Overview

DECT standard
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DECT standard

Although DECT does not address high-speed data communications, it is a good technology for data communications in SOHO applications. Using symmetric connections 288kbps can be attained and with asymmetric connections even 552kbps are possible. Therefore they are adequate for ISDN and ISDN, or ADSL. Even when ADSL will attain higher bit rates in residential applications, new modulation schemes will keep DECT on the market.

Frequency band          : 1880-1900MHz
Number of carriers : 10
Carrier spacing : 1728MHz
Access technique : TDMA, TDD, FDMA
Traffic duplex channels : 12
Data rate per channel : 32kbps
Range : 30m or 300m
Modulation : GFSK (BT = 0.5)
Sensitivity : -86dBm at 0.01BER
Average RF power/ slot : 10mW
Peak RF power : 250mW

Table 1 - DECT basic characteristics

DECT supports voice and data communications, and addresses a large range of applications, such as telephones WLL (Wireless Local Loop), PBX (Private Branch Exchange), etc. In the first years of its existence, DECT was mainly used in residential cordless voice applications. Now DECT began to be successfully used in data communications. Moreover, DECT has also been accepted as one of the five air interfaces of IMT 2000. In summary, DECT is an evolutionary and low cost mass-market technology for wireless voice/data communications. Also, DECT is only a radio access technology permitting data transmission, both in local area networks and public networks. Adopting DECT for local area networks, in spite of its current speed limitations, means to offer today, an additional feature to SOHO applications. In a more long-term scenario, with the foreseen improvements, DECT will continue to be competitive.

DECT characteristics

Table 1 presents some characteristics of the DECT standard.

DECT relies on a decentralised channel allocation procedure, called Dynamic Channel Selection (DCS). Instead of having fixed channels, the portable terminal is continuously scanning the available channels, trying to use the best one. The channel to be used is not defined by the base station. The DECT portable terminal chooses the channel among the 120 existing channels, taking the least interfered channel from its channel list. The list is periodically updated. The set-up of new channel takes in account the local interference situation. Set-up is achieved for new connections or for handovers. DECT has the ability to handover time slots, in a seamless way, to improve communication quality.

Within the 10ms TDMA frame (Figure 1) there are 24 slots. The 24 slots are divided into two groups of 12 slots. One group is employed in forward transmission and the other in reverse transmission. Channel data rate is 32kbps. ADPCM (ADaptative Pulse Code Modulation) for voice communication uses the 32kbps. For data transmission, a CRC is introduced and only 24kbps, per channel, are affordable. Therefore, multiple channels are needed to improve data rate. Using symmetric connections 12 channels (12×24 = 288kbps) is the limit. To attain higher data rates, asymmetric connections can be used. Using asymmetric connections, at least one slot is necessary to receive acknowledgements. In this situation, using 23 slots, 264kbps (23×24 = 532kbps) can be achieved.


Figure 1 - DECT basic characteristics The architecture of DECT protocol is closely related to the lower layers of the OSI reference model. DECT has a physical layer, a data link layer, and a network layer. The data link layer is composed of a MAC (Medium Access Control) layer and a DLC (Data Link Control) layer.

See also: DECT The Standard Explained (pdf-File)

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