Music Hit In your life

วันเสาร์ที่ 3 เมษายน พ.ศ. 2553

Satellite


In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.

The first artificial satellite, Sputnik 1, was launched by the Soviet Union in 1957. By 2010 thousands of satellites have been launched into orbit around the Earth. These originate from more than 50 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as space debris. A few space probes have been placed into orbit around other bodies and become artificial satellites to the Moon, Venus, Mars, Jupiter and Saturn.

Satellites are used for a large number of purposes. Common types include military and civilian Earth observation satellites, communications satellites, navigation satellites, weather satellites, and research satellites. Space stations and human spacecraft in orbit are also satellites. Satellite orbits vary greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low Earth orbit, polar orbit, and geostationary orbit.

Satellites are usually semi-independent computer controlled systems. Satellite subsystems attend many tasks, such as power generation, thermal control, telemetry, attitude control and orbit control.



http://en.wikipedia.org/wiki/Satellite

GSM frequency bands

GSM frequency bands or frequency ranges are the cellular frequencies designated by the ITU for the operation of the GSM for mobile phones.

GSM-900, GSM-1800 and EGSM-900
GSM-900 and GSM-1800 are used in most parts of the world: Europe, Middle East, Africa, Oceania and most of Asia. In South and Central America the following countries use the following:

Costa Rica - GSM-1800
Brazil - GSM-850, 900 and 1800
Guatemala - GSM-850, GSM-900 and 1900
El Salvador - GSM-850, GSM-900 and 1900
GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink), providing 124 RF channels (channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used. Guard bands 100 kHz wide are placed at either end of the range of frequencies

E-GSM
In some countries the GSM-900 band has been extended to cover a larger frequency range. This 'extended GSM', E-GSM, uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel numbers 975 to 1023 and 0) to the original GSM-900 band. The GSM specifications also describe 'railways GSM', GSM-R, which uses 876–915 MHz (uplink) and 921–960 MHz (downlink). Channel numbers 955 to 1023. GSM-R provides additional channels and specialized services for use by railway personnel.

Phones described as having "EGSM" or "EGSM 900" support both the original GSM 900 band and the extended band. Older phones with "GSM 900" may not support EGSM. Most newer phones with "GSM 900" do support EGSM, it is just not listed that way since it is assumed that newer phones support it..

All these variants are included in the GSM-900 specification


GSM-1800
GSM-1800 uses 1710–1785 MHz to send information from the mobile station to the base tranceiver station (uplink) and 1805–1880 MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz.

GSM-1800 is also called DCS (Digital Cellular Service) in the United Kingdom, while being called PCS in Hong Kong[2] (not to mix up with GSM-1900 which is commonly called PCS in the rest of the world.)


GSM-850 and GSM-1900
GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas.

GSM-850 uses 824–849 MHz to send information from the mobile station to the base station (uplink) and 869–894 MHz for the other direction (downlink). Channel numbers are 128 to 251.
GSM-850 is also sometimes called GSM-800 because this frequency range was known as the "800 MHz band" (for simplification) when it was first allocated for AMPS in the United States in 1983.
The term Cellular is sometimes used to describe the 850 MHz band, because the original analog cellular mobile communication system was allocated in this spectrum.
GSM-1900 uses 1850–1910 MHz to send information from the mobile station to the base station (uplink) and 1930–1990 MHz for the other direction (downlink). Channel numbers are 512 to 810.
PCS is the original name in North America for the 1900 MHz band. It is an initialism for Personal Communications Service.

GSM-450
Another less common GSM version is GSM-450[3]. It uses the same band as, and can co-exist with, old analog NMT systems. NMT is a first generation (1G) mobile phone system which was primarily used in Nordic countries, Benelux, Alpine Countries, Eastern Europe and Russia prior to the introduction of GSM. It operates in either 450.4–457.6 MHz paired with 460.4–467.6 MHz (channel numbers 259 to 293), or 478.8–486 MHz paired with 488.8–496 MHz (channel numbers 306 to 340). GSM Association claims one of its around 680 operator-members has a license to operate a GSM 450 network in Tanzania. However, currently all active public operators in Tanzania use GSM 900/1800 MHz. Overall, where the 450 MHz NMT band exists, it either still runs NMT, or its been replaced by CDMA. GSM-450 is a provision, it has not seen commercial deployment.


