The Digital Enhanced Cordless Telecommunications

The Digital Enhanced Cordless Telecommunications (DECT) standard defines a protocol for secure digital telecommunications and is intended to offer an economical alternative to existing cordless and wireless solutions. DECT uses Time Division Multiple Access (TDMA) technology to provide ten 1.75-MHz channels in the frequency band between 1.88 and 1.90 GHz.

Each channel can carry up to 12 simultaneous twoway conversations. Speech quality is comparable to conventional land-based phone lines. Frequency bands have been made available for DECT in more than 100 countries. Whereas conventional analog cordless phones have a range of about 100 meters, the DECT version can operate reliably up to 300 meters.

What started out as a European standard for replacing analog cordless phones has been continually refined by the European Telecommunication Standards Institute (ETSI) to become a worldwide standard that provides a platform for wireless local loops (WLL), wireless LANs, and more recently, wireless Internet access. In addition, DECT services are compatible with GSM and ISDN, and dual-mode DECT/GSM handsets are available.

A key advantage of DECT is dynamic reconfiguration, which means that implementation does not require advance load, frequency, or cell planning. Other wireless architectures require a predetermined frequency-allocation plan. Conventional analog cellular networks, for example, are organized as cells in honeycomb fashion. To avoid conflict from adjacent cells, each base station is allotted only a fraction of the allowable frequencies.

Changing a particular station’s frequency band to accommodate the addition of more base stations to increase network capacity entails an often difficult and expensive hardware upgrade. However sparsely the base stations are constructed at the start of an installation, all possible base stations must be assigned frequencies before any physical systems are put into place.

In a DECT system, planning for uncertain future growth is unnecessary. This is because a DECT base station can dynamically assign a call to any available frequency channel in its band. The 12 conversations occurring at any one time can take place on any of the 10 channels in any combination. The handset initiating a call identifies an open frequency and time slot on the nearest base station and grabs it.

DECT systems also can reconfigure themselves on the fly to cope with changing traffic patterns. Therefore, adding a base station requires no modification of existing base stations and no prior planning of channel allocations. Compared to conventional analog systems, DECT systems do not suffer from interference or cross-talk.

Neither different mobile units nor adjacent DECT cells can pose interference problems because DECT manages the availability of frequencies and time slots dynamically. This dynamic reconfiguration capability makes DECT useful also as a platform for WLLs. DECT allows the deployment of a few base stations to meet initial service demand, with the easy addition of more base stations as traffic levels grow.

Voice compression [i.e., Adaptive Differential Pulse Code Modulation (ADPCM)] and the higher levels of the DECT protocol are not implemented at the base stations but are handled separately by a concentrator. The concentrator routes calls between the WLL network and the public switch telephone network (PSTN). This distributed architecture frees up base station processing power so that it can better handle the up to 12 concurrent transmission and reception activities.

For high-end residential and small-business users, DECT permits wireless versions of conventional PBX equipment, supporting standard functions such as incoming and outgoing calls, call hold, call forwarding, and voice mail without having to install new wiring. In this application, DECT dynamic reconfiguration means that implementation does not require advance load, frequency, or cell planning.

Users can begin with a small system and then simply add components as needs change. The DECT/GSM Interworking Profile allows a single handset to address both DECT systems and conventional cellular networks. This allows users to take advantage of the virtually free wireless PBX service within a corporate facility and then seamlessly switch over to GSM when the handset passes out of range of the PBX base station. When the call is handled by GSM, appropriate cellular charges accrue to the user.

If the call cannot get through on either type of network, it is diverted to a voice mailbox.

Wireless Local Loops

Although residential cordless communication represents the largest current market for DECT-based products, other applications look promising for the future. In developing countries, where lack of a universal wired telecommunications infrastructure can limit economic growth, DECT permits the creation of a wireless local loop (WLL), thereby avoiding the considerable time and expense required to lay wire lines.

WLLs can be implemented in several ways, which are summarized in Figure D-1. In a small cell installation in densely populated urban or downtown areas, the existing telephone network can be used as a backbone that connects the base stations for each DECT cell. These DECT base stations may be installed on telephone poles or other facilities.

Customer boxes (i.e., transceivers) installed on the outsides of houses and office buildings connect common phone, fax, and modem jacks inside. Through the transceivers, customers use their telephone, fax, and modem equipment to communicate with the base stations outside. In addition, customers can use DECTcompliant mobile phones, which can receive and transmit calls to the same base station.

In larger cell installations, such as suburban or rural areas, fiberoptic lines may provide the backbone that connects local relay stations to the nearest base station. These relay stations transmit and receive data to and from customer boxes. In these installations, the customer box must have a direct line of sight to the relay station.

Network feeds over long distances may be accomplished via microwave links, which is more economical than having to install new copper or fiber lines. Large cells can be converted easily into smaller cells by installing additional base units or relay stations. Since DECT system has a self-organizing air interface, no top-down frequency planning is necessary, as is the case with other wireless connection techniques such as GSM or its derivative Digital Cellular System 1800 (DCS 1800).

While most WLL installations focus on regular telephone and fax services, DECT paves the way for enhanced services. Multiple channels can be bundled to provide wider bandwidth, which can be tailored for each customer and billed accordingly. Among other things, this allows the mapping of ISDN services all the way through the network to the mobile unit.

Wireless LANs

In many data applications with low bit rate requirements, DECT can be a cost-effective solution. One example is remote wireless access to corporate LANs. By bundling channels, full-duplex transmission of up to 480 kbps per frequency carrier is theoretically possible.

For multiple data links, a DECT base station can be complemented by additional DECT base stations controlled by a DECT server. This forms a multicell system for higher traffic requirements. With a transparent interface to ISDN, data access and videoconferencing through wireless links can be realized. Such installations also may include such services as voice mail, automatic call back, answering and messaging services, data on demand, and Internet access.

DECT is a radio access technology. As such, it has been designed and specified to work with many other types of networks, including the PSTN, ISDN, GSM, and the Internet, as well as LANs and telephone systems in office buildings and homes. DECT modules incorporated into building control and security systems provide intelligent systems that allow automatic control and alerting to augment or replace today’s customized telemetry and wired systems.

DECT also may find its way into the home, providing automatic security alerting in the event of unauthorized entry, fire, or flood; remote telephone control of appliances; and return channels for interactive television. While DECT is an international standard, it has been adapted only recently for use in North America, where it operates in the unlicensed 2.4-GHz ISM (industrial, scientific, and medical) band. The standard in North America is known as Worldwide Digital Cordless Telephone (WDCT), which is based on DECT.