block diagram of digital communication system
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A digital signal is a physical signal that is a representation of a sequence of discrete values (a quantifieddiscrete-time signal), for example of arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal. The term digital signal can refer to
- a continuous-time waveform signal used in any form of digital communication.
- a pulse train signal that switches between a discrete number of voltage levels or levels of light intensity, also known as a a line coded signal, for example a signal found in digital electronics or in serial communications using digital baseband transmissionin, or a pulse code modulation (PCM) representation of a digitized analog signal.
A signal that is generated by means of a digital modulation method (digital passband transmission), produced by a modem, is in the first case considered as a digital signal, and in the second case as converted to an analog signal.
Waveforms in digital systems
In computer architecture and other digital systems, a waveform that switches between two voltage levels representing the two states of a Boolean value (0 and 1) is referred to as a digital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.
The clock signal is a special digital signal that is used to synchronize digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.
The given diagram is an example of the practical pulse and therefore we have introduced two new terms that are:
- Rising edge: the transition from a low voltage (level 1 in the diagram) to a high voltage (level 2).
- Falling edge: the transition from a high voltage to a low one.
Although in a highly simplified and idealised model of a digital circuit we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finite transition time the output may not properly reflect the input, and indeed may not correspond to either a logically high or low voltage.
Logic voltage levels
The two states of a wire are usually represented by some measurement of an electrical property: Voltage is the most common, but current is used in some logic families. A threshold is designed for each logic family. When below that threshold, the wire is "low," when above "high." Digital circuits establish a "no man's area" or "exclusion zone" that is wider than the tolerances of the components. The circuits avoid that area, in order to avoid indeterminate results.
It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid, and occur only in a fault condition or during a logic level transition. However, few logic circuits can detect such a condition and most devices will interpret the signal simply as high or low in an undefined or device-specific manner. Some logic devices incorporate schmitt trigger inputs whose behaviour is much better defined in the threshold region, and have increased resilience to small variations in the input voltage.
The levels represent the binary integers or logic levels of 0 and 1. In active-high logic, "low" represents binary 0 and "high" represents binary 1. Active-low logic uses the reverse representation.
Power line communication or power line carrier (PLC), also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or Broadband over Power Lines (BPL) are systems for carrying data on a conductor also used for electric power transmission.
Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Powerline communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires (for example, premises wiring), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically the transformer prevents propagating the signal, which requires multiple PLC technologies to be used to form very large networks.
All power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of powerline communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power wiring system was originally intended for transmission of AC power, in conventional use, the power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications. A new discovery called E-Line that allows a single power conductor on an overhead power line to operate as a waveguide to provide low attenuation propagation of RF through microwave energy lines while providing information rate of multiple Gbps is an exception to this limitation.
Data rates over a power line communication system vary widely. Low-frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second; however, these circuits may be many miles long. Higher data rates generally imply shorter ranges; a local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates installation of dedicated network cabling.
Ultra-High-frequency communication (â‰¥100 MHz)
The highest information rate transmissions over power line use RF through microwave frequencies transmitted via a transverse mode surface wave propagation mechanism that requires only a single conductor (). An implementation of this technology called E-Line has been demonstrated using a single power line conductor. These systems have demonstrated symmetric and full duplex communication [http://www.computingunplugged.com/issues/issue200608/00001828001.html well in excess of 1 Gbit/s] in each direction. Multiple Wi-Fi channels with simultaneous analog television in the 2.4 and 5.3 GHz unlicensed bands have been demonstrated operating over a single medium voltage line conductor. Because the underlying propagation mode is extremely broadband, it can operate anywhere in the 20 MHz - 20 GHz region. Also since it is not restricted to below 80 MHz, as is the case for high-frequency BPL, these systems can avoid the need to share spectrum with other licensed or unlicensed services and can completely avoid the interference issues associated with use of shared spectrum while offering complete flexibility for modulation and protocols of an [http://www.corridor.biz/FullArticle.pdf RF-microwave system].
