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From Wikipedia

Power line communication

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.

Basics

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 (≥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 (≥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 home’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-T’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

Digital signal

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

  1. a continuous-time waveform signal used in any form of digital communication.
  2. 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.



From Yahoo Answers

Question:communications

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.

Question:

Answers:Much faster info transfer speeds, cheaper, less errors.

Question:

Answers:Advantages of digital communication: 1. It is fast and easier. 2. No paper is wasted. 3. The messages can be stored in the device for longer times, without being damaged, unlike paper files that easily get damages or attacked by insects. 4. Digital communication can be done over large distances through internet and other things. 5. It is comparatively cheaper and the work which requires a lot of people can be done simply by one person as folders and other such facilities can be maintained. 6. It removes semantic barriers because the written data can be easily chaned to different languages using software. 7. It provides facilities like video conferencing which save a lot of time, money and effort. Disadvantages: 1. It is unreliable as the messages cannot be recognised by signatures. Though software can be developed for this, yet the softwares can be easily hacked. 2. Sometimes, the quickness of digital communication is harmful as messages can be sent with the click of a mouse. The person oes not think and sends the message at an impulse. 3. Digital Communication has completely ignored the human touch. A personal touch cannot be established because all the computers will have the same font! 4. The establishment of Digital Communication causes degradation of the environment in some cases. "Electronic waste" is an example. The vibes given out by the telephone and cell phone towers are so strong that they can kill small birds. Infact the common sparrow has vanished due to so many towers coming up as the vibrations hit them on the head. 5. Digital Communication has made the whole wordl to be an "office." The people carry their work to places where they are supposed to relax. The whole world has been made into an office. Even in the office, digital communication causes problems because personal messages can come on your cell phone, internet, etc. 6. Many people misuse the efficiency of Digital Communicatio. The sending of hoax messages, the usage by people to harm the society, etc cause harm to the society on the whole. Hope this is of some help. Do let me know! Merry Christmas!

Question:Communication details

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.

From Youtube

ADVANTAGES OF DIGITAL TECHNOLOGY - DICTATION INDUSTRY (ANALOG VS DIGITAL) :In the analogue era, before dictation became a simple data file, recordings often had to go a long way.. With digital technology, sending cassettes is a thing of the past. You can dictate the document in any location you wish and transmit the speech files immediately to the office from your laptop via data or network card. You can also benefit from the enhanced functionality of digital: the means to insert sentences afterwards into existing dictations, the ability to locate dictations easily, archiving capability (for saving space!) and the bonus of no wear on the machine (due to the absence of mechanical parts). Digital dictation systems offer numerous benefits over conventional analogue equipment. By going digital, the time required for producing a typed document can be significantly reduced because dictations are stored as a speech file on the PC, in the internal memory of the dictation device or on additional memory cards (DigtaCards) - instead of on audio tape as was the case previously. Other advantages are that extra sentences can be inserted afterwards into existing speech files; dictations can be located quickly; to save space, they can also be archived and sent to an office instantly via electronic means. This allows you to save time and, above all, costs. Because the different stages - dictation, typing, printing and correction - are all brought closer together you'll find you can react more quickly to the needs of your customers, patients or clients. To sum up ...

NZ Police Switch To Digital Radio :The New Zealand Police have begun there move away from there current analogue radio channels and radios to a new Tait manufactured P-25 encrypted network that is near impossible to scan even with new P-25 capable scanners. Currently only the Wellington region has the technology for general duties police work, but the technology is being released in Auckland and Christchurch next year before being rolled out across the country. The Police want to have Digital radio in the main centres before the Rugby World Cup Here in 2011. Tech stuff The radio network is an American developed standard called Apco Project 25 (P-25) Digital system. The P-25 standard is used mainly in North and South America, parts of Asia and Australia and now New Zealand for digital radio. P-25 can be used with or without Encryption. Only non Encryped P-25 can be scanned with a P-25 capable scanner. Europe mainly uses a standard called TETRA for its digital radio for Emergency Services. The radios have been manufactured by Christchurch based company Tait communications The NZ Fire Service, Ambulance, Customs, MAF and Fisheries have all agreed to move from there analogue systems they run now to the new P-25 in the future although there have been no dates set for the change over for these agencies. Advantages of digital two way radio -Easy to encrypt -Better sound quality -Better coverage in City areas Disadvantages -Doesnt work as well as analogue in rural area when it comes to reception. -equipment is ...