled with the broadband. So if you don't have a BT line already or don't need or use one very much it is a good choice.  However, check the phone tariffs available - they are not the same as BT tariffs and may not be as advantageous depending on who you phone and when.

What is 21CN?  Go to top of page

21CN is the name given to BT's next generation network programme, under which it is creating an IP-based national fibre network to enable communication in a range of formats and support the demand for voice and data convergence. One of the most significant benefits of the network is the flexibility of choice and control that customers will gain in the way they use the connectivity services made available. 21CN will enable you to provide faster and more stable broadband at ADSL2+ speeds as well as Quality of Service (QoS) options for customers who wish to be able to prioritise traffic.

BT is enabling its exchanges for 21CN on a roll-out programme starting June 2008. Not all new services will be available right away from all exchanges. Existing ADSL customers shouldn't notice any difference to service.

Note that those who have been migrated to 21CN exchanges are not automatically upgraded to ADSL2+. They shouldn't notice any difference and indeed most customers have not been aware of the change in technology providing their broadband.

What is ADSL2+ Go to top of page

ADSL2+ is an enhanced version of ADSL. Its principal characteristics are increased performance in terms of improved stability and data transfer speeds.

While ADSL provides speeds of 'up to 8Mbps', ADSL2+ is capable of delivering speeds of 'up to 24Mbps'. Furthermore, while ADSL provides a maximum upstream speed of 448Kbps (832Kbps in the case of ADSL MAX Premium), ADSL2+ allows an uncapped upstream experience, potentially up to 1.4Mbps.

However, like traditional ADSL, it should be noted that ADSL2+ is a rate adaptive product whose speed performance is dependent on factors such as the distance of customer premises from the exchange and the quality of the line connecting them to the exchange. Customers currently experiencing speeds of over 4Mbps on ADSL MAX should see an increase in speed. This isn't going to help customers who are located far from exchanges. It will offer better facilities for the lucky ones who get reasonable speeds already. Also only a few exchanges are ADSL2+ enabled at the moment. It's going to widen the gap even further between the broadband haves and have-nots.

It will provide additional features such as the facility to prioritise types of traffic on a QoS (Quality of Service) basis which is going to be more and more important with the convergence of voice and data and the availability of low-cost VOIP systems.

ADSL2+ services, under the banner of 2Plus Broadband, are available from ENTAnet via resellers like Realcom.

What is Annex M?  Go to top of page

Annex m is a service available only on ADSL2+ which allows the subscriber to increase the upstream speed of their connection by trading some of their download speed. However, it will only work successfully where there is a good line and close to the exchange. Could be usefull where you are using broadband to upload data to websites, off-site back-up or are working remotely from a central office. See more from ENTAnet.

What is a contention ratio? Go to top of page

Contention is basically sharing. With ADSL you are sharing the resource with a number of other users. Most of the time the sharers are not using it at the same time but in theory you don't actually get all the bandwidth.

To buy internet access with no contention - effectively a leased line - costs several thousand pounds a year.

Contention ratio refers to the maximum number of users you could be sharing your bandwidth with. There are 2 contention ratios in general use: 50:1 (home) and 20:1 (business / office / premium). If you have a 50:1 contention ratio then in theory another 49 users could be competing for the 2Mb bandwidth you have. In practice this very rarely happens and even on the slowest ADSL product users typically see a 10x improvement over a 56k modem.

However, with the new Max services contention is more complex but also less definite. Contention for service occurs at the BT exchanges and various points in their network and also at the ISP. Different ISPs may implement contention ratios in different ways. In general, remember your ADSL connection is a shared service and the performance may go up and down depending on how busy the various connections in the service are.

Why is the download speed greater than the upload speed? Go to top of page

ADSL is Asymmetric which means it runs at different speeds for upload and download.

There is also a service called SDSL (Symmetric Digital Subscriber Line) which runs at the same speed for both upload and download with a contention ration of 10:1. (If you are interested in this service please send an email with your post code and we will check availability)

What is ALT? Go to top of page

ALT (Anti-Loss Tool) is an traffic management tool that was created by ENTAnet to maximise performance for all users and to ensure latency and packet loss is kept to a minimum at times of stress on the network. Read more about how this works in ENTAnet's Customer FAQ document >>

What type of phone line is required for ADSL service? Go to top of page

A standard BT PSTN analogue line is required for ADSL. Certain BT services and/or configurations are NOT compatible with ADSL, these include:-

  • PBX Switchboard
  • ISDN 30(PRI)
  • Surftime
  • BT Data Stream
  • BT Video Stream
  • Non-BT Telephone Line e.g supplied by cable co.
  • Certain Redcare Alarm Monitoring - although mostly OK now
  • Pulse Metered Line
  • Coin/Pay Phone

It is possible to convert a ISDN 2/2e, Business Highway and Home Highway service into ADSL. This does however mean that the ISDN service is lost as the line has to be converted back to a single analogue line. Most ISDN/Highway circuits have 3 phone numbers attached, please note that the 2 additional numbers that came with the ISDN/Highway will be lost. This is a consideration if any of the additional numbers are in use and well known (i.e. Fax Number). more info>>

Is ADSL available everywhere? Go to top of page

ADSL is available in nearly all of BT Exchanges. However, the distance from the exchange must be less than a certain figure and quality of wiring and connections can affect connectivity and speed. Email us with your post code and if possible the telephone number you wish to use (see stipulations above) and we will check it out for you, or try the availability checker at Samknows.

If ADSL is not available at your location there are other options for broadband e.g. cable modem from NTL or Telewest and satellite but these services are not always suitable for business use.

How can I find out about my local BT exchange? Go to top of page

There are nearly 5600 BT exchanges in the UK, each in a different phase of upgrade and with different facilities. Nearly all can provide basic ADSL, some host equipment for other ISPs, some have already been upgraded to 21CN. To check out your exchange try Samknows exchange search.