High-frequency communication (â‰¥MHz)
High frequency communication may (re)use large portions of the radio spectrum for communication, or may use select (narrow) band(s), depending on the technology.
Home networking (LAN)
Power line communications can also be used in a home to interconnect home computers (and networked peripherals), as well as any home entertainment devices (including TVs, Blu-ray players, game consoles and Internet video boxes such as Apple TV, Roku, Kodak Theatre, etc.) that have an Ethernet port. Consumers can buy powerline adapter sets at most electronics retailers and use those to establish a wired connection using the existing electrical wiring in the home. The powerline adapters plug into a wall outlet (or into an extension cord or power strip, but not into any unit with surge suppression and filtering, as this may defeat the signal) and then are connected via CAT5 to the homeâ€™s router. Then, a second (or third, fourth, fifth) adapter(s) can be plugged in at any other outlet to give instant networking and Internet access to an Ethernet-equipped Blu-ray player, a game console (PS3, Xbox 360, etc.) a laptop or an Internet TV (also called OTT for Over-the-Top video) box that can access and stream video content to the TV.
The most established and widely deployed powerline networking standard for these powerline adapter products is from the HomePlug Powerline Alliance. HomePlug AV is the most current of the HomePlug specifications (HomePlug 1.0, HomePlug AV and the new HomePlug Green PHY for smart grid comprise the set of published specifications) and it has been adopted by the IEEE P1901 group as a baseline technology for their standard, due to be published and ratified in September or October of 2010. HomePlug estimates that over 45 million HomePlug devices have been deployed worldwide. Other companies and organizations back different specifications for power line home networking and these include the Universal Powerline Association, the HD-PLC Alliance and the ITU-Tâ€™s G.hn specification.
Internet access (broadband over powerlines)
Broadband over power lines (BPL), also known as power-line Internet or powerband, is the use of PLC technology to provide broadband Internet access through ordinary power lines. A computer (or any other device) would need only to plug a BPL "modem" into any outlet in an equipped building to have high-speed Internet access. International Broadband Electric Communications or IBEC and other companies currently offer BPL service to several electric cooperatives.
BPL may offer benefits over regular cable or DSL connections: the extensive infrastructure already available appears to allow people in remote locations to access the Internet with relatively little equipment investment by the utility. Also, such ubiquitous availability would make it much easier for other electronics, such as televisions or sound systems, to hook up. Cost of running wires such as ethernet in many buildings can b
An enterprise digital assistant (EDA) is a handheld computer, born of the personal digital assistant popularity which was adapted for extensive, more robust usage within the SME (Small to Medium Enterprise) and Enterprise business applications as a Data Capture mobile device.
Over the years, these business applications have grown from simple batched data collection using barcode readers to include extensive usage of other expanding business technologies within the areas of WLANs (Wireless Local Area Networks), GPRS Edge Communications, Biometrics, Magnetic Stripe, Smart Card and RFID data capture technologies.
EDAs have many uses in many types of business: Warehouse management, Inventory control and Field Services to mentions a few.
EDAs are to the business sector what the personal digital assistant was to the domestic sector, allowing business applications to combine a portable device with their data collection requirements.
As is the case with PDAs, EDAs may include a touch screen, IrDA, Bluetooth, a memory card slot, however, EDAs may also include one or more data capture devices.
As the gray area between a PDA, Smartphone and EDA can be blurred when comparing the wide array of common features and functions, where EDAs attempt to define a difference is within business mobility applications which normally bring a pre-define requirement for long term constant daily operation (Normally allowing a minimum of 8 hours), and a higher than normal IP rating (impact rating / Drop Test) of no less than an IP54 rating, when compared to PDAs, combined with a minimum of a least one Data Collection function i.e. a Barcode or RFID Reader etc.
An EDA is designed to withstand all-day everyday use in more harsh or hazardous environments, these rugged compact devices can deliver wireless WAN/LAN/PAN voice and data communications, including VOIP functionality.
