cordect wireless in local loop system
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Telecom infrastructure is a key catalyst for economic growth, and is one of the key focus areas for infrastructure development in developing countries. Telecom growth in these countries is hampered by the high capital cost of telecom equipment. There is a requirement for new telecom systems to be developed, keeping in view the affordability and the services required in these countries. corDECT is one such product, which is designed to be affordable to the common man in the developing economies, and provides all the services that is expected from a state-of-the-art telecom network.
corDECT is based on ETSI's DECT air interface standard and supports toll quality voice, voice band FAX / DATA, and, above all, a facility to simultaneously provide 35/70 KBPS of internet connectivity. The corDECT subscriber terminal has two interfaces, one for the standard two-wire telephone, so that a subscriber can connect any standard telephone FAX / MODEM / payphone, and an Internet port for connecting directly to a computer's serial ports without a modem. The subscriber has virtually two lines, one dedicated for Internet and the other for voice / FAX / payphone / data. The best part of corDECT is that both these lines can be used simultaneously.
The corDECT system has all the features of an Exchange (switch). corDECT exchange caters to 1000 lines, and can be connected to the PSTN on standard interfaces like V5.2. A key feature of the corDECT Switch is that it has a built-in RAS to segregate all the Internet traffic from the voice traffic and the Internet traffic is taken directly to the ISP Router. This prevents the Internet traffic from congesting the telephone network.
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 Telecommunication 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.
corDECT Wireless Access System
The corDECT Wireless Access System (WAS) is designed to provide simultaneous circuit-switched voice and medium-rate Internet connectivity at homes and offices.
A. Conceptual Access System
In this conceptual model, there is a Subscriber Unit (SU) located at the subscriber premises. The SU has a standard two-wire interface to connect a telephone, cordless phone, or modem. It also provides direct (without modem) Internet connectivity to a standard PC, using either a serial port (RS-232 or USB) or Ethernet. The access system allows simultaneous telephone and Internet connectivity. The SUâ„¢s are connected to an Access Centre (AC) using any convenient technology like wireless, plain old copper, coaxial cable, optical fibre, or even power lines.
The AC must be scalable, serving as few as 200 subscribers and as many as 2000 subscribers. In urban areas, the AC could be located at a street corner, serving a radius of 700 m to 1 km. This small radius in urban areas is important for wireless access, in order to enable efficient reuse of spectrum. When cable is used, the small radius ensures low cost and higher bitrate connectivity. However in rural areas, the distance between the AC and the SU could easily be 10 km even go up to 25 km in certain situations.
The AC is thus a shared system catering to multiple subscribers. The voice and Internet traffic to and from subscribers can be concentrated here and then carried on any appropriate backhaul transport network to the telephone and Internet networks respectively. At the AC, the telephone and Internet traffic is separated. The telephone traffic carried to the telephone network on E1 links using access protocols such as V5.2. the Internet traffic from multiple subscribers is statically multiplexed, taking advantage of the bursty nature of the Internet traffic, and carried to the Internet network. As use of Voice-over-IP (VoIP) grows, voice traffic from subscriber traffic could also be sent to the Internet, gradually making the connectivity to the telephone network redundant. However, for connecting to the legacy telephone network, the voice port of the AC may be required for some time to come. An AC could also incorporate switching and maintenance functions when required.
B. corDECT Wireless Access System
Following the conceptual model, the corDECT Wireless Access System uses a similar architecture to provide telephone and Internet service to a subscriber, as shown in fig.2.
In this the subscriber premises equipment, Wallset IP (WS-IP) or Wallset (WS), has a wireless connection through a Compact Base Station (CBS) to an access switch, called a DECT Interface Unit (DIU). The air interface is compliant to the DECT standard. The DIU switches the voice traffic to the telephone network using the V5.2 protocol to connect to an exchange. It also switches the Internet call to a built-in Remote Access Switch (RAS) which then routes the traffic to the Internet network. The RAS has an Ethernet interface, which is connected to the Internet using any suitable routing device.
The CBS is normally connected to the DIU using three twisted-pair wires, which carry signals as well as power from the DIU to the CBS. Alternatively, it can be connected to the DIU using a standard E1 interface (on radio, fibre, or copper) as shown in fig.3. A BSD can support up to 4 CBSâ„¢s.
For long-range communication, a WS-IP or WS can also be connected to the CBS using a two-hop DECT wireless link, one between WS-IP or WS and a Relay Base Station (RBS) and another between the RBS and CBS, as shown in fig.4. The wireless range supported between a WS-IP or WS and CBS or RBS is 10 km in Line of Sight (LOS) conditions. The range supported between a CBS and RBS is 25 km in LOS conditions.
