The DECT Standard
The DECT Standard
Introduction to DECT standardisation
The members of the European Telecommunications Standards Institute (ETSI)
have developed the DECT standard. In ETSI Sub Technical Committee Radio
Equipment and Systems 03 (STC RES-03), European telecommunications
equipment manufacturers, system operators and regulators work together on
the definition and evolution of the DECT standards.
In addition to ETSI, several other bodies are involved in the DECT
standardisation process.
The Commission of the European Community provides considerable support by
providing the legislation needed to establish (in conjunction with CEPT ERC) a
common frequency allocation and (in conjunction with the ACTE committee) by
enabling European wide harmonisation of the regulatory environment for DECT
products.
After the first edition of the DECT standard was available in 1992, the DECT
standardisation work concentrated on the definition of the Generic Access
Profile (GAP) and other interworking profiles (DECT/GSM, DECT/ISDN,
DECT/Radio Local Loop, CTM and several data profiles). This work and
additional demands from the DECT market initiated several extensions and
enhancements to the base standard enabling even more effective application
of DECT products which led to the 2nd edition of the base standard being
finalised by the end of 1995. Some examples of this are:
inclusion of emergency call procedures to aid acceptance of DECT for public
access applications,
definition of the Wireless Relay Station (WRS) as a new system component
to enable more cost efficient infrastructures and
description of the optional direct portable to portable communication feature
for DECT.
The DECT common interface standard has a layered structure and is contained
in ETS 300 175, Parts 1 to 8 [1] to [8]. It is a comprehensive set of
requirements, protocols and messages providing implementers with the ability
to create network access profiles (protocol subsets) to be able to access
virtually any type of telecommunications network.
To stimulate interoperability between DECT equipment from different
manufacturers ETSI members started to work on the definition of standard
interworking profiles by the end of 1993. The Generic Access Pro-file GAP [9]
was the first profile, completed in 1994. It contains the protocol subset
required for the basic telephony service in residential cordless telephones,
business wireless PABX, and public access applications; it provides the basis
for all other DECT speech profiles. Interoperability testing for GAP has been
finished successfully.
Part Title
Description
1
Overview
General introduction to the other parts of ETS 300 175
2
Physical layer Radio requirements of DECT, e.g. carrier frequency
allocation, modulation method, transmission frame
structure, transmitted power limits, spurious emission
requirements etc.
3
Medium
Description of procedures, messages, and protocols for
Access
radio resource man-cess Control agement i.e. link set-up,
Control Layer channel selection, handover, link release and link layer
quality maintenance etc.
4
Data Link
Description of provisions to secure a reliable data link to
the network layer Control layer
5
Network
layer
Description of the signalling layer with call control and
mobility management functions and protocols.
6
Identities and Description of the portable and fixed part identities
Adressing
requirements for all Addressing DECT application
environments.
7
Security
aspects
Procedures to prevent eavesdropping, unauthorised
access and fraudulent use. pects
8
Telephony
Telephony requirements for systems supporting the 3.1
kHz speech service to ensure proper interworking with
public telecommunications networks. De-fines
transmission levels, loudness ratings, sidetone levels,
frequency re-sponse, echo control requirements etc.
Table 1. Parts 1 to 8 of the DECT CI standard ETS 300 175
Basic Operating Principles
The principles as applied in the DECT standard have been designed to meet the
following objectives:
high capacity cellular structured network access
allowing for network wide mobility
Flexible and powerful identities and addressing
high spectrum efficiency
reliable - high quality and secure - radio access
robustness even in hostile radio environments
speech transmission quality comparable to the wired telephony service
enabling cost efficient implementations of system components
allowing for implementation of a wide variety of terminals like e.g. small
pocketable handsets
flexibility towards varying bandwidth needs (which is bandwidth on demand
e.g. for ISDN and data applications)
Furthermore, the standard reflects a high degree of flexibility in the protocols
to enable future extension.
Mobility
Although network-wide mobility is outside the scope of the DECT standard, the
mobility functions in the DECT standard provide the ability to access the
mobility capabilities of telecommunications networks through a (multi) cellular
infrastructure giving tremendous flexibility to users roaming across their
residence or business site.
Wireless users with authorised access to the network (subscribed users) can
make and receive calls at any location covered by the DECT infrastructure (if
the infrastructure supports mobility) and move around in this area even when
in active communication. When the radio channel is interfered, the seamless
handover capability of DECT assures an unnoticeable escape to a newly
selected non-interfered radio channel.
The MC/TDMA/TDD principle
The DECT radio interface is based on the Multi Carrier, Time Division Multiple
Access, Time Division Duplex (MC/TDMA/TDD) radio access methodology.
