You can get VoIP to work quite well with a surprisingly low spec’d broadband service. The minimum you really need is 128/64 (ie 128kbs downstream and 64kbps upstream). The most popular codec is G.729 which uses 31.2 Kbps downstream and 8 Kbps upstream. So as long as you are not thrashing the internet at the same time as making a phone call, you should be fine.
You can get Quality of Service QoS software in your router that can prioritise the upstream leg, but it won’t have any impact on the downstream leg. The only way therefore to successfully be able to surf the net and make a VoIP call is to ensure you have enough downstream bandwidth to cope with both. You can read up more about QoS on the router page.
VoIP works just fine on ADSL 1 and ADSL 2. You will need a line filter between your PSTN wall outlet and the VoIP ATA if you are using a SPA-3000. You do not need a line filter between your ATA and your phone, nor a line filter between your ATA and your Modem.
If you want to split the phone line for 2 handsets or a fax machine, you should do this after you have been through the SPA. ie Wall=>ADSL Filter=>SPA=>fax/phone splitter.
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Both Telstra and Optus cable are great choices for both internet and VoIP.
Telstra cable requires you to login to bigpond with your bigpond username and password before you can use the service. If you don’t have a router, then this logon procedure is done by your PC when you boot up. When you install VoIP, you will need to purchase a router and configure the router to login rather than the PC. This is because there is no way that an ATA can login to a broadband service for you. If you have Bigpond, make sure you purchase a router that allows you to login to your ISP.
Optus Cable limits the downstream bandwidth when you exceed your monthly limit. If you are using the G.729 codec, you will still be able to successfully use VoIP when you have been limited by Optus, but you won’t be able to surf the net at the same time.