Broadband & Digital subscriber line

Broadband in telecommunications is a term that refers to a signaling method that includes or handles a relatively wide range of frequencies, which may be divided into channels or frequency bins. Broadband is always a relative term, understood according to its context. The wider the bandwidth, the greater the information-carrying capacity. In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still broader band is required to carry music without losing the high audio frequencies required for realistic sound reproduction. A television antenna described as "normal" may be capable of receiving a certain range of channels; one described as "broadband" will receive more channels. In data communications a modem will transmit a bandwidth of 64 kilobits per seconds (kbit/s) over a telephone line; over the same telephone line a bandwidth of several megabits per second can be handled by ADSL, which is described as broadband (relative to a modem over a telephone line, although much less than can be achieved over a fibre optic circuit, for example).[citation needed]
Broadband in data communications can refer to Broadband Networks or Broadband Internet and may have the same meaning as above, so that data transmission over a fiber optic cable would be referred to as broadband as compared to a telephone modem operating at 600 bits per second.[citation needed]
However, broadband in data communications is frequently used in a more technical sense to refer to data transmission where multiple pieces of data are sent simultaneously to increase the effective rate of transmission, regardless of actual data rate. In network engineering this term is used for methods where two or more signals share a medium.[citation needed]
The various forms of Digital Subscriber Line (DSL) services are broadband in the sense that digital information is sent over a high-bandwidth channel above the baseband voice channel on a single pair of wires.[citation needed]
A baseband transmission sends one type of signal using a medium's full bandwidth, as in 100BASE-T Ethernet. Ethernet, however, is the common interface to broadband modems such as DSL data links, and has a high data rate itself, so is sometimes referred to as broadband. Ethernet provisioned over cable modem is a common alternative to DSL.
Digital subscriber line
DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many[attribution needed] have adopted digital subscriber line as a more marketing-friendly term for the most popular version of consumer-ready DSL, ADSL. DSL uses high frequency; regular telephone uses low frequency.
Typically, the download speed of consumer DSL services ranges from 512 kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology, line conditions and service level implemented. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for Symmetric Digital Subscriber Line (SDSL).
Voice and data
Some variants of DSL connections, like ADSL and very high speed DSL (VDSL), typically work by dividing the frequencies used in a single phone line into two primary 'bands'. The ISP data is carried over the high frequency band (25 kHz and above) whereas the voice is carried over the lower frequency band (4 kHz and below). (See the ADSL article on how the high frequency band is sub-divided). The user typically installs a DSL filter on each phone. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies (the human voice). The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other.
History and science
Digital subscriber line technology was originally implemented as part of the ISDN specification, which is later reused as IDSL. Higher speed DSL connections like HDSL and SDSL have been developed to extend the range of DS1 services on copper lines. Consumer oriented ADSL is designed to operate also on a BRI ISDN line, which itself is a form of DSL, as well as on an analog phone line.
DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.
U.S. telephone companies promote DSL to compete with cable modems. DSL service was first provided over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers ILECs to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element , this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.
Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Distances greater than 2 km (1.25 miles) significantly reduce the bandwidth usable on the wires, thus reducing the data rate. By using an ADSL loop extender, these distances can be increased substantially.
In 2007, Dr. John Papandriopoulos, a University of Melbourne engineering researcher, patented algorithms that can potentially boost DSL line speeds to a maximum of 250 Mbit/s.
Operation
The local loop of the public switched telephone network (PSTN) was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth.
At the local telephone exchange (United Kingdom) or central office (United States) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).
The laws of physics, specifically the Shannon limit, cap the speed of data transmission. For a long time, it was believed that a conventional phone line couldn't be pushed beyond the low speed limits (typically under 9600 bit/s). In the 1950s, 4 MHz television signals were often carried between studios on ordinary twisted pair telephone cable, suggesting that the Shannon Limit would allow transmitting many megabits per second. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. In the 1980s techniques were developed for broadband communications that allowed the limit to be greatly extended.
The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances). The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable.
One of Lechlider's greatest contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of synchronous DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.
Because DSL operates at above the 3.4 kHz voice limit, it cannot be passed through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, nor can voice service be maintained past a certain distance without such coils. Some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines not to need them through the use of fiber to the neighborhood or node FTTN.
The commercial success of DSL and similar technologies largely reflects the advances made in electronics, that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularization of DSL technology:
Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of VLSI technology, the cost of the equipment associated with a DSL deployment (a DSLAM at one end and a DSL "modem" at the other end) lowered significantly.
A DSL line can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations.
In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical when compared to dial up access. Telephone companies were pressured into moving to ADSL largely due to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to popularize ADSL technology.
Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL "modem" can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer's premises.
Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.
Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, to consumers.
Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US has dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Optical communication

An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal.
Forms of optical communication
There are many forms of non-technological optical communication, including body language and sign language.
Techniques such as semaphore lines, ship flags, smoke signals, and beacon fires were the earliest form of technological optical communication.
The heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signaling code. Navy ships often use a signal lamp to signal in Morse code in a similar way.
Distress flares are used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards.
Aircraft use the landing lights at airports to land safely, especially at night. Aircraft landing on an aircraft carrier use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relative to the best landing glideslope.
Optical fiber is the most common medium for modern digital optical communication.
Free-space optical communication is also used today in a variety of applications.
Optical fiber communication
Optical fiber is the most common type of channel for optical communications, however, other types of optical waveguides are used within communications gear, and have even formed the channel of very short distance (e.g. chip-to-chip, intra-chip) links in laboratory trials. The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation, although historically optical phase and frequency modulation have been demonstrated in the lab. The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at significantly lower cost.
Free-space optical communication
IrDA is an example of low-data-rate, short distance free-space optical communications using LEDs. RONJA is an example of 10Mbit/s 1.4 km full-duplex optical point-to-point link.

Laser

The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.A typical laser emits light in a narrow, low-divergence monochromatic (single-coloured, if the laser is operating in the visible spectrum), beam with a well-defined wavelength. In this way, laser light is in contrast to a light source such as the incandescent light bulb, which emits light over a wide area and over a wide spectrum of wavelengths.
The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories. Recently, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in optical storage devices such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers and laser pointers.
In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are used by the military for target identification and illumination for weapons delivery. Lasers used in medicine are used for internal surgery and cosmetic applications.
Design
Although the word light in the acronym Light Amplification by Stimulated Emission of Radiation is typically used in the expansive sense, as photons of any electromagnetic energy; it is not limited to photons in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.
A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, stimulating the gain material continuously. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. A typical pump source is a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
Laser physics
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ("excited") quantum states. Particles can interact with light both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. Strictly speaking, these are the essential ingredients of a laser. However, usually the term laser is used for devices where the light that is amplified is produced as spontaneous emission from the same gain medium as where the amplification takes place. Devices where light from an external source is amplified are normally called optical amplifiers.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design.
The optical cavity, a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, the transverse modes of the beam can be described as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams (for stable-cavity lasers). Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well.
The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10-15 s).
Although the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than other light sources. The operation of a free electron laser can be explained without reference to quantum mechanics.
Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.