Basic DECT frequency allocation uses 10 carrier frequencies (MC) in the 1880
to 1900 MHz range. The time spectrum for DECT is subdivided into timeframes
repeating every 10 ms. Each frame consists of 24 timeslots each individually
accessible (TDMA) that may be used for either transmission or reception. For
the basic DECT speech service two timeslots - with 5 ms separation - are
paired to provide bearer capacity for typically 32 kbit/s (ADPCM G.726 coded
speech) full duplex connections. To simplify implementations for basic DECT
the 10 ms timeframe has been split in two halves (TDD); where the first 12
timeslots are used for FP transmissions (downlink) and the other 12 are used
for PP transmissions (uplink).
The TDMA structure allows up to 12 simultaneous basic DECT (full duplex)
voice connections per transceiver providing a significant cost benefit when
compared with technologies that can have only one link per transceiver (e.g.
CT2). Due to the advanced radio protocol, DECT is able to offer widely varying
bandwidths by combining multiple channels into a single bearer. For data
transmission purposes error protected net throughput rates of n x 24 kbit/s
can be achieved, up to a maximum of 552 kbit/s with full security as applied
by the basic DECT standard.
Use of the radio spectrum
Using the MC/TDMA/TDD principle for basic DECT (utilising both frequency and
time dimensions) a total spectrum of 120 duplex channels is available to a
DECT de-vice at any instant location. When adding the third dimension (space)
to the principle -given the fact that the capacity of DECT is limited by
interference from adjacent cells and Carrier over Interference ratios of C/I =
10 dB can be achieved - a very low channel reuse factor (1) can be obtained.
Different communication links in adjacent cells can use the same channel
(frequency/timeslot combi-nation. Therefore dense packing of DECT base
stations (e.g. at a distance of 25 m in an ideal hexagonal coverage model) will
allow for a traffic capacity of the basic DECT tech-nology up to approx. 10000
Erlang/km 2 /floor (see Note 1 below) without the need for fre-quency
planning. Installation of DECT is reasonably simple since one only needs to
consider radio coverage and traffic needs. Note 1: 1 Erlang represents an
average traffic load caused by one basic DECT speech connection -using one
frequency/timeslot pair - for 100% of time.
Continuous broadcast service
A DECT base station is continuously transmitting on - at least - one channel,
thus providing a beacon function for DECT portables to lock-on to. The
transmission can be part of an active communication link with a portable or a
dummy bearer transmission. The base station’s beacon transmission carries
broad-cast information - in a multi-frame multiplexed structure - on base
station identity, system capabilities, RFP status and paging information for
incoming call set-up. Port- ables locked-on to a beacon transmission will
analyse the broadcast information to find out if the portable has access rights
to the system (only portables with access rights are allowed to set-up a
communication link), determine whether system capabilities match with the
services required by the portable and - if a communication is required whether the base station has free capacity for a radio link with the portable.
Figure 5. The DECT fixed part beacon function
Dynamic Channel Selection and Allocation
DECT features continuous Dynamic Chan-nel Selection and Allocation. All DECT
equipment is obliged to regularly scan - its local radio environment - at least
once every 30 seconds. Scanning means receiving and measuring local RF
signal strength on all idle channels. Scanning is done as a background process
and produces a list of free and occu-pied channels (RSSI list; RSSI = Received
Signal Strength Indication), one for each idle timeslot/carrier combination, to
be used in the channel selection process. An idle time-slot is (temporarily) not
in use for transmis-sion or reception. Within the RSSI list, low signal strength
values represent free and non-interfered channels, while high values represent busy or interfered channels. With the aid of the RSSI information, a DECT
PP or FP is capable of selecting the most optimal (least interfered) channel to
set-up a new commu-nication link.
In a DECT portable part, the channels with highest RSSI values are
continuously ana-lysed to check if the transmission originates from a base
station to which the portable has access-rights. The portable will lock onto the
strongest base station, as mandated by the DECT standard. Channels with
lowest RSSI value are used to set-up a radio link with the base station if the
portable user decides to establish a communication or when an in-coming call
is signalled to the portable through the reception of a paging message.
In a DECT base station the channels with low RSSI values are used when
selecting a channel to set-up a beacon transmission (dummy bearer).
The Dynamic Channel Selection and Alloca-tion mechanism guarantees that
radio links are always set-up on the least interfered channel available.
Call set-up
Portable user originated call set-up
The initiative to set-up radio links in basic DECT applications is always taken by
the portable part. The portable selects (using its Dynamic Channel Selection)
the best channel available for set-up, and accesses the fixed part on this
channel. To be able to detect the PP’s set-up attempts the fixed part must be
receiving on the channel when the PP transmits its access request. To allow
portables to use all 10 DECT RF carriers, the fixed part continuously scans its
idle receive channels for portable setup attempts in a sequential way. Portables
synchronise to this sequence by means of the information transmitted through
the FP continuous broadcast service. From this information portables can
deter-mine the exact moment when successful ac-cess the FP is possible on
the selected chan-nel.
Network originated call set-up
When a call comes in for a DECT portable, the access network will page the
portable by sending a page message - containing the PP’s identity - through its
continuous broadcast service. A portable receiving a paging mes-sage with its
identity included will set-up a radio link - to serve the incoming call - using the
same procedure as used for the PP origi-nated link set-up.
