Technology in Capsules

Article Index

2006-12-31T23:40:00.000+05:30

Latest

VoIP

Communication Related Articles

Home Networking Related Articles

HomePNA

Computer Related Articles

Miscellaneous Articles

World Of Transforms

WI-MAXIMUM

2006-09-15T11:42:00.000+05:30

WiMax ou interopérabilité sans fil pour l'Access de micro-onde, est un IEEE 802.16 a basé la norme pour les réseaux sans fil de zone métropolitaine (WMANs). IEEE 802.16 est groupe de travail le numéro 16 d'IEEE 802, se spécialisant dans l'accès sans fil à bande large point-à-multipoint. WiMax est soutenu par l'industrie elle-même, y compris Intel, Dell, Motorola, Fujitsu, AT&T, British Telecom, France Telecom, confiance Infocomm, Siemens, Sify, tonneliers de Pricewatehouse et Tata Teleservices - formant une alliance appelée le forum de WiMax. Il représente la prochaine génération de la gestion de réseau sans fil. Le protocole de WiMax est une norme qui apporte la gestion de réseau capable calculant des dispositifs ensemble. Par exemple. Pour fournir l'Internet/Intranet accéder d'une manière semblable à WiFi. Tandis que la chaîne de WiFi est limitée jusqu'à 150 pieds, WiMax peut transmettre des données jusqu'à une distance de 48km. Il offre également des vitesses se reliantes mieux par rapport à d'autres protocoles. Il peut se relier à une vitesse maximum de 70Mbps. WiMax n'est pas conçu pour s'opposer avec WiFi, mais pour coexister avec lui. L'assurance de WiMax est mesurée en kilomètres carrés, alors que cela de WiFi est mesuré en mètres carrés. La norme originale de WiMax (IEEE 802.16) propose l'utilisation du spectre de fréquence de gigahertz 10-66 pour la transmission de WiMax, qui est bien au-dessus de la chaîne de WiFi (jusqu'à maximum 5GHz). Mais 802.16a a ajouté le soutien de la fréquence de gigahertz 2-11 également. Une station de base de WiMax peut être accédée par plus de 60 utilisateurs. WiMax peut également fournir des services de radiodiffusion également. Les caractéristiques de WiMax fournit également des équipements bien meilleurs que WiFi, fournissant une largeur de bande plus élevée et une sécurité de données élevée par l'utilisation des arrangements augmentés de chiffrage. WiMax peut également fournir le service dans la ligne de la vue (LOS) et la Non-Ligne des endroits de la vue (NLOS), mais la gamme changera en conséquence. WiMax permettra l'interpénétration pour la fourniture à bande large de service de VoIP, de vidéo, et d'accès d'Internet - simultanément. WiMax peut également fonctionner avec les réseaux mobiles existants. Les antennes de WiMax mettent en boîte la « part » une tour de cellules sans compromettre la fonction des rangées cellulaires déjà en place. Par conséquent la diversification des services peut être accomplie facilement. Puisque WiMax soutient plusieurs protocoles de transmission, ce réseau peut agir en tant qu'épine dorsale pour une ISP et un fournisseur de service de telecom. WiMax a été félicité comme solution pour le problème de connectivité de dernier-mille, dans des secteurs ruraux et urbains.

VoIP

2006-03-05T16:22:00.000+05:30

VoIP – An Introduction

VoIP stands for Voice over Internet Protocol. It provides a means of transmitting voice over Internet Protocol (IP). But for that purpose, voice is to be converted into some packet format so that transmission over IP is possible. VoIP technology helps to make calls between PC and PC, PC and telephone etc. The obvious advantage is the reduction in cost.

How It Works?

First, the speech signals are sampled at or above Nyquist Rate and the samples are quantized into discrete levels. These are then converted into digital format. For the above given process, we can use any of the standard like Pulse Code Modulation (PCM), Adaptive Differential PCM (ADPCM), MP-MLQ LPC-10 etc. At the receiving end you will have to reverse the things. Decompress the encoded data, reassemble packets in order, and convert these digital values into analog using a Digital to Analog Converter (DAC). Once data is converted into digital form, we have to think about transmitting it over IP. Here comes the greatest difficulty in VoIP – the real time nature of voice is very crucial, otherwise the sound would not be so easy to understand. But normal TCP/IP implementations are not real time, just like the nature of internet. You may have to wait until each packet is routed correctly through appropriate machines. But such things should not disturb the normal voice, since we want continuous speech from the other end. Hence VoIP implementations use UDP protocol instead of TCP. VoIP data packets live in RTP (Real-Time Transport Protocol) packets which are inside UDP-IP packets.

