DECT or Digital Enhanced Cordless Telecommunications (formerly
Digital European Cordless Telephone[citation needed]) is an
ETSI standard for digital portable phones (cordless home telephones),
commonly used for domestic or corporate purposes. DECT can also
be used for wireless broadband data transfers. DECT is recognised
by the ITU as fulfilling the IMT-2000 requirements and thus
qualifies as a 3G system. Within the IMT-2000 group of technologies,
DECT is referred to as IMT-FT (Frequency Time).
DECT was developed by ETSI but has since been adopted by many
countries all over the world. The original DECT frequency band
(1880 MHz–1900 MHz) is used in all countries in Europe.
Outside Europe, it is used in most of Asia, Australia and South
America. In the United States, the Federal Communications Commission
in 2005 changed channelization and licensing costs in a nearby
band (1920 MHz–1930 MHz, or 1.9 GHz), known as Unlicensed
Personal Communications Services (UPCS), allowing DECT devices
to be sold in the U.S. with only minimal changes. These channels
are reserved exclusively for voice communication applications
and therefore are less likely to experience interference from
other wireless devices such as baby monitors and wireless networks.
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:
* 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 has
also had huge 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.
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, some manufacturers such as Ericsson , Ascom 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. 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. 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.
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 nor 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
Technical details
Some DECT properties:
* Audio codec: G.726
* Net bit rate: 32 kbit/s
* Frequency: 1880 MHz–1900 MHz in Europe, 1900 MHz-1920
MHz in China, 1910 MHz-1930 MHz in Latin America and 1920 MHz–1930
MHz in the US
* 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 is the layer which 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 network
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[3]. 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 slow uptake 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.
Radio links using DECT
DECT operates in 1880-1900 MHz band and defines ten channel
from 1881.792 MHz to 1897.344 MHz with 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 Multiple
Access 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.
DECT 6.0
DECT devices made for use in the U.S. 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.
