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1 .Explain the main differences between VoIP service and PSTN service.

VoIP stands for Voice over Internet Protocol while PSTN stands for Public Switched Telecommunications Network.

PSTN is the same as the Plain Old Telephone Systems but a worldwide wired telephone network while VOIP uses digital data. PSTN relies on circuit switching to connect one phone to the other and the call is routed through numerous switches operating on a local, regional national or international level. These means that it sets up a dedicated channel between two points for the duration of the call and is based on copper wires carrying analog voice data. VOIP on the other hand uses packet switching telephony and takes analog signals and turn them into digital data that can be transmitted over the internet.

So the other differences are:

• VoIP uses internet connectivity while PSTN uses a dedicated telephone line(s)
• VoIP requires about 10 kbps in each direction due to compression, while PSTN typically requires 64 Kbps in each direction.
• When it comes to pricing then you can get free VoIP TO VoIP calling both local and international except when calling to mobile and landlines. On the other hand there are no free calls when it comes to PSTN.
• Service terminates when internet connectivity is lost while PSTN service usually remains active during power outages.
• Emergency services are usually not provided by VoIP and untraceable while in PSTN emergency calling is enabled and traceable to location.
• Remote extensions are a standard feature with VoIP while with PTSN these are at an added cost.
• When it comes to upgrades, VoIP usually requires software upgrades and additional bandwidth compared to PTSN may involve several additions of hardware and adding lines.

2. Can we make calls between VoIP and PSTN subscribers? If yes, explain how?

With VoIP, you use an IP network, through an ADSL or other Internet connection, to make/receive phone calls to/from PSTN landline networks. You can even talk to people with a landline phone number through your computer, for example using Skype In and Skype Out service. PSTN has phone numbers and IP networks have IP addresses, this is through address translation. Your computer or VoIP device connected to the Internet has an IP address assigned to it. When you call a person on a PSTN landline phone, your call is handed over to the PSTN network through the person’s phone number, which you dial. And vice-versa.

In VoIP, every phone number has an IP address to which it maps. Each time a device (PC, IP phone, ATA etc.) engages in a VoIP call, its IP address is translated into the phone number, which is then handed over to the PSTN network. This is analogous to the way web addresses (domain names) and email addresses are mapped to IP addresses.

In other words, What makes phone calls like this work is an extra piece of equipment known as a gateway, which acts as a bridge between the Internet (on one hand) and the PSTN (on the other). You can think of a gateway as a kind of translator that converts telephone calls in IP-format into traditional signals that ordinary phones can understand (and vice versa). It’s also involved in call signaling, so when you dial a landline from a VoIP phone, the gateway converts the call-signaling data into a formaT that the PSTN can understand (and rings the landline the old-fashioned way).