:: wikimiki.org ::

Time-division Multiplexing

Time-division multiplexing

Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels. In some TDM systems, successive pulses represent bits from successive channels, e.g., voice channels in a T1 system. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot"). What distinguishes coarse time-division multiplexing from packet switching is that the time-slots are pre-allocated to the channels, rather than arbitrated on a per-time slot basis. Uses of time-division multiplexing:
- The PDH and SDH network transmission standards
- The GSM telephone system Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time . Interleaving can be done at bits or blocks of bytes . This enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network . This article consists of two sections, namely, Transmission using TDM and Synchronous Digital Hierarchy (SDH). The first section examines the basic principles underlying TDM, while the second section discusses how SDH is used to switch TDM frames.

History

TDM was a synchronous technique invented during World War II to encrypt transatlantic radio conversation between Churchill and Roosevelt. By the early 60's, engineers from Bell Labs had developed the first T1 Channel Banks, that combined 24 digitised voice calls over a 4 wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signal of 64 Kbit/s. Channel banks used a byte's fixed position (temporal alignment) in the frame to determine which call it belonged to .

Transmission using Time Division Multiplexing (TDM)

In circuit switched networks such as the Public Switched Telephone Network (PSTN) there exists the need to transmit multiple subscribers’ calls along the same transmission medium . To accomplish this, network designers make use of TDM. TDM allows switches to create channels, also known as tributaries, within a transmission stream . A standard voice signal has a bandwidth of 64 kbit/s, determined using Nyquist’s Sampling Criterion . TDM takes frames of the voice signals and multiplexes them into a TDM frame which runs at a higher bandwidth. So if the TDM frame consists of n voice frames, the bandwidth will be n
- 64 kbit/s . Each voice frame in the TDM frame is called a channel or tributary . In European systems, TDM frames contain 30 digital voice frames and in American systems, TDM frames contain 24 digital voice frames . Both of the standards also contain extra space for signalling and synchronisation data . Multiplexing more than 24 or 30 digital voice frames is called Higher Order Multiplexing . Higher Order Multiplexing is accomplished by multiplexing the standard TDM frames . For example, a European 120 channel TDM frame is formed by multiplexing four standard 30 channel TDM frames . At each higher order multiplex, four TDM frames from the immediate lower order are combined, creating multiplexes with a bandwidth of n x 64 kbit/s, where n = 120, 480, 1920, etc. .

Synchronous Digital Hierarchy (SDH)

Plesiochronous Digital Hierarchy (PDH) was developed as a standard for multiplexing higher order frames . PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames . This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy (SDH). The requirements which drove the development of SDH were as follows :
- Be synchronous – All clocks in the system must align with a reference clock.
- Be service-oriented – SDH must route traffic from End Exchange to End Exchange without worrying about exchanges in between, where the bandwidth can be reserved at a fixed level for a fixed period of time.
- Allow frames of any size to be removed or inserted into an SDH frame of any size.
- Easily manageable with the capability of transferring management data across links.
- Provide high levels of recovery from faults.
- Provide high data rates by multiplexing any size frame, limited only by technology.
- Give reduced bit rate errors. SDH has become the primary transmission protocol in most PSTN networks . It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, so as to create larger SDH frames known as Synchronous Transport Modules (STM) . The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame . SDH can also multiplex packet based frames such as Ethernet, PPP and ATM . While SDH is considered to be a transmission protocol (Layer 1 in the OSI Reference Model), it also performs some switching functions, as stated in the third bullet point requirement listed above . The most common SDH Networking functions are as follows:
- SDH Crossconnect – The SDH Crossconnect is the SDH version of a Time-Space-Time crosspoint switch. It connects any channel on any of its inputs to any channel on any of its outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and outputs are connected to other exchanges .
- SDH Add-Drop Multiplexer – The SDH Add-Drop Multiplexer (ADM) can add or remove any multiplexed frame down to 1.544Mb. Below this level, standard TDM can be performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used in End Exchanges where the channels from subscribers are connected to the core PSTN network . SDH Network functions are connected using high-speed Optic Fibre. Optic Fibre uses light pulses to transmit data and is therefore extremely fast . Modern optic fibre transmission makes use of Wavelength Division Multiplexing (WDM) where signals transmitted across the fibre are transmitted at different wavelengths, creating additional channels for transmission . This increases the speed and capacity of the link, which in turn reduces both unit and total costs .

Statistical Time-division Multiplexing (STDM)

STDM is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over 1 line. Many college and corporate campuses use this type of TDM to logically distribute bandwidth. If there is one 10MBit line coming into the building, STDM can be used to provide 178 terminals with a dedicated 56k connection (178
- 56k = 9.96Mb). A more common use however is to only grant the bandwidth when that much is needed. STDM does NOT reserve a time slot for each terminal, rather assign a slot when the terminal is requiring data to be sent/received.

Digital

:For other uses, see Digital (disambiguation) A digital system is one that uses numbers, especially binary numbers, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (an analog system) or non-numeric symbols such as letters or icons. The distinction of "digital" versus "analog" or "symbolic" can refer to method of input, data storage and transfer, the internal working of an instrument, and the kind of display. The word comes from the same source as the word digit and digitus: the Latin word for finger (counting on the fingers) as these are used for discrete counting. The word digital is most commonly used in computing and electronics, especially where real-world information is converted to binary numeric form as in digital audio and digital photography. Such data-carrying signals carry either one of two electronic or optical pulses, logic 1 (pulse present) or 0 (pulse absent). The term is often meant by the prefix "e-", as in e-mail and ebook, even though not all electronics systems are digital.

Digital noise

When data is transmitted using analog methods, a certain amount of noise enters into the signal. This can have myriad causes: data transmitted by radio may be received badly, suffer interference from other radio sources, or pick up background radio noise from the rest of the universe. Electric pulses being sent down wires are attenuated by the resistance of the wire, and dispersed by its capacitance, and heat variations can increase or reduce these effects. While digital transmissions are also degraded, any slight variations can be safely ignored. Any variance could provide a great amount of distortion in an analog signal. In a digital signal, these variances can be overcome, as any signal close to a particular value will be interpreted as that value. Care must be taken when connected digital and analog systems; tolerable variances for the digital part can leak into the analog part and become intolerable.

Analog, symbolic, and digital displays; ease of reading

For human readable information, digital, analog, and symbol display methods can all be useful. Should an instant impression be required, analog meters and indicator lights often give information quickly. Many people glance quickly at their analog watch and know roughly what the time is or at an automobile dashboard and know that a door is ajar. When accuracy is required, however, digital displays are preferred. Reading analog meters requires time and a little bit of skill, whereas writing down the value on a digital display is merely a case of copying down the numbers. In cases where both accuracy and quick reckoning are both required, dual displays are often used. A needle (analog) just touching onto the bottom of an orange shaded area is much different to a needle almost touching into the red area, but an indicator lamp (symbol) would just glow orange and a numeric (digital) display, although it could be colored orange, would not indicate the relative level of danger to an untrained operator.

