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Digital Enhanced Cordless Telecommunications
Application
The DECT standard fully specifies a means for a portable unit, such as a cordless telephone, to access a fixed telecoms network via radio. But, unlike the GSM standards, does not specify any internal aspects of the fixed network itself. Connectivity to the fixed network (which may be of many different kinds) is done through a base station or "Radio Fixed Part" to terminate the radio link, and a gateway to connect calls to the fixed network. In most cases the base station connection is to the public switched telephone network or telephone jack, although connectivity with newer technologies such as Voice over IP has become available. There are also other devices such as some baby monitors utilizing DECT, and in these devices there is no gateway functionality.
The DECT standard originally envisaged three major areas of application:[citation needed]
domestic cordless telephony, using a single base station to connect one or more handsets to the public telecoms network, which is now available
enterprise premises cordless PABXs and wireless LANs, using many base stations for coverage. Calls continue as users move between different coverage cells, through a mechanism called handover. Traffic both within the system and to the public telecoms network
public access, using large numbers of base stations to provide building or urban area coverage as part of a public telecoms network.
Of these, the domestic application (cordless home telephones) has been extremely successful. The enterprise PABX market had some success, and all the major PABX vendors have offered DECT access options. The public access application has not succeeded, since public cellular networks have fulfilled the need better. The one major application of DECT for public access, the Telecom Italia's FIDO network covering major cities in Italy, lasted about a year and was shut down in 1997.
DECT has also been used for Fixed Wireless Access as a substitute for copper pairs in the "last mile" in countries such as India and South Africa.[citation needed] By using directional antennas and sacrificing some traffic capacity, cell coverage could extend to over 10 km. In Europe, the power limit laid down for use of the DECT spectrum (250 mW peak) was expressed in ERP, rather than the more commonly-used EIRP, thus permitting the use of powerful directional antennas to produce much higher EIRP and hence long ranges.
VoIP/IP-DECT
In business, DECT has become an essential part of many PABX installations with manufacturer's proprietary methods of supporting PABX features over the DECT standard.[citation needed] Since the onset of the migration from TDM PBXs to VoIP and VoIP hybrid solutions, manufacturers such as Ascom Wireless Solutions, Aastra Technologies, Philips, Avaya, RTX Telecom, and Polycom have developed IP-DECT solutions where the backhaul from the base station is VoIP (H323 or SIP) while the handset loop is still DECT. These solutions are sometimes restricted by the cost of the base station but may be economic where the concentration of users is high.[citation needed] PBX networking vendors such as Cisco promote the adoption of WIFI-VoIP handsets as the replacement for DECT, but this imposes significant overhead on the design and complexity of the WIFI network in order to provide roaming, coverage and reservation of bandwidth, not to mention QOS.[citation needed] The question of Voice Mobility in commercial environments is still very open. DECT is robust but needs its own radio infrastructure. WIFI is deployed as an ad-hoc network environment. The new entrant into the field will be 3G-nano Cell technology where VoIP sessions would be supported as a private connection onto a 3G handset/PDA.
Features
Typical abilities of a domestic DECT Generic Access Profile (GAP) system include:
Multiple handsets to one base station and one phone line socket. This allows several cordless telephones to be placed around the house, all operating from the same telephone jack. Additional handsets usually have a battery charger station instead of a base station. The additional handsets do not require additional telephone sockets or additional transceivers
Interference-free wireless operation to around 100 metres outdoors. Operates clearly in common congested domestic radio traffic situations. For instance, generally immune to interference from Wi-Fi networks or video senders, Bluetooth technology, baby monitors and other wireless devices.
Ability to make internal (intercom) calls between handsets.
