Twisted pair
Twisted pair cabling is a type of wiring in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted balanced pair, a twisted pair reduces electromagnetic radiation from the pair and crosstalk between neighboring pairs and improves rejection of external electromagnetic interference, it was invented by Alexander Graham Bell. In a balanced line, the two wires carry equal and opposite signals, the destination detects the difference between the two; this is known as differential signaling. Noise sources introduce signals into the wires by coupling of electric or magnetic fields and tend to couple to both wires equally; the noise thus produces a common-mode signal which can be canceled at the receiver when the difference signal is taken. Differential signaling starts to fail; this problem is apparent in telecommunication cables where pairs in the same cable lie next to each other for many miles.
Twisting the pairs counters this effect as on each half twist the wire nearest to the noise-source is exchanged. Provided the interfering source remains uniform, or nearly so, over the distance of a single twist, the induced noise will remain common-mode; the twist rate makes up part of the specification for a given type of cable. When nearby pairs have equal twist rates, the same conductors of the different pairs may lie next to each other undoing the benefits of differential mode. For this reason it is specified that, at least for cables containing small numbers of pairs, the twist rates must differ. In contrast to shielded or foiled twisted pair, UTP cable is not surrounded by any shielding. UTP is the primary wire type for telephone usage and is common for computer networking; the earliest telephones used open-wire single-wire earth return circuits. In the 1880s electric trams were installed in many cities. Lawsuits being unavailing, the telephone companies converted to balanced circuits, which had the incidental benefit of reducing attenuation, hence increasing range.
As electrical power distribution became more commonplace, this measure proved inadequate. Two wires, strung on either side of cross bars on utility poles, shared the route with electrical power lines. Within a few years, the growing use of electricity again brought an increase of interference, so engineers devised a method called wire transposition, to cancel out the interference. In wire transposition, the wires exchange position once every several poles. In this way, the two wires would receive similar EMI from power lines; this represented an early implementation of twisting, with a twist rate of about four twists per kilometre, or six per mile. Such open-wire balanced lines with periodic transpositions still survive today in some rural areas. Twisted-pair cabling was invented by Alexander Graham Bell in 1881. By 1900, the entire American telephone line network was either twisted pair or open wire with transposition to guard against interference. Today, most of the millions of kilometres of twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice service, only handled or seen by telephone workers.
Unshielded twisted pair cables are found in many Ethernet networks and telephone systems. For indoor telephone applications, UTP is grouped into sets of 25 pairs according to a standard 25-pair color code developed by AT&T Corporation. A typical subset of these colors shows up in most UTP cables; the cables are made with copper wires measured at 22 or 24 American Wire Gauge, with the colored insulation made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is divided into small but identical bundles; each bundle consists of twisted pairs. The bundles are in turn twisted together to make up the cable. Pairs having the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are selected to minimize crosstalk within a large cable. UTP cable is the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, can use UTP cables.
Twisted pair cabling is used in data networks for short and medium length connections because of its lower costs compared to optical fiber and coaxial cable. UTP is finding increasing use in video applications in security cameras. Many cameras include a UTP output with screw terminals; as UTP is a balanced transmission line, a balun is needed to connect to unbalanced equipment, for example any using BNC connectors and designed for coaxial cable. Twisted pair cables incorporate shielding in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield; such shielding can be applied to individual quads. Individual pairs are foil shielded, while an overall cable may use any of braided screen or foi
Distribution frame
In telecommunications, a distribution frame is a passive device which terminates cables, allowing arbitrary interconnections to be made. For example, the Main Distribution Frame located at a telephone central office terminates the cables leading to subscribers on the one hand, cables leading to active equipment on the other. Service is provided to a subscriber by manually wiring a twisted pair between the telephone line and the relevant DSL or POTS line circuit. In broadcast engineering, a distribution frame is a location within an apparatus room through which all signals pass, with the ability to arbitrarily route and connect sources and destinations between studios and other internal and external points. Connections can either be made using terminal blocks; because the frame may carry live broadcast signals, it may be considered part of the airchain. In data communication, a building distribution frame houses etc.. Distribution frames for specific types of signals have specific acronyms: DDF - digital distribution frame IDF - Intermediate distribution frame MDF - Main distribution frame ODF or OFDF - optical fiber distribution frame VDF - voice distribution frame Distribution frames may grow to large sizes.
