Selasa, 30 Oktober 2012

CPU cache

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"Cache memory" redirects here. For the general use in computing, see Cache (computing).
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations. As long as most memory accesses are cached memory locations, the average latency of memory accesses will be closer to the cache latency than to the latency of main memory.

Contents

Overview

When the processor needs to read from or write to a location in main memory, it first checks whether a copy of that data is in the cache. If so, the processor immediately reads from or writes to the cache, which is much faster than reading from or writing to main memory.
Most modern desktop and server CPUs have at least three independent caches: an instruction cache to speed up executable instruction fetch, a data cache to speed up data fetch and store, and a translation lookaside buffer (TLB) used to speed up virtual-to-physical address translation for both executable instructions and data. The data cache is usually organized as a hierarchy of more cache levels (L1, L2, etc.; see Multi-level caches).

Cache Entries

Data is transferred between memory and cache in blocks of fixed size, called cache lines. When a cache line is copied from memory into the cache, a cache entry is created. The cache entry will include the copied data as well as the requested memory location (now called a tag).
When the processor needs to read or write a location in main memory, it first checks for a corresponding entry in the cache. The cache checks for the contents of the requested memory location in any cache lines that might contain that address. If the processor finds that the memory location is in the cache, a cache hit has occurred (otherwise, a cache miss). In the case of:
  • a cache hit, the processor immediately reads or writes the data in the cache line.
  • a cache miss, the cache allocates a new entry, and copies in data from main memory. Then, the request is fulfilled from the contents of the cache.

Cache Performance

The proportion of accesses that result in a cache hit is known as the hit rate, and can be a measure of the effectiveness of the cache for a given program or algorithm.
Read misses delay execution because they require data to be transferred from memory much slower than the cache itself. Write misses may occur without such penalty, since the processor can continue execution while data is copied to main memory in the background.
Instruction caches are similar to data caches, but the CPU only performs read accesses (instruction fetches) to the instruction cache. (With Harvard architecture and modified Harvard architecture CPUs, instruction and data caches can be separated for higher performance, but they can also be combined to reduce the hardware overhead.)

Replacement Policies

Main article: cache algorithms
In order to make room for the new entry on a cache miss, the cache may have to evict one of the existing entries. The heuristic that it uses to choose the entry to evict is called the replacement policy. The fundamental problem with any replacement policy is that it must predict which existing cache entry is least likely to be used in the future. Predicting the future is difficult, so there is no perfect way to choose among the variety of replacement policies available.
One popular replacement policy, least-recently used (LRU), replaces the least recently accessed entry.
Marking some memory ranges as non-cacheable can improve performance, by avoiding caching of memory regions that are rarely re-accessed. This avoids the overhead of loading something into the cache, without having any reuse.
  • Cache entries may also be disabled or locked depending on the context.

Write Policies

If data is written to the cache, at some point it must also be written to main memory. The timing of this write is known as the write policy.
  • In a write-through cache, every write to the cache causes a write to main memory.
  • Alternatively, in a write-back or copy-back cache, writes are not immediately mirrored to the main memory. Instead, the cache tracks which locations have been written over (these locations are marked dirty). The data in these locations are written back to the main memory only when that data is evicted from the cache. For this reason, a miss in a write-back cache may sometimes require two memory accesses to service: one to first write the dirty location to memory and then another to read the new location from memory.
There are intermediate policies as well. The cache may be write-through, but the writes may be held in a store data queue temporarily, usually so that multiple stores can be processed together (which can reduce bus turnarounds and improve bus utilization).
The data in main memory being cached may be changed by other entities (e.g. peripherals using direct memory access or multi-core processor), in which case the copy in the cache may become out-of-date or stale. Alternatively, when the CPU in a multi-core processor updates the data in the cache, copies of data in caches associated with other cores will become stale. Communication protocols between the cache managers which keep the data consistent are known as cache coherence protocols.

