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[+44 330 383 8000](tel:+443303838000 "+44 330 383 8000") ![](/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](/gb/request-information "Write to us") ### SDSL / SHDSL [How is SDSL/SHDSL different to normal internet connections?](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdsl-shdsl#section-3) [What is SDSL used for?](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/sdsl-shdsl#section-4) # SDSL / SHDSL **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/290788/46969bd118/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](/gb/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)](/gb/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](/gb/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) The abbreviation FMC stands for the technical phrase Fixed Mobile Convergence and describes the integration of mobile and [fixed-line](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/the-fixed-line-network) communication services within a complete solution. Whilst mobile and fixed-line telephone networks were technically entirely separate until just a few years ago, nowadays they are increasingly merging. It is therefore possible today to reach a modern FMC solution both on a mobile basis with its landline [number](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/national-international-phone-numbers), and on a fixed-line basis using the mobile number. Furthermore, the use of the telephone service is entirely device-independent. Companies that use FMC solutions are therefore more reachable and thus also benefit from the increased efficiency of their personnel. Additionally, costs are also minimised because the most economical type of connection at the respective time can be selected automatically. Mobile devices such as the smartphone or PDA therefore become valuable extension units, also when carried around the office in the trouser pocket. When the user returns to the office from a business trip, their smartphone books them into the internal company network automatically via WLAN. Without the user even perceiving this action they then use the more economical telephone service of the office system and avoid costly [mobile network connections](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/mobile-phone-network). If you are in the GSM network during an ongoing call with the cell phone (FMC) and switch to a WLAN network that is already defined as an access point on the cell phone, the FMC client automatically changes the connection type from GSM to WLAN (seamless handover). Conversely, this is the same. If you are in the WLAN network with your mobile phone during an ongoing call and leave it, there is an automatic handover from the WLAN to the GSM network. The handover is signaled acoustically on the mobile phone, and there may be a brief interruption. As a rule, the caller does not notice anything about the handover. The FMC client can be used with different SIM cards thanks to the "SIM change" function. This is particularly useful when commuting frequently between different countries. Someone who lives in Austria and works in Germany, for example, can use SIM cards for both countries (German providers such as Vodafone and Austrian providers such as Orange) with the FMC client. If you insert a corresponding SIM card into the mobile phone, for which the GSM number has previously been stored in the telephone system, you only have to select "SIM change" in the configuration menu of the FMC client and the FMC client will connect to it automatically the telephone system. While FMC solutions were primarily tailored to consumer households or small office environments due to the WiFi connectivity, small and medium-sized companies will also adapt their network environment in the future and benefit from the advantages of the flexible and resource-saving solutions. The FMC solution from NFON not only enables the office number to be used on WiFi, but also on the move in the GSM network if WiFi is not available. A connection to the telephone system is established simply by making a call. This brings with it several advantages. On the one hand, you can be reached on the go via the landline number, but you can also signal the office number from your cell phone. This means that you only communicate with one phone number and can be reached from anywhere under this number (one-number concept). If you want to save yourself the landline call to connect to the telephone system while on the go, the callback function can be used. You simply tell the telephone system by pressing a button that it should call you back. The system then calls back and a connection to the destination number is established. Domestically, this function is used to shift the costs from the mobile phone user to the company, e.g. B. the employee uses a private mobile phone tariff. When using the callback function, costs arise exclusively on the part of the telephone system, which leads to a clear cost separation. This function can also be useful abroad. As a rule, roaming charges for outgoing calls (outbound) are more expensive than roaming charges for incoming calls (inbound). If this is the case, you can simply use callback to tell the telephone system abroad that it should call you back. This means that there are no expensive outbound roaming costs and you save money. In addition, you can also save money on expensive forwarding from the telephone system to the cell phone: If you leave the office in the evening and activate fixed call forwarding to your private cell phone, all calls to the office number can be forwarded to the cell phone, which means expensive forwarding costs on the part of the telephone system develop. Anyone who uses the NFON FMC solution on their cell phone saves these costs by simply redirecting to the FMC extension and using the reverse callback function. The reverse callback function avoids expensive forwarding fees. When a call comes in, you first get a call from the telephone system and immediately afterwards a connection is established from the mobile phone to the telephone system – the telephone system then merges the connection from the FMC client and the calling connection. No difference to any other connection is noticeable for the call participants. However, it could be that the call setup takes a few seconds. In the case of a landline flat rate on the mobile phone, there are no additional costs. The prerequisite for using the reverse callback function is a data package in the mobile phone contract and the availability of the data network (GPRS, UMTS, etc.) during an incoming call. [Fixed Mobile Convergence (FMC)](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/fmc) ### Voice over Internet Protocol (VoIP) **What is VoIP?** The **Voice over Internet Protocol,** also called IP telephony, is a method of communication for making calls over a broadband Internet connection as an alternative to regular calls made over an analogue phone using the [PSTN](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/pstn). These VoIP calls can happen entirely over the Internet but where required, gateways also link to the PSTN. This means that VoIP users can still connect to anyone with a telephone number including local, mobile, and international numbers. VoIP services can be accessed from a computer with a so-called [softphone,](/gb/get-started/cloud-telephony/lexicon/knowledge-base-detail/softphones) from an IP phone, from a smartphone or from a conventional landline phone that is connected to a VoIP adapter. 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