Rechnernetze

DECT

Introduction

The Digital Enhanced Cordless Telecommunications (DECT) standard provides a general radio access technology for wireless tele-communications, operating in the preferred 1880 to 1900 MHz band using GFSK (BT = 0.5) modulation.

DECT has been designed to provide access to any type of telecommunication network thus supporting numerous different applications and services. The range of DECT applications includes residential, PSTN and ISDN access, wireless PABX, GSM access, Wire-less Local Loop, Cordless Terminal Mobility CTM, Local Area Network access supporting voice telephony, fax, modem, E-mail, Inter-net, X.25 and many other services in a cost efficient manner.

The variety of applications supported by DECT finds its origin in the flexibility of the advanced protocol ‘toolbox’ that allows implementers to assemble virtually any application subset required. Mobility functions in the DECT protocol provide cordlessness to roaming users through a pico-cellular infra- structure of DECT base stations.

Figure 2. The pico-cellular structure of the DECT access network

A DECT system comprises a DECT Fixed Part (FP), utilising one or more base stations (RFPs), and one or more DECT Portable Parts (PPs). There is no limit to the size of the infrastructure as far as the number of base stations and cordless terminals is concerned. Infrastructures using the DECT technology can support traffic densities up to 10000 Erlang/ km², which is comparable to 100000 users in office environments.

In principle the DECT base standard (of which all parts are shown in Table 1) only covers the “air interface” between a DECT Fixed Part (FP) and a DECT Portable Part (PP), it provides a toolbox with protocols and messages from which selections can be made (profiles) to access any specific type of network. In addition to cordlessness, DECT makes available the network specific services and features (including mobility) to the user through the DECT common air interface transparently.

Its Multi Carrier, Time Division Multiple Access, Time Division Duplex (MC/TDMA/TDD) radio access method and continuous Dynamic Channel Selection and Allocation capability enable high capacity, pico-cellular systems, being utilised even in busy or hostile radio environments. These methods enable DECT to offer excellent quality of service without the need for frequency planning. DECT makes efficient use of the assigned radio spectrum, even when multiple operators and applications share the same frequency spectrum.

Figure 3. The DECT access technology using the GAP Common Air Interface

Standardised profiles have been defined for e.g. Generic Access (GAP; which is mandatory as a minimum requirement for all DECT voice telephony equipment as from October 1997), Radio in the Local Loop applications (RAP), ISDN and GSM interworking (GIP). Standard profiles encourage DECT equipment manufacturers to implement interworking with the network in a harmonised way. This creates interoperability between DECT equipment from different manufacturers and directs competition towards differentiation on non-technological features, providing consumers, and network operators with the luxury to choose from a variety of standard products. Standardisation of interworking also allows for mass production of system components, which in its turn provides significant cost benefits enabling highly attractive price/performance ratios for DECT equipment.

The North American Personal Wireless Telecommunications standards PWT and PWT/E (TIA) are based on DECT. PWT and PWT/E provide the same services as DECT; they use the same framing structure MAC, DLC, NWK layer and identities but an alternative modulation scheme and frequency allocation. The PWT operates in the US unlicensed band 1910 to 1920 MHz. PWT/E is an extension into the licensed bands 1850 - 1910 MHz and 1930 - 1990 MHz.