A typical system consists of one DIU with one or two RAS units, up to 20 CBSâ„¢s, and up to a 1000 WS-IPâ„¢s or WSâ„¢s. The BSD and RBS units are used as required by the deployment scenario.
C. Sub-systems of corDECT Wireless Access System
1. Wallset IP and Wallset
As shown in fig.5, the Wallset with Internet port (WS-IP) provides voice connectivity to the subscriber using a RJ-11 interface enabling one to connect a standard DTMF or decadic telephone, G3 fax machine, PCO, speaker phone, cordless phone, or modem. In addition the WS-IP has a RS-232 port to directly connect a PC (obviating the need for a telephone modem). The PC establishes a dial-up PPP (Point-to-Point Protocol) Internet connection using a standard dial-up utility. Internet access is supported at 35 or 70 kbps. In fact, WS-IP can support simultaneous voice 35 kbps Internet connections.
Besides these 2 user interfaces, the WS-IP has an antenna port where either a whip antenna, or an externally mounted antenna (through cable), can be connected. The power to the WS-IP is provided by a 12 V adapter connected to the AC mains and optionally by a solar panel which can be connected in parallel. The WS-IP has a built-in battery and battery charger. The built-in battery provides 16 hours stand-by time and more than 3 hours talk time for voice calls. A Wallset (WS) is a similar terminal without the Internet port.
The multiwallset (MWS), shown in fig.6 provides simultaneous voice service to 4 subscribers. It has all the features of WS, but at a significantly lower per-line cost.
The MWS has a DECT transceiver module (DTM), which is an outdoor unit with a built-in antenna with 7.5 dB gain. It is connected to an indoor subscriber interface module (SIM) unit, which has 4 RJ-11 ports for telephones. Each port supports all the terminals a WS supports.
The connection between the DTM and the SIM uses a single twisted-pair wire, obviating the need for RF cable and connectors. The MWS has a built-in battery for backup and is powered through the AC mains.
3. Multiwallset IP
The Multiwallset with Internet port (MWS-IP) is a MWS with 4 telephones and additional Ethernet interface to provide dial-up Internet connectivity. Multiple PCâ„¢s can be connected to the Ethernet port and provide a shared 35/70 kbps Internet connections. The PPP-over Ethernet protocol is used to set up individual connections.
4. Compact Base Stations
The CBS shown in fig.7 provides the radio interface between the DIU and the corDECT subscriber terminals. It supports up to 12 simultaneous voice calls. It is a small unobtrusive, weather proof unit that is remotely powered from the DIU or a BSD.
The CBS has two antennas for diversity. A directional antenna with significant gain can be used when coverage is required to be confined to certain directions. The maximum LOS range between a subscriber unit and a CBS is 10 km. The CBS is connected to a DIU or a BSD with 3 twisted-pair copper wires, each of which carry voice or data traffic, signaling and power. The maximum loop length, with 0.4 mm diameter wire, can be 4km between the DIU and the CBS and 1 km between the BSD and the CBS.
5. DECT Interface Unit
The DIU shown in fig.8 implements the functions of a switch (or a RLU), Base Station Controller, and the Operations and Maintenance Console (OMC). System reliability is guaranteed by a redundant, hot stand-by architecture. The OMC allows exhaustive real-time monitoring and management of the entire corDECT system. A fully- configured DIU with an in-built RAS only occupies a single 28 U, 19 cabinet and consumes less than 600 W.
Up to 20 CBSâ„¢s can be supported by a DIU, directly or through the BSD . The DIU provides up to 8 E1 links to the telephone network and/or RAS. The signaling protocol used is either V5.2, which parents the DIU to an exchange, or R2-MF, in which case the DIU acts as a 1000 â€œline exchange
6. Remote Access Switch
The RAS ,shown in fig.9,is a 19 1U unit normally integrated within the DIU cabinet .It terminates the PPP connections from Internet subscribers using corDECT WS-IP or MWS-IP .It is connected to the DIU using up to 2 E1 ports and does IP based routing for up to 60 simultaneous corDECT Internet calls. The RAS has a 10BaseT Ethernet port to connect to the Internet.
7. Base Station Distributor
The BSD is a compact, remotely located, locally powered, rack-mountable unit that supports up to 4 CBSâ„¢s. The E1 interface between a DIU and the BSD can be on copper, fiber, or radio and link distance depends only on the link design. The BSD is designed to extend corDECT coverage to packets of subscribers located far away from the DIU.