CPU stalls

The time taken to fetch one cache line from memory (read latency) matters because the CPU will run out of things to do while waiting for the cache line. When a CPU reaches this state, it is called a stall.
As CPUs become faster, stalls due to cache misses displace more potential computation; modern CPUs can execute hundreds of instructions in the time taken to fetch a single cache line from main memory. Various techniques have been employed to keep the CPU busy during this time.
  • Out-of-order CPUs (Pentium Pro and later Intel designs, for example) attempt to execute independent instructions after the instruction that is waiting for the cache miss data.
  • Another technology, used by many processors, is simultaneous multithreading (SMT), or — in Intel's terminology — hyper-threading (HT), which allows an alternate thread to use the CPU core while a first thread waits for data to come from main memory.
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Bus

From Wikipedia, the free encyclopedia
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This article is about Road vehicles designed to carry passengers. For longer distance passenger vehicles, see Coach (vehicle). For other uses, see Bus (disambiguation).
"Busing (transportation)" redirects here. For student busing in the United States, see Desegregation busing in the United States.
An Arriva double-decker bus, running route 102 on the London Buses network.
Volvo B10M articulated bus in Chiba, Japan.
Police bus in Australia
A school bus in Kuala Lumpur.
A bus (play /ˈbʌs/; plural "buses", /ˈbʌsɨz/, archaically also big car, omnibus, multibus, or autobus) is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers.[1] The most common type of bus is the single-decker rigid bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are used for longer distance services. Bus manufacturing is increasingly globalised, with the same design appearing around the world.
Buses may be used for scheduled bus transport, scheduled coach transport, school transport, private hire, tourism; promotional buses may be used for political campaigns and others are privately operated for a wide range of purposes.
Horse drawn buses were used from the 1820s, followed by steam buses in the 1830s, and electric trolleybuses in 1882. The first buses powered by internal combustion engines were used in 1895[citation needed] and this is still the most common power source. Recently there has been growing interest in hybrid electric buses, fuel cell buses, electric buses as well as ones powered by compressed natural gas or bio-diesel.

Contents

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System Clocks
Every modern PC has multiple system clocks. Each of these vibrates at a specific frequency, normally measured in MHz (megahertz, or millions of cycles per second). A clock "tick" is the smallest unit of time in which processing happens, and is sometimes called a cycle; some types of work can be done in one cycle while others require many. The ticking of these clocks is what drives the various circuits in the PC, and the faster they tick, the more performance you get from your machine (other things being equal).
The original PCs had a unified system clock; a single clock (running at a very low speed like 8 MHz) drove the processor, memory (there was no cache back then) and I/O bus. As PCs have advanced and different parts have gained in speed more than others, the need for multiple clocks has arisen. A typical modern PC now has either four or five different clocks, running at different (but related) speeds. When the "system clock" is referred to generically, it normally refers to the speed of the memory bus running on the motherboard (and not usually that of the processor).
The various clocks in the modern PC are created using a single clock generator circuit (on the motherboard) to generate the "main" system clock, and then various clock multiplier or divider circuits to create the other signals. The table below shows the typical arrangement of clocks in a 266 MHz Pentium II PC, and how they relate to each other:
Device / Bus Clock
Speed (MHz)
Generated As
Processor
266
System Clock * 4
Level 2 Cache
133
System Clock * 2
(or Processor / 2)
System (Memory) Bus
66
  
PCI Bus
33
System Clock / 2
ISA Bus
8.3
PCI Bus / 4
The entire system is tied to the speed of the system clock. This is why increasing the system clock speed is usually more important than increasing the raw processor speed; the processor spends a great deal of time waiting on information from much slower devices, especially the system buses. While a faster processor will have greater performance, this increase in speed will not lead to nearly as much performance improvement if the processor is spending a great deal of time sitting idle waiting for other, slower parts of the system.
Next: Multiprocessing
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Memori akses acak