VoIP signaling Protocols

VoIP mainly uses two signaling protocols: H.323 and SIP (Session Initiation Protocol).
H.323 is an complex suite of protocols that provides specifications for real-time, interactive videoconferencing, data sharing, and audio applications such as IP telephony. SIP is smaller, more efficient, and takes advantage of existing protocols to handle certain parts of the process. MGCP (Media Gateway Control Protocol), for example, is used by SIP to establish a gateway connecting to the PSTN.

VoIP Advantage

Reduced Cost.
Can handle multiple calls at a time, like we browse many sites at a time.

VoIP Limitations

Unless the delay is overcome and a real time streamlining is obtained voice quality would be just unbearable.

Insecure lines - meaning anyone can listen to your conversation because of shared servers or open platforms.

TiVo

2005-09-16T23:20:00.000+05:30

TiVo is a type of personal video recorder similar to VCR. It is a device which allows its users to capture TV programs and view them later. Hence they allow the recording of programs and allows them to be viewed later. There is an internal hard disk for capturing programs. TiVo makes sense in the fact that it allows replay of live programs.

TiVo allows to record programs based on time, programs titles, and even based on the artists and popularity. Programs may be stored on the internal hard-disk untill it is full, and when it is full the first program recorded is deleted. It can also record progrmas based on the viewing habits of the user. The playback can be done while recording another program. This is unlike the traditional tape recorders which allows either playback or recording at a time. TiVo device also allows interface with personal computer and even with home networks.Standalone TiVo systems can only record one channel at a time. Even with digital cable or satellite services that may provide multiple simultaneous signals, these units rely on an external convertor to select and decode digital signals.

TiVo was started in 1997 by veterans of Silicon Graphics and Time Warner. TiVo unit is built by TiVo Inc. They provide harware and linux based software for TiVo.TiVo systems are based on PowerPC or MIPS processors, connected to MPEG-2 encoder/decoder chips and high-capacity IDE/ATA hard drives. Early TiVo units used one or two 13GB drives; typical units have a drive of 40-140GB in size. Although not supported by TiVo or equipment manufacturers, larger drives can be added.

TiVo service is available only in United States and United Kingdom, at present. But there has been work arounds which makes it to work in Australia, New Zealand, Canada, and the Netherlands.

HomePNA

2005-08-21T00:15:00.000+05:30

Ethernet has been the de-facto standard for local area communication for a long time. Initially there was 10Mbit, then came 100Mbit and now its not rare to find 1GBit Ethernet. But the need for the communication medium has also been changed. Now a days the trend is to become wireless. But the often forgotten phone line can also provide a decent performance when it comes to a communication that is not very bandwidth hungry. During the mid 90’s a company provided its own technology for providing data access through telephone lines at 1Mbps and it became very popular. Then different companies like AMD formed an alliance named Home Phone-line Network Alliance (Home PNA) to provide standardization for the telephone line communication. They developed a data transfer standard based on the Tut Systems' technology for telephone lines and called it HomePNA 1.0. The first version of this standard was identical to the Tut Systems' technology - 1 Mbit/s, 25 computers in the HomePNA 1.0 network and a communication range about 150 m. Then they released network cards (PCI and USB), different communicators, Ethernet-to-HomePNA bridges etc. The HomePNA technology is a usual Ethernet with 1 Mbit/s (HomePNA 1.0) and 10 Mbit/s (HomePNA 2.0) in all aspects. The CSMA/CD, IEEE-802.3, MAC addresses are applicable not only for the Ethernet but also for both HomePNA standards. This technology differs from the Ethernet only on a physical level. And installation of HomePNA cards doesn't differ from that of HomePNA adapters. Operating systems operate with these adapters as with usual Ethernet ones.

HomePNA 1.0 is used successfully in office buildings - practically all of them have their own telephone network, which can be used for the Internet as well. It is very convenient both for clients and providers. HomePNA networks can be built in those buildings which have phone jacks. I.e. you don't need hubs and switches, but only HomePNA cards. Theoretically, the HomePNA 2.0 standard has every chance to reach 100 Mbit/s speed!

Near Field Communication (NFC)

2005-08-15T12:38:00.000+05:30

Near Filed Communication (NFC) is a close range radio communication protocol used for very sensitive applications. It was jointly developed by Sony and Philips. The standard specifies ways to establish P2P(Peer-to-Peer) communication links for data exchange. After the P2P network has been configured, another wireless communication technology, such as Bluetooth or Wi-Fi, can be used for longer range communication or for transfering larger amounts of data. Its development was parallel to RFID (Radio Frequency Identification), but both differ in many ways. NFC offers a very short range as compared to RFID. This is an added advantage in the sense that it requires very little transmission power and cheap transmitters can be used for the purpose. Hence it is very suitable for Smartcard like applications. It can also work in both active and passive modes. NFC works on a frequency range of 13.56 MHz. It offers a baud rate of 106 kbps to 424kbps. The transmission is made from a frequency of 13.56MHz inductively, hence it uses high magnetic field. At a transaction only two participants can be involved - one transmitter (initiator) and one receiver (target). The transmission can be either in active fashion or passive fashion. Both have their own merits and demerits. The NFC transmission runs helping duplex, i.e. that one of the two devices can send only in each case or receive at a time.