http://en.wikipedia.org/wiki/GSM_frequency_bands

Roaming

In wireless telecommunications, roaming is a general term that refers to the extending of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device keeps connected to the network, without losing the connection. The term "roaming" originates from the GSM (Global System for Mobile Communications) sphere; the term "roaming" can also be applied to the CDMA technology, see CDMA Roaming. Traditional GSM Roaming is defined (cf. GSM Association Permanent Reference Document AA.39) as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. This can be done by using a communication terminal or else just by using the subscriber identity in the visited network. Roaming is technically supported by mobility management, authentication, authorization and billing procedures

Roaming is divided into "SIM-based roaming" and "Username/password-based roaming", whereby the technical term "roaming" also encompasses roaming between networks of different network standards, such as e.g. WLAN (Wireless Local Area Network) or GSM. Device equipment and functionality, such as SIM card capability, antenna and network interfaces, and power management, determine the access possibilities.

Using the example of WLAN/GSM roaming, the following scenarios can be differentiated (cf. GSM Association Permanent Reference Document AA.39):

SIM-based roaming: GSM subscriber roams onto a Public WLAN operated by:

  • their GSM Operator, or
  • another Operator who has a roaming agreement with their GSM Operator.

Username/password based roaming: GSM subscriber roams onto a Public WLAN operated by:

  • their GSM Operator, or
  • another Operator who has a roaming agreement with their GSM Operator.

Although these user/network scenarios focus on roaming from GSM Network Operator's network(s), clearly roaming can be bi-directional, i.e. from Public WLAN Operators to GSM Networks. Traditional roaming in networks of the same standard, e.g. from a WLAN to a WLAN or a GSM network to a GSM network, has already been described above and is likewise defined by the foreignness of the network based on the type of subscriber entry in the home subscriber register.

In terms of user service scenarios, the user can have access to the same set of services, irrespective of access type. However, differentiation also exists. Service scenarios may include access to a range of different services, including:

  • Access to corporate Intranet services;
  • Access to operator walled garden services; and
  • Access to public Internet.


In the case of session continuity, seamless access to these services across different access types is provided.


http://en.wikipedia.org/wiki/Roaming

International Roaming (IR)


บริการ International Roaming (IR) คืออะไร มีประโยชน์อย่างไร?


บริการ International Roaming (IR) หรือที่เรียกกันทั่วไปว่า บริการโทรข้ามแดนอัตโนมัติ คือบริการที่อำนวยความสะดวกให้ผู้ใช้บริการโทรศัพท์มือถือ สามารถนำโทรศัพท์มือถือเครื่องเดิม หมายเลขเดิม ที่ใช้อยู่ที่เมืองไทย ไปใช้โทรออก หรือรับสายโทรเข้า ในต่างประเทศได้ แทบจะทุกประเทศที่สำคัญๆ ทั่วโลกเลยทีเดียว

ซึ่งขณะนี้ผู้ให้บริการเครือข่ายแทบทุกระบบในบ้านเรา ก็เปิดให้ใช้บริการนี้กันอย่างเป็นทางการแล้ว แต่ละค่ายก็จะมีโปรโมชันที่แตกต่างกันออกไป รวมถึงจำนวนของประเทศที่สามารถนำไปใช้งานได้ด้วย และนอกจากจะสามารถโทรสนทนากันข้ามแดนทั่วโลกได้แล้ว ขณะนี้ผู้ให้บริการเครือข่ายบางราย ก็ยังเปิดให้ใช้บริการเชื่อมต่อกับระบบ GPRS หรือส่งข้อความแบบ MMS ได้อีกด้วย ซึ่งถือว่าเป็นความสะดวกสบายสำหรับผู้ที่ต้องเดินทางไปต่างประเทศอย่างมากเลยทีเดียว

อย่างไรก็ตาม ก่อนที่จะใช้งานบริการ International Roaming ก็ควรจะศึกษาและสอบถามรายละเอียดกับผู้ให้บริการให้ดีเสียก่อน

ที่มา http://www.thaimobilecenter.com