home
	Content: • General • DECT technology properties • How a DECT system really works • What could you do with DECT • Links for more information General The Digital Enhanced Cordless Telecommunications (DECT) technology is standardised by the European Telecommunications Standards Institute (http://www.etsi.org). First terminals implementing DECT were available on the market as early as in 1992. Swissvoice, at that time Ascom Terminals, took very active role into the development of the extensive set of DECT standards and had one of the first working DECT prototypes. Throughout the time Swissvoice employees have been editors of a number of DECT standards and have chaired various DECT standardisation committees. The DECT technology is a general digital radio access technology. As a radio access technology its basic aim is to provide cordless access to a core network, PSTN, ISDN, IP or LAN. A DECT communication system comprises of at least 2 terminals: a Fixed Part (e.g. Base station) and a Portable Part (e.g. a Handset). Additionally, a range extender can be added to the system called Wireless Relay Station. The Fixed part is normally connected with a wire to a public telephone network (e.g. PSTN, ISDN, IP) or may be a part of another terminal (e.g. a PC). A Fixed Part may comprise one or more radio end points and hence cover a larger area and provide more capacity (usually such are the base stations used in large office environment, whereas base stations used in residential environment normally comprise of only one radio end point). The Portable Part is the terminal used by the end user to access cordlessly the services provided by the external network or application. As with the fixed part, the portable part could be a part from another terminal or device. Portable Parts may communicate with other telephones through connections provided via the DECT base station and the external network, or, with other portable parts via the base station only (known as internal calls and free of charge). Special DECT modes have been developed that allow direct communication between portable parts without the need of a Base station. When a WRS is involved, the communication is made between the Portable Part and the WRS and then relayed to the Fixed Part by the WRS. Currently one DECT Radio end point can handle up to 12 in parallel communication channels. Some DECT systems on the market, however can handle up to 6, that is due mainly to cost reduction issues. The access range in door is up to 50 m; larger areas can be covered depending on the walls and room space. In free areas, e.g. being out in your garden, handset may go as far as 300 m from the base station providing that there are no many walls in between. A WRS can double this range on the expense of the number communication channels it can handle in parallel (twice as less). More than one WRS can be added to a Base station. In most of the cases movement throughout the whole area covered by the FP and the WRSs can be done without interruption of the communication (called seamless handover). DECT Technology properties Like any other radio technology, DECT uses radio waves for communication and a frequency to transmit on. In Europe the 1880 to 1900 MHz band has been preferentially allocated to DECT and no other radio technologies are allowed to be implemented in this band. World wide, due to the fact that in some countries the band has been used by the military or has been already allocated to other technologies, parts of this or other bands have been allocated for DECT. Furthermore, because DECT has been approved by the ITU as one of the 5 members of the IMT-2000 family (http://www.imt-2000.org) -- the so called Third Generation (3G) family where for example UMTS is a member as well, additional frequency bands have been designated for possible DECT allocation; whether they will be allowed or not for DECT depends on the frequency allocation in each particular country. Generally speaking DECT is a FD (frequency division) TDMA (called time-division multiple access) / TDD (time-division duplex) technology. What all this means is that: &Mac183; a DECT radio channel is defined comprising part of the radio spectrum (1/10 from the band allowed in Europe - hence 10 separate DECT radio channel); &Mac183; over each radio channel, a time division is introduced called a "time slot" with 10 ms duration (this allows different pieces of information to be transmitted and distinguished one from another on one and the same radio channel); &Mac183; the time slots have been further numbered from 0 to 23 and tied in pairs allowing for multiple access to one and the same radio channel (this allows up to 12 calls to be carried out over a single radio channel, which multiplied by 10 available in Europe radio channels makes 120 in parallel calls possible to take place in a close location). To take advantage of these 120 channels available avoiding the necessity of frequency planning a special mechanism is introduced called Dynamic Channel Selection (DCS). The DCS is one of the distinctive DECT features that makes the technology perfect for areas with high user density allowing for a DECT system dynamically to find and utilise the slot (or set of slots) with the best quality. Before a voice or data information is to be transmitted over the air it needs to be digitally encoded. Different coding technologies may be used with various pros and cons. The main difference in the results from those different codings is that different transmission data rates that can be ensured. For voice and low data transmission each DECT time slot can carry information with around 32 kbps data rate. Other coding schemes, specially designed for multimedia traffic allow for data rates up to 6.9 Mbps or more per radio. An extensive set of procedures and techniques are defined to fight the possibility of fraud and prevent unauthorised users to take advantages of someone's else DECT system, among them: granting access rights, authentication, encryption, etc. How does a DECT system really work Of course for start what you will need is a pair of one DECT base station and at least one DECT portable part. If you want to talk with the external world you will need as well a public network connection/line, e.g. a PSTN or an ISDN, IP network can do as well too. If you want only internal connections, e.g. between 2 or more DECT portable parts you may skip the external network - you will need the DECT terminals only and the communication between them will be for free. First you have to connect the Base Station to the telephone line and to a power supply. As soon as your DECT base station is powered up it will measure all the available DECT radio channels and time slot pairs and establish a list with the best few (less interference - good quality). Then the FP will start transmitting short beams (signals) on the time slot found to be of best quality and will, on regular bases, update its list with best communication channels and if necessary may move its beam to another time slot. When could this happen? For example if another DECT base station, your neighbour's one, starts operation in the area - you need not worry, there 120 possible slots available. Each beam carries information about the capabilities of the Base station and will allow a Portable Part to find a Fixed Part. Beams are transmitted every 10 ms (milliseconds 1s = 1000 ms) for a duration of 0.052 ms which make all in total transmission for approximately 7.5 minutes within 24 hours. Now when you have your Base station working, you shall power up your Portable Part and start talking... Well, not exactly. In order a DECT portable part to operate with a DECT fixed part it is not enough that it is powered up and the FP beam is present - the Portable Part needs to be known to the FP (or in technical terms, the PP needs to be subscribed to the FP). Often, when you buy your DECT set the PP is already subscribed to the FP, however it needs not to be always the case and especially if you buy separately a DECT Handset. Let us assume here that your PP is not known to the FP. When such PP is powered up it may start searching the air for FP beams transmission into the allowed DECT frequency band. This activity of course is invisible for you and you will not notice it unless the phone manufacturer has decided to show some message on the display telling what is going on. When the PP finds one FP, the PP will synchronise to it - that is, to recognise when exactly valid information on the time slot starts and finishes. From now on the PP shall start reading the information carried by the beams and extract important details - this will allow the PP to establish whether it could try and request rights for communication through this FP. Normally the subscription process needs to be started by the User and how this should be done will be described in the User Manual. Usually, after pressing a number of keys on the PP the procedure for requesting access rights will be started. During this procedure important security information will be partially exchanged and more will be calculated. In order the procedure to succeed at least a common key sequence (something like a password) needs to be known by the FP and the PP - this normally will be provided by the user himself. After the procedure is completed successfully, the PP is known to that FP and visa versa. "Known" means that important identification information, as well as security keys known only by the two, will be stored into their memories. It may happen that the first base station found was not your own one but that of your neighbour. Not to worry - normally, in order to allow subscription base stations need to be prepared, perhaps by pushing a button, and secondly you remember they need to know that key (the "password"). If these two requirements are not satisfied the base station will simply reject the attempt of the Portable Part and the Portable part will start searching for another Base Station. All the procedure will be repeated until the right base station is found. Normally finding whether the base station allows subscription will be invisible to the user and will not require any intervention, however, if the procedure is started and then rejected because the key did not match it is very likely that the you will be indicated in one way or another (e.g. with a message on your display). Now when your Base Station and Portable part know each other they can start talking one with another. And you can make calls. In fact from now on the PP will listen continuously to that FP beam for two reasons: first to stay synchronised and be able to communicate, and second, if an incoming from the external network call comes to the FP, the FP will send special information on its beam to alert the PP that there is a call coming in. For all communications between a PP and FP, invisible to the user, the PP will measure all available communication time slot pairs and will establish its own best-channels list. Whenever a communication is needed, the PP will start transmitting on one of the best time slots - not on any one, but on the one on which exactly the FP will be listening at this time (information when and on which radio channel the FP listens will be retrieved from the FP regular beams). During this establishment phase the FP will check whether it knows the PP requesting connection and if it does not recognise the PP, connection will not be allowed. When the communication is established (i.e. the FP recognises the PP) before any important information is transmitted, normally the connection will be ciphered, that is to make it impossible for unwanted ears to spy on it. Be aware, ciphering is a feature that is not by default implemented in every DECT system however - if you are interested in this you shall consult your user manual or ask before you buy - all swissvoice terminals, see PSTN/DECT or ISDN/DECT, do provide this feature. After all this has been done, do not worry it takes again some milliseconds, now you can talk. And you can walk too - keep to the 50 - 300 meters, or your call will be dropped - most of the terminals will provide you with some indication if you are going too far - swissvoice terminals at least will do. If during communication the communication channel quality changes, these neighbours again, the PP will automatically switch to another slot pair - you will not notice. Ciphering is another mean for preventing pretending-to-be-known PPs from being capable of using your FP - hidden keys, often updated, are used which are known only to PPs and FPs that really know each other - if those keys do not match after the ciphering the two sides will not understand each other - the intruder will here just noise. What if your handset powers down - you forgot to charge it last week? Of course you need to recharge first. As soon as enough power is in your PP will start looking again for a Base station. Do not worry, you need not to repeat the subscription procedure. This time as soon as a Base station is found the PP, automatically, will perform the so-called location registration procedure (if supported) thereby announcing itself to the Base station. Now you can make and receive calls again. What could you do with DECT Now when you know how DECT technology works you may want to check out what could it be used for in or around your home. Here some ideas. You got that nice spacious house, finally. No more drama when you want to read that new book, your son plays guitar, your husband experiments with that new pasta recipe he got from his secretary, and your little daughter watches her favourite cartoon series on the TV - all these in one and the same living room ... now every one has its own room ... You have your study in the Summer house; your older son exercise on his guitar in the cellar of the main house, two floors above him is your husband's study, actually called kitchen; and your daughter watch those cartoons in the company of her best friend, Teddy the Big Bear... Life is beautiful isn't it ... until one of you wants to say something to any one else ... "Maaaaamyyyy!" shouts out of her window your daughter. "Whaaat?" you shout back from the Summerhouse ... then, your neighbour shout back "Cant youuuu be shut up pleeeease!" ... Well, there are other means for cordless communication, don't you know? One of them is called DECT! One DECT base station can handle few portable parts and you can talk one to each other without shouting - and all this free of charge. On top of this, if you are lucky with your network operator, and chose the right DECT phone -- check out the swissvoice ones PSTN/DECT or ISDN/DECT, you may be able for example to: see who is calling you before answering (the so called CLIP service - internal and external); forward a call from one handset to another, or during a call with your mother consult your husband (on his handset) whether you could visit her next Sunday; ... You can even send and receive SMS, yes, just like with those mobile phones. Actually, these are not all specific DECT features, what DECT provides you is the cordless, all around the house and garden, communication and provision of many nice services and features your network or service provider has on offer. If you want handsets from different manufacturers with the same base station you can have this with DECT. Before buying check whether they comply with the GAP standard (should be indicated on the box or in the user manual) which ensures that it all works. Be aware however, that many of the small nice non-DECT features may not work - the GAP standard does not cover non-DECT features unfortunately. You do not have such big house? Well, all this work in an apartment too. You want to call all your kids, and husband, and ... whoever else somewhere in the apartment for dinner? Just press a button on the Base station and all portable parts will start ringing (this is called Paging). You can use this as well if you do not remember where you left your DECT handset last night (well these are the challenges of the cordless things ...). You are visiting your neighbour friend and left your little son sleeping - you can put close by a DECT portable and listen from time to time on the second portable you took with you whether he is still sleeping ... and even he could call you back by pressing just a single key on the portable (the so called Baby phone feature) - Life gets easier, isn't? In fact, that is what DECT is all about ... You are one of those Internet addicts. You managed some how to live with the long, always in your way, cable that connects your PC to the telephone line, or your DSL modem. Now you got a Laptop and you were told that they were mobile, at least as long as their battery lasts... And you plan to go to internet from your bed ... And again cable ... Well, here DECT comes again. You have not forgotten it is a cordless access technology, have you? All you need is a DECT enabled Card for your Laptop and a DECT base station that can carry data. You are rights, you can do the same for your old PC - no cables in your way, unless you have started liking them ... The fact is that there are many other possibilities that the DECT technology opens for users, operators, service providers, developers and alike. Unfortunately this web site is too small to describe all of it. Links for more information If you need more information on the DECT technology here are some links ETSI Project DECT http://portal.etsi.org/dect/Summary.asp DECT Forum http://www.dectweb.com/DECTForum/Default.htm DECT web http://www.dectweb.com/ To find out more about the extensive swissvoice DECT products portfolio go to PSTN/DECT or ISDN/DECT.