Ultra-Wideband (UWB) Technology

2005-08-15T12:37:00.000+05:30

Ultra-Wideband (UWB) Technology is a technology designed for short range, wireless personal area networks. It offers people freedom from wires, ie let them go wireless. It brings the convenience and mobility of wireless communications to high-speed interconnects in devices throughout the digital home and office. This enables wireless connection of multiple devices for transmission of video, audio and other high-bandwidth data.

UWB, short-range radio technology, complements other longer range radio technologies such as Wi-Fi*, WiMAX, and cellular wide area communications. It is used to relay data from a host device to other devices in the immediate area (up to 10 meters, or 30 feet).

ZigBee™

2005-08-15T12:36:00.000+05:30

ZigBee™ is the name of an alliance of companies formed around IEEE's 802.15.4 specification for low data rates in the Industrial, Scientific and Medical (ISM) radio bands. The ZigBee protocol promises to provide longer battery life and to be a lower-cost alternative to Bluetooth for wireless sensing and control applications. Companies looking for reliable and secure wireless monitoring and control solutions have a new alternative –- ZigBee™ (www.zigbee.org). ZigBee provides the network, security and application profiles layers for the IEEE 802.15.4 global standard for reliable, low-power, wireless data communications.

HSDPA

2005-08-15T12:34:00.000+05:30

HSDPA(High Speed Downlink Packet Access) improves system capacity and increases user data rates in the downlink direction, that is, transmission from the Radio Access Network to the mobile terminal. This module, which is designed to supplement the UMTS Air Interface tutorial, explores the HSDPA channel architecture along with the additional functionality required in the UMTS MAC (Medium Access Control) layer. The processes of adaptive modulation and coding along with procedures by which HSDPA data is scheduled for transmission across the air interface and the re-transmission patterns that are employed to ensure the high data rates supported by this technique are maintained.

Unlicensed Mobile Access

2005-08-15T12:33:00.001+05:30

Unlicensed Mobile Access (UMA) technology grants access to GSM and GPRS mobile services over unlicensed spectrum technologies, including Bluetooth and WiFi. Service providers, by enabling UMA technology, can enable subscriber roam and smoothly handover between cellular networks and between private and public unlicensed wireless networks using dual-mode mobile handsets. In the usual handover process between networks the user experience is not so good, but the UMA technology makes this process simple and provides the user with a nice experience during handover in both voice and data transfers. the convergence of mobile, fixed and Internet telephony (fixed-mobile convergence).

The core components of the system are:

UMA enabled dual mode handset.
UMA Network Controller (UNC) with broadband access
Unlicensed Mobile Access Network (UMAN)

The roaming process is completely transparent to the user. The handover process is also transparent.

In order to promote the use of UMA, a number of leading companies like Kineto, Motorola, Alcatel etc. have developed an open standard. These standards are available to vendors and carriers of the mobile communications systems to develop and deploy their own solutions. addition to developing and maintaining the initial specifications, the participating companies are actively working with the 3GPP standards organization to use the specifications as the basis for the development of a formal standard.

Wi-MAX

2005-08-15T12:31:00.000+05:30

WiMax or Wireless Interoperability for Microwave Access, is an IEEE 802.16 based standard for Wireless Metropolitan Area Networks (WMANs). IEEE 802.16 is working group number 16 of IEEE 802, specializing in point-to-multipoint broadband wireless access. WiMax is supported by the industry itself, including Intel, Dell, Motorola, Fujitsu, AT&T, British Telecom, France Telecom, Reliance Infocomm, Siemens, Sify, Pricewatehouse Coopers and Tata Teleservices – forming an alliance called WiMax Forum. It represents the next generation of wireless networking.

The WiMax protocol is a standard that brings networking capable computing devices together. Eg. To provide internet/intranet access in a way similar to WiFi. While the range of WiFi is limited up to 150 feet, WiMax can transmit data up to a distance of 48km. It also offers better connecting speeds as compared to other protocols. It can connect at a maximum speed of 70Mbps. WiMax is not designed to clash with WiFi, but to coexist with it. WiMax coverage is measured in square kilometers, while that of WiFi is measured in square meters. The original WiMax standard (IEEE 802.16) proposes the usage of 10-66 GHz frequency spectrum for the WiMax transmission, which is well above the WiFi range (up to 5GHz maximum). But 802.16a added support for 2-11 GHz frequency also. One WiMax base station can be accessed by more than 60 users. WiMax can also provide broadcasting services also.