An extended range between the telephone and base (allowing greater physical distance between the two devices)
Extended battery talk-time, sometimes up to 24 hours[citation needed]
Technical details
Some DECT properties:[citation needed]
Audio codec: G.726
Net bit rate: 32 kbit/s
Frequency: 1880 MHz1900 MHz in Europe, 1900 MHz-1920 MHz in China, 1910 MHz-1930 MHz in Latin America and 1920 MHz1930 MHz in the US and Canada
Carriers: 10 (1,728 kHz spacing) in Europe, 5 (1,728 kHz spacing) in the US
Time slots: 2 x 12 (up and down stream)
Channel allocation: dynamic
Average transmission power: 10 mW (250 mW peak) in Europe, 4 mW (100 mW peak) in the US
The DECT physical layer uses:
Frequency division multiple access (FDMA),
Time division multiple access (TDMA) and
Time division duplex (TDD)
This means that the radio spectrum is divided into physical channels in two dimensions: frequency and time.
The maximum allowed power for portable equipment as well as base stations is 250 mW. A portable device radiates an average of about 10 mW during a call as it is only using one of 24 time slots to transmit.
The DECT media access control layer controls the physical layer and provides connection oriented, connectionless and broadcast services to the higher layers. It also provides encryption services with the DECT Standard Cipher (DSC). The encryption is fairly weak, using a 35-bit initialization vector and encrypting the voice stream with 64-bit encryption. There are reports that the security algorithm has been broken.
The DECT data link layer uses LAPC (Link Access Protocol Control), a specially designed variant of the ISDN data link protocol called LAPD. They are based on HDLC.
The DECT network layer always contains the following protocol entities:
Call Control (CC)
Mobility Management (MM)
Optionally it may also contain others:
Call Independent Supplementary Services (CISS)
Connection Oriented Message Service (COMS)
Connectionless Message Service (CLMS)
All these communicate through a Link Control Entity (LCE).
The call control protocol is derived from ISDN DSS1, which is a Q.931 derived protocol. Many DECT-specific changes have been made.[specify] The mobility management protocol includes many elements similar to the GSM protocol, but also includes elements unique to DECT.
Unlike the GSM protocol, the DECT network specifications do not define cross-linkages between the operation of the entities (eg Mobility Management and Call Control). The architecture presumes that such linkages will be designed into the interworking unit that connects the DECT access network to whatever mobility-enabled fixed network is involved. By keeping the entities separate, the handset is capable of responding to any combination of entity traffic, and this creates great flexibility in fixed nework design without breaking full interoperability.
DECT GAP is an interoperability profile for DECT. The intent is that two different products from different manufacturers that both conform not only to the DECT standard, but also to the GAP profile defined within the DECT standard, are able to interoperate for basic calling. The DECT standard includes full testing suites for GAP, and GAP products on the market from different manufacturers are in practice interoperable for the basic functions.
DECT for Data Networks
Other interoperability profiles exist in the DECT suite of standards, and in particular the DPRS (DECT Packet Radio Services) bring together a number of prior interoperability profiles for the use of DECT as a wireless LAN and wireless internet access service. With good range (up to 200 m indoors and 6 km using directional antennae outdoors), dedicated spectrum, high interference immunity, open interoperability and data speeds of around 500 kbit/s, DECT appeared at one time to be a superior alternative to Wi-Fi. The protocol capabilities built into the DECT networking protocol standards were particularly good at supporting fast roaming in the public space, between hotspots operated by competing but connected providers. The first DECT product to reach the market, Olivetti's Net3, was a wireless LAN, and German firms Dosch & Amand and Hoeft & Wessel built niche businesses on the supply of data transmission systems based on DECT.
However, the timing of the availability of DECT, in the mid 1990s, was too early to find wide application for wireless data outside niche industrial applications. Whilst contemporary providers of Wi-Fi struggled with the same issues, providers of DECT retreated to the more immediately lucrative market for cordless telephones. A key weakness was also the inaccessibility of the U.S. market, due to FCC spectrum restrictions at that time. By the time mass applications for wireless Internet had emerged, and the U.S. had opened up to DECT, well into the new century, the industry had moved far ahead in terms of performance and DECT's time as a wireless data transport was past.