In major installations, audio distribution frames can have as many as 10,000 incoming and outgoing separate copper wires. Telephone signals do not use a separate earth ground wire, but some urban exchanges have about 250,000 wires on their MDF. Installing and rewiring these jumpers is a labour-intensive task, leading to attempts in the industry to devise so-called active distribution frames or Automated Main Distribution Frames; the principal issues which stand in the way of their widespread adoption are reliability. Newer digital mixing consoles can act as control points for a distribution frame or router, which can handle audio from multiple studios at the same time. Multiple smaller frames, such as one for each studio, can be linked together with fibre-optics, or with gigabit Ethernet; this has the advantage of not having to route dozens of feeds through walls to a single point. Intermediate distribution frame Main distribution frame Patch panel Splicebox Wiring closet
United States Department of Defense
The Department of Defense is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States Armed Forces. The department is the largest employer in the world, with nearly 1.3 million active duty servicemen and women as of 2016. Adding to its employees are over 826,000 National Guardsmen and Reservists from the four services, over 732,000 civilians bringing the total to over 2.8 million employees. Headquartered at the Pentagon in Arlington, just outside Washington, D. C. the DoD's stated mission is to provide "the military forces needed to deter war and ensure our nation's security". The Department of Defense is headed by the Secretary of Defense, a cabinet-level head who reports directly to the President of the United States. Beneath the Department of Defense are three subordinate military departments: the United States Department of the Army, the United States Department of the Navy, the United States Department of the Air Force.
In addition, four national intelligence services are subordinate to the Department of Defense: the Defense Intelligence Agency, the National Security Agency, the National Geospatial-Intelligence Agency, the National Reconnaissance Office. Other Defense Agencies include the Defense Advanced Research Projects Agency, the Defense Logistics Agency, the Missile Defense Agency, the Defense Health Agency, Defense Threat Reduction Agency, the Defense Security Service, the Pentagon Force Protection Agency, all of which are under the command of the Secretary of Defense. Additionally, the Defense Contract Management Agency provides acquisition insight that matters, by delivering actionable acquisition intelligence from factory floor to the warfighter. Military operations are managed by ten functional Unified combatant commands; the Department of Defense operates several joint services schools, including the Eisenhower School and the National War College. The history of the defense of the United States started with the Continental Congress in 1775.
The creation of the United States Army was enacted on 14 June 1775. This coincides with the American holiday Flag Day; the Second Continental Congress would charter the United States Navy, on 13 October 1775, create the United States Marine Corps on 10 November 1775. The Preamble of the United States Constitution gave the authority to the federal government to defend its citizens: We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America. Upon the seating of the first Congress on 4 March 1789, legislation to create a military defense force stagnated as they focused on other concerns relevant to setting up the new government. President George Washington went to Congress to remind them of their duty to establish a military twice during this time.
On the last day of the session, 29 September 1789, Congress created the War Department, historic forerunner of the Department of Defense. The War Department handled naval affairs until Congress created the Navy Department in 1798; the secretaries of each of these departments reported directly to the president as cabinet-level advisors until 1949, when all military departments became subordinate to the Secretary of Defense. After the end of World War II, President Harry Truman proposed creation of a unified department of national defense. In a special message to Congress on 19 December 1945, the President cited both wasteful military spending and inter-departmental conflicts. Deliberations in Congress went on for months focusing on the role of the military in society and the threat of granting too much military power to the executive. On 26 July 1947, Truman signed the National Security Act of 1947, which set up a unified military command known as the "National Military Establishment", as well as creating the Central Intelligence Agency, the National Security Council, National Security Resources Board, United States Air Force and the Joint Chiefs of Staff.
The act placed the National Military Establishment under the control of a single Secretary of Defense. The National Military Establishment formally began operations on 18 September, the day after the Senate confirmed James V. Forrestal as the first Secretary of Defense; the National Military Establishment was renamed the "Department of Defense" on 10 August 1949 and absorbed the three cabinet-level military departments, in an amendment to the original 1947 law. Under the Department of Defense Reorganization Act of 1958, channels of authority within the department were streamlined, while still maintaining the ordinary authority of the Military Departments to organize and equip their associated forces; the Act clarified the overall decision-making authority of the Secretary of Defense with respect to these subordinate Military Departments and more defined the operational chain of command over U. S. military forces as running from the president to the Secretary of Defense and to the unified combatant commanders.
Provided in this legislation was a centralized research authority, the Advanced Research Projects Agency known as DARPA. The act was written and promoted by the Eisenhower administration, was signed into law 6 August 1958; the Secretary of Defense, appointed by the president with the advice and consent of the Senate, is by federal law (1
Electric power distribution
Electric power distribution is the final stage in the delivery of electric power. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment or household appliances. Several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level; the transition from transmission to distribution happens in a power substation, which has the following functions:Circuit breakers and switches enable the substation to be disconnected from the transmission grid or for distribution lines to be disconnected.
Transformers step down 35 kV or more, down to primary distribution voltages. These are medium voltage circuits 600-35,000 V. From the transformer, power goes to the busbar that can split the distribution power off in multiple directions; the bus distributes power to distribution lines. Urban distribution is underground, sometimes in common utility ducts. Rural distribution is above ground with utility poles, suburban distribution is a mix. Closer to the customer, a distribution transformer steps the primary distribution power down to a low-voltage secondary circuit 120/240 V in the US for residential customers; the power comes to the customer via an electricity meter. The final circuit in an urban system may be less than 50 feet, but may be over 300 feet feet for a rural customer. Electric power distribution became necessary only in the 1880s when electricity started being generated at power stations. Before that electricity was generated where it was used; the first power distribution systems installed in European and US cities were used to supply lighting: arc lighting running on high voltage alternating current or direct current, incandescent lighting running on low voltage direct current.
Both were supplanting gas lighting systems, with arc lighting taking over large area and street lighting, incandescent lighting replacing gas for business and residential lighting. Due to the high voltages used in arc lighting, a single generating station could supply a long string of lights, up to 7-mile long circuits; each doubling of the voltage would allow the same size cable to transmit the same amount of power four times the distance for a given power loss. Direct current indoor incandescent lighting systems, for example the first Edison Pearl Street Station installed in 1882, had difficulty supplying customers more than a mile away; this was due to the low 110 volt system being used throughout the system, from the generators to the final use. The Edison DC system needed thick copper conductor cables, the generating plants needed to be within about 1.5 miles of the farthest customer to avoid excessively large and expensive conductors. Transmitting electricity a long distance at high voltage and reducing it to a lower voltage for lighting became a recognized engineering roadblock to electric power distribution with many, not satisfactory, solutions tested by lighting companies.
The mid-1880s saw a breakthrough with the development of functional transformers that allowed the AC voltage to be "stepped up" to much higher transmission voltages and dropped down to a lower end user voltage. With much cheaper transmission costs and the greater economies of scale of having large generating plants supply whole cities and regions, the use of AC spread rapidly. In the US the competition between direct current and alternating current took a personal turn in the late 1880s in the form of a "War of Currents" when Thomas Edison started attacking George Westinghouse and his development of the first US AC transformer systems, pointing out all the deaths caused by high voltage AC systems over the years and claiming any AC system was inherently dangerous. Edison's propaganda campaign was short lived with his company switching over to AC in 1892. AC became the dominant form of transmission of power with innovations in Europe and the US in electric motor designs and the development of engineered universal systems allowing the large number of legacy systems to be connected to large AC grids.
In the first half of the 20th century, in many places the electric power industry was vertically integrated, meaning that one company did generation, distribution and billing. Starting in the 1970s and 1980s, nations began the process of deregulation and privatisation, leading to electricity markets; the distribution system would remain regulated, but generation and sometimes transmission systems were transformed into competitive markets. Electric power begins at a generating station, where the potential difference can be as high as 33,000 volts. AC is used. Users of large amounts of DC power such as some railway electrification systems, telephone exchanges and industrial processes such as aluminium smelting use rectifiers to derive DC from the public AC supply, or may have their own generation systems. High-voltage DC can be advantageous for isolating alternating-current systems or controlling the quantity of electricity transmitted. For example, Hydro-Québec has a direct-current line which g
Communications satellite
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder. Communications satellites are used for television, radio and military applications. There are 2,134 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,200 miles above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it; the high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between separated geographical points. Communications satellites use a wide range of microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use.
This allocation of bands minimizes the risk of signal interference. The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke, along with Vahid K. Sanadi building on work by Konstantin Tsiolkovsky. In October 1945 Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World; the article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus, Arthur C. Clarke is quoted as being the inventor of the communications satellite and the term'Clarke Belt' employed as a description of the orbit. Decades a project named Communication Moon Relay was a telecommunication project carried out by the United States Navy, its objective was to develop a secure and reliable method of wireless communication by using the Moon as a passive reflector and a natural communications satellite. The first artificial Earth satellite was Sputnik 1. Put into orbit by the Soviet Union on October 4, 1957, it was equipped with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz.
Sputnik 1 was launched as a major step in the exploration of rocket development. However, it was not placed in orbit for the purpose of sending data from one point on earth to another; the first satellite to relay communications was an intended lunar probe. Though the spacecraft only made it about halfway to the moon, it flew high enough to carry out the proof of concept relay of telemetry across the world, first from Cape Canaveral to Manchester, England; the first satellite purpose-built to relay communications was NASA's Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from U. S. President Dwight D. Eisenhower. Courier 1B, built by Philco, launched in 1960, was the world's first active repeater satellite; the first artificial satellite used to further advances in global communications was a balloon named Echo 1. Echo 1 was the world's first artificial communications satellite capable of relaying signals to other points on Earth.
It soared 1,600 kilometres above the planet after its Aug. 12, 1960 launch, yet relied on humanity's oldest flight technology — ballooning. Launched by NASA, Echo 1 was a 30-metre aluminized PET film balloon that served as a passive reflector for radio communications; the world's first inflatable satellite — or "satelloon", as they were informally known — helped lay the foundation of today's satellite communications. The idea behind a communications satellite is simple: Send data up into space and beam it back down to another spot on the globe. Echo 1 accomplished this by serving as an enormous mirror, 10 stories tall, that could be used to reflect communications signals. There are two major classes of communications satellites and active. Passive satellites only reflect the signal coming from the source, toward the direction of the receiver. With passive satellites, the reflected signal is not amplified at the satellite, only a small amount of the transmitted energy reaches the receiver. Since the satellite is so far above Earth, the radio signal is attenuated due to free-space path loss, so the signal received on Earth is very weak.
Active satellites, on the other hand, amplify the received signal before retransmitting it to the receiver on the ground. Passive satellites are little used now. Telstar was the second direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, the French National PTT to develop satellite communications, it was launched by NASA from Cape Canaveral on July 10, 1962, in the first sponsored space launch. Relay 1 was launched on December 13, 1962, it became the first satellite to transmit across the Pacific Ocean on November 22, 1963. An immediate antecedent of the geostationary satellites was the Hughes Aircraft Company's Syncom 2, launched on July 26, 1963. Syncom 2 was the first communications satellite in a geosynchronous orbit, it revolved around the earth once per day at constant speed, but because it still had north-south motion, special equipment was needed to track it. Its successor, Syncom 3 was the first geostationary communications satellite.
Syncom 3 obt
Telephone exchange
A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems human operators that interconnect telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. In historical perspective, telecommunication terms have been used with different semantics over time; the term telephone exchange is used synonymously with central office, a Bell System term. A central office is defined as a building used to house the inside plant equipment of several telephone exchanges, each serving a certain geographical area; such an area has been referred to as the exchange. Central office locations may be identified in North America as wire centers, designating a facility from which a telephone obtains dial tone. For business and billing purposes, telephony carriers define rate centers, which in larger cities may be clusters of central offices, to define specified geographical locations for determining distance measurements.
In the United States and Canada, the Bell System established in the 1940s a uniform system of identifying central offices with a three-digit central office code, used as a prefix to subscriber telephone numbers. All central offices within a larger region aggregated by state, were assigned a common numbering plan area code. With the development of international and transoceanic telephone trunks driven by direct customer dialing, similar efforts of systematic organization of the telephone networks occurred in many countries in the mid-20th century. For corporate or enterprise use, a private telephone exchange is referred to as a private branch exchange, when it has connections to the public switched telephone network. A PBX is installed in enterprise facilities collocated with large office spaces or within an organizational campus to serve the local private telephone system and any private leased line circuits. Smaller installations might deploy a PBX or key telephone system in the office of a receptionist.
In the era of the electrical telegraph, post offices, railway stations, the more important governmental centers, stock exchanges few nationally distributed newspapers, the largest internationally important corporations and wealthy individuals were the principal users of such telegraphs. Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same schema and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone. A telephone exchange is a telephone system located at service centers responsible for a small geographic area that provided the switching or interconnection of two or more individual subscriber lines for calls made between them, rather than requiring direct lines between subscriber stations; this made it possible for subscribers to call each other at businesses, or public spaces.
These made telephony an available and comfortable communication tool for everyday use, it gave the impetus for the creation of a whole new industrial sector. As with the invention of the telephone itself, the honor of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was Hungarian Tivadar Puskás in 1877 while he was working for Thomas Edison; the first experimental telephone exchange was based on the ideas of Puskás, it was built by the Bell Telephone Company in Boston in 1877. The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to Berlin under the direction of Heinrich von Stephan. George W. Coy designed and built the first commercial US telephone exchange which opened in New Haven, Connecticut in January, 1878; the switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations. Charles Glidden is credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878.
In Europe other early telephone exchanges were based in London and Manchester, both of which opened under Bell patents in 1879. Belgium had its first International Bell exchange a year later. In 1887 Puskás introduced the multiplex switchboard.. Exchanges consisted of one to several hundred plug boards staffed by switchboard operators; each operator sat in front of a vertical panel containing banks of ¼-inch tip-ring-sleeve jacks, each of, the local termination of a subscriber's telephone line. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a cord circuit; when a calling party lifted the receiver, the local loop current lit a signal lamp near the jack. The operator responded by inserting the rear cord into the subscriber's jack and switched her headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair into the called party's local jack and started the ringing cycle. For a long distance call, she plugged into a trunk circuit to connect to another operator in another bank of boards or at a remote central office.
In 1918, the average time to complete the connection for a long-distance call was 15 minutes. Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatic
Cable television headend
A cable television headend is a master facility for receiving television signals for processing and distribution over a cable television system. The headend facility is unstaffed and surrounded by some type of security fencing and is a building or large shed housing electronic equipment used to receive and re-transmit video over the local cable infrastructure. One can find head ends in power-line communication substations and Internet communications networks. Most cable TV systems carry local over-the-air television stations for distribution. Since each terrestrial channel represents a defined frequency, a dedicated commercial-grade receiving antenna is needed for each channel that the cable company wishes to receive and distribute. Smaller systems may use a broadband antenna to share several channels; these antenna are built into a single tower structure called a master antenna television structure. Commercial TV pre-amplifiers strengthen the weakened terrestrial TV signals for distribution.
Some cable TV systems receive the local television stations' programming by dedicated coaxial, microwave link or fiber-optic line, installed between the local station and the headend. A device called a modulator at the local station's facilities feed their programming over this line to the cable TV headend, which in turn receives it with another device called a demodulator, it is distributed through the cable TV headend to subscribers. This is more reliable than receiving the local stations' broadcasts over the air with an antenna. However, off-air reception is used as a backup by the headend in case of failure. In some cases systems receive local channels by satellite. Other sources of programming include those delivered via fiber optics, telephone wires, the Internet, microwave towers and local public-access television channels that are sent to the cable headend on an upstream frequency over the cable system itself, or via a dedicated line set up by the cable company, as mentioned earlier for reception of local television stations' programming by the headend.
Once a television signal is received, it must be processed. For digital satellite TV signals, a dedicated commercial satellite receiver is needed for each channel, to be distributed by the cable system, they output stereo audio signals as well as a digital signal for digital plants. Analog terrestrial TV signals require a processor, a RF receiver that outputs video and audio. In some cases the processor will include a built-in modulator. Digital terrestrial TV signals require a special digital processor. Digital channels are received on an L band QAM stream from a satellite, which uses multiplexing. Using special receivers such as the Motorola MPS, the signal can be demultiplexed or "Demuxed" to extract specific channels from the multiplexed signal. At this point, local insertion may be performed to add content targeted to the local geographic area. Cable television signals are mixed in accordance with the cable system's channel numbering scheme using a series of cable modulators, in turn fed into a frequency multiplexer or signal combiner.
The mixed signals are sent into a broadband amplifier sent into the cable system by the trunk line and continuously re-amplified as needed. Modulators take an input signal and attach it to a specific frequency. For example, in North America, NTSC standards dictate that CH2 is a 6 MHz wide channel with its luminance carrier at 55.25 MHz, so the modulator for channel 2 will impose the appropriate input signal on to the 55.25 MHz frequency to be received by any TV tuned to Channel 2. Digital channels are modulated as well. Using QAM, a CATV operator can place up to eight subchannels on each channel so channel 2 may be carrying channels 1 - 8 in a viewer's city. Set-top boxes or CableCards are required to receive these digital signals and are provided by the cable operator themselves. Many modern cable systems are now "all digital" meaning analog video signals have been discontinued in order to reuse spectrum; the RF channels analog used to occupy are now open for a cable system to reuse most as High Speed Data channels to increase subscriber download/upload internet speeds.
Analog video removal essentially eliminates cable theft since analog signals were transmitted unencrypted. Most digital video signals are compressed to MPEG-2 and MPEG-4 formats in order to combine multiple video streams into a QAM making the most efficient use of spectrum which a customer cable set top box receives, demodulates, de-encrypts and displays as a virtual channel number that the viewer recognizes. In many cases the same TV network may appear multiple times in a local channel lineup as a different channel the viewer sees this is due to previous generations of channel lineups kept in service and intended to not confuse viewers who are familiar with the network appearing on a number they are used to. Although a channel may be in a line up multiple times the RF QAM it is combined or "muxed" into is modulated and compressed just once. A set top box tunes to that same QAM. Virtual channeling allows the cable operator to change the physical frequency a QAM is on without the viewer noticing the channel number changing in their lineup.
Most digital cable systems enc
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).
This article incorporates 