WiMax specifications also provides much better facilities than WiFi, providing higher bandwidth and high data security by the use of enhanced encryption schemes. WiMax can also provide service in both Line Of Sight (LOS) and Non-Line Of Sight (NLOS) locations, but the range will vary accordingly. WiMax will allow the interpenetration for broadband service provision of VoIP, video, and internet access – simultaneously. WiMax can also work with existing mobile networks. WiMax antennas can "share" a cell tower without compromising the function of cellular arrays already in place. Hence diversification of services can be accomplished easily. Since WiMax supports several communication protocols, this network can act as the backbone both for an ISP and telecom service provider. WiMax has been praised as the solution for the last-mile connectivity problem, in both rural and urban areas.

MIMO (Multiple Input Multiple Output)

2005-08-15T12:30:00.000+05:30

The dream of every communication designer is to provide good quality service to each of his customer within the limited available bandwidth. But in the recent years, the amount of data flowing through the channel has increased. But there has been no such high increase in the available bandwidth. So it is a challenge to effectively utilize the channel spectrum along with providing good quality of service across wireless links. One of the possible ways is to use multiple antennas at both the ends of the transmission link. MIMP exploits natural phenomena like multi-path propagation to increase throughput, reduce error rates etc. rather than trying to eliminate them.

The main motive behind the MIMO was to increase the user data rates within the constrained spectrum. The initial application of MIMO was proposed for indoor wireless LAN, fixed wireless access networks etc. But now the aim to widen its applications to cover other areas also. The core idea behind the system is to exploit the de-correlation of multiple received signals in the presence of multi-path propagation, there by separating the data streams occupying same bandwidth. Hence Rayleigh fading and constrained total power comes into the figure.

Data is transmitted in busts and the receiver knows the channel pattern through the usage of training algorithms; but not necessarily the transmitter. The training sequence enables the receiver to predict the channel coefficients and extract the data coming in multiple streams. To maximize the effect of transmitting rate half of the time interval is used for data transmission and half for training. The adaptive transmission is possible only if the transmitter knows the channel coefficients in advance, so that the more data can be sent through good channels. In case of time division duplex channels, this requires the channel to be stationary and hence channel details need to be fed back at the same rate as the channel characteristics are changing. In case of the frequency division duplex channels, the coefficients should be transmitted at a different frequency. To overcome the fast feedback requirement, the spatial mean of channel coefficients has been proposed; instead of the instantaneous values. This greatly enhances the channel capacity, in case of correlated channels.

There is an ongoing effort to standardize the MIMO standard under the name IEEE 802.11n. It will offer up to eight times coverage and about six times data rates, of current 802.11g networks.

USTS (Uplink Synchronous Transmission Scheme)

2005-08-15T12:29:00.000+05:30

To maximize the usage of 3G uplink capacity the 3GPP (Third Generation Partnership Project) is exploring techniques in which channel redundancy can be exploited. One approach is to implement USTS in which signals from mobiles within the cell are orthogonalized. That is, a number of mobiles are allocated the same scrambling code, but different channelization codes. This paper explores aspects of USTS including OVSF (Orthogonal Variable Spreading Factor) code allocation and timing both for initial synchronization and closed loop timing in order to maintain the channel timing reference within tolerable limits.

MBMS (Multimedia Broadcast / Multicast Service)

2005-08-15T12:28:00.000+05:30

The MBMS is a unidirectional point to multipoint bearer service in which data is transmitted from a single source entity to multiple recipients. These services will typically be in the form of streaming video and audio and should not be confused with the CBS (Cell Broadcast Service) that is currently supported. This paper describes the architecture of the MBMS along with its functional notes and integration into 3G and GERAN (GSM & EDGE Radio Access Network) with Core Network, UTRAN (UMTS Terrestrial Radio Access Network) and radio aspects being explained.

Interworking 2G and 3G Networks

2005-08-15T12:27:00.000+05:30

Ensuring seamless handover between 2G networks such as GSM and 3G networks such as UMTS will be vital if next generation cellular networks are to be rolled out successfully. This module explores many of the mobility management procedures that will need to be implemented to support effective 2G/3G interworking both in the Circuit Switched and Packet Switched domains. The paper focuses on the signalling required in order to facilitate interworking as well as issues regarding networks operating at different releases of the 3GPP specification, and how these shortfalls may be overcome.