What is NAT/No-NAT? Go to top of page

NAT (Network Address Translation) is a technology that allows you to share a single IP address assigned by a ISP among a number of computers or pieces of apparatus (e.g. printers/cameras ) on a LAN. Most ADSL hardware supports NAT out of the box.

NO-NAT means that instead of having a single IP address to share you get a subnet of 8 or 16 IP's that are routable and can be assigned to your computers. Please note that because a subnet is assigned it means you do not have access to all the 8 or 16 IP addresses. On a 8 IP connection 5 IP addresses are available to you and on a 16 IP connection 13 IP addresses are available. This is because a IP is reserved for the network address, broadcast address and router respectively.

Do I get a fixed IP address? Go to top of page

All ENTAnet ADSL accounts come with a fixed IP address on both NAT and No-NAT. There are low-cost options for additional IP addresses. Some ISPs don't offer this - so if you need it e.g. for VPN access then make sure you ask before placing the order.

A fixed IP address is required if you are using ADSL for inter-branch communications, if you have home-workers who need to access your company systems, VOIP and is useful for remote troubleshooting and support.

What equipment to I need? Go to top of page

Apart from a BT analogue telephone line you will need an ADSL router if you wish to connect a server or several computers on a LAN to the internet or an ADSL modem - usually a USB connection - if you just have a single PC to connect. Routers come complete with a small switch for connecting other PCs and printers in the office with Cat5e cables and a wireless hub which will connect PCs, printers and lap-tops with compatible wireless interface cards.

Zyxell p660hwRouters provide better performance than USB modems and don't impact on performance of the PC. A Router also includes a firewall.

For example, Zyxel 660HW

  • Embedded Firewall
  • Up to 100M Wireless
  • NAT and DHCP
  • Supports PPPoE, PPPoA and RFC1483
  • 4-Port Ethernet 10/100Mbps Switch
  • Easy web-based configuration and management

What is a microfilter? Go to top of page

microfilterA microfilter is an adapter that plugs into the telephone socket the ADSL service is provided on. The microfilter separates the ADSL signal from the voice signal. You plug your phone and your ADSL hardware into the microfilter and the microfilter into the BT phone socket.

SplitterAn alternative is a "splitter" which doesn't have a "tail" but this can be awkward to fit if the phone socket is close to the floor.

Note: if you have more than one BT extension you must put a microfilter or splitter on each one and run the phone from that even if you aren't running a data device from that extension. ADSL will not work if you don't do this.

I used to turn off the computer Go to top of page

We have just got broadband. When we have used our computer in the past, we have always closed it down and then turned it off at the wall. My husband says that now we are on broadband and a router, we can close it down, but not turn it off at the wall. Is this correct?

Your husband is talking about the communications connection, i.e. modem & phone socket, rather than the power. When you used a dial-up internet connection you may have wanted to disconnect the modem from the phone socket when you closed down your computer, especially if you were using a single phone line for internet and for voice. Then you could use the line for ordinary telephone and also you can be confident that the internet connection has finished and you are not being billed for time you were not really using.

With broadband you get two connections on the same phone line - one for ordinary telephone and the other for the internet connection. You don't get charged for the time you use on the internet - just for the connection (although some service providers charge if you do a large amount of downloading). The router takes care of the connection and you don't have to do anything when you have finished your internet session or close down your computer. Actually, it is not a good thing to turn the power off your router because it will take a few minutes to "find" the connection again when powered on again. When you have your router set up correctly just leave it powered on. It won't matter if you turn the power off your computer - when you turn it on again it will connect to your router and thence to the internet automatically.

If you have broadband from your television company - e.g. NTL / Virgin - you will have a different kind of router but the story is the same - just leave the router powered on and it will look after your internet connection for you.

I have an ISDN2 line. How do I get ADSL? Go to top of page

ADSL works over PSTN (analogue) lines. Your ISDN2 line must be converted back to PSTN for ADSL to be installed.

An ISDN line will normally have 2-3 line numbers allocated to it. ADSL runs over a PSTN line which only has one number associated with it. You will only retain the line number that is specified in the ADSL order, the other numbers will be lost. Bear this in mind if you need to have a separate number for incoming faxes.

It is necessary for a BT engineer to visit to remove the ISDN and convert this to a PSTN, ADSL will then be put on the PSTN line the same day (unless circumstances dictate otherwise)

The lead time for an ISDN conversion is two weeks approx, BT will charge the a nominal fee on your BT phone bill for the conversion work.

If the you miss the BT appointment or turn the engineer away BT may make a charge.

What is IPv6?

Since 1984 IPv4 (Internet Protocol version 4) addresses which consist of four groupings of numbers (e.g. have been used to access the Internet. Twenty five years on and unsurprisingly the 4.3 billion addresses originally available with that scheme of numbering are now running out. A new scheme is being implemented and rolled out now alongside the old. IPv6 as it is known uses hexadecimal format and more numbers and will keep the world going for a while longer.

Entanet's network is enabled to support IPv6. However, their DSL, colocation and leased line services are set to use IPv4 as default to ensure that customers' setup is correct prior to adopting IPv6, for example in terms of having IPv6 supporting hardware and firewalls in place. If ENTAnet customers would like to upgrade to IPv6 you can do so easily and for no extra charge by emailing

Want to know more about ADSL & broadband? Go to top of page

Visit the ADSL Guide "How it works"

Also check out and


If you have any further questions or would like to discuss your requirements and internet access options please telephone our office on the number below or email

Contact Realcom Applications:   email -  telephone - 01530 440000
Froggarts Cottage, School Lane, Coleorton, Leicestershire, LE67 8HT

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