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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") ### SDSL / SHDSL [How is SDSL/SHDSL different to normal internet connections?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdsl-shdsl#section-3) [What is SDSL used for?](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdsl-shdsl#section-4) What is SDSL and SHDSL? The terms Symmetric Digital Subscriber Line (SDSL) and Single-Pair High-Speed Digital Subscriber Line (SHDSL) mean the same thing. They refer to a specific type of internet connection where the same bandwidth is given to both uploads and downloads. Note: The two different names exist because two separate organisations introduced their own standards for this technology. The SDSL standard was approved by the European Telecommunications Standards Institute (ETSI), and SDSL was approved by the International Telecommunication Union (ITU). ### sdsl ![](https://a.storyblok.com/f/290788/a5db81b457/sdsl_shdsl.jpg) ### How is SDSL/SHDSL different to normal internet connections? When we talk about standard internet connections, we are usually talking about Digital Subscriber Line (DSL) connections. DSL allows data to be transferred over existing wired telephone lines and is allocated to higher frequency bands to avoid conflicts with data from analog calls. The term DSL most commonly refers to Asynchronous Digital Subscriber Line ([ADSL](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/adsl)) technology. In this type of connection, more bandwidth is allocated for downloads from the internet service provider to the user, making downloads quicker than uploads. ADSL works like this because most users download more than they upload. In fact most common activities online, including web browsing and media streaming, only require a downstream data flow, making ADSL ideal for these use cases. SDSL is different from ADSL in that bandwidth is split evenly between uploading and downloading flows. ### What is SDSL used for? SDSL is used for applications where data has to flow quickly in both directions. A classic use case is [Voice-over-IP (VoIP)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/voice-over-ip) telephony. Here, large amounts of data are being constantly downloaded and uploaded as the callers speak to one another. Assuming that both callers talk the same amount, the same amount of data needs to be sent and received. By providing equal speeds for data uploads and downloads, and with connection speeds that are typically much faster than ADSL, SDSL connections can greatly improve the quality of VoIP calls. This technology is used in the leading VoIP solutions, including the [NFON Cloud Telephone System](/en/products/cloudya). To find out more about analog and VoIP telephone technologies and to understand the capabilities of the NFON Cloud Telephone System, please review the articles below: ### Related Articles ### Fixed Mobile Convergence (FMC) [Fixed Mobile Convergence (FMC)](/en/get-started/cloud-telephony/lexicon/knowledge-base-detail/fmc) ### 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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