TDD (Time Division Duplex)

2005-08-15T12:26:00.000+05:30

The use of TDD (Time Division Duplex) across the UMTS air interface to offer low range asymmetric data flows appear an attractive option to operators who are planning to offer 3G data hotspots to subscribers. This module explores UMTS TDD operation in detail and discusses the frame formats and procedures that may be used across the air interface. The module also describes the issues surrounding TDD operation such as cell blocking and power control as well as the advantages that can be leveraged from such a system.

EDGE (Enhanced Data rates for Global Evolution)

2005-08-15T12:25:00.001+05:30

This paper examines the rational behind EDGE and the modifications required within the network to support it. Concentrating on GSM (Global System for Mobile Communications), the principles of 8PSK (Eight Phase Shift Keying) are discussed with regards power control and planning etc. The module also examines the modifications required by GPRS (General Packet Radio Service) to support EDGE and in particular the protocols of RLC (Radio Link Control) and MAC (Medium Access Control).

The principles of convolutional coding are also examined with regards the nine modulation and coding schemes in addition to describing the significance of the EDGE families and the use of puncturing schemes.

Location Services - 2G/3G Implementation

2005-08-15T12:25:00.000+05:30

Location based architecture may be implemented into mobile networks in a number of ways including TA (Time Advance), TOA (Time of Arrival) EOTD (Enhanced Observed Time Difference) and GPS. This module describes the concepts of these positioning methods. It also explores the location based network functions and their interaction with the cellular network nodes such as the HLR, the packet and circuit switching functions as, well as the RAN (Radio Access Networks).

Assisted GPS

2005-08-15T12:24:00.000+05:30

Assisted GPS works by providing a GPS reference network in which a series of receivers have a clear view of the sky and can operate continuously. This reference information is then made available to cellular networks such as GSM and UMTS to refine location accuracy within the sphere of location based services. This module discusses the interaction between the reference network and the Location functions within a cellular network and how these combine to offer accurate location information.

MExE (Mobile Execution Environment)

2005-08-15T12:23:00.000+05:30

MExE provides a standardized execution environment within the UE, and enables the negotiation of capabilities with the MExE service provider. This paper describes the types of service that can typically be delivered using MExE and the architecture of the MExE environment. It then goes on to discuss how MExE services may be accessed such as remote service access or application download. Finally the concepts of MExE and UE classmarks are discussed along with an overview of MExE Classmarks 1, 2 and 3.

W-AMR (Wideband - Adaptive Multi Rate)

2005-08-15T12:22:00.001+05:30

Wideband variable rate speech codecs such as the W-AMR codec used in UMTS systems offers superior wideband speech quality at data rates comparable to current narrowband speech codecs. This paper does not focus on the actual operation of the W-AMR speech codec itself, but its impact on the UTRAN (UMTS Terrestrial Radio Access Network) and air interface channels that must support its operation.

SIM Application Toolkit

2005-08-15T12:22:00.000+05:30

The SAT (SIM Application Toolkit) provides a flexible interface through which developers can build services and MMI (Man Machine Interface) in order to enhance the functionality of the mobile. This module is not designed for service developers, but network engineers who require a grounding in the concepts of the SAT and how it may impact on network architecture and performance. It explores the basic SAT interface along with the architecture required in order to deliver effective SAT based services to the handset.

RADIUS (Remote Access Dial In User Service)

2005-08-15T12:21:00.000+05:30

<>Radius is an acronym that stands for Remote Authentication Dial In User Service is a protocol used for applications such as IP mobility and network access –both for local access and roaming access. It is a protocol for Authorization, Authentication and Accounting. RADIUS was originally developed by Livingston Enterprises. But now there exist many commercial and open source RADIUS servers. RADIUS server is responsible for the verification of the login information (Username & Password) that is passed over the RADIUS protocol. It uses any of the authentication protocols like PAP, CHAP or EAP to authenticate this information and then provided the desired level of access if things were right. The login information is passed on to a Network Access Server (NAS) over Point-to-Point Protocol (PPP). RADIUS also provides billing information and the servers are notified when a session starts and stops so that the user is billed accordingly. The billing information is also kept as a log in the servers so that they can be used accordingly in case of any analysis is required.

RADIUS servers can also read and write information from various data sources like text files and database etc. Simple Network Management Protocol (SNMP) is used for the remote monitoring of RADIUS servers. RADIUS is extensible, and many of the commercial products have their own hardware and software implementations for their own dialects.

<>RADIUS has a limitation when coming to the mobile communications involving 3G devices. They require highly sophisticated billing functions as they provide more services. Hence, a replacement for RADUIS, DIAMETER is also under consideration. The DIAMETER protocol provides full backward compatibility with RADIUS protocol and is expected to solve the current issues related to RADIUS.