Handover
Due to the powerful Dynamic Channel Se-lection and Allocation and seamless
handover capabilities of DECT, portables can escape from an interfered radio
connection by estab-lishing a second radio link - on a newly se-lected channel to either the same (intracell handover as shown in figure 6) or to another base
station (intercell handover as shown in figure 7). The two radio links are
temporarily maintained in parallel with identical speech information being
carried across while the quality of the links is being analysed. After some time
the base station determines which radio link has the best quality and releases
the other link.
Figure 6. The DECT intracell handover function
If the DECT portable is moving from one cell area into another, the received
signal strength - as measured by the portable’s Dynamic Channel Selection
and Allocation functions - of the base station will reduce gradually. The signal
strength of the base station serving the cell towards which the portable is
moving will gradually increase. At the moment the new base station’s signal
becomes stronger than the signal from the old base station, a seamless
handover (as described above) will be performed to the new base station (see
figure 7.). The seamless handover is a fully autonomous initiative from the
DECT portable part, which the user will not notice. Although a handover is
always initiated by the DECT portable part, it may also be the uplink (from PP
to FP) that suffers from poor quality. For this case, DECT has signalling
protocols that enable the fixed part to signal the perceived link quality to the
PP, that can subsequently initiate the handover.
Figure 7. The DECT intercell handover function
Diversity
Handover in DECT is a mechanism to escape from interfered or channels with
low signal level. Handover is however not sufficiently fast to counteract fast
fading situations. For this purpose the DECT base station can be equipped with
antenna diversity (see figure 8). A signalling protocol is available in the
standard to control FP antenna diversity from the portable. Due to the TDD
nature (symmetry) of the radio link between the FP and PP, the FP antenna
diversity not only improves the uplink quality but also the downlink quality, at
slow speed.
Figure 8. Antenna diversity from 1 to 2
Coexistence
The coexistence properties of radio access technology mainly rely on the ability
to escape (handover) - in the frequency domain -from the interfered radio link,
not relying on information transferred over the original (interfered) channel.
MC/TDMA/TDD, continuous Dynamic Channel Selection and Allocation and the
handover procedures in the DECT standard show excellent coexistence
properties even under heavy interference conditions.
Security
The use of a radio access technology providing mobility includes considerable
risks with respect to security. The DECT standard provides the measures to
counteract the natural security flaws that generally appear when applying
cordlessness. Effective subscription and authentication protocols have been
included to prevent unauthorised access and an advanced ciphering concept
provides protection against eavesdropping.
Subscription
The subscription process is the process by which the network opens its service
to a particular portable.
The network operator or service provider provides the portable user with a
secret subscription key (PIN code), that will be entered into both the fixed and
the portable part before the procedure starts. Before the handset initiates the
actual subscription procedure it should also know the identity of the fixed part
to subscribe to (for security reasons the subscription area could even be
limited to a single designated - low power - base station of the system). The
time to execute the procedure is usually limited and the subscription key can
only be used once, this to further minimise the risk of misuse. Subscription in
DECT can be done “over the air,“ a radio link is set-up and both ends verify
that they use the same subscription key. Handset and network identities are
exchanged, and both sides calculate a secret authentication key to be used for
authentication at every call set-up. The secret authentication key is not
transferred over the air.
A DECT portable may have multiple subscriptions. With every subscription
session, the portable will calculate a new secret authentication key associated
with the network to which it subscribes. New keys and network identities are
added to a list - kept in the portable - which is used in the locking process.
Portables will only lock to a network where it has access rights (network
identity is contained in the list).
Authentication
Authentication of a handset may be done as a standard procedure at every call
set-up. During the authentication session, the base station checks the secret
authentication key without sending it over the air. The principle for hiding the
identity information in the air is as follows: the base station sends a random
number to the handset that is called the ‘challenge’. The handset calculates a
‘response’ by combining the authentication key with the random information
and transmits the ‘response’ to the base station. The base station also
calculates the expected ‘response’ and compares it with the received
‘response’. Figure 9 illustrates the authentication mechanism. The comparison
results into a continuation of the call set-up or a re-lease. If somebody is
eavesdropping on the air interface, in order to steal the authentication key he
needs to know the algorithm to recalculate the key from the ‘challenge’ and
the ‘response’. This ‘reverse’ algorithm demands for a huge amount of
computing power. So the cost of retrieving the key by eavesdrop-ping of the
authentication procedure is made extremely high.
Figure 9. DECT authentication : challenge and response
Encryption
The authentication process uses an algorithm to calculate the ‘response’ from a
‘challenge’ and the authentication key in handset and base station. This is in
fact a way to send the identity of the user in an encrypted form over the air in
order to preventing theft of the identity. Looking at user data (e.g. speech) the
same principle can be applied. During authentication, both sides also calculate
a cipher key. This key is used to cipher the data sent over the air. At the
receiving side the same key is used to decipher the information (see figure
10.). In DECT, the ciphering process is part of the standard (however not
mandatory).
Figure 10. The DECT cyphering function
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