8. Relay Base Station
A RBS as shown in fig.11 extends the range of the corDECT system by relaying DECT packets between the CBS and the subscriber units. The RBS can handle 11 calls simultaneously. The RBS consists of 2 units. The RBS Air Unit is typically mounted on a tower/mast and houses the baseband and the RF sub-system. The RBS Ground Unit supplies power and provides maintenance supports to the Air Unit and is mounted at the bottom of the tower.
The maximum LOS range between a CBS and a RBS is 25 km while the maximum LOS range between the RBS and corDECT subscribers is 10 km.
9. Network Management
corDECT provides comprehensive operation and maintenance through the corView OMC console. Itâ„¢s repertoire includes h/w and s/w configuration, subscriber administration, accounting, fault notification, and traffic management.
This easy to use, menu-driven console can be run either locally or remotely. When used remotely, a single corView workstation serves as an NMS for a number of corDECT systems. corView can also be used with the CygNet NMS to provide integrated management of a network of corDECT and other systems. corView supports the SNMP protocol and can be connected to the corDECT system by any IP network.
corDECT Features at a Glance
The corDECT WLL system provides features and services comparable to the best wire line systems.
1. Voice Quality
corDECT delivers the same toll-quality speech performances as a good copper-based local loop. Toll-quality voice is ensured by using 32 kbps ADPCM for voice digitization as per the ITU-T G.726 standard. ADPCM also ensures transparency to DTMF signals for interactive Voice Response Systems.
2. Data Services
The employment of 32 kbps ADPCM permits all voice-band data services available from a conventional wired connection. It is also possible to occupy a double time slot on air to transmit at 64 kbps with error correction. This can be used for data connectivity at speeds similar to the best wireline speed.
3. Internet Access Speed
Internet Access is possible simultaneously with a voice call using the WS-IP. There are two access rates: 35 kbps and 70 kbps, using one and two time slots respectively.
The system supports payphone with battery reversal as well as 12 kHz/16 kHz metering pulses. The pulses are provided by the WS for an external charge meter.
5. System Capacity
Each corDECT system supports up to 1000 subscribers. Itâ„¢s base stations can evacuate more than 150 E of traffic and funnel it to the telephone network and Internet using up to 8 E1 links.
6. Air Interface Transmit Power
The power transmitted by a WS or Base Station nominally is 250 mW during the burst or about 10 mW on the average. This ties in with the need for small cells to enhance frequency re-use and also conserves battery power.
7. Typical CBS Coverage
The coverage achieved by corDECT is 10 km in LOS conditions, made possible by enhanced receiver sensitivity, a patented timing adjustment feature and compact high gain antennas. The non-LOS(N-LOS) coverage varies from 400 m to 1 km depending on the way the CBSâ„¢s are installed.
8. Typical RBS Coverage
The RBS can be at a maximum distance of 25 km from the CBS and it can serve subscribers in a 10 km radius around it. The RBS is primarily meant to be used in rural or sparsely populated areas. It also finds occasional use in urban areas for covering regions in shadow.
9. Authentication and Subscription
Authentication is the process by which a corDECT subscriber terminal is positively verified as belonging to a legitimate subscriber of a particular DIU. It is invoked during call setup for every call. It can also be invoked during other circumstances like termination of access of a WS by the DIU. Authentication involves an Authentication Key which is never transmitted on air .The keys are maintained securely in the system and are inaccessible to anyone.
Subscription is the process by which a subscriber is added/deleted from the system and the features the subscriber desires to have are enabled. It is also the process by which the system formally transfers the identity, such as subscriber number, to the WS.
The WS can use this to:
(i) gain access to the system and make calls and
(ii) recognize the system in order to receive calls
The DIU can use this to:
(i) validate service requests from WS,
(ii) limit access to classes of service, and
(iii) recognizes calls for valid WSs in order to route calls to them
10. Major Subscriber Services
The corDECT system when operating in Switch Mode provides all the
services of a large modem exchange. Some of the important services are:
Â¢ Standing Alarm Call Service
Â¢ Occasional Alarm Call Service
Â¢ Call Completion Supplymentary Services
- Absent subscriber
- Call waiting
11. OMC Features
The corDECT systemâ„¢s OMC supports the following:
Â¢ System Administration Features
- Subscriber administration
- E1 line administration
- Billing databases
- PSTN ports and CBS administration
Â¢ Traffic Analysis
- Exchange traffic
- CBS traffic
- Subscriber traffic
1. Towards Always-on Internet Access
Internet access is characterized by bursts of packets with long periods of inactivity. If the wireless connection is suspended during inactive periods and resumed quickly when there is a burst of traffic, the available wireless channels can be used by a much larger number of subscribers. DECT provides for such suspension and quick resumption of connections, using its powerful control-plane signaling protocols.