Analog to digital conversion

:Main article: Analog-to-digital converter Converting an analog source to digital data is done with two steps: sampling, which changes the source to a series of discrete values (called samples), and quantization, which converts each sample to a number. For example, the sensor of a digital camera contains millions of sensing elements (one for each pixel). When an exposure is made, the light focused on the array is converted into millions of electric charges (sampled). These charges are then amplified and converted to numbers (quantized). The resulting digital image is then processed and stored in the camera's memory card. The samples in this case are spatial. In contrast, converting an audio source to digital requires temporal samples: it is converted to an electrical signal using a microphone, and the voltage of this signal is sampled thousands of times per second (the sampling frequency). Each sample is then quantized to form the digital audio data. Both sampling and quantization will result in a loss of data. Changes in the original data that occur between the samples will not appear in the digital data (or worse, will cause aliasing, the appearance of data not present in the original source). And while a voltage can be any of a seemingly unlimited number of values between its minimum and maximum (limited only by quantum mechanics), a digital representation using

n

bits can have only

2^n

possible values. While this information will be preserved in future transmission, the data has been lost. The amount of information that can be stored in a digital representation is called its resolution. And since the conversion to digital is a two step process, there are two types of resolution: sampling resolution and quantization resolution. Sampling resolution can be either spatial (expressed in pixels per inch) or temporal (expressed as samples per second) or both (for example, a video). Quantization resolution is usually expressed as the number of bits used to represent each sample and is thus often called the bit depth or (for pictures) the color depth. The best resolution for a given set of digital data depends on the processing it will undergo and its ultimate purpose. For example, compact discs use a sampling resolution of 44,100 samples/second, which is sufficient for audio in the range of human hearing. Most digital photographs use a bit depth of 8 bits/color, which produces more colors than the human eye can discern. However many photographers use camera raw with 12 bits/color to allow for more accuracy during processing before producing a final photograph at 8 bits/color for display or printing. Scientific photography may also require greater bit depth. If sufficient resolution is used, the data loss caused by the conversion to digital is offset by the accuracy of digital processing. When analog signals are transmitted and stored, accuracy is lost due to noise and distortion. So neither digital nor analog offer perfect fidelity; resolution is sacrificed for accuracy with digital and vice versa for analog. When both high resolution and high accuracy are needed, either a high resolution digital system or a high accuracy analog system must be used (with a correspondingly high cost).

Symbol to digital conversion

Since symbols are not continuous, converting symbols to digital is simpler and less prone to data loss than analog to digital conversion. Instead of sampling and quantization, similar steps are used: polling and encoding. A symbol input device usually consists of a number of switches that are polled at regular intervals to see which switches are pressed. Data will be lost if, within a single polling interval, two switches are pressed, or a switch is pressed, released, and pressed again. This polling can be done by a specialized processor in the device to prevent burdening the main CPU. When a new symbol has been entered, the device sends an interrupt to alert the CPU to read it. For devices with just a few switches (such as the buttons on a joystick), the status of each can be encoded as bits (usually 0 for released and 1 for pressed) in a single word. This is very useful when combinations of key presses are meaningful, and is sometimes used for passing the status of modifier keys on a keyboard (such as shift and control). But it does not scale to support more keys than the number of bits in a single byte or word. Devices with many switches (such as a computer keyboard) usually arrange these switches in a scan matrix, with the individual switches on the intersections of x and y lines. When a switch is pressed, it connects the corresponding x and y lines together. Polling (often called scanning in this case) is done by activating each x line in sequence and detecting which y lines then have a signal, thus which keys are pressed. When the keyboard processor detects that a key has changed state, it sends a signal to the CPU indicating the scan code of the key and its new state. The symbol is then encoded, or converted into a number, based on the status of modifier keys and the desired character encoding. Using a custom encoding for a specific application can be done with no loss of data. However, using a standard encoding such as ASCII is problematic if a symbol such as 'ß' needs to be converted but is not in the standard.

Historical digital systems

Although digital signals are generally associated with the binary electronic digital systems used in modern electronics and computing, digital systems are actually ancient, and need not be binary nor electronic.
- A beacon is perhaps the simplest non-electronic digital signal, with just two states (on and off). In particular, smoke signals are one of the oldest examples of a digital signal, where an analog "carrier" (smoke) is modulated with a blanket to generate a digital signal (puffs) that conveys information.
- DNA comprises a long sequence of four digits (denoted A, C, G, and T), effectively a base-four numeral system. (In fact, in the double helix structure, there are two strands, but one of them is never read.) Each of these digits is an organic molecule, known as a nucleotide. DNA is the major system of information transfer from one generation to another, and evolution has developed its digital properties into a robust method of communication.
- Morse code uses five digital states—dot, dash, short gap (between each letter), medium gap (between words), and long gap (between sentences)—to send messages via a variety of potential carriers such as electricity or light, for example using an electrical telegraph or a flashing light.
- Semaphore signalling uses rods or flags held in particular positions to send messages to the receiver watching them some distance away.
- International maritime signal flags have distinctive markings that represent letters of the alphabet to allow ships to send messages to each other.
- More recently invented, a modem modulates an analog "carrier" signal (such as sound) to encode binary electrical digital information, as a series of binary digital sound pulses. A slightly earlier, surprisingly reliable version of the same concept was to bundle a sequence of audio digital "signal" and "no signal" information (i.e. "sound" and "silence") on magnetic cassette tape for use with early home computers.

Frequency

: Frequency is the measurement of the number of times that a repeated event occurs per unit time. It is also defined as the rate of change of phase of a sinusoidal waveform.

Measurement

To calculate the frequency of an event, the number of occurrences of the event within a fixed time interval are counted, and then divided by the length of the time interval. In SI units, the result is measured in hertz (Hz), named after the German physicist Heinrich Rudolf Hertz. 1 Hz means that an event repeats once per second, 2 Hz is twice per second, and so on. This unit was originally called a cycle per second (cps), which is still used sometimes. Other units that are used to measure frequency include revolutions per minute (rpm) and radians per second (rad/s). Heart rate and musical tempo are measured in beats per minute (BPM). An alternative method to calculate frequency is to measure the time between two consecutive occurrences of the event (the period) and then compute the frequency as the reciprocal of this time: :

f = \frac

where T is the period. A more accurate measurement takes many cycles into account and averages the period between each.

Frequency of waves

Measuring the frequency of sound, electromagnetic waves (such as radio or light), electrical signals, or other waves, the frequency in hertz is the number of cycles of the repetitive waveform per second. If the wave is a sound, frequency is what mainly characterizes its pitch. Frequency has an inverse relationship to the concept of wavelength. The frequency f is equal to the speed v of the wave divided by the wavelength λ (lambda) of the wave: :

f = \frac

In the special case of electromagnetic waves moving through a vacuum, then v = c, where c is the speed of light in a vacuum, and this expression becomes: :

f = \frac

NOTE: When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes.

Invariance

Apart from its being modified by Doppler effect, frequency is an invariant quantity in the universe. That is, it cannot be changed by any physical process unlike velocity of propagation or wavelength.

Examples

- The frequency of the standard pitch A above middle C is usually defined as 440 Hz, that is, 440 cycles per second () and known as concert pitch, to which an orchestra tunes.
- A baby can hear tones with oscillations up to approximately 20,000 Hz, but these frequencies become more difficult to hear as people age.
- In Europe, the frequency of the alternating current in mains is 50 Hz (close to the tone G).
- In North America, the frequency of the alternating current is 60 Hz (close to the tone B flat — that is, a minor third above the European frequency). The frequency of the 'hum' in an audio recording can show where the recording was made — in Europe or in America.

