What do you do when your ADSL2+ connection has been working fine for weeks and then all of a sudden, when you need to make an online booking or transfer some bank funds, you find that the lights on your modem are flashing and that the status of your ADSL connection says anything but 'connected'? The natural tendency is for your blood to boil, anxiety to set in, and eventually to scramble to find the number for tech support. But before talking to someone on another continent about what is wrong with the connection in the next room, it's worth taking a few steps to resolve the problem yourself.
The first thing you should do when your ADSL modem's synchronisation is unsuccessful — that is, DSL light flashes and the PPP light (if you have one) is off — is to the switch off the router for a few minutes and then switch it back on. This is the first thing that tech support will tell you to do. If that doesn't resolve your problem, then you will need to check your phone line.
Pick up your phone and make sure there is no noise on the line. You should not be able to hear any white noise or crackling. If you do, then there is a problem with your line or your filters. Change filters, and make sure that you get ADSL2+ rated filters if you have an ADSL2+ connection. If the noise continues, you'll need to get your supplier to check your line.
If there is noise on the line but your modem still can't connect, proceed to an isolation test. Disconnect all the devices from the phone line and plug the modem directly into your wall socket. If your modem still isn't giving you any love, then you need to fiddle with its connection speed. Where you will find these speeds varies depending on the modem brand, but generally look in the Advanced section and then the DSL (or ADSL) section.
If your modem has all ADSL modes selected — G.DMT (ADSL), G992.3 or G.DMT.bis (ADSL2), or G992.5 (ADSL2+) and any other modes, such as Annex.M — try deselecting all of them except for G.DMT (or ADSL). This will only allow your modem to go up to a maximum speed of 8Mbps. In many cases, this solves the problem of connecting to your ISP — the modem will be able to synchronise with your ISP — but you will not be able to connect at speeds faster than 8Mbps.
You will probably have to give your ISP a call in this situation anyway, and see if they can offer a solution for your speed woes, but at least you will be able to get online in the interim.