Development is in progress to build this new capability into the corDECT system. When it is available, a very large fraction of the 1000 subscribers in each system can be logged onto the Internet simultaneously and remain logged on for as long as desired.
2. Packet-Switched High Speed Internet Downloading
It is highly desirable for a user to have the ability to download from the Internet at a high peak bitrate, even if the download-channel is shared by many users, each accessing it when needed. The high bitrate of the DECT air interface is eminently suited for providing this type of service. A major new development of the corDECT system underway is a packet-switched shared downlink Internet channel at 384 kbps.
3. More Integration for Cost-Effectiveness
A next-generation subscriber terminal is under development which is more integrated and compact. It will provide several options: one voice line, two voice lines, or one voice line + one Internet port.
4. VoIP in corDECT
The corDECT system employ DSPâ„¢s extensively. As there is a powerful DSP in every WS, the voice signals can be converted to/from packets at the WSs themselves, transmitted on air in packetized form and hence to the Internet through a gateway at the DIU. Thus, the corDECT system can be made VoIP-compatible in a very efficient and cost-effective manner.
The corDECT development team will continue to make available new products to take corDECT to ever higher levels of performance. At the same time, existing versions will be maintained and upgraded. This will ensure that corDECT remains the most versatile and cost-effective WLL system with the best suite of features and services.
APPENDIX Digital Enhanced Cordless Telecommunications
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 LANâ„¢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. DECT has been specified to make possible lowcost subscriber terminals, high subscriber density with heavy call-traffic levels, wirelinequality 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 and (iv) cost-effective modulation/demodulation techniques. The next two sections focus on some of the key features of the DECT standard.
DECT: Some Salient Features
i. Frequency Band: The RF band originally allotted to DECT is 1880 â€œ 1900 MHz, though the entire 20 MHz need not be employed by each system. All DECT-based systems including private and public systems operate on the common band with no requirement for regulation. An extended DECT band that includes the band 1900 â€œ 1935 MHz is also
ii. Mode of Access: The DECT standard employs a version of Time Division Multiple Access (TDMA). There are 10 frequencies of operation in a 20 MHz band, with a spacing of 1.728 MHz. The burst-rate is 1.152 Mbps, accommodating 24 slots. The communication is Time Division Duplex (TDD). This not only ensures that propagation conditions are identical at any time in both directions of transmission, but also simplifies transceiver design. The 24 slots in a TDMA frame are divided into two groups of 12 slots each, one group for each direction of transmission. The frame structure is shown in Figure A.1. The frame duration is 10 ms and a TDD slot-pair is separated by 5 ms.
DECT frame structure
iii. Multi-Carrier TDMA: A very important difference that sets DECT apart from conventional TDMA systems is that all the slots in a TDMA frame need not be transmitted on the same frequency. Each of the 12 slots could be on a different frequency, though the pair of slots used for each TDD link must be on the same frequency. This variation of TDMA is called Multi-Carrier TDMA (MC-TDMA) and is the key to the high DECT frame structure capacity achieved by DECT. The 12 slot-pairs and 10 frequencies give rise to 120 channels, as if they were independent of one another. A Wallset can operate on one or more of these 120 channels, while a Base Station receives and transmits on a maximum of 12 of them at a given time. The concept of MCTDMA is illustrated in Figure A.2 for a hypothetical frame of three slots, with each
slot employing a different frequency.
iv. Transmit Power: The power transmitted by Wallset or Base Station is 250 mW during the burst, or about 10 mW average power. This ties in with the need for small cells to increase frequency re-use and conserves battery power.
v. Voice Digitization: DECT employs 32 kbps ADPCM. This ensures toll quality and permits all the data (fax/modem) services available from a conventional wired connection. It is also possible to occupy a double-slot to transmit at 64 kbps with error connection. This can be used for PCM or for data connectivity.
vi. Modulation: DECT employs Gaussian Frequency Shift Keying (GFSK) with a Gaussian Filter (BT=0.5). Only 75% of the burst rate of 1.152 Mbps is used for voice. DECT employs ADPCM for its high voice quality and GFSK because transceiver cost is reduced. By throwing in generous signaling capacity, DECT is able to employ a very sophisticated channel selection procedure. This is the most important aspect of DECT which sets it apart from existing cellular systems and is discussed below.
