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25-02-2009, 12:24 AM
This report describes a new wireless local loop system for rapid expansion of telecom services developed under a joint project and implimentation 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
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corDECT is an advanced, field proven, Wireless Access System developed by Midas Communication Technologies and the Indian Institute of Technology, Madras, in association with Analog Devices Inc., USA. corDECT provides a complete wireless access solution for new and expanding telecommunication networks with seamless integration of both voice and Internet services. It is the only cost-effective Wireless Local Loop (WLL) system in the world today that provides simultaneous toll-quality voice and 35 or 70 kbps Internet access to wireless subscribers.
corDECT is based on the DECT standard specification from the European Tele- communication Standards Institute (ETSI). In addition, it incorporates new concepts and innovative designs brought about by the collaboration of a leading R & D company, a renowned university, and a global semiconductor manufacturer. This alliance has resulted in many breakthrough concepts including that of an Access Network that segregates voice and Internet traffic and delivers each, in the most efficient manner, to the telephone network and the Internet respectively, without the one choking the other.
The DECT standard proposed by the European Telecommunication Standards Institute (ETSI) is meant for providing wireless access to networks of various types, from the PSTN to LAN’s. It deals only with the task of defining the air interface between subscriber terminal and Base Station. The mode of connecting the DECT-based Wireless Local Loop system to the PSTN and Internet is left to the service provider.
The corDECT wireless in local loop (WLL) technology has been chosen to connect 5.77 lakh consumers spread across the country by Bharat Sanchar Nigam Limited (BSNL). With this the biggest dream of Ashok Jhunjhunwala, Professor of Electrical Engineering Department of IIT, Madras to telecom network the neglected smaller towns is coming true. It has also vindicated his claim that corDECT is a less expensive and superior technology for the smaller towns and rural areas.
The corDECT technology developed indigenously provides the last mile wireless connection between the telephone exchange and end consumer. Conventionally copper wires achieved this.
This is not the first time that corDECT technology has been chosen by the government. In 2001 the Department of Telecommunications opted for this technology to network 24 cities. However, the number of lines was limited to 25,000.
The good news is that many major telecom players who have entered the fray to provide basic telephone services are seriously looking at using corDECT. The Chennai based Internet service provider Satyam Infoway Ltd (Sify) has not ruled out the possibility of using it to connect its `i ways' (cyber cafés) in smaller towns.
Its relevance is not restricted to rural areas alone. For the urban areas where the telephone penetration is better the use of corDECT will go a long way in meeting the specific requirement of providing faster Internet connectivity.
The corDECT technology provides 70 kilo bits per second (kbps) connectivity speed. This drops to 35 kbps when Internet and phone are used simultaneously.
Now compare it with 40-50 kbps speed when Internet is accessed using a dial up line within one km radius from the telephone exchange and 25-38 kbps when more than one km radius. Smaller towns have speeds anywhere between 14.4-19.6 kbps and rural areas have pitiable speeds of 4.8-9.6 kbps.
Though access speed seems comparable in the urban areas, it is common knowledge that the access speed reduces when more subscribers log on to the Net in the case of dial up lines. But this is not the case with corDECT as the speed remains constant. This is because dedicated pipelines are provided to each customer that are not shared with other subscribers.
Bandwidth sharing is not unique to dial up connection — both GSM (Global Service for Mobile communication) and CDMA (Code Division Multiple Access) based WLL technology use the same principle. GSM and CDMA are primarily designed to support only voice and not data (Internet). But the use of certain technologies makes it possible for GSM and CDMA to transfer data. But this data transfer is typically for transactional applications and cannot meet the requirements of a typical home user looking at sustained browsing. Even data transfer in spurts suffers from slow speed due to bandwidth sharing.
This is not the case with CorDECT as it works on the principle of reusing spectrum very often by using microcellular architecture. The use of multiple smaller cell sites enables a wise utilisation of bandwidth. The technology does not require frequency planning. It is designed to allow multiple operators to use the same spectrum in the same area without having to coordinate on spectrum division.
The corDECT combines 20 frequencies and 10 time slots to make available 120 channels in every sector at any given point of time. The subscriber terminal continuously scans for channels and chooses the quietest channel and a base station delivering the strongest signal. These channels can be reused and are limited only by the number of installed base stations.
But will a situation not arise when there are more subscribers in a given area posing a challenge to availability of channels and hence the assurance of `dedicated pipelines? We solve this by first increasing the number of base stations. By doing this we are effectively reducing the number of subscribers per cell site. Alternatively we could use special antennas or network planning to overcome capacity saturation. A cell site in urban area can serve customers in a 2 km radius and this can go up to 10 km radius in rural areas.
Several special antennas with narrow beam width can be mounted in different directions on a single base station. This way the antennas essentially divide the area served many fold by using a single base station.
Ironically, the government's decision to use corDECT technology on a large-scale vis-à-vis other competing technologies like CDMA's WLL comes long after it has been accepted and deployed in other countries. "Contrast this with the way the U.S. banned GSM technology (being used in Europe) for nine long years to help its industry develop a competing technology.
DECT has been specified to make possible low-cost subscriber terminals, high subscriber density with heavy call-traffic levels, wireline- quality voice, modem/fax capability, 32/64 kbps and higher-rate data services, all with a modest spectral allocation of 20 MHz. The key technical advances incorporated in DECT when compared to prior standards that make all this possible are:
(i) dynamic channel selection
(ii) microcellular architecture
(iii) channels with multiple data rates
(iv) cost-effective modulation/demodulation techniques.
The system enables wireless subscribers to be connected to the PSTN in a cost effective manner. The system is easy to deploy and maintain and is readily adaptable to both high and low subscriber densities. The architecture and interfaces have been carefully planned to enable easy adaptation of the system to any network-specific or country- specific requirements
2. Literature Survey
2.1 Access Network
Fortunately, an uncelebrated but major technological innovation changed the Access Network from the mid-eighties onwards. As shown in Figure 2.1, the AN now consists of an optical fibre from an exchange to a Remote Line Unit (RLU) and typically a 3 - 4 km copper loop from the RLU to the subscriber premises. The signals carried on the fibre are time-multiplexed digital voice and signaling information. A RLU typically serves 1000 to 4000 subscribers and the links from RLU to exchange consist of 4 - 16 E1’s. Since the loop length is reduced to 3 - 4 km, a wire gauge of 0.4 mm is sufficient and this brings down the cost considerably. The rising cost of copper, however, continues to push up the cost of even this solution every year. Today, the per-line copper cost (3 - 4 km long, including laying charges) and the shared cost of fibre and RLU, again amounts to almost two-thirds of the total per-line cost.
The signaling protocol on the AN (in the signaling slots on the E1 links between the RLU’s and exchange) initially remained proprietary. However, access signaling was standardized internationally in the early nineties, in the form of the V5.1 and V5.2 protocols. The V5.2 interface makes the AN appear as an RLU to any exchange, overcoming the earlier restriction of having a proprietary RLU for each exchange.
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