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") ![](/images/getstarted-writetous-navbar-int_ZY7MOf.webp) ### Write to us Fill out our request form. Our experts will respond as soon as possible. [Write to us](/en/request-information "Write to us") ### S0 bus [Are S0 interfaces secure?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/s0-bus#section-3) [Why might companies want to keep their S0 interfaces?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/s0-bus#section-4) [Do S0 interfaces have a future?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/s0-bus#section-5) **What is an S0 interface?** An S0 interface (sometimes also called an S interface or S0 bus) allows traditional telephone users to connect to the [Integrated Services Digital Network (ISDN)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/isdn) and place calls over the [Public-Switched Telephone Network (PSTN)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). It is is similar to a [router](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/router) or modem that allows users to connect to the Internet, except that it provides connectivity into the traditional telephone network instead. Note: Even though ISDN is digital, it does not make use of the internet. ### Are S0 interfaces secure? S0 interfaces and ISDN were developed in the 1980s, when voice and data security were not as critically important as they are today. Neither of these technologies uses data encryption, which creates potential security vulnerabilities and increases the risk of data breaches. The lack of security with SO interfaces and ISDN are in stark contrast to [Voice-over-IP (VoIP)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) technology, which uses [advanced encryption methods](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/aes) to protect voice calls routed over the internet. ### Why might companies want to keep their S0 interfaces? If you are ready to switch to VoIP but still want to keep your existing ISDN technology, you can use a router with an S0 interface built in. This may help you extend returns on legacy telephony investments, but there are some known issues to be aware of, including poor or unreliable functioning of fax machines. ### Do S0 interfaces have a future? No, not really. With the internet able to [route voice calls](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/call-routing) worldwide, there is no longer any need for the PSTN or for ISDN technology that routes calls across it. As a result, ISDN is being decommissioned across Europe in favour of [all-IP telephony](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/internet-telephony) solutions and standards and, when that happens, SO interfaces will become redundant as well. The benefits of migrating from ISDN to all-IP include lower calling costs, simpler telephony services and reduced in-house hardware, software and support requirements. For more information about VoIP and telephony in the [cloud](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/cloud), enabled by technology such as the [NFON Cloud Telephony System](/en/products/cloudya), please refer to the articles below: ### Related Articles ### Voice over Internet Protocol (VoIP) PSTN stands for Public Switched Telephone Network and is generally known as the traditional phone network. It has been connecting people since the late 19th century, at first over copper telephone lines and later using fibre optic cables, satellites, and undersea cables. All of these are interconnected by switching centres which establish a dedicated connection – or circuit – between callers. Voice signals can then travel over this circuit. When the caller speaks into his handset, the caller’s phone translates the sound waves into electrical signals. These are transmitted over the PSTN (converted to optical signals for part of the way, so they can travel via fibre optic cables), and the phone at the other end of the line then translates the received signals back into sound. The arrival of **Voice over IP (VoIP)** technology – essentially, transmitting voice calls over the Internet rather than the PSTN – challenged traditional telephony and fundamentally changed how people communicate. 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