External links

- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion: frequency to wavelength and back]
- [http://www.sengpielaudio.com/calculator-period.htm Conversion: period, cycle duration, periodic time to frequency] Category:Physical quantity Category:Sound Category:Wave mechanics ko:진동수 ja:周波数 th:ความถี่

Bit

:This article is about the unit of information. See Bit (disambiguation) for other meanings. :For the use of binary numbers in computer systems, please see the article binary arithmetic. A bit refers to a digit in the binary numeral system (base 2). For example, the number 1001011 is 7 bits long. The unit is sometimes abbreviated to b (but see below). Binary digits are almost always used as the basic unit of information storage and communication in digital computing and digital information theory.

History and explanation

Claude E. Shannon first used the word bit in a 1948 paper. Shannon's bit is a portmanteau word for binary digit (or possibly binary unit). He attributed its origin to John W. Tukey. A bit is like a light switch; it can be either on or off. A single bit is a one or a zero, a true or a false, a "flag" which is "on" or "off", or in general, the quantity of information required to distinguish two mutually exclusive states from each other. The bit is the smallest unit of storage currently used in computing, although much research is ongoing in quantum computing with qubits.

More than one bit

A byte is a collection of bits, originally variable in size but now almost always eight bits. Eight-bit bytes, also known as octets, can represent 256 values (2⁸ values, 0–255). A four-bit quantity is known as a nibble, and can represent 16 values (2⁴ values, 0–15). "Word" is a term for a slightly larger group of bits, but it has no standard size. In the IA-32 architecture, 16 bits are called a "word" (with 32 bits being a "double word" or dword), but other architectures have word sizes of 32, 64 or others. Terms for large quantities of bits can be formed using the standard range of prefixes, e.g., kilobit (kbit), megabit (Mbit) and gigabit (Gbit). Note that much confusion exists regarding these units and their abbreviations, see binary prefixes. It has often been recommended to use "bit" for the bit and "b" for the byte, to prevent confusion with the unit bel, B. However, "b" is often used for bit and "B" for byte. The IEC recommends to use only "bit" and "B" for maximum disambiguation. Since the bel is almost never used by itself (only used as a decibel, dB) the chances of conflict are small. Certain bitwise computer processor instructions (such as xor) operate at the level of manipulating bits rather than manipulating data interpreted as an aggregate of bits. Telecommunications or computer network transfer rates are usually described in terms of bits per second (not to be confused with baud).

T-carrier

In telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America and Japan. The basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 kbit/s, and is commonly used for one voice circuit. The E-carrier system, where 'E' stands for European, is incompatible with the T-carrier and is used just about everywhere else in the world besides North America and Japan. It typically uses the E1 line rate and the E3 line rate. The E2 line rate is less commonly used. See the table below for bandwidth comparisons.

T1

The most common legacy of this whole system is the line rate designations. A "T1" now seems to mean any data circuit that runs at the original 1.544 Mbit/s line rate. Originally the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s. Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground, and cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the capacity was increased until the failure rate was unacceptable, then reduced. A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest frequency at which voice communications occurs is at 4000 Hz, one needs, when converting analog voice to digital data, at least double that frequency for the sample rate. This yields the number 8000. Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Since each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).

Digital signal crossconnect

DS1 signals are frequently used to connect equipment within a facility. In this case, a low-level signal (6 volts peak-to-peak differential) called the DSX1 is used. DSX refers to a digital signal crossconnect, and it is essentially a patch panel allowing easy interconnection. When a DS1 leaves the building, it becomes a T1 and is referred to as a span. The signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal. DS3 signals are almost exclusively used within buildings, for interconnections and as an intermediate step before being muxed onto a SONET circuit. This is because a T3 circuit can only go about 600 feet between repeaters. When a customer orders a DS3, they usually get a (much faster) SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS3 is delivered in its familiar form, two coax cables with BNC connectors on the ends.

Bit robbing

The T-carrier system traditionally uses in-band signalling or bit robbing, resulting in lower transmission rates than the E-carrier system. This resulted in many US ISDN installations only having an effective data rate of 56 kbit/s over a nominal 64 kbit/s channel. See also A&B. This depends on the framing format used, and almost all systems are now capable of transmitting a "clear" 64 kbit/s channel, despite the failure of providers to sell such services.

Notes

Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy. Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission. Note 3: Historically, if an "F" precedes the "T", optical fiber cables are utilized at the same rates. Note 4: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 32 voice-frequency channels and multiples thereof. See table below.

T-Carrier Systems	North American	Japanese	European (CEPT)
Level zero (Channel data rate)	64 kbit/s (DS0)	64 kbit/s	64 kbit/s
First level	1.544 Mbit/s (DS1) (24 user channels) (T1)	1.544 Mbit/s (24 user channels)	2.048 Mbit/s (32 user channels) (E1)
(Intermediate level, US. hierarchy only)	3.152 Mbit/s (DS1C) (48 Ch.)	-	-
Second level	6.312 Mbit/s (DS2) (96 Ch.)	6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.)	8.448 Mbit/s (128 Ch.) (E2)
Third level	44.736 Mbit/s (DS3) (672 Ch.) (T3)	32.064 Mbit/s (480 Ch.)	34.368 Mbit/s (512 Ch.) (E3)
Fourth level	274.176 Mbit/s (DS4) (4032 Ch.)	97.728 Mbit/s (1440 Ch.)	139.268 Mbit/s (2048 Ch.) (E4)
Fifth level	400.352 Mbit/s (5760 Ch.)	565.148 Mbit/s (8192 Ch.)	565.148 Mbit/s (8192 Ch.) (E5)

Note 1: The DS designations are used in connection with the North American hierarchy only. Technically a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but the terms are almost always used interchangeably. Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using Synchronous optical networking, SONET or Synchronous digital hierarchy, SDH.

References

- ANSI T1.403-1999 - Network and Customer Installation Interfaces
- Federal Standard 1037C
- [http://www.oreilly.com/catalog/t1survival/chapter/ch05.html T1: A Survival Guide Chapter 5] Category:Computer_and_telecommunication_standards

Packet switching

In computer networking and telecommunications, packet switching is the now-dominant communications paradigm, in which packets (units of information carriage) are individually routed between nodes over data links which might be shared by many other nodes. This contrasts with the principal other paradigm, circuit switching, which sets up a dedicated connection between the two nodes for their exclusive use for the duration of the communication. Packet switching is used to optimize the use of the bandwidth available in a network, to minimize the transmission latency (i.e. the time it takes for data to pass across the network), and to increase robustness of communication. Basically, a file is broken up into smaller groups of data known as packets. Such "packets" carry with them information with regard to their origin, destination and sequence within the original file. This sequence is needed for re-assembly at the file's destination.

Packets

A packet is a block of data (called a payload) with address and administrative information attached to allow a network of nodes to deliver the data to the destination, e.g., as in IPv6. A packet is analogous to a letter sent through the mail with the address written on the outside.

Packet routing

Packets are routed to their destination through the most expedient route (as determined by some routing algorithm). Not all packets travelling between the same two nodes will necessarily follow the same route. One data connection will usually carry a stream of packets from several nodes. The destination node reassembles the packets into their appropriate sequence. Consequently packets sent between a pair of nodes may arrive in an order different from the order in which they were sent. Packet switching influenced the development of the Actor model of concurrent computation in which messages sent to the same address may be delivered in an order different from the order in which they were sent.