Engineering the GRX (GPRS Roaming eXchange)

2005-08-15T12:20:00.001+05:30

This paper describes the rational behind the GRX in terms of worldwide access, security, reliability and scalability. It explores the structure of commercial, wide area, IP networks and the typical security and routing protocols employed within this environment. It is designed enable engineers who are working in this area to gain a firm understanding of the modern mobile packet switching architecture as well as a detailed insight into the requirement for, and operation of, the GRX.

SCTP (Stream control transmission protocol)

2005-08-15T12:20:00.000+05:30

SCTP (Stream control transmission protocol) is a transport level protocol providing end-to-end communication between two or more applications running in separate hosts. SCTP operates on top of the connectionless packet network supported by IP (Internet Protocol) offering a connection oriented, reliable transport mechanism for independently sequenced message streams. It was originally designed to provide a general purpose transport for message-oriented applications transporting signalling data. This paper describes the architecture and operation of SCTP including its features, the setting up of SCTP associations, data delivery and shutdown. Its position within the Sigtran architecture as a whole is also included.

TMN (Telecommunication Management Network)

2005-08-15T12:19:00.000+05:30

This paper provides detailed information on the ITU (International Telecommunications Union)
management model - TMN. It describes the TMN framework in terms of the managed object model in addition to describing the relationship between Manager and Agent.The module also explains the relationship between the TMN 5 layer model and standard network elements found with both fixed and wireless networks. This includes a description of the TMN functions; Operation System, Mediation Device, Q Adapter, Network Element, Work Station and Data Communication Network.The protocol CMIP (Common Management Information Protocol) is also discussed in addition to CORBA (Common Object Resource Broker Architecture) and the IP based protocol - SNMP (Simple Network Management Protocol).

SIGTRAN (Signalling Transport)

2005-08-15T12:16:00.000+05:30

This paper provides an overview of Sigtran by giving a basic introduction into the architecture of an all IP telecommunications network along with the operation of SCTP (Stream Control Transmission Protocol), M3UA (MTP3 User Adaptation) and M2UA (MTP2 User Adaptation) and where they may be utilized within the evolved 3G Core Network.

Signal Code Modulation

2005-08-14T18:39:00.000+05:30

This method present ways to eliminate the inherent quantization noise component in digital communications, instead of conventionally making it minimal. The primary analog signal is represented by a sample which is quantized and encoded digitally, and an analog component, which is a function of the quantization component of the digital sample. The advantages of such a system are two sided offering advantages of both analog and digital signaling. The presence of the analog residual allows for the system performance to improve when excess channel SNR is available. The digital component provides increased SNR and makes it possible for coding to be employed toachieve near error-free transmission.

Linux Filesystem

2005-07-17T21:29:00.000+05:30

The concept of file system is entirely diffrent in linux and windows. Windows uses drives to organise your files and directories. Linux uses a single root directory (represented as / ) to organise files. Other things are organised as sub-directories of this root folder. In windows, you need to know the drive in which a file resides to access that file. But under linux, you can access that file by reffering to the root folder. Even if you have multiple hard disks, everything is under the root folder /. The following list gives a list of second level directories under linux.


/bin/ ---- system binaries, user programs with normal user permissions
/sbin --- executables that need root permission
/data/ --- a user defined directory
/dev/ ---- system device tree
/etc/ ---- system configuration
/home/ --- users' subdirectories
/home/{username} akin to the Windows My Documents folder,
where files owned by a specific person live
/tmp/ ---- system temporary files
/usr/ ---- application software
/usr/bin - executables for programs with user permission
/var/ ---- system variables
/lib/ ---- libraries needed for installed programs to run
/mnt/ ---- mounted partitions, such as those on CD-ROMs and floppies

Each and every device (including removable devices and fixed hard disks) is
represented in linux as a sub directory under the root folder.

Hidden files are also treat with some diffrence under linux. Windows keeps this information under the file metadata or file attributes. But under linux hidden files start with a period (aka - dot eg: '.myfile' is a hidden file).

Shells supported by Linux Operating System

2005-07-15T18:31:00.000+05:30

The following are the most commonly used shells on linux platform:

bash (Bourne Again SHell)
sh (Bourne Shell, standard on many UNIX systems)
csh (C Shell, with a syntax similar to that of C language)
pdksh(Piblic Domain Korn Shell)
tcsh(Tiny C Shell, coomonly used for small systems)
sash(stand-alone shell, could be used when libraries are not available)
ash
zsh

A shell is a program that translates your actions sensed via input devices, or by other means, to the operating system. The default shell on linux machines is bash

Diffrence between linux and DOS(Windows)

2005-07-15T18:19:00.000+05:30

Please note that all the diffrence between linux and MS-DOS applies to MS Windows also unless stated otherwise.

Linux is case sensitive, whereas DOS is not.

Linux filename can contain a maximum of 256 letters including special characters, where as DOS can contain only 11 characters (8 for name+3 for extension). Windows is a special case here, it may contain 256 characters.