EDAs are available in a number of form factors, for example Handheld or Wearable and can be further extended to include figure barcode scanners, RFID panel antennas, Swipe card readers, External Battery Packs and Printer carry cases.
Where PDAs are intended as single person devices for personal applications and usage, EDAs are very often intended as multi-person devices for business applications and usage.
PDAs can be expanded to add-on data capture functionality, whereas EDAs very often are designed without the need for additions and can include a combination of built in data capture functionality. Barcode scanner or RFID Reader, 802.11b/g Wi-Fi, GSM, GPRS and VOIP are the most common standards of functionality within EDA mobile devices.
An information system (IS) - or application landscape - is any combination of information technology and people's activities using that technology to support operations, management, and decision-making. In a very broad sense, the term information system is frequently used to refer to the interaction between people, algorithmic processes, data and technology. In this sense, the term is used to refer not only to the information and communication technology (ICT) an organization uses, but also to the way in which people interact with this technology in support of business processes.
Some make a clear distinction between information systems,and computer systems ICT, and business processes. Information systems are distinct from information technology in that an information system is typically seen as having an ICT component. Information systems are also different from business processes. Information systems help to control the performance of business processes.
Alter argues for an information system as a special type of work system. A work system is a system in which humans and/or machines perform work using resources (including ICT) to produce specific products and/or services for customers. An information system is a work system whose activities are devoted to processing (capturing, transmitting, storing, retrieving, manipulating and displaying) information.
Part of the difficulty in defining the term information system is due to vagueness in the definition of related terms such as system and information. Beynon-Davies argues for a clearer terminology based in systemics and semiotics. He defines an information system as an example of a system concerned with the manipulation of signs. An information system is a type of socio-technical system. An information system is a mediating construct between actions and technology.
As such, information systems inter-relate with data systems on the one hand and activity systems on the other. An information system is a form of communication system in which data represent and are processed as a form of social memory. An information system can also be considered a semi-formal language which supports human decision making and action.
It consists of computers, instructions, stored facts, people and procedures.
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Answers:better verificiation of infromation transmitted. usually in digital comm there are extra bits (or digits) added to verifiy the accuracy of the information in the packet transmitted. If there is a loss of connection or signal and the packet is corrupted, the receiver can run a CRC and know it didn't get all the information needed and request it be sent again--well, in a duplex system anyway. With analog communications, like tranceiver radios the only way for one party to know information was lost is to be human and just know info received doesn't make sense or that the signal dropped and the other party just faded out. Plus the whole speed thing due to various compression algorithms/forms.
Answers:Bandwidth, and Security. You can cram a lto more Zero's and Ones in a single period then analog data. For radios especially. As far as security, you can modulate those Zero's and Ones according to the DIG/ANLG converter on both ends. If the other person doesn't have the same converter then they can't demodulate the data.
Answers:Since the male reproductive system consists of the penis, two testicles, two epididymides, two vas deferentia, two seminal vesicles, and the prostate gland, you actually have ten items you can model. Go to a art supply store & get modeling clay or go to the grocery store & get several packages of canning wax it comes in a box shaped like a pound of butter. You want the kind that is a solid block, not the kind that has several smaller brick in the box. Either medium is relatively easy to work with, but if you do not have to color the models, the wax would probably easier to work with if you have never tried sculpting before. Look at both , then decide. While you are looking around the art supply store, you need to look at the sculpting tools. Looking at them should give you ideas of what you might be able to use from normal kitchen utensils or shop tools you might have available. To put a slight bend in the penis, use hot water to soften the wax if you use the modeling clay you can put the bend in any time before the clay sets up.
Answers:Information density. Digital systems can carry far more information in the same channel. This also implies that this information can be stored in less space. Security - digital systems can be encrypted, and in fact, the modulation itself offers some degree of encryption. For CDMA systems, the intrinsic encryption is of such a high order that it rivals sophisticated encryption schemes. That should get you started!