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.
DECT Interface Unit (DIU)
The DIU is heart of the CorDECT system It serves as an interface between the PSTN and CBS.The DIU is connected to the CBS by three cables that carry data in ISDN format and device and derive the power to CBS. Each DIU can be connected to a maximum of 20 CBS. Each DIU is connected to PSTN through standard E1 lines. Each CBS can be located up to four kilometers from a DIU.
Network Layer Protocol
The DECT network protocol is defined for the control plane (C plane) only. It provides the means to request, allocate, manage, and deallocate key resources in the central system and the portable. The capabilities of the DECT system in terms of its flexibility to support a wide range of applications and interface to diverse networks is determined by the characteristics of the network protocol were drawn from the ISDN user-network protocol at layer 3 (ITU-T Recommendation Q.931) and to some extend from the GSM layer 3 protocol. Since, however, the DECT architecture and requirements differ considerably from ISDN and GSM, DECT network layer protocol is essentially modu8lar and it is intended to support a number of protocol functions. Some of the protocols, which the network layer supports, are given below.
The main functions of the DIU are
- Enabling cell formatting.
- Administration and monitoring of the network.
- Interfacing between PSTN and CBS, and delivering signals without the use of wires to the subscriber.
- Providing power to the base station.
Management Entity (MGE)
The management entity (MGE) is responsible for a number of functions in DECT that involve only one side of the communication link and as such do not appear in an OSI layer. These include such functions as radio resource control (choice of free channels, assessment of channel quality), mobility management (registration of DECT portables), and error handling (call termination at radio link interruption).
Radio Aspects Of CorDect
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.
Radio Specifications of CorDECT
Access method : TDMA
Spectrum allocation : 1880-1900MHz
Carrier spacing : 1728 KHz
Number of carriers : 10
Channels/Carrier : 12
Modulation : GFSK
Transmission rate : 1152Kb/s
Frame duration : 10ms
Speech coding : 32Kb/s ADPCM
Peak output power : 250mW
The corDECT wireless local loop system offers relatively low cost and rapid installation of telecom services in areas with even high subscriber density environments. This microcellular system relies on a modest bandwidth of 20MHz for the entire country. Two or three DECT wireless local loop systems and DECT based local area networks could coexist in the same area without interfering with one another -- all operating on the same 20MHz bandwidth. The high capacity is achieved not by using low bit-rate compression techniques but with toll quality ADPCM or PCM voice coding. The microcellular system can provide ADPCM (32kbps) or PCM (64kbps) and even ISDN (128kbps) services to users. Its developers describe it as a "future proof" system.