Shell for linux is bash(Bourne Again SHell)-most of the time and that for DOS is COMMAND.COM. For Windows it is EXPLORER.EXE

Linux supports file system security by default. DOS and Windows 9x does not support file system security. Only Windows NT/2K/XP allows file system security.

Linux is a multi user operating system. DOS is not. Windows is a multi user operating system.

Linux represents all the devices connected to your computer as files. For eg: your monitor is a file. This is not the case for DOS.

Linux supports networking by itself. But DOS does not. Windows also supports networking.

Linux is a multi-tasking OS, whereas DOS is not. Windows is also Multi-tasking.

These are the some of the more pronounced diffrences. More may folllow.

Oops!, I forgot the major diffrence, Linux is free and open source. But for M$ Windows or M$ DOS, you need to spend money.

CDMA Technology

2005-07-10T14:36:00.000+05:30

In usual TDMA, the entire frequency spectrum available is shared among the users. Each user is granted an access for a particular time, then the next gains access, after which another. After all currently using subscribers had a chance, the cycle repeats itself. The above case can be represented pictorially as:

TDMA requires synchronization among the users to achieve good performance.

In FDMA the entire frequency spectrum is divided into a number of units and allocated to each user. Hence, each user can use the allocated frequency spectrum for entire time period. It could be compared to AM or FM radio stations for which a particular operating frequency is assigned.

CDMA (Code Division Multiple Access) is different in that it neither assigns a particular time slot for each user nor it splits the frequency among the users. It allows all the users to use the entire frequency spectrum for all the time. This is achieved by what is called spreading. Each user is assigned a unique code, which allows them to distinguish between others. The base band data is spread with this unique code and a receiver having the same code can only receive the data. The code spreads the bandwidth of the user.

Spread Spectrum

Spread spectrum is a method of spreading the bandwidth such that the data occupies much more bandwidth than actually needed. Bandwidth spreading is achieved by using the above mentioned code. This code is independent of the base band data. The same code is used to despread the data at the receiver. The signal is transmitted in a channel which lies below the noise channel. The receiver then uses a correlator to despread the data and the resulting signal is passed through a narrow band pass filter. Unwanted signals are not despread and hence will not pass through the filter. Bits of spreading signal are known as chips. The rate of spreading is referred to as chip rate rather than bit rate.

Codes

CDMA codes are not supposed to provide call security, rather they are meant to provide a uniqueness which facilitates call identification. Codes should not correlate among themselves nor should they be time shifted versions of each other. Spreading codes should be like noise i.e. they should be random. If the narrow band interference is to be overcome, then the signal should have a balanced number of ones and zeros and it should have a single peak auto correlation function. Channel codes are designed to be maximum separated from each other and cell information codes are to be balanced not to correlate to other codes of itself. Codes are generated using modulo 2 adders, shift registers and feedback loops.

CDMA Advantage

Besides the above mentioned factors, it has the capability of using signals that arrive in the receivers, at different time. This phenomenon is called multipath. This is a problem for TDMA and FDMA. But in the case of CDMA, it is a blessing in disguise. CDMA combines multipath signals and make them stronger to provide a better signal. CDMA user units use rake receivers. This is a set of receivers which work together to provide a much better signal. One of the receivers (fingers) constantly searches for multipath signals and feeds the information back to the other fingers. Each receiver then demodulates the signal corresponding to a strong multipath. The combined results are then processed to get a better quality base band data.

World Of Transforms!

2005-07-09T13:43:00.000+05:30

How many transforms can you name? One, Three or more?
Here's a list

Fourier Transforms
Sine and Cosine Transforms
The Hartley Transform
Laplace Transforms
The Z-Transform
Hilbert Transforms
Radon and Abel Transforms
The Hankel Transform
Wavelet TransformThe Mellin Transform
Mixed Time-Frequency Signal Transformations
Fractional Fourier Transforms
Lapped Transforms
Discrete Time and Discrete Fourier Transforms

What is .NET?

2005-07-05T17:19:00.000+05:30

.NET (dot NET) is one of the words heard too often these days connected to the computer arena. What exactly is it? I will try to answer the above question in a simple and a meaningful way. While talking about the difference between C and C++ I have heard many people saying â€“ â€˜The difference is that C uses printf and C++ uses coutâ€?. This is not the difference. Likewise, there is a fallacy that .NET is Microsoft way of implementing Java. This is not the case. While .NET is providing all the features provided by Java, it does provide something extra. Sure, .NET should have been inspired by the phenomenal success of Java.