Dari Wikipedia bahasa Indonesia, ensiklopedia bebas
Langsung ke: navigasi, cari
Memori akses acak (bahasa Inggris: Random access memory, RAM) adalah sebuah tipe penyimpanan komputer yang isinya dapat diakses dalam waktu yang tetap tidak memperdulikan letak data tersebut dalam memori. Ini berlawanan dengan alat memori urut, seperti tape magnetik, disk dan drum, di mana gerakan mekanikal dari media penyimpanan memaksa komputer untuk mengakses data secara berurutan.
Pertama kali dikenal pada tahun 60'an. Hanya saja saat itu memori semikonduktor belumlah populer karena harganya yang sangat mahal. Saat itu lebih lazim untuk menggunakan memori utama magnetic.
Perusahaan semikonduktor seperti Intel memulai debutnya dengan memproduksi RAM , lebih tepatnya jenis DRAM.
Biasanya RAM dapat ditulis dan dibaca, berlawanan dengan ROM (read-only-memory), RAM biasanya digunakan untuk penyimpanan primer (memori utama) dalam komputer untuk digunakan dan mengubah informasi secara aktif, meskipun beberapa alat menggunakan beberapa jenis RAM untuk menyediakan penyimpanan sekunder jangka-panjang.
Tetapi ada juga yang berpendapat bahwa ROM merupakan jenis lain dari RAM, karena sifatnya yang sebenarnya juga Random Access seperti halnya SRAM ataupun DRAM. Hanya saja memang proses penulisan pada ROM membutuhkan proses khusus yang tidak semudah dan fleksibel seperti halnya pada SRAM atau DRAM. Selain itu beberapa bagian dari space addres RAM ( memori utama ) dari sebuah sistem yang dipetakan kedalam satu atau dua chip ROM.

Daftar isi

Tipe umum RAM

Beberapa jenis RAM. Dari atas ke bawah: DIP, SIPP, SIMM 30 pin, SIMM 72 pin, DIMM, DDR DIMM.

Tipe tidak umum RAM


Produsen peringkat atas RAM

Pranala luar

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Register prosesor

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Register prosesor, dalam arsitektur komputer, adalah sejumlah kecil memori komputer yang bekerja dengan kecepatan sangat tinggi yang digunakan untuk melakukan eksekusi terhadap program-program komputer dengan menyediakan akses yang cepat terhadap nilai-nilai yang umum digunakan. Umumnya nilai-nilai yang umum digunakan adalah nilai yang sedang dieksekusi dalam waktu tertentu.
Register prosesor berdiri pada tingkat tertinggi dalam hierarki memori: ini berarti bahwa kecepatannya adalah yang paling cepat; kapasitasnya adalah paling kecil; dan harga tiap bitnya adalah paling tinggi. Register juga digunakan sebagai cara yang paling cepat dalam sistem komputer untuk melakukan manipulasi data. Register umumnya diukur dengan satuan bit yang dapat ditampung olehnya, seperti "register 8-bit", "register 16-bit", "register 32-bit", atau "register 64-bit" dan lain-lain.
Istilah register saat ini dapat merujuk kepada kumpulan register yang dapat diindeks secara langsung untuk melakukan input/output terhadap sebuah instruksi yang didefinisikan oleh set instruksi. untuk istilah ini, digunakanlah kata "Register Arsitektur". Sebagai contoh set instruksi Intel x86 mendefinisikan sekumpulan delapan buah register dengan ukuran 32-bit, tapi CPU yang mengimplementasikan set instruksi x86 dapat mengandung lebih dari delapan register 32-bit.

Jenis register

Register terbagi menjadi beberapa kelas:
  • Register data, yang digunakan untuk menyimpan angka-angka dalam bilangan bulat (integer).
  • Register alamat, yang digunakan untuk menyimpan alamat-alamat memori dan juga untuk mengakses memori.
  • Register general purpose, yang dapat digunakan untuk menyimpan angka dan alamat secara sekaligus.
  • Register floating-point, yang digunakan untuk menyimpan angka-angka bilangan titik mengambang (floating-point).
  • Register konstanta (constant register), yang digunakan untuk menyimpan angka-angka tetap yang hanya dapat dibaca (bersifat read-only), semacam phi, null, true, false dan lainnya.
  • Register vektor, yang digunakan untuk menyimpan hasil pemrosesan vektor yang dilakukan oleh prosesor SIMD.
  • Register special purpose yang dapat digunakan untuk menyimpan data internal prosesor, seperti halnya instruction pointer, stack pointer, dan status register.
  • Register yang spesifik terhadap model mesin (machine-specific register), dalam beberapa arsitektur tertentu, digunakan untuk menyimpan data atau pengaturan yang berkaitan dengan prosesor itu sendiri. Karena arti dari setiap register langsung dimasukkan ke dalam desain prosesor tertentu saja, mungkin register jenis ini tidak menjadi standar antara generasi prosesor.