vii. Channel Allocation: Mobile Cellular Systems hitherto employ the so-called Fixed Channel Allocation (FCA) approach. Here, the available channels are distributed among neighboring cells in a planned fashion, depending on traffic needs. Channels are reused at appropriate distances based on the terrain, transmit-power, antenna height, etc. Channels are allocated from the allotted set to users on demand by the Base Stations and hand-off is controlled by the network of Base Stations as the mobile user crosses over into neighboring cells. Systems like GSM employ Mobile-Assisted Hand-Off (MAHO) but the hand-off is still centrally controlled. When deciding the reuse distance in an FCA-based system, one needs to make allowance for shadowing (due to obstructions). Re-use is decided based on worst-case scenarios, assuming the best propagation path for the interference and worst-case shadowing of the desired signal. The DECT standard employs a completely decentralized channel allocation procedure called Dynamic Channel Selection (DCS) or Adaptive Channel Allocation (ACA). In this approach, the available set of channels is not distributed a priori among the cells. Any Wallset can set up a call on any of the channels, deciding on the one it will use at a given time by measuring the signal strength in that channel at its geographical location.
The so-called received signal strength indication (RSSI) is used for this purpose. Based on a table of RSSI measurements for all channels, which is continuously updated, MC-TDMA the Wallset selects the strongest Base Station signal received at the given location at that time to lock onto, and the quietest channel to communicate with the Base Station. This scheme requires that Base Stations transmit some signal even if no calls are in progress, i.e., a beacon, or dummy bearer in DECT parlance, is a must when the Base Station is idle.
viii. Encryption and Authentication: DECT provides encryption of the voice signal or data, to prevent eavesdropping. Authentication allows one to curb unauthorized use of the Wallset.
AC Access Centre
ADPCM Adaptive Differential Pulse Code Modulation
ARQ Automatic Repeat Request
BSD Base Station Distributor
CBS Compact Base Station
CCB Coin Collection Box
CDMA Code Division Multiple Access
CSMUX Concentrating Subscriber Mux
DCS Dynamic Channel Selection
DECT Digital Enhanced Cordless Telecommunications
DID Direct In-Dialing
DIU DECT Interface Unit
DPRS DECT Packet Radio Service
DSL Digital Subscriber Line
DSP Digital Signal Processor
DTMF Dual Tone Multi-Frequency
ETSI European Telecommunication Standards Institute
FCA Fixed Channel Allocation
FDMA Frequency Division Multiple Access
FTP File Transfer Protocol
GFSK Gaussian Frequency Shift Keying
HDSL High-speed Digital Subscriber Line
IS-95 CDMA Cellular Standard
ITU-T International Telecommunication Union - Telecommunication
Standardization Sector LOS Line-of-Sight
MC-TDMA Multi-Carrier TDMA
MWS-IP Multiwallset with Internet Port
NAT Network Address Translation
N-LOS Non Line-of-Sight
NMS Network Management System
OMC Operation and Maintenance Console
PAP Password Authentication Protocol
PBX Private Branch Exchange
PPP Point to Point Protocol
PSTN Public Switched Telephone Network
RADIUS Remote Access Dial-in User Service
RAS Remote Access Switch
RBS Relay Base Station
RLU Remote Line Unit
RSSI Received Signal Strength Indicator
RSU Remote Switching Unit
SNMP Simple Network Management Protocol
STD/ISD Subscriber Trunk Dialing/International Subscriber Dialing
TCP/IP Transmission Control Protocol/Internet Protocol
TDD Time Division Duplex
TDM Time Division Multiplexing
TDMA Time Division Multiple Access
V5.2 Interface protocol for connecting an access network to a PSTN
WAS Wireless Access System
WS-IP Wallset with Internet Port
1. Information Technology Magazine, May 2003.
2. Electronics for you Magazine.
4. Computer Networks, Tanenbaum.
5. Mobile Communications, Schiller J
I express my sincere gratitude to Dr. Agnisarman Namboodiri, Head of Department of Information Technology and Computer Science , for his guidance and support to shape this paper in a systematic way.
I am also greatly indebted to Mr. Saheer H. and
Ms. S.S. Deepa, Department of IT for their valuable suggestions in the preparation of the paper.
In addition I would like to thank all staff members of IT department and all my friends of S7 IT for their suggestions and constrictive criticism.
1. INTRODUCTION 1
2. corDECT WIRELESS ACCESS SYSTEM 2
A. CONCEPTUAL ACCESS SYSTEM
B. corDECT WIRELESS ACCESS SYSTEM
C. SUBSYSTEMS OF corDECT WIRELESS ACCESS SYSTEM
3. corDECT FEATURES AT A GLANCE 11
4. corDECT FUTURE 14
5. CONCLUSION 16
6. APPENDIX : DIGITAL ENHANCED CORDLESS TELECOMMUNICATIONS 17
A. DECT: SOME SALIENT FEATURES
7. ABBREVIATIONS 21
8. REFERENCES 23
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