Packet switching in the Internet

The most well-known use of the packet switching model is the Internet, which is a packet-switched network, running the Internet Protocol layer 3 protocol over a variety of other network technologies. Ethernet, X.25 and Frame relay are international standard layer 2 packet switching networks. Newer mobile phone technologies such as GPRS and i-mode also employ packet switching. Packet switching is also called connectionless networking, because it is the opposite of circuit switched or connection-oriented networking, although technologies such as MPLS are beginning to blur the boundaries between the two. ATM is another hybrid technology, which uses cell relay instead of packet switching. Fast packet switching is a packet switching technique that increases the throughput by eliminating overhead.

Controversy over invention of packet switching

There is controversy over the invention of packet switching.

Work at MIT

In Internet Society publication A Brief History of the Internet, Barry M. Leiner, Vinton G. Cerf, David D. Clark, Robert E. Kahn, Leonard Kleinrock, Daniel C. Lynch, Jon Postel, Larry G. Roberts, Stephen Wolff stated :"Leonard Kleinrock at MIT published the first paper on packet switching theory in July 1961 and the first book on the subject in 1964. Kleinrock convinced Roberts of the theoretical feasibility of communications using packets rather than circuits, which was a major step along the path towards computer networking. The other key step was to make the computers talk together. To explore this, in 1965 working with Thomas Merrill, Roberts connected the TX-2 computer in Mass. to the Q-32 in California with a low speed dial-up telephone line creating the first (however small) wide-area computer network ever built. The result of this experiment was the realization that the time-shared computers could work well together, running programs and retrieving data as necessary on the remote machine, but that the circuit switched telephone system was totally inadequate for the job. Kleinrock's conviction of the need for packet switching was confirmed."

Work at RAND

According to Larry Roberts in The Evolution of Packet Switching, :"The first published description of what we now call packet switching was an 11-volume analysis, On Distributed Communications, prepared by Paul Baran of the Rand Corporation in August 1964. This study was conducted for the Air Force, and it proposed a fully distributed packet switching system to provide for all military communications, data, and voice. The study also included a totally digital microwave system and integrated encryption capability. The Air Force's primary goal was to produce a totally survivable system that contained no critical central components. Not only was this goal achieved by Rand's proposed packet switching system, but even the economics projected were superior, for both voice and data transmissions. Unfortunately, the Air Force took no follow-up action, and the report sat largely ignored for many years until packet switching was rediscovered and applied by others." Baran described two key ideas: first, use of a decentralized network with multiple paths between any two points; and second, dividing complete user messages into what he called message blocks (later called packets) before sending them into the network. This first allowed the elimination of single points of failure, and enabled the network to automatically and efficiently work around any failures. A summary paper describing the entire scheme was published in 1964.

Work at National Physical Laboratory

According to Larry Roberts in The Evolution of Packet Switching, :"Almost immediately after the 1965 meeting, Donald Davies conceived of the details of a store-and-forward packet switching system, and in a June 1966 description of his proposal coined the term "packet" to describe the 128-byte blocks being moved around inside the network. Davies circulated his proposed network design throughout the U.K. in late 1965 and 1966. It was only after this distribution that he discovered Paul Baran's 1964 report. :The first open publication of the NPL proposal was in October 1967 at the A.C.M. Symposium in Gatlinburg, TN. In nearly all respects, Davies' original proposal, developed in late 1965, was similar to the actual networks being built today. His cost analysis showed strong economic advantages for the packet approach, and by all rights, the proposal should have led quickly to a U.K. project. However, the communications world was hard to convince, and for several years, nothing happened in the U.K. on the development of a multi-node packet switching network. :Donald Davies was able, however, to initiate a local network with a single packet switch at the NPL. By 1973 this local network was providing an important distribution service within the laboratory. This project, plus the strong conviction and continued effort by those at NP1. (Davies, Barber, Scantlebury, Wilkinson, and Bartlett), did gradually have an effect on the U.K. and much of Europe." Davies had begun working with related concepts in 1965, after a conference in the United Kingdom on time-sharing brought up the inadequacies of existing circuit-switched networks. His work was originally carried out independently from Baran's work, although Davies learned of it after he gave a seminar on his ideas at NPL in 1966. Prior to his death, Davies contested the claim that Leonard Kleinrock invented packet switching pointing out that Kleinrock's research was actually in queueing theory, which is a key theoretical underpinning to packet switching. Davies claimed that Kleinrock's published works nowhere prominently mention breaking a user's message up into segments, and sending the segments through the network separately, which he said was the key innovation.

Simultaneous Development

As with many other inventions, it can be said that the research groups developed simultaneously. Thus, the ideas that were to become the ARPANET came from three research centers: MIT, the RAND Corporation, and National Physical Laboratory.

Reference

- Leonard Kleinrock, [http://www.lk.cs.ucla.edu/LK/Bib/REPORT/PhD/ Information Flow in Large Communication Nets], (MIT, Cambridge, May 31, 1961) Proposal for a Ph.D. Thesis
- Leonard Kleinrock. Information Flow in Large Communication Nets (RLE Quarterly Progress Report, July 1961)
- Leonard Kleinrock. Communication Nets: Stochastic Message Flow and Delay (Mcgraw-Hill, New York, 1964)
- Paul Baran et al., [http://www.rand.org/publications/RM/baran.list.html On Distributed Communications, Volumes I-XI] (RAND Corporation Research Documents, August, 1964)
- Paul Baran, [http://www.rand.org/publications/RM/RM3420/ On Distributed Communications: I Introduction to Distributed Communications Network]] (RAND Memorandum RM-3420-PR. August 1964)
- Paul Baran, On Distributed Communications Networks (IEEE Transactions on Communications Systems, March 1964)
- D. W. Davies, K. A. Bartlett, R. A. Scantlebury, and P. T. Wilkinson, A digital communications network for computers giving rapid response at remote terminals (ACM Symposium on Operating Systems Principles. October 1967)
- R. A. Scantlebury, P. T. Wilkinson, and K. A. Bartlett, The design of a message switching Centre for a digital communication network (IFIP 1968)
- Larry Roberts and Tom Merrill, Toward a Cooperative Network of Time-Shared Computers (Fall AFIPS Conference. October 1966)
- Lawrence Roberts, [http://www.packet.cc/files/ev-packet-sw.html The Evolution of Packet Switching] (Proceedings of the IEEE, November, 1978)

External links

- [http://www.rand.org/about/history/baran.html Paul Baran and the Origins of the Internet]
- [http://www.computer.org/internet/v1n3/kleinrock9702.htm Len Kleinrock on the Origins] (subscribers only)
- [http://www.isoc.org/internet/history/brief.shtml A Brief History of the Internet]
- [http://www.ziplink.net/~lroberts/InternetChronology.html Internet Chronology by Larry Roberts]
- [http://www.zakon.org/robert/internet/timeline/ Hobbes' Internet Timeline v7.0] Category:Computer networks Category:Packets Category:Telecommunications ja:パケット通信

SDH

The Synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built on experience in the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation. SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus an entire network can operate synchronously, though the presence of different timing sources allow for different circuits within an SONET signal to be timed off of different clocks (through the use of pointers and buffers.) SDH was made possible by the existence of atomic clocks. Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is innaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.