Ukuran register

Tabel berikit berisi ukuran register dan padanan prosesornya
Register Prosesor
4-bit Intel 4004
8-bit Intel 8080
16-bit Intel 8086, Intel 8088, Intel 80286
32-bit Intel 80386, Intel 80486, Intel Pentium Pro, Intel Pentium, Intel Pentium 2, Intel Pentium 3, Intel Pentium 4, Intel Celeron, Intel Xeon, AMD K5, AMD K6, AMD Athlon, AMD Athlon MP, AMD Athlon XP, AMD Athlon 4, AMD Duron, AMD Sempron
64-bit Intel Itanium, Intel Itanium 2, Intel Xeon, Intel Core, Intel Core 2, AMD Athlon 64, AMD Athlon X2, AMD Athlon FX, AMD Turion 64, AMD Turion X2, AMD Sempron
Nuvola apps mycomputer.png  Artikel bertopik komputer ini adalah sebuah rintisan. Anda dapat membantu Wikipedia dengan mengembangkannya.
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Broadband

From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about telecommunications signalling methods. For high-speed Internet access, see Internet access.
Look up broadband in Wiktionary, the free dictionary.
The term broadband refers to the wide bandwidth characteristics of a transmission medium and its ability to transport multiple signals and traffic types simultaneously. The medium can be coax, optical fiber, twisted pair or wireless. In contrast, baseband, describes a communication system in which information is transported across a single channel.[1]
Prior to the invention of home broadband, dial-up Internet access was the only means by which one could access the Internet and download files such as songs, movies, e-mails, etc. It would take anywhere from 10–30 minutes to download one song (3.5 MB) and over 28 hours to download a movie (700 MB). Dial-up Internet was also considered very inconvenient as it would impair the use of the home telephone line, and users would contemplate whether or not to get a second line, and if doing so was worth the cost.
In 1997, the cable modem was introduced, although the common use of broadband didn't begin rising until 2001. Having a broadband connection enabled one to download significantly faster than on dial-up. As with many new technologies, most consumers were unable to afford the cost of faster Internet service. However, high costs weren't a factor for long as by 2004, most average American households considered home broadband service to be affordable. Since its inception, broadband has continually strengthened and available connection speeds continue to rise.
Different criteria for "broad" have been applied in different contexts and at different times. Its origin is in physics, acoustics and radio systems engineering, where it had been used with a meaning similar to wideband.[2][3] However, the term became popularized through the 1990s as a vague marketing term for Internet access.

Contents

In telecommunication

Broadband refers to a communication bandwidth of at least 256 Kbps. Each channel is 6 MHz wide and it uses an extensive range of frequencies to effortlessly relay and receive data between networks. [4] In telecommunications, a broadband signaling method is one that handles a wide band of frequencies. Broadband is a relative term, understood according to its context. The wider (or broader) the bandwidth of a channel, the greater the information-carrying capacity, given the same channel quality.
In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still broader band will carry music without losing the high audio frequencies required for realistic sound reproduction. This broad band is often divided into channels or frequency bins using passband techniques to allow frequency-division multiplexing, instead of sending a higher-quality signal.
A television antenna may be described as "broadband" because it is capable of receiving a wide range of channels; while a single-frequency or Lo-VHF antenna is "narrowband" since it receives only 1 to 5 channels. The US federal standard FS-1037C defines "broadband" just as a synonym for wideband.[5]
In data communications a 56k modem will transmit a data rate of 56 kilobits per second (kbit/s) over a 4 kilohertz wide telephone line (narrowband or voiceband). The various forms of digital subscriber line (DSL) services are broadband in the sense that digital information is sent over a high-bandwidth channel. This channel is at higher frequency than the baseband voice channel, so it can support plain old telephone service on a single pair of wires at the same time.[6]
However when that same line is converted to a non-loaded twisted-pair wire (no telephone filters), it becomes hundreds of kilohertz wide (broadband) and can carry up to 60 megabits per second using very-high-bitrate digital subscriber line (VDSL or VHDSL) techniques.
In the late 1980s, the Broadband Integrated Services Digital Network (B-ISDN) used the term to refer to a broad range of bit rates, independent of physical modulation details.[7]