Ironically, the failure of DECT as a data protocol became a strength when DECT 6.0 phones finally appeared in the U.S. in late 2005. By this time, the ISM bands had become crowded in the U.S., especially the 2.4 GHz band which is used by both the most common variants of Wi-Fi, 802.11b and 802.11g, and many cordless phones; thus interference between unlicensed devices has become common in these bands. However, because Wi-Fi does not operate in the UPCS band and DECT devices negotiate with each other for the available spectrum, not only are DECT 6.0 phones immune from this type of interference, their operation does not impair other nearby devices operating on the same frequency, which is a common issue with 2.4 GHz cordless phones.[citation needed]
Radio links
DECT operates in the 1880-1900 MHz band and defines ten channels from 1881.792 MHz to 1897.344 MHz with a band gap of 1728 kHz. Each base station frame provides 12 duplex speech channel with each time slot occupying any of channel. DECT operates in multicarrier/TDMA/TDD structure. DECT also provides Frequency-hopping spread spectrum over TDMA/TDD structure. If frequency hopping is avoided then each base station can provide up to 120 channels in the DECT spectrum before frequency reuse. Each time slot can be assigned to a different channel in order to exploit advantages of frequency hopping and avoid interference from other users in asynchronous fashion.[citation needed]
DECT 6.0
DECT devices made for use in the U.S. and Canada use the term DECT 6.0 to distinguish them from both DECT devices used elsewhere and U.S. cordless equipment operating in the 900 MHz, 2.4 GHz and 5.8 GHz ISM bands. It is a marketing term coined by Rick Krupka, Director of Cordless Products at Siemens, when he was pushing the FCC to accept DECT in the US and is not a spectrum band reference. The term "6.0 GHz" for DECT 6.0 phones is incorrect as they operate at 1.9 GHz, but the term DECT 1.9 might have confused customers, as they may solely interpret larger numbers as signifying a better (or later) product.
See also
Generic access profile (GAP)
GSM Interworking Profile (GIP)
IP-DECT
CorDECT
Unlicensed Personal Communications Services
Mobile phone radiation and health
Wireless local loop
Microcell
CT2 (DECT's predecessor)
WDECT
Net3
Cat-iq
Cordless telephone
References
General references:
Tuttlebee, Wally H.W. (1996). Cordless Telecommunications Worldwide. Springer. ISBN 978-3540199700.
Phillips, John A.; Mac Namee, Gerard (1998). Personal Wireless Communication with DECT and PWT. Artech. ISBN 978-0890068724.
Prof. Dr. W. Kowalk (2007-03-13). "Rechnernetze The DECT Standard". http://einstein.informatik.uni-oldenburg.de/rechnernetze/seite24.htm. Retrieved 2008-12-29. A summary of the DECT standard.
Specific references:
^ DECT Forum (2007-03). "DECT Operation and Evolution" (Microsoft Powerpoint (ZIP archive)). pp. 5. http://www.dect.org/userfiles/file/General/DECT Background/DECT General Information.zip. Retrieved 2009-06-01.
^ "Serious security vulnerabilities in DECT wireless telephony". Heise. 2008-12-29. http://www.heise-online.co.uk/security/25C3-Serious-security-vulnerabilities-in-DECT-wireless-telephony--/news/112326.
^ Schuler, Andreas; Tews, Erik; Weinmann, Ralf-Philipp (2008-12-29). deDECTed.org. https://dedected.org/cgi-bin/trac.cgi/attachment/wiki/25C3/talk-25c3.pdf?format=raw.
^ "Wireless LANs: developments in technology and standards". IEE Journal of Computing and Control Engineering. 1994-10. http://scitation.aip.org/vsearch/servlet/VerityServlet?KEY=CCEJEL&smode=strresults&sort=chron&maxdisp=25&threshold=0&possible1=flatman&possible1zone=article&OUTLOG=NO&viewabs=CCEJEL&key=DISPLAY&docID=1&page=1&chapter=0.
External links
Wikimedia Commons has media related to: DECT telephones
DECT Forum at dect.org
DECT information at ETSI
DECTWeb.com
DECT reaches a New Generation at dect.org
Project for an open source implementation of DECT
Categories: Local loop | Broadband | Mobile telecommunications standardsHidden categories: Articles needing additional references from March 2009 | All articles needing additional references | All articles with unsourced statements | Articles with unsourced statements from April 2009 | Articles needing more detailed references | Articles with unsourced statements from October 2008
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