Dissecting the DECT Option

One alternative that is being bandied about for local mobile voice is the European-developed Digital Enhanced Cordless Telephone (DECT) technology. While the US market interest has focused on voice over Wireless LAN (VoWLAN) on Wi-Fi networks, in the European market the installed base for local voice is roughly 95% DECT versus 5% Wi-Fi. Several US suppliers including Siemens, NEC, Nortel, Mitel, and Polycom offer DECT-based solution so they are being proposed in a number of installations. For an enterprise buyer looking for a solution for local mobility, it is important to be aware of the pros and cons of the two solutions.

The primary advantage of Wi-Fi is that it’s probably installed already, though the network may require a significant upgrade to support voice. Generally a Wi-Fi voice network requires a denser deployment with more access points and coverage that extends throughout the facility. About half of existing WLAN deployments cover only common areas like conference rooms and cafeterias; voice users can roam anywhere within the facility so we have to ensure service availability, and that may include coverage in stairwells and rest rooms.

The Wi-Fi technology is derived from data-oriented Ethernet technology, which means we have to take additional measures to ensure adequate performance. Along with providing adequate network capacity, it is important to utilize Wi-Fi Multi-Media (WMM) the QoS standard specified in IEEE 802.11e. Security measures like WPA and WPA2 should be adequate to provide privacy, but the power-hungry Wi-Fi interface makes battery life a challenge.

DECT on the other hand is based on a voice-oriented TDM technology. Unlike WLAN where all of the stations vie for access to a shared radio channel, DECT systems assign a channel to each user for the duration of the call so QoS is assured. Further, DECT systems operate in a different frequency band than wireless LANs so both can be installed in the same building with no interference concerns from either WLANs. DECT is also immune to interference from cordless phones, microwave ovens, and other devices that use the 2.4 GHz ISM band.

In North America, the DECT frequency band runs from 1920 to 1930 MHz, which allows for 5 DECT carrier channels; each DECT carrier channel can support 12 simultaneous voice connections. A wider frequency band is assigned in Europe and South America allowing for 10 DECT carrier channels. For larger deployments, channels can be reused in different parts of the coverage area.

The carrier channels are spaced 1.728 MHz apart and support a transmission rate of 1.15 Mbps; that is rather slow compared to the typical Wi-Fi rate of 11 M or 54 Mbps. In DECT, voice signals are encoded using 32 Kbps ADPCM, and over-the-air transmissions are encrypted using the DECT Standard Cipher Algorithm (DSCA). Call signaling is based on ISDN’s Q.931 protocol. The radio carriers use a Time Division Duplex (TDD) technique where they operate alternately inbound and outbound. There is also a hand off mechanism so users can roam throughout the DECT coverage area.

There are some other advantages as well, particularly in terms of cost and battery life. Given its greater transmission range, DECT will allow you to cover an area with 1/3 to 1/2 as many access points as with Wi-Fi. It was designed with the idea of mobile handsets in mind, so the battery life for DECT devices is considerably better than Wi-Fi.

The downside for DECT is that it is fundamentally a voice technology with limited data transmission capabilities; if the entire carrier channel supporting 12 users operates at only 1.15 Mbps, you’re not going to be getting the type of data rates you see on Wi-Fi. There is a new version of DECT called CAT-iq for “Cordless Advanced Technology- internet and quality”. It uses the same basic DECT transmission and multiplexing formats, but allows multiple time slots to be grouped together. The “quality” moniker refers to the ability to clump two 32 Kbps voice timeslots to support 64 Kbps G.722 wideband voice. The “internet” reference is to the ability to support data services, though they still top out at around 384 Kbps.

Given its lower cost and better battery life, there may be a role for DECT in the US market, particularly in applications that require only voice service. What we have yet to see is any move to add mobile UC capabilities to DECT devices; of course, we haven’t seen much of that on the Wi-Fi voice front either. DECT could offer a cost effective way to provide voice-only coverage more cheaply that upgrading an existing wireless LAN, and it could also be an option in greenfield deployments with voice-only requirements. The big drawback is that DECT is now and likely always be a basic voice technology, which doesn’t bode well with a world that’s aiming for smartphones and mobile UC capabilities.