In computer networks

Many computer networks use a simple line code to transmit one type of signal using a medium's full bandwidth using its baseband (from zero through the highest frequency needed). Most versions of the popular Ethernet family are given names such as the original 1980s 10BASE5 to indicate this. Networks that use cable modems on standard cable television infrastructure are called broadband to indicate the wide range of frequencies that can include multiple data users as well as traditional television channels on the same cable. Broadband systems usually use a different radio frequency modulated by the data signal for each band.[8] The total bandwidth of the medium is larger than the bandwidth of any channel.[9]
The 10BROAD36 broadband variant of Ethernet was standardized by 1985, but was not commercially successful.[10][11] The DOCSIS standard became available to consumers in the late 1990s, to provide Internet access to cable television residential customers. Matters were further confused by the fact that the 10PASS-TS standard for Ethernet ratified in 2008 used DSL technology, and both cable and DSL modems often have Ethernet connectors on them.
Power lines have also been used for various types of data communication. Although some systems for remote control are based on narrowband signaling, modern high-speed systems use broadband signaling to achieve very high data rates. One example is the ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring (including power lines, but also phone lines and coaxial cables).

In video

Broadband in analog video distribution is traditionally used to refer to systems such as cable television, where the individual channels are modulated on carriers at fixed frequencies.[12] In this context, baseband is the term's antonym, referring to a single channel of analog video, typically in composite form with separate baseband audio.[13] The act of demodulating converts broadband video to baseband video.
However, broadband video in the context of streaming Internet video has come to mean video files that have bitrates high enough to require broadband Internet access for viewing.
Broadband video is also sometimes used to describe IPTV Video on demand.[14]

Internet access

Main article: Internet access
The standards group CCITT defined "broadband service" in 1988 as requiring transmission channels capable of supporting bit rates greater than the primary rate which ranged from about 1.5 to 2 Mbit/s.[15] The US National Information Infrastructure project during the 1990s brought the term into public policy debates.[16]
Broadband became a marketing buzzword for telephone and cable companies to sell their more expensive higher data rate products, especially for Internet access. In the US National Broadband Plan of 2009 it was defined as "Internet access that is always on and faster than the traditional dial-up access".[17] The same agency has defined it differently through the years.[18]
Even though information signals generally travel nearly the speed of light in the medium no matter what the bit rate, higher rate services are often marketed as "faster" or "higher speeds".[19] (This use of the word "speed" may or may not be appropriate, depending on context. It would be accurate, for instance, to say that a file of a given size will typically take less time to finish transferring if it is being transmitted via broadband as opposed to dial-up.) Consumers are also targeted by advertisements for peak transmission rates,[20] while actual end-to-end rates observed in practice can be lower due to other factors.[21]

See also

Nation specific:

References

  1. ^ Glen Carty. Broadband Networking. McGraw Hill Osborne. p. 4. ISBN 007219510X.
  2. ^ Keith Attenborough (1988). "Review of ground effects on outdoor sound propagation from continuous broadband sources". Applied Acoustics 24 (4): 289–319. doi:10.1016/0003-682X(88)90086-2.
  3. ^ John P. Shanidin (September 9, 1949). "Antenna". US Patent 2,533,900. Issued December 12, 1950.
  4. ^ "Broadband evolution, types and features".
  5. ^ "Definition: broadband". Federal Standard 1037C, Glossary of Telecommunication Terms. 1996. Retrieved July 19, 2011.
  6. ^ Broadband – Definitions from Dictionary.com
  7. ^ Ender Ayanoglu; Nail Akar. "B-ISDN (Broadband Integrated Services Digital Network)". Center for Pervasive Communications and Computing, UC Irvine. Retrieved July 12, 2011.
  8. ^ Carl Stephen Clifton (1987). What every engineer should know about data communications. CRC Press. p. 64. ISBN 978-0-8247-7566-7. "Broadband: Modulating the data signal onto an RF carrier and applying this RF signal to the carrier media"
  9. ^ Mary Louise Hollowell (1983). The cable/broadband communications books. 3. Knowledge Industry Publications. p. 64. ISBN 978-0-86729-042-4. "Broadband: relative term referring to a systemm which carries a wide frequency range."
  10. ^ "802.3b-1985 – Supplement to 802.3: Broadband Medium Attachment Unit and Broadband Medium Specifications, Type 10BROAD36 (Section 11)". IEEE Standards Association. 1985. Retrieved July 12, 2011.
  11. ^ Paula Musich (July 20, 1987). "Broadband user share pains, gains". Network World: pp. 1, 8. Retrieved July 14, 2011. "Broadband networks employ frequency-division multiplexing to divide coaxial cable into separate channels, each of which serves as an individual local network."
  12. ^ Home Technology Integration and Cedia Installer I By Ron Gilster, Helen Heneveld
  13. ^ Cabling Installation & Maintenance
  14. ^ The Guardian: BT Vision boasts 150,000 customers
  15. ^ "Recommendation I.113, Vocabulary of Terms for Broadband aspects of ISDN". ITU-T. June 1997 (originally 1988). Retrieved 19 July 2011.
  16. ^ Jeffrey A. Hart; Robert R. Reed; François Bar (November 1992). "The building of the internet: Implications for the future of broadband networks". Telecommunications Policy 16 (8): 666–689. doi:10.1016/0308-5961(92)90061-S.
  17. ^ "What is Broadband?". The National Broadband Plan. US Federal Communications Commission. Retrieved July 15, 2011.
  18. ^ "Inquiry Concerning the Deployment of Advanced Telecommunications Capability to All Americans in a Reasonable and Timely Fashion, and Possible Steps to Accelerate Such Deployment Pursuant to Section 706 of the Telecommunications Act of 1996, as Amended by the Broadband Data Improvement Act". GN Docket No. 10-159, FCC-10-148A1. Federal Communications Commission. August 6, 2010. Retrieved July 12, 2011.
  19. ^ "Virgin Media delivers world's fastest cable broadband". News release (Virgin Media). July 22, 2011. Retrieved July 24, 2011.
  20. ^ "Virgin Media’s ultrafast 100Mb broadband now available to over four million UK homes". News release. Virgin Media. June 10, 2011. Retrieved August 18, 2011.
  21. ^ Tom Phillips (August 25, 2010). "'Misleading' BT broadband ad banned". UK Metro. Retrieved July 24, 2011.
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Wireless LAN

From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the radio stations in Lancaster, Pennsylvania, see WLAN (AM) and WLAN-FM.
This notebook computer is connected to a wireless access point using a PC card wireless card.
An example of a Wi-Fi network.
A wireless local area network (WLAN) links two or more devices using some wireless distribution method (typically spread-spectrum or OFDM radio), and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network. Most modern WLANs are based on IEEE 802.11 standards, marketed under the Wi-Fi brand name.
Wireless LANs have become popular in the home due to ease of installation, and in commercial complexes offering wireless access to their customers; often for free. Large wireless network projects are being put up in many major cities: New York City, for instance, has begun a pilot program to provide city workers in all five boroughs of the city with wireless Internet access.[1]
An embedded RouterBoard 112 with U.FL-RSMA pigtail and R52 mini PCI Wi-Fi card widely used by wireless Internet service providers (WISPs)