Structure of SONET/SDH signals

The basic SONET signal operates at 51.840 Mbit/s and is designated STS-1 (Synchronous Transport Signal one). The STS-1 frame is the basic unit of transmission in SONET. The Synchronous Transport Module level 1 (STM-1) is the basic signal rate of SDH. The two major components of the STS-1 frame are the transport overhead and the synchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the section overhead and line overhead. These bytes are used for signalling and measuring transmission error rates. The SPE comprises the payload overhead (9 bytes, used for end to end signalling and error measurement) and the payload of 774 bytes. The STS-1 payload is designed to carry a full DS-3 frame. When the DS-3 enters a SONET network, path overhead is added, and that SONET network element is said to be path terminating. Where multiple DS-3 paths are multiplexed, the SONET NE is said to be line terminating. The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125 microseconds on a fiber-optic circuit designated OC-1 (optical carrier one). In practice the terms STS-1 and OC-1 are sometimes used interchangeably, though the OC-N format refers to the signal in its optical form. It is therefore incorrect to say that an OC-3 contains 3 OC-1s: An OC-3 can be said to contain 3 STS-1s. Three OC-1 (STS-1) signals are multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbit/s. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2430 bytes and transmitted in 125 microseconds. The STS-3 signal is also used as a basis for the SDH hierarchy, where it is designated STM-1. Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four OC-3 or STM-1 circuits can be aggregated to form a 622.08 Mbit/s circuit designated as OC-12 or STM-4. The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbit/s. Speeds beyond 10 Gbit/s are technically viable and are under evaluation. Where fiber exhaust is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of Dense Wave Division Multiplexing (DWDM). Such circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.

SONET/SDH and relationship to 10 Gigabit Ethernet

Another fast growing circuit type amongst data networking equipment is 10 Gigabit Ethernet - while similar in rate to OC-192/STM-64, and, in its wide area variant, encapsulating its data using a light-weight SDH/SONET frame so as to be compatible at low level with equipment designed to carry those signals, it does not provide any interoperability at the bitstream level with other SDH/SONET systems.

SONET/SDH data rates

SONET/SDH system management protocols

SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom language for managing and reconfiguring SONET network elements. TL1 (or whatever command language a SONET Network Element utilizes) must be carried by other management protocols, including SNMP, CORBA, and XML. SONET Network Management is a large, difficult, and arcane subject, but there are some features that are fairly universal. First of all, most SONET NEs have a limited number of management interfaces defined. These are:
- Electrical Interface. The electrical interface (often 50 ohm)sends SONET TL1 commands from a local management network physically housed in the Central Office where the SONET NE is located. This is for "local management" of that NE and, possibly, remote management of other SONET NEs.
- Craft Interface. Local "craftspersons" can access a SONET NE and issue commands through a dumb terminal or terminal emulation program running on a laptop.
- SDH has dedicated Data Communciation Channels DCC for management traffic. According to ITU-T G. 7712 there are three modes used for management:
- IP only stack, using PPP as data-link
- OSI only stack, using LAP-D as data-link
- Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks. An interesting fact about modern SONET NEs is that, to handle all of the possible management channels and signals, most NEs actually contain a router for routing the network commands and underlying (data) protocols.

SONET Network Architectures

Currently, SONET (and SDH) have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection, and are key in understanding the almost worldwide usage of SONET and SDH for moving digital traffic. The three main architectures are:
- Linear APS (Automatic Protection Switching) SONET Linear APS Networks contain 4 fibers: 2 working in each direction, and two protect.
- UPSR (Unidirectional Path Switched Ring)In a UPSR two (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress determines the higher-quality copy and decides to switch, if deterioration in one copy occurs. UPSRs tend to sit nearer to the edge of a SONET network and, as such, are sometimes called "collector rings".
- BLSR (Bidirectional Line Switched Ring) BLSR comes in two varieties, a 2-fiber BLSR and 4-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring. BLSRs trade cost and complexity for bandwdith efficiency as well as the ability to support "extra traffic", which can be pre-empted when a protection switching event occurs. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities.

SONET Synchronization

Like management, Synchronization of SONET and SDH networks is a difficult and arcane subject. Remember that a SONET NE will transport and/or multiplex traffic that has originated from a variety of different Synch sources. In addition, a SONET NE may have a number of different synchronization options to choose from, which in some cases it will do so dynamically based on Synch Status Messages and other indicators. As for Synchronization sources available to a SONET NE, these are:
- Local External Timing. This is generated by an atomic Cesium clock or a satellite-derived clock by a device located in the same central office as the SONET NE. the interface is often a DS1, with Synch Status Messages supplied by the clock and placed into the DS1 overhead.
- Line-derived timing. a SONET NE can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 Synch Status bytes to ensure quality.
- Holdover. As a last resort, in the absence of higher quality timing, a SONET NE can go into "holdover", until higher quality external timing becomes available again. In this mode a SONET NE uses its own timing circuits to time the SONET signal. An interesting and hard-to-troubleshoot issue in SONET Networks is the existence of "timing loops". With a timing loop, SONET NEs in a network are each deriving their timing from another NE, and back again to initial NE, like a snake biting it's own tail. This network loop will eventually see it's own timing "float away" from any external SONET networks, causing mysterious bit errors, the source of which can be hard to find (unless the presence of the timing loop is detected). In general, a SONET Network that has been properly configured will never find itself in a timing loop, but it is sometimes hard to avoid this without sophisticated network management tools.

Next Generation SDH

SONET/SDH was originally developed primarily to transport multiple DS1s (ie T1s), DS3s (ie, T3s), and other groups of multiplexed 64kbit/s pulse-code modulated voice traffic. The ability to transport ATM (Asynchronous Transfer Mode) traffic was another early application. In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller SONET multiplexing containers (eg, STS-1) are inversely multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes. (Another example is STS-3c Packet-over-SONET.)SONET was therefore able to transport both voice and data simultaneously. One problem with the traditional concatentation, however, is inflexibility. Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left over, due to the fixed sizes of concatenated containers. Virtual Concatentation allows for a more arbitrary gluing-together of lower order multiplexing containers to build larger containers of fairly arbitrary size, without the need for intermediate SONET NEs to support that particular form of concatenation. Virtual Concatenation now often leverages X.86 or Generic Framing Procedure(GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container. Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamically virtually concatenating multiplexing containers based (ostensibly) on short-term bandwidth needs in the network.

External links

- [http://www.commsdesign.com/design_corner/OEG20020425S0003 Ethernet-over-Sonet Tutorial: Part 2]
- [http://fibers.org/articles/fs/9/3/3/1 Next-generation SDH and MSPP]
- [http://img.lightreading.com/heavyreading/pdf/hr20031114_esum.pdf The Future of SONET/SDH] (pdf)
- [http://www.corrigent.com/CS_products_SONET.html SONET/SDH]
- [http://comm.disa.mil/itu/r_g0700.html ITU-T defining standards]
- [http://www.pcc.qub.ac.uk/tec/courses/network/SDH-SONET/SDH-SONET.html The Queen's University of Belfast SDH/SONET Primer]
- [http://www.acterna.com/united_kingdom/technical_resources/pocket_guides/sdh_guide1.html SDH Pocket Handbook from Acterna]
- [http://www.acterna.com/united_kingdom/technical_resources/pocket_guides/sonet_guide.html SONET Pocket Handbook from Acterna]
- [http://www.ertyu.org/steven_nikkel/netspeeds.html Network Connection Speeds Reference]
- [http://rfc-ref.org/RFC-TEXTS/4207/index.html Synchronous Optical Network / Synchronous Digital Hierarchy Encoding for Link Management Protocol Test Messages] Category:Fiber optics Category:Network protocols ja:Synchronous Digital Hierarchy

Global System for Mobile Communications

The Global System for Mobile Communications (GSM) is the most popular standard for mobile phones in the world. GSM phones are used by over a billion people across more than 200 countries. The ubiquity of the GSM standard makes international roaming very common between mobile phone operators which enables phone users to access their services in many other parts of the world as well as their own country. GSM differs significantly from its predecessors in that both signalling and speech channels are digital, which means that it is seen as a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from very early on. GSM is an open standard which is currently developed by the 3GPP. From the point of view of the consumer, the key advantage of GSM systems has been higher digital voice quality and low cost alternatives to making calls such as text messaging. The advantage for network operators has been the ability to deploy equipment from different vendors because the open standard allows easy inter-operability. Also, the standards have allowed network operators to offer roaming services which mean subscribers can use their phone all over the world. GSM retained backward-compatibility with the original GSM phones as the GSM standard continued to develop, for example packet data capabilities were added in the Release '97 version of the standard, by means of GPRS. Higher speed data transmission have also been introduced with EDGE in the Release '99 version of the standard.

History

In spite of its current popularity, the history of mobile phones began long before GSM was conceived. Throughout the evolution of cellular telecommunications, various systems were developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard was intended to address these problems. The GSM group ("Groupe Spéciale Mobile" (French) 1, 2, 3 and 4) was founded in 1982. The name of the system comes from the name of this group, though later the decision was made to keep the initials but to change what they stood for. Originally the group was hosted by CEPT. From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen. The technical fundamentals of the GSM system were defined in 1987. In 1989, ETSI took over control and by 1990 the first GSM specification was completed, amounting to over 6,000 pages of text. Commercial operation began in 1991 with Radiolinja in Finland. In 1998, the 3rd Generation Partnership Project (3GPP) was formed. Originally it was intended only to produce the specifications of the next (third, 3G) generation of mobile networks. However, 3GPP also took over the maintenance and development of the GSM specification. ETSI is a partner in 3GPP. GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to not limit the designers yet still make it possible for the operators to buy equipment from different suppliers.

Market situation

3G]] More than one billion people use GSM phones as of 2005, making GSM the dominant mobile phone system worldwide with about 70% of the world's market. GSM's main competitor, CDMA2000, is used primarily in North America and [http://www.eurotechnology.com/3G/ Asia ]. CDMA2000 was seeing increased worldwide adoption when WCDMA did not appear to be commercially available. CDMA2000 also benefited from increased radio [http://www.techno-preneur.net/new-timeis/News-section/cdma_gsm.html spectrum efficiencies] as compared to the more common GSM networks and the not quite available WCDMA. Roaming with GSM phones is a major advantage over the competing technology as roaming across CDMA networks from different operators is difficult or impossible, depending on the handset and operators concerned. Another major reason for the growth in GSM usage, particularly between 1998 to 2002, was the availability of prepaid calling from mobile phone operators. This allows people who are either unable or unwilling to enter into a contract with an operator to have mobile phones. Prepaid also enabled the rapid expansion of GSM in many developing countries where large sections of the population do not have access to banks or bank accounts and countries where there are no effective credit rating agencies. (In the USA, starting a non-prepaid contract with a cellular phone operator is almost always subject to credit verification through personal information provided by credit rating agencies). The largest North American GSM carrier is Cingular Wireless. Other North American GSM carriers include T-Mobile USA and Cincinatti Bell Wireless.

Radio interface

GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate at various different radio frequencies. Most GSM networks operate at 900 MHz or 1800 MHz. The exception to the rule are networks in parts of the Americas (including the USA and Canada) that operate at 850 MHz or 1900 MHz. In the 900 MHz band the uplink frequency band is 933-960 MHz, and the downlink frequency band is 890-915 MHz. This 25 MHz bandwith is subdivided into 124 carrier frequencies, each spaced 200 kHz apart. Time division multiplexing is used to allow eight speech channels per Radio frequency channel. There are eight burst periods grouped into what is called a TDMA frame. The channel data rate is 270.833 kb/s, and the frame duration is 4.615 ms. There are four different cell sizes in a GSM network - macro, micro, pico and umbrella cells. The coverage area of each cell is different in different environments. Macro cells can be regarded as cells where the base station antenna is installed in a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose diameter is a few dozen metres; they are mainly used indoors. On the other hand, umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells. Cell radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 km. There is also a concept of an extended cell, where the cell radius could be double or even more. Indoor coverage is also supported by GSM and is achieved by using power splitters to deliver the radio signal from the antenna outdoors to a separate indoor antenna distribution system. This is typically deployed when a lot of call capacity is needed indoors, for example in shopping centres or airports. However, this is not a pre-requisite, since indoor coverage is also provided by in-building penetration of the radio signal. The modulation used in GSM is Gaussian minimum shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal being modulated is smoothened with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels.

Network structure

interference The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
- the Base Station Subsystem (the base stations and their controllers).
- the Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
- the GPRS Core Network (the optional part which allows packet based Internet connections).
- all of the elements in the system combine to produce many GSM services such as voice calls and SMS.

Subscriber Identity Module

One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smartcard containing the user's subscription information and phonebook. This allows the user to retain his information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. In the USA and Europe, most operators lock the mobiles they sell. This is done because the price of the mobile phone is usually subsidised with revenue from subscriptions and operators want to try to avoid subsidising competitor's mobiles. A subscriber can usually contact the provider to remove the lock for a fee (which operators sometimes try to claim to be ignorant of), utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. Some providers in the USA, such as T-Mobile and Cingular, will unlock the phone for free if the customer has held an account for a certain period. Third party unlocking services exist that are often quicker and lower cost than that of the operator. In most countries removing the lock is legal.

GSM security

GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using shared-secret cryptography. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user - whereas GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in countries that may not be able to support the infrastructure necessary for A5/1. A large security advantage of GSM is that the Ki, the crypto variable stored on the SIM card that is the key to any GSM ciphering algorithm, is never sent over the air interface. Serious weaknesses have been found in both algorithms, and it is possible to break A5/2 in real-time in a ciphertext-only attack. The system supports multiple algorithms so operators may replace that cipher with a stronger one.

Patent issues

In 2005, a number of companies (including Cisco Systems and Ericsson) were sued for infringement of U.S. Patent No. 5,561,706 for offering products alleged to be compliant with the GSM 3.60 standard.

External links

- [http://www.corscience.de/en-location-based-services.html GSM tracking-systems]
- [http://www.3gpp.org 3GPP The current standardisation body for GSM with free standards available].
- [http://www.openmobilealliance.org OMA The current standardisation body for services aspects of mobile networks - some free standards available].
- [http://www.etsi.org/ ETSI the original GSM standardisation body]
- [http://www.gsmworld.com/ GSM Association - the group representing GSM operators (official site)] - includes coverage maps for all members
- [http://www.gsmworld.com/news/statistics/substats.shtml Number of GSM Subscribers]
- [http://www.tele-servizi.com/janus/engfield1.html List of acronyms of GSM network parameters]
- [http://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html Overview of GSM] by John Scourias
- [http://www.bryte.net/ Tutorial on GSM from Bryte.net]
- [http://www.visualgsm.com/gsm_index.htm Visualtron's tutorial on GSM]
- [http://www.radio-electronics.com/info/cellulartelecomms/gsm_technical/gsm_introduction.php GSM technical overview / tutorial] from Radio-Electronics.Com
- [http://www.telenor.com/telektronikk/volumes/index.php?page=seksjon&id1=26&id2=38&select=00-04 GSM - ideas, origin and milestones - a Norwegian perspective] from Telenor's journal of technology [http://www.telektronikk.com Telektronikk]
- [http://www.gsm-security.net/gsm-security-faq.shtml GSM Security FAQ]
- [http://www.gsmprofile.com GSM PROFILE - its all about your cell phone]
- [http://hellomobile.com/gsm-triband-phones-sim-faqs.htm GSM Service and TriBand International Roaming FAQs]
- [http://hellomobile.com/cell-phone-safety.htm Cell Phone Safety and Wireless Facts]
- [http://hellomobile.com/gsm-networks.htm GSM Phone World Network List]
- [http://www.eventhelix.com/RealtimeMantra/Telecom/ GSM Call Flow Diagrams]
- [http://www.lodgephoto.com/articles/cellphones_europe.htm Article for Americans on using GSM cellphones in Europe] and ROW Category:Audio codecs Category:GSM Standard Category:Mobile telephony standards ko:GSM ja:GSM

Time division multiple access

Time Division Multiple Access (TDMA) is a technology for shared medium (usually radio) networks. It allows several users to share the same frequency by dividing it into different time slots. The users transmit in rapid succession, one after the other, each using their own timeslot. This allows multiple users to share the same transmission medium (e.g. radio frequency) whilst using only the part of its bandwidth they require. Used in the GSM, PDC and iDEN digital cellular standards, among others. TDMA is also used extensively in satellite systems, local area networks, physical security systems, and combat-net radio systems. :The name "TDMA" is also commonly used in America to refer to a specific second generation (2G) mobile phone standard, more properly referred to as IS-136 or D-AMPS, which uses the TDMA technique to timeshare the bandwidth of the carrier wave. :The two different uses of this term can be confusing. TDMA (the technique) is used in the GSM standard. However, TDMA (the standard, i.e. IS-136) has been competing against GSM and systems based on CDMA modulation for adoption by the carriers, although it is now being phased out in favor of GSM technology. carrier wave TDMA is a type of Time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing offset required to make its transmission match the gap in transmission from its peers. In the GSM system, the synchronisation of the mobile phones is achieved by sending timing offset commands from the base station which instructs the mobile phone to transmit earlier or later. The mobile phone is not allowed to transmit for its entire timeslot, but there is a guard period at the beginning and end of the timeslot. As the transmission moves into the guard period, the mobile network adjusts the timing offset to re-center the transmission. Initial synchronisation of a phone requires even more care. Before a mobile transmits there is no way to actually know the offset required. For this reason, an entire timeslot has to be dedicated to mobiles attempting to contact the network (known as the RACH in GSM). The mobile attempts to broadcast at the beginning of the timeslot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35km from the base station, the time delay will mean the mobile's broadcast arrives at the very end of the timeslot. In that case, the mobile will be instructed to broadcast its messages starting a whole timeslot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighboring time slot and be ignored. It is this feature, rather than limitations of power which limits the range of a GSM cell to 35 kilometers when no special tricks are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome. In radio systems, TDMA is almost always used alongside FDMA (Frequency division multiple access) and FDD (Frequency division duplex); the combination is referred to as FDMA/TDMA/FDD. This is the case in both GSM and IS-136 for example. The exceptions to this rule include WCDMA-TDD which combines FDMA/CDMA/TDMA and TDD instead. A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for its own timeslot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies. This allows safe inter frequency handovers, something which is difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in UMTS. This in turn allows for co-existence of microcell layers with macrocell layers. But, CDMA supports "soft hand-off" which allows a mobile phone to be in communication with up to 6 base stations simultaneously, a type of "same-frequency handover". The incoming packets are compared for quality, and the best one is selected. This enables CDMA to perform in areas where TDMA calls would be dropped. A disadvantage of TDMA systems is that they create interference at a frequency which is directly connected to the time slot length. This is the irritating buzz which can sometimes be heard if a GSM phone is left next to a radio. Another disadvantage is that the "dead time" between time slots limits the potential bandwidth of a TDMA channel. This is why early efforts to incorporate timeslots into UMTS failed, leaving UMTS as a purely CDMA technology. The only country to continue pursuing TD-SCDMA (time division synchronous CDMA) is mainland China, because the government does not want to pay patent royalties to Qualcomm of the USA or licensing fees to the mainly European UMTS consortium.

TDMA features

- Shares single carrier frequency with multiple users
- Non-continuous transmission makes handoff simpler
- Slots can be assigned on demand ⇒ BW on demand
- Less stringent power control due to reduced interuser interference
- Higher synchronization overhead
- Equalization is necessary for high data rates
- Frequency/slot allocation complexity
- Pulsating power envelop: Interference with other devices

Category:Multiplexing

Category:Telecommunications

Wikipédia:Éditeurs problématiques/Campagne de propagande organisée

Déplacé depuis Wikipédia:Éditeurs problématiques Fred.th 18 octobre 2005 à 12:45 (CEST) Il existe une campagne de propagande concertée organisée à partir du site [http://www.liberaux.org/index.php?showtopic=3985&st=0 liberaux.org]] pour donner une définition très partisane du mot libéralisme. Un tout petit groupe d'éditeurs liés à cette mouvance a fait de cette page sa "page-territoire". La section "Critiques" (obtenue de haute lutte et non sans avoir essuyé pas mal d'injures de bas étage) est régulièrement vandalisée. Le contenu global de l'article est d'un piètre niveau, trop biaisé par le parti-pris, sans doute. Je pense qu'il serait sain qu'un ou plusieurs administrateurs prennent l'initiative de restaurer un contenu à la fois neutre et encyclopédique et de geler l'édition le temps que les excités de liberaux.org apprennent le sens du pluralisme, celui du mot encyclopédie, et celui de la politesse qui devrait pourtant découler "naturellement" de leur philosophie ;-) :je suis bien content de n'avoir jamais vu ici un libéral rechercher si telle ou telle association squatte tel ou tel article, ni appeller à la censure contre cette origine : par contraste, ça prouve bien où se trouve le pluralisme et la tolérance. Et la neutralité. Pour la politesse, je veux bien des leçons, mais de la part de quelqu'un qui a la politesse élémentaire de signer et d'utiliser les pages de discussion avant d'appeller à la censure. Et je suis également bien content d'avoir demander (il y a longtemps et pas que pour ce sujet) à mes amis libéraux de participer à wikipedia (cerains ne m'ayant pas attendus, connaissant WP depuis plus longtemps que moi !), quand je lis ce genre de choses, ça donne une idée de ce qu'on pourrait trouver dans la page visée et dans wikipedia en général si ils n'étaient pas là :-P gem 13 avr 2005 à 16:49 (CEST) ::euh, je confirme, l'article libéralisme n'est pas très neutre pour l'instant. je ne connais pas les rédacteurs, mais je ne pense pas a priori qu'ils doivent être pires que la moyenne. m'enfin, faut juste espérer qu'il pourra se neutrifier, c important sur la wiki.eek 17 avr 2005 à 20:38 (CEST) Euh, je veux bien que l'article ait des problèmes de neutralité, mais en quoi la définition pose un problème ? « Le libéralisme est un courant de pensée, ou un ensemble de courants de pensée, visant à faire reconnaître la primauté de l'individu sur les groupes (et leurs chefs), suivant l'idée qu'ils tendent à l'annexer et lui imposer son comportement et que cela ne serait pas acceptable. » Turb 17 avr 2005 à 20:41 (CEST) :en rien. la définition est parfaitement neutre. c'est après que ça se gâte. il y a des critiques non-argumentées d'autres idéologies. je ne pense pas que l'article doive traiter de ça. elles devraient plutôt figurer sur les pages en question. mais j'ai l'impression qu'en ce moment l'article est sur le bon chemin, au niveau neutrification. eek 18 avr 2005 à 02:00 (CEST) :::Je confirme que le problème s'est répété sur la page anarcho-capitalisme. 3 éditeurs jugent que la critique sur l'anarcho-capitalisme n'est pas à sa place. Ils n'hésitent donc pas à infléchir l'article dans la direction qu'is veulent, et sont impolis. Il faudrait pourtant que différentes opinions s'expriment sur le sujet. De plus, ces éditeurs semblent avoir du mal à prendre un peu de recul vis à vis de cette doctrine. Ils en prennent une défense partisane. Actuellement, la page est non-neutre, et ca ne s'améliore pas recyclage 30 août 2005 à 11:59 (CEST) ::::Pareillement ! (voir ici) Le fait que je mette le bandeau de désaccord de neutralité, avec des points non neutres exprimés clairement (afin de neutraliser l'article), est considéré par cet éditeur (Mélodius) comme une politique de sabotage, il menace insidieusement, au cas où je ne retire pas le bandeau et mes demandes, d'aller débattre sur la page anarchisme, avec beaucoup de monde, afin de ... (oui, il peut pas dire, c'est un intriguant, prêt à faire propagande de masse). Et l'autre (Dilbert) confirme comme quoi il ne veut pas prendre en compte mes demandes de neutralité, en faisant croire qu'il y a déjà répondu. Donc voilà quelques éléments confirmant ce qui a déjà été écrit précédemment. Libre 1 septembre 2005 à 22:17 (CEST) ::::Je confirme à nouveau que ces trois éditeurs (Dilbert - Mélodius - gadrel) de wikilibéral sont des commentateurs acquis dans le lien [http://www.liberaux.org/index.php?showtopic=3985&st=0 liberaux.org] énoncé par une personne au début, au sujet d'une campagne organisée. L'article anarcho-capitalisme subit la même préssion que l'article libéralisme a pu subir (article que j'ai suivi de loin) ces dérniers mois. Je ne suis pas adepte de l'idée de "Campagne de propagande organisée", mais là j'ai plutôt l'impréssion que ça se confirme et que la proposition de Gem, sur liberaux.org, de s'organiser collectivement sur tel ou tel article a été suivi par certains éditeurs énoncés au début. Il y a des choses certainement qui se passent en coulisses (d'aprés ce que j'ai pu lire de leur échanges)... La manipulation est une arme qu'ils usent intentionellement. La fin semble justifier leurs moyens, mais qu'elles sont leurs fins si ce n'est de libéraliser fr.wikipédia.org ? Hors ce n'est pas le but définit dans la wikipédia. ::::Mr Mélodius s'attache à m'attribuer toutes les modifications faites sur leur article AC ou sur l'article anarchisme, et donc, des attaques incéssantes, des procés d'intentions fusent (technique ad hominemique demandant justification à l'attaqué. Pour lui faire perdre du temps, ainsi il ne travaille pas sur autre chose pendant ce temps là). ::::Dans la page de discussion de anarcho-capitalisme on peut lire ceci : ::::
- "on va poursuivre le débat sur la page "anarchisme" qui, ABUSIVEMENT, prétend monopoliser le terme en faveur de l'anarchisme de gauche, qui déjà aux Etats-Unis est un phénomène minoritaire et le sera bientôt également en Europe. Jusqu'à présent, j'ai recherché un modus vivendi, mais si l'esprit de conciliation n'est pas réciproque, il n'y a plus d'accord qui tient. Et je ne serai pas tout seul." ::::Ils refusent tout débat coopératifs, lancent des trolls, ne répondent pas aux demandes de neutralité (si ce n'est en partie pour faire croire que...). Et lorsqu'ils remarquent qu'ils manquent de consistance, aprés avoir insulté et traité leurs adversaires, ils modifient finalement la partie de maniére +/- neutrement (ce qui améne à nouveau des précisions - ils usent des contre temps; procés d'intentions; afin d'embrouiller le débat.). :::: Libre 3 septembre 2005 à 11:44 (CEST) C'est ennuyeux, une propagande qui ne soit pas le fait de "libre" ; ça le choque. Que l'article anarcho-capitalisme ne soit pas le reflet de ses a priori et de sa méconnaissance CRASSE du sujet, c'est intolérable n'est-ce pas. Le but de wikipédia, c'est d'offrir des articles NEUTRES et INFORMÉS. Or, "libre" n'est ni l'un ni l'autre. Incapable d'argumenter, car il ne sait rien sur l'anarchie en général et encore moins sur l'anarchie capitaliste, il s'entête à vandaliser les articles. Fort heureusement, un administrateur est venu l'arrêter. Depuis, on n'entend plus parler de lui. Car "libre" ne sait pas parler ou écrire : il ne sait que vandaliser et pleurnicher, petit Caliméro de cour de récré. Gadrel. :T'as quel âge ? J'ai l'impréssion de lire le discours que ferait un adolescent attardé dans sa cours de récré. Libre 5 septembre 2005 à 22:06 (CEST) Je confirme l'attitude exécrable de l'imbécile (au sens étymologique du terme) de service, le dénommé Libre. Impossible de dialoguer avec cet énergumène, il ne comprend décidément rien au sens du mot "neutralité" : pour lui, la neutralité de point de vue revient à affirmer ses propres opinions, en décidant par exemple de juger et de sélectionner personnellement telle ou telle forme d'anarchisme, afin de faire de cette encyclopédie le reflet de ses propres opinions politiques. Par exemple, Libre est allé jusqu'à inventer des termes sortis tout droit de son imaginaire (panarcho-libéral et autres joyeuses inventions dans le même genre) pour exclure de manière irrévocable la philosophie anarcho-capitaliste de l'anarchisme. Que le nécessaire soit donc fait pour éviter le vandalisme auquel cet ignare de mauvaise foi s'est habitué. DocMacToast 5 septembre 2005 à 22:34 (CEST) : Apparemment, j'ai mis le doigt dans l'engrenage que certains ont lancés pour une "supposé" propagande organisé (voir la page de discussion anarcho-capitalisme, dilbert, gadrel et mélodius, leurs attitudes agréssives, méprisantes, vindicatives, manipulatoires, mensongéres et sectaires, et pour le moins non neutre), je ne vais donc pas être épargné, ici ou ailleurs (D'aprés mon éxpérience de ces gens là, en quelque sorte je suis maintenant l'utilisateur à abattre ou à décridibiliser à tout prix/prime, et quand on parle de prix/prime, les libéraux sont pas loin), avec leur