Published on Mar 12, 2016
This report describes a new wireless local loop system for rapid expansion of telecom services developed under a joint project involving Indian scientists form Indian Institute Of Technology, Chennai, Midas technology and Analog Devices Inc., USA. The new system, called corDECT, is based on microcellular architecture and uses a modest bandwidth of 20MHz to provide voice, fax, and data communication in low as well as very high subscriber density environments.
The high capacity at a modest bandwidth is made possible without prior frequency planning through a completely decentralized channel allocation procedure called dynamic channel selection. This technology provides cost-effective simultaneous high quality voice and data connectivity, a voice communication using 32Kbps ADPCM and Internet connectivity at 35\70 Kbps. This report discusses the relevance of corDECT in the context of current trends towards wireless systems, contrasts the microcellular architecture of corDECT with existing wireless systems based on macrocellular architectures, and outlines its market potential.
A new wireless local loop system to eliminate the physical connections between telephone exchanges and subscribers has just hit the market after a two-year long joint research effort by Indian and US engineers. The new system, called corDECT, is said to offer significant cost-savings, rapid installation, and improved reliability over traditional connections based on copper cables. It is based on a microcellular architecture that is said to offer cost and operational advantages over wireless/mobile telephone systems based on macrocellular architectures. The corDECT system is based on the European Digital Enhanced Cordless Telecommunications standard that uses a modest bandwidth of 20MHz in the 1880-1900 MHz range and does not require prior frequency planning necessary in conventional mobile cellular systems.
The corDECT technology uses relatively low-cost, easy-to-install subsystems and can serve relatively high subscriber density environments -several thousands of subscribers per square kilometer. Four Indian companies have bought the technology for domestic manufacture. Its developers believe there is a large market potential in the Asia-Pacific region and in other developing countries. This report will describe the CorDECT wireless local loop system and its subsystems and compare the microcellular architecture of corDECT with macrocellular architectures employed in many wireless telephone systems.
Need for wireless local loop system:
The telephone and the Internet have changed the way we deliver and receive information and the way we use it for business, entertainment, planning and living. Unfortunately, only 15 percent of the world's population is believed to have access to the Internet. And more than 80 percent of people in the world are believed to have never even heard a dial tone. This Digital divide -between the information 'haves' and 'have nots' -is widening. In India, the problem is acute. Among 1000 million people. There are fewer than 35 million people; there are fewer than 35 million phones connections and around two million Internet connections. There is an urgent need to bridges the gap.
The biggest reason for is high cost. The existing per line cost of a telephone network is Rs.30, 000, which most people in India cannot afford; this has to be reduced by a factor of three to four. In order to reduce the cost, we must consider the factors responsible for overall system cost. The telecom network consists of two components.
1.A backbone network consisting of routers, switches and interconnection of exchanges and routers, including intercity and international connections.
2.An access network that includes the connection of the exchange to the office and home.
Fortunately, the cost of the backbone network is reducing rapidly each successive year with improvements in technology. In order to reduce overall costs. There is a need to focus on the cost of the access networks- that is, the cost of the local loop. By reducing this cost, it is possible to reduce overall per line cost by Rs12, 000 to Rs.16, 000.
For nearly a century, these connections have relied on pairs of copper cables. But laying out wired local loops has been an expensive, time-consuming process that also requires detailed planning and intensive labor costs. According to a projection by the International Telecommunications Union, developing countries alone will require 35-million pair kilometers of copper cable by the turn of the century just to maintain existing waiting lists. The increasing cost of copper, the operational problems associated with wired lines, and the demands for mobility are factors fueling the move towards wireless local loops.
Importance of CorDECT in present scenario:
Internet connections today, for the most part, use a modem to connect a computer to a telephone line. In this case, Internet traffic passes through the telecom network, which overloads the telecom network. It is necessary to develop an access network technology, which separates Internet data form the voice and prevent it from interfering with the telephone network. This would also make it possible to use the telephone and the Internet on the same line simultaneously.
DECT (DIGITAL ENHANCED CORDLESS TELECOMMUNICATIONS)
The European Digital Enhanced Cordless Telecommunications standard was developed by ETSI (European Telecommunication Standard Institute.) to serve a range of applications that included the following
Small business systems (single site, single cell)
Large business systems (multisite, multicell)
Public cordless access systems
Wireless data (LAN) access systems
Evolutionary systems (fixed wireless access, cordless access to cellular)
The DECT standard defines interworking profiles for interoperability with different digital networks and between equipment. The standard defines the air interface for users. When DECT is used for wireless local loop, it specifies the interface between the subscriber terminal and the base stations. It covers generic access profile, the radio access profile, the DECT/GSM and the DECT/ISDN interworking profiles, the DECT data profiles and cordless terminal mobility. The DECT frequency band is 1880-1900MHz. The standard uses 32kbps ADPCM allowing toll quality voice as well as fax and data transmission and is up gradable to ISDN. The key to the high capacity at modest bandwidth is an automatic channel allocation procedure called dynamic channel selection.
The DECT system has 10 radio frequency carriers in a modest 20 MHz bandwidth -- 1880-MHz to 1900MHz. Each carrier is divided into frames of 24 timeslots, 12 in one direction, and 12 in the other. A DECT channel is defined by a time slot and frequency pair. So the 12 slot pairs and 10- frequency carriers give rise to 120 channels. The available set of channels is not distributed between individual cells. Instead, DECT terminals can select any channel at any given time from among the 120, selecting the best channel available. The terminals continuously scan the channels to select the one with the strongest signal or the least interference and use that channel for communication.
A functional or conceptual view for a DECT system is as shown in fig.2.1
2.1Functional model for DECT system:
FIG.2.1.Functional Model for DECT
The DECT system is composed of a number of functional entities, including the following
The DECT handset of terminal which may include a Cordless Terminal adapter (CTA) to support more general applications (e.g.fax, data)
The DECT Radio Fixed Part (RFP), which supports the physical layer of the DECT common air interface.
The Cordless Cluster Controller (CC), which supports the MAC, DLC, and network layers for one or more clusters of RFP’s and, in a multicell environment, handles intercell handoffs.
The Network Interface Unit (IU) or interworking unit provides required interfaces to connect to specific networks (PSTN, ISDN, GSM etc.) and, in a multicell environment, may also provide call/connection control for DECT terminals.
The Mobility Management function, which supports centralized authentication for public access or telepoint applications or other mobility management functions in multilocation PBX-based networks.
2RADIO ASPECTS OF DECT
In Europe the 1880-1900 MHz band has been set-aside for DECT. To utilize the available 20 MHz band in an efficient and flexible manner for supporting voice and data applications, the DECT standard provides for space, frequency, and time distribution, Space dispersion in DECT is supported through the frequency reuse feature based on the cellular concept. To provide frequency distribution, the available spectrum is segmented into 10 carrier frequencies (frequency channels) from 1881.792MHz with separation of 1.728MHz [i.e. by deploying frequency division multiple accesses (FDMA)]. Time distribution is achieved by using time division multiple accesses (TDMA), where by each frequency channel supports 12 duplex time slots or 32 Kb/s channels. FDMA/TDMA structure of DECT channels
Thus a total of 120 channels, each with a 32 Kb/s data rate, are available in the FDMA/TDMA structure to carry voice and data traffic; additional capacity required of business systems is provided by invoking frequency reuse in a cellular configuration. In the cellular configuration used in business systems and telepoint, DECT also supports handoffs across cells. The multiplex burst structure used in DECT for carrying voice and signaling information
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