Contents

History

Norman Abramson, a professor at the University of Hawaii, developed the world’s first wireless computer communication network, ALOHAnet, using low-cost ham-like radios. The system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines.[2]
"In 1979, F.R. Gfeller and U. Bapst published a paper in the IEEE Proceedings reporting an experimental wireless local area network using diffused infrared communications. Shortly thereafter, in 1980, P. Ferrert reported on an experimental application of a single code spread spectrum radio for wireless terminal communications in the IEEE National Telecommunications Conference. In 1984, a comparison between infrared and CDMA spread spectrum communications for wireless office information networks was published by Kaveh Pahlavan in IEEE Computer Networking Symposium which appeared later in the IEEE Communication Society Magazine. In May 1985, the efforts of Marcus led the FCC to announce experimental ISM bands for commercial application of spread spectrum technology. Later on, M. Kavehrad reported on an experimental wireless PBX system using code division multiple access. These efforts prompted significant industrial activities in the development of a new generation of wireless local area networks and it updated several old discussions in the portable and mobile radio industry.
The first generation of wireless data modems was developed in the early 1980's by amateur communication groups. They added a voice band data communication modem, with data rates below 9600 bps, to an existing short distance radio system such as a walkie talkie. The second generation of wireless modems was developed immediately after the FCC announcement in the experimental bands for non-military use of the spread spectrum technology. These modems provided data rates on the order of hundreds of Kbps. The third generation of wireless modem now aims at compatibility with the existing LANs with data rates on the order of Mbps. Currently, several companies are developing the third generation products with data rates above 1 Mbps and a couple of products have already been announced. "[3]
54 Mbit/s WLAN PCI Card (802.11g)
"The first of the IEEE Workshops on Wireless LAN was held in 1991. At that time early wireless LAN products had just appeared in the market and the IEEE 802.11 committee had just started its activities to develop a standard for wireless LANs. The focus of that first workshop was evaluation of the alternative technologies. By 1996, the technology was relatively mature, a variety of applications had been identified and addressed and technologies that enable these applications were well understood. Chip sets aimed at wireless LAN implementations and applications, a key enabling technology for rapid market growth, were emerging in the market. Wireless LANs were being used in hospitals, stock exchanges, and other in building and campus settings for nomadic access, point-to-point LAN bridges, ad-hoc networking, and even larger applications through internetworking. The IEEE 802.11 standard and variants and alternatives, such as the wireless LAN interoperability forum and the European HiperLAN specification had made rapid progress, and the unlicensed PCS Unlicensed Personal Communications Services and the proposed SUPERNet, later on renamed as U-NII, bands also presented new opportunities."[4]
WLAN hardware was initially so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible. Early development included industry-specific solutions and proprietary protocols, but at the end of the 1990s these were replaced by standards, primarily the various versions of IEEE 802.11 (in products using the Wi-Fi brand name). An alternative ATM-like 5 GHz standardized technology, HiperLAN/2, has so far not succeeded in the market, and with the release of the faster 54 Mbit/s 802.11a (5 GHz) and 802.11g (2.4 GHz) standards, it is even more unlikely that it will ever succeed. Since 2002 there has been newer standard added to 802.11; 802.11n which operates on both the 5Ghz and 2.4Ghz bands at 300 Mbit/s, most newer routers can broadcast a wireless network on both wireless bands, this is called dualband. Because of the crowded 2.4Ghz band and the interference with other services like Bluetooth, more and more routers are available as dualband version. The extension of the 5Ghz to 5.8Ghz leaves space for a lot of more stations. A HomeRF group was formed in 1997 to promote a technology aimed for residential use, but disbanded at the end of 2002.[5]

Architecture

Stations

All components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped with wireless network interface controllers (WNICs). Wireless stations fall into one of two categories: access points, and clients. Access points (APs), normally routers, are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with. Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones and other smartphones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface.

Basic service set

The basic service set (BSS) is a set of all stations that can communicate with each other. Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS.
There are two types of BSS: Independent BSS (also referred to as IBSS), and infrastructure BSS. An independent BSS (IBSS) is an ad-hoc network that contains no access points, which means they can not connect to any other basic service set.

Extended service set

An extended service set (ESS) is a set of connected BSSs. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string.

Distribution system

A distribution system (DS) connects access points in an extended service set. The concept of a DS can be used to increase network coverage through roaming between cells.
DS can be wired or wireless. Current wireless distribution systems are mostly based on WDS or MESH protocols, though other systems are in use.

Types of wireless LANs

Peer-to-peer

Peer-to-Peer or ad-hoc wireless LAN
An ad-hoc network also called WiFi Direct network is a network where stations communicate only peer to peer (P2P). There is no base and no one gives permission to talk. This is accomplished using the Independent Basic Service Set (IBSS).
In a Wi-Fi P2P group the group owner operates as an access point and all other devices are clients. There are two main methods to establish a group owner in the Wi-Fi Direct group. In one approach user sets up a P2P group owner manually. This method is also known as Autonomous Group Owner (autonomous GO). In the second method also called negotiation-based group creation two devices compete based on the group owner intent value. The device with higher intent value becomes a group owner and the second device becomes a client. Group owner intent value can depend on whether the wireless device performs a cross-connection between an infrastructure WLAN service and a P2P group, remaining power in the wireless device, whether the wireless device is already a group owner in another group and/or a received signal strength of the first wireless device.
A peer-to-peer (P2P) network allows wireless devices to directly communicate with each other. Wireless devices within range of each other can discover and communicate directly without involving central access points. This method is typically used by two computers so that they can connect to each other to form a network.
If a signal strength meter is used in this situation, it may not read the strength accurately and can be misleading, because it registers the strength of the strongest signal, which may be the closest computer.
Hidden node problem: Devices A and C are both communicating with B, but are unaware of each other
IEEE 802.11 defines the physical layer (PHY) and MAC (Media Access Control) layers based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The 802.11 specification includes provisions designed to minimize collisions, because two mobile units may both be in range of a common access point, but out of range of each other.
The 802.11 has two basic modes of operation: ad hoc mode and infrastructure mode. In ad hoc mode, mobile units transmit directly peer-to-peer. In infrastructure mode, mobile units communicate through an access point that serves as a bridge to a wired network infrastructure. Since wireless communication uses a more open medium for communication in comparison to wired LANs, the 802.11 designers also included shared-key encryption mechanisms: Wired Equivalent Privacy (WEP), Wi-Fi Protected Access (WPA, WPA2), to secure wireless computer networks.