The .NET framework provides the following:

1. It provides a new way to access the Windows core functions. Traditionally this was done with C/C++. Visual Basic (before the introduction of .NET) used a different method to access the windows core API (Application Program Interface). So the type of access depends on the platform you are using. C/C++, VB, Delphi all had its own way of doing it. This is where .NET comes in. It provides a uniform way of accessing the operations provided by Windows, whatever be the programming language you are using. This portion of .NET is commonly referred to as the .NET Framework class library.

2. .NET provides a new infrastructure for application management. It provides numerous features for the same purpose â€“ code-level security, cross language class level inheritance, cross language type compatibility, hardware and platform level independence etc. Microsoft developed a new way of implementing all these â€“ The Common Language Runtime (CLR). The CLR includes the Common Type System (CTS) for cross-language type compatibility and the Common Language Specification (CLS) for ensuring that third-party libraries can be used from all .NET-enabled languages. Hardware and software independence is achieved by using Microsoft Intermediate Language (MSIL or just IL). It is similar to Java bytecode; all the .NET programs are compiled into IL. IL programs are converted into native machine language at runtime (JIT compiling). IL is never interpreted.

3.Greater Support for Web: The classic Active Server Pages (ASP), has been replaced by ASP.NET. While classic ASP was an interpreted one, the .NET version is a compiled one like mentioned above. This increases the execution speed greatly. In addition, the .NET developer can access all the .NET objects supported in web pages, compared to the six objects available for the classic ASP developer.

4. A new focus on distributed-application architecture. Visual Studio .NET provides top-notch tools for creating and consuming web services -- vendor- independent software services that can be invoked over the Internet.

The Story behind Java

2005-07-05T12:56:00.000+05:30

Sun Microsystems, the creators of the Java Programming Language says that with Java, you can "write once, run anywhere". What exactly makes this possible? The answer to this question lies in the fact that when you compile a java program, it is not converted into the native machine language as the other compilers like C/C++ does. But the program you have written is converted to what is called a â€˜Java Bytecodeâ€™. Your program is converted into this bytecode, whether you compile the program in UNIX or Windows. When the program is executed, this bytecode is converted to the native machine code by Java Virtual Machine (JVM). JVM is an interpreter that is used to convert bytecode to platform specific instructions. By using JVM you are decreasing the speed of execution by 10 folds. But this has an added advantage that the program can run in any platform.

Have you ever noticed an error occurring for a program written in C/C++ due to lack of some run-time files? No. But if you try to run the Java Program on a machine that has no JVM installed, it will not run. The secret behind platform independency also lies here. Whether you are running on a UNIX machine or a Windows machine, you need to install JVM targeted for the same operating system to get your program working. JVM provides the platform independency. There are separate downloads for each platform available on the Sun website.

Speed penalty of the Java programs can be overcome using JIT (Just In Time) compilers. These are compilers which compile the bytecode to native instruction set of the computer running the Java program. This will overcome the problem of speed penalty when using JVM.

DTMF Signalling

2005-07-01T22:13:00.000+05:30

DTMF stands for Dual Tone Multiple Frequency. It is a tone consisting of two frequencies superimposed. Individual frequencies are chosen such that it is easy to design filters and easy to transmit the tones through a telephone line having bandwidth of approximately 3.5 kHz. DTMF was not intended to be used for data transfer, it was meant to be used for sending the control signals along the telephone line. With standard decoders it is possible to send 10 beeps per second i.e., five bits per second. DTMF standard specifies 50ms tones and 600ms duration between two successive tones.

Note that the last column is not commonly seen in the telephones that we used, but telephone exchanges use them quite often. Nowadays, DTMF is used for dialing the numbers in telephones, configuring telephone exchanges etc. A CB transceiver of 2.7 MHz is normally used to send floating codes. DTMF was designed to be able to send the codes using microphone. Each beep (or digit you dial on the telephone) is composed of two concurrent frequencies, which are superimposed on amplitude. The higher of the two frequencies is normally aloud by 4dB, and this shift is termed as twist. If the twist is equal to 4dB, the higher frequency is loud by 4dB. If the lower frequency is loud, then the twist is said to be negative.

Generating DTMF

DTMF signals can be generated through dedicated ICs or by using RC networks connected to a microprocessor. MT8880 is an example of a dedicated IC. But getting the latter method work is a bit difficult if high accuracy is needed. The crystal frequency needs to be sacrificed for a non standard cycle length. Hence this method is used for simple applications. Most often, a PIC micro could be used for the above purpose.

Decoding DTMF

Detecting DTMF with satisfactory precision is a hard thing. Often, a dedicated IC such as MT8870 is used for this purpose. It uses two 6th order band-pass filters using switched capacitor filters and it suppresses any harmonics. Hence they can produce pretty good sine waves from distorted input. Hence it is preferred. Again microprocessors can also be used, but their application is limited.