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The term SCADA describes centralized systems which monitor and control sites, or complexes of systems distributed over considerable areas in other words anything between an industrial plant and a country. A majority of the control actions are executed automatically by Remote Terminal Units also known as RTUs”or by Programmable Logic Controllers also known as PLCs. Host control functions are typically restricted to basic overriding or supervisory level intervention. For instance, a PLC may control the flow of cooling water in a section of an industrial process, but the SCADA system may enable operators to alter the set points for the flow,and permit alarm conditions to be exhibited and recorded. The feedback control loop goes through the RTU or PLC, while the SCADA system monitors the complete operations of the loop.

A SCADA System is typically comprised of subsystems which include:

• A Human-Machine Interface or HMI which is the apparatus that presents process data to a human operator. In doing this, the human operator monitors and controls the process.
• A supervisory or computer system, which acquires data on the process and sends commands to the process.
• Remote Terminal Units or RTUs which connects to sensors in the process, converting sensor signals to digital data then sending digital data to the supervisory system.
• Programmable Logic Controller or PLCs which is used as field devices because they are more affordable, multi-purpose, flexible, and modifiable in comparison to special-purpose RTUs.
• Communication infrastructure which connects the supervisory system to the Remote Terminal Units.

There is some amount of bewilderment related to the disparities between SCADA systems and Distributed control systems or DCS. Essentially, a SCADA system normally refers to a system that coordinates, but does not actually control a set of processes in real time. The idea of real-time control is clouded in some way by newer telecommunications technology, enabling reliable, low latency, high speed communications over enormous areas. The disparities between SCADA and DCS are culturally determined and can generally be overlooked. As communication infrastructures with greater capacity are made available, the differences between SCADA and DCS will fail to be.

SCADA systems primarily incorporate a distributed database which is principally referred to as a tag database. This database is composed of data elements called tags or points. A point is representative of a single input or output value monitored or controlled by the system. Points can either be soft or hard. A soft point is the result of logic and mathematical operations applied to other points while hard point is representative of an actual input or output within the system. Points are generally stored as value-timestamp pairs: a value, and the timestamp when it was recorded or calculated. A series of value-timestamp pairs will give the history of that point. It is also typical to store extra metadata with tags, inclusive of the path to a field device or PLC register, design time comments, and alarm information.

The development of mobile technology can be traced back as early as the 1940s. Originally, two-way radios or mobile rigs were put in vehicles such as taxicabs, police cruisers and ambulances, but were not actually mobile phones because they were not generally connected to a telephone network. Essentially, the users of this early form of mobile technology were could not dial phone numbers from their vehicles. At first, mobile two-way radios were permanently installed in vehicles, but proceeding versions such as the transportables or “bag phones” were manufactured with a cigarette lighter plug so that they could also be carried, and therefore could be used as either a mobile or as a portable two-way radio. In the early part of the 1940s, Motorola developed a backpacked two-way radio, the Walkie-Talkie and later on created a large hand-held two-way radio for the United States military. The battery powered “Handie-Talkie” or HT was approximately the size of a man’s forearm. The technology would soon transcend the analogue Motorola DynaTAC prototype first used in 1973 to the three distinct generations of mobile phones that would each improve upon the technology. The generations are classified as 1G, 2G and 3G.

1G or 1-G specifically describes the first-generation of wireless telephone technology, mobile telecommunications. These were the analogue telecommunications standards that were put in place in the 1980s and continued until they were replaced by 2G digital telecommunications. The principal difference between the two mobile telephone systems, 1G and 2G, is that the radio signals that the 1G networks used were analogue, while 2G networks use digital radio signals.

In the 1990s, the second generation or 2G mobile phone systems including GSM, IS-136 (“TDMA”), iDEN and IS-95 or “CDMA” were introduced. In 1991 the first GSM network, Radiolinja, was established in Finland. 2G phone systems were characterized by digital circuit switched transmission and the use of advanced and fast phone-to-network signalling. For the most part, the frequencies that were used by 2G systems in Europe were higher than those in America. For instance, the 900 MHz frequency range was used for both the 1G and 2G systems in Europe, so the 1G systems were promptly shut down to make room for the 2G systems. In America the IS-54 standard was used in the same band as AMPS and displaced some of the existing analogue channels. The implementation of the 2G technology saw the introduction of smaller, more compact mobile phones. This modification was enabled because of technological improvements including more advanced batteries and more energy-efficient electronics.

SMS text messaging became a reality with the 2G network, initially on GSM networks and then on all digital networks. In 1991, the first machine-generated SMS message was sent in the United Kingdom. In 1993, the first actual person-to-person SMS text message was sent in Finland. SMS would very soon after its introduction, become the prefered method of communication for the youth. In fact, in most markets, a text message is prefered to using voice mail. The second generation also enabled users to access media content on mobile phones, when Radiolinja, now Elisa, in Finland introduced the downloadable ring tone as paid content.

Following the success of the 2G network the development of the third generation or 3G technology began. The creation of this newer technology paved the way for a myriad of different standards with different contenders promoting their own technologies. Unlike the 2G systems, the meaning of 3G has been standardized in the IMT-2000 standardization processing. This process did not necessarily standardize on a technology, but instead on a set of requirements (2 Mbit/s maximum data rate indoors, 384 kbit/s outdoors, for example). From then on, the original ideal of a single unified worldwide standard was demolished and several different standards have since then been introduced.

The first pre-commercial trial network using 3G technology was launched by NTT DoCoMo in Japan in the Tokyo region in May 2001. By late 2007 there were 295 Million subscribers on 3G networks globally, which represented 9% of the total number of people using mobile phones.

Stonehenge is likely considered the most significant prehistoric monument in Britain. It has attracted visitors for many years. It is a testament to the skill and dedication of the people who builtit many centuries ago.

The Stonehenge that stands in its present location is the final phase of its construction that was completed about 3500 years ago. The final stage was initiated soon after 1500 BC when the bluestones were altered into the horseshoe and circle that is viewable in the present. The original number of stones in the bluestone circle has been approximated to be around 60, many of these have been removed or destroyed. Some are currently mere stomps in the ground.

To Go To Stonehenge by Car

From Amesbury the directions are as follows: go 2 miles west on the junction of A303 and A344/360.

From London Gatwick Airport: take the M23 motorway and join the M25 motorway, following the signs for Heathrow Airport.

From London Heathrow Airport: follow signs to the M4 West then continue for about 2 miles and come off at junction 4b onto the M25 South bound.

From the M25, exit at junction 12 for the M3 motorway towards Basingstoke. Once you get on the M3 follow it to junction 8 signed A303 Andover. Then continue on the A303 all the way until you arrive at a roundabout. Go straight once you reach this point and 2 miles on bear right onto the A344 and the car park is on the right hand side about 500 metres on.

To Go To Stonehenge by Train

The closest train station to Stonehenge is Salisbury which is about 9.5 miles away. From London the trains depart from Waterloo Station to Salisbury. You should check for times and prices as these may be subject to change, additionally, the trains depart approximately every hour. The journey takes about an hour and a half.

To Go To Stonehenge by Bus

The buses depart from Heathrow Airport and from Victoria Coach Station in the centre of London. The journey takes just about 2 hours. You can get off at Amesbury. From there you can either walk (about 2 miles), catch a local bus, or get a taxi.

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If you have a desire to visit the White House. There are a few things you will be mustdo and know when making your preliminary preparations.

Public tours of the White House are readily available to anyone who is a citizen of the United States. First a request must be made through your Member of Congress. Any request of this sort may be accepted as much as months in advance. These tours are self guided and are available from 7:30 a.m. to 11:00 a.m. Tuesday through Thursday, 7:30 a.m. to 12:00 p.m. Fridays, and 7:30 a.m. to 1:00 p.m. Saturdays. Tours, however, are not accommodated on federal holidays. Tour hours may sometimes be lengthened whenever it is feasible based on the official White House schedule. The tours are scheduled on a first come, first served basis. These requests, as mentioned earlier, can be submitted up to six months in advance but no less than 30 days in advance. You are unquestionably encouraged to submit your request as early as you can since a limited number of tours are available in any given period. All White House tours are one hundred percent free of charge.

If you are a not a United States citizen, then you will be expected contact your embassy in Washington, DC for more assistance in submitting a tour request.

It is said that all tours are substantially made that much better if you stop by the White House Visitor Center located at the southeast corner of 15th and E Streets, before or after you have completed your tour. The Center is open seven days a week from as a early as 7:30 a.m. Right up until 4:00 p.m. and features many details about the White House, including its architecture, furnishings, first families, social events, and relations with the press and world leaders, as well as a thirty-minute video. You will possibly need anywhere between 20 minutes to one hour to explore the exhibits in their entirety. The White House Historical Association also sponsors a sales area. Rest rooms are provided but there are no food services offered.

If you are scheduled for a tour and require the loan of a wheelchair you should inform the officer at the Visitors Entrance upon your arrival. However it is of importance to note that reservations are not possible. If you are using a wheelchair, or have any other mobility impairments, you will still use the same Visitors Entrance but will be escorted by ramp from the entrance level to the Ground floor, and then by the elevator from the Ground floor to the State floor.

There are special tours that have been established to accommodate the hearing and visually impaired.

If you have any questions or wish to find out about any last minute modifications to your schedule then you may call the 24-hour Visitors Office information line at 202-456-7041.

You will not be allowed to take certain items with you. These prohibited items include, but are not necessarily limited to, the following: handbags, book bags, backpacks, purses, food and beverages of any kind, strollers, cameras, video recorders or any type of recording device, tobacco products, personal grooming items (make-up, hair brush or comb, lip or hand lotions, etc.), any pointed objects (pens, knitting needles, etc.), aerosol containers, guns, ammunition, fireworks, electric stun guns, mace, martial arts weapons/devices, or knives of any size. The United States Secret Service maintains the right to restrict any other personal items. Umbrellas, wallets, cell phones and car keys are allowed.