Bridge

A bridge can be used to connect networks, typically of different types. A wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to a wireless network. The bridge acts as the connection point to the Wireless LAN.

Wireless distribution system

Main article: Wireless Distribution System
A Wireless Distribution System enables the wireless interconnection of access points in an IEEE 802.11 network. It allows a wireless network to be expanded using multiple access points without the need for a wired backbone to link them, as is traditionally required. The notable advantage of WDS over other solutions is that it preserves the MAC addresses of client packets across links between access points.[6]
An access point can be either a main, relay or remote base station. A main base station is typically connected to the wired Ethernet. A relay base station relays data between remote base stations, wireless clients or other relay stations to either a main or another relay base station. A remote base station accepts connections from wireless clients and passes them to relay or main stations. Connections between "clients" are made using MAC addresses rather than by specifying IP assignments.
All base stations in a Wireless Distribution System must be configured to use the same radio channel, and share WEP keys or WPA keys if they are used. They can be configured to different service set identifiers. WDS also requires that every base station be configured to forward to others in the system as mentioned above.
WDS may also be referred to as repeater mode because it appears to bridge and accept wireless clients at the same time (unlike traditional bridging). It should be noted, however, that throughput in this method is halved for all clients connected wirelessly.
When it is difficult to connect all of the access points in a network by wires, it is also possible to put up access points as repeaters.

Roaming

Roaming among Wireless Local Area Networks
There are two definitions for wireless LAN roaming:
  • Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to another AP within a home network because the signal strength is too weak. An authentication server (RADIUS) presumes the re-authentication of MS via 802.1x (e.g. with PEAP). The billing of QoS is in the home network. A Mobile Station roaming from one access point to another often interrupts the flow of data among the Mobile Station and an application connected to the network. The Mobile Station, for instance, periodically monitors the presence of alternative access points (ones that will provide a better connection). At some point, based on proprietary mechanisms, the Mobile Station decides to re-associate with an access point having a stronger wireless signal. The Mobile Station, however, may lose a connection with an access point before associating with another access point. In order to provide reliable connections with applications, the Mobile Station must generally include software that provides session persistence.[7]
  • External Roaming (2): The MS (client) moves into a WLAN of another Wireless Internet Service Provider (WISP) and takes their services (Hotspot). The user can independently of his home network use another foreign network, if this is open for visitors. There must be special authentication and billing systems for mobile services in a foreign network.

References

  1. ^ "NY Muni Wireless Network Launch in Sight". Internet News. Retrieved 2010-11-03.
  2. ^ "History of Wireless". Johns Hopkins Bloomberg School of Public Health. Archived from the original on 2007-02-10. Retrieved 2007-02-17.
  3. ^ "The First IEEE Workshop on Wireless LANS: Preface". Worcester Polytechnic Institute. Retrieved 2008-03-16.
  4. ^ "The Second IEEE Workshop on Wireless LANS: Summary". Worcester Polytechnic Institute. Retrieved 2008-03-16.
  5. ^ Wayne Caswell (November 17, 2010). "HomeRF Archives". Retrieved July 16, 2011.
  6. ^ Wireless Distribution System Linked Router Network DD-WRT Wiki. Retrieved December 31, 2006.
  7. ^ "How Wi-Fi Roaming Really Works". Retrieved 2008-10-09.
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