PSTN - Public Switched Telephony Network / POTS - Plain Old Telephony Service
Both PSTN and POTS refer to the same technology. It is a basic pair of copper wires used to connect directly into standard telephones, fax machines and dial up modems. Almost every home around Australia has them, and quite a few businesses.
ISDN (BRI) - Integrated Services Digital Network (Basic Rate ISDN)
Basic rate ISDN (also known as OnRamp 2 or ISDN2 if you are purchasing from Telstra) is a service delivered via standard copper cable. What is referred to as an NTE (Network Terminal Equipment) or NTU (I forget which is the current one) is installed at the end of the copper line in your house, and it converts the signals to give two outputs allowing the connection of either two analogue telephones or a digital telephone system. Basic rate ISDN services are fairly common in small businesses but Telstra has recently dropped internet plans for use with certain ISDN services raising questions over their future plans to continue support the technology. In Australia, you can not use a BRI service with ADSL; if you wish to use ADSL, you will need to have an additional PSTN line installed or order an ADSL2 service from a provider such as iiNet who provide 'Naked DSL' which means you don't require an active PSTN style phone line for them to install the service.
ISDN (PRI) - Integrated Services Digital Network (Primary Rate ISDN)
Primary rate ISDN (also known as an OnRamp 10/20/30 if you are purchasing from Telstra) provides 10, 20 or 30 channels of voice communication. Depending on your carrier, you can have the channels increased in blocks of 5 once you have a minimum 10. This means you can have an ISDN 10, 15, 20, 25 or 30. PRI is terminated in an RJ-45 style connector and can be delivered by your chosen carrier using a fibre network or an SHDSL style internet service. In cases where PRI is delivered over SHDSL, an adapter (generally a box which requires external power and is the size of an ordinary ADSL modem) is installed at your premises which connects to the SHDSL network and then provides an output for ISDN. Companies such as PowerTel in Australia deliver these style of services.
Now, if you aren't already confused by all the different names for the same product, your mind might just blow apart at this point. When ordering hardware for you Asterisk system (or any other telephone system for that matter) an E1/T1 line is a PRI line. The E or the T says which type of ISDN you are using. This is determined by the country you live in. Basically, all PRI lines in Europe, India and Australia are E1 with 30 voice channels. If you live in North America or Japan, you will have a T1 with 24 channels. Both E1 and T1 systems use a set number of channels for voice or data, but then they also use what is called a 'D' channel. The 'D' channel transmits data to your phone system telling it what number was dialled by the caller, their identification, which channel the calls is on and a few other bits of information related to setting up the call.
Don't worry; if you're still reading this, you are still alive at this point. If you're new to this, there is a lot to take in. The following information will hopefully help you to cement the information above and also understand some more of the 'ins and outs of ISDN'.
ISDN vs PSTN - What Is The Difference?
Other than the number of channels, ISDN in Australia delivers quite a few advantages over PSTN. A significant one is that ISDN will support what is called Direct Inward Dialling or In Dials. You will probably have seen the referred to as DID's as well; this is just an abbreviation for Direct Inward Dialling. When you get your PRI connected, you will generally be given a 100 number DID range which you won't receive with PSTN. BRI is slightly different; for some reason you have to pay extra to get DID's. With PRI you can also add as many 100 number ranges to your service as you like. The advantage of having these numbers is you can then give staff a 'direct number' which when the system is programmed to do so, will allow external callers to bypass reception and ring directly on the desired extension.
ISDN also generally includes caller line identification (or Caller ID) free of charge. This is an extra cost option for PSTN services. Caller ID is good for integration with CRM systems and can now also be used to route incoming calls to different extensions.
In Australia, it is generally very expensive to make digital data calls via ISDN. If you are using the lines purely for data, such as accessing the internet, you may find it cheaper to stay with PSTN. ISDN does provide you with capacity to transfer data at 64 k/bits per second instead of the slower speeds on PSTN, but the cost may outweigh the benefit.
Which One Should I Choose?
When it comes to Asterisk, you should immediately discount BRI lines. The drivers and interface cards traditionally not been as good as PRI and don't forget that Telstra's pricing model generally makes BRI 'expensive' compared with other technologies. This leaves two choices: PSTN or PRI. The basic rule of thumb: once you have 6 PSTN lines, you are paying as much as you would need to for PRI but getting less functionality. In practice, you may be paying more for 4 or 5 PSTN lines than ISDN depending on what deals are available for primary rate ISDN when you look at signing up for it. Do be aware that most carriers will charge a connection fee for PRI and the cards in your phone system will need to be changed before your system will support PRI if you already have PSTN.
Line Interface Cards / Connecting Asterisk To The Real World
Digium make several different types of cards to connect your Asterisk server to a telephone line. The important thing to note is that there are several different variations of the same card. These variations are due to different mother boards and support for hardware echo cancellation. Always select a card with a hardware echo cancellation module. It will save you lots of frustration and will reduce echo on your phones calls to next to nothing. Also, you should check the Asterisk server and find out if the PCI slots on the motherboard support PCI-E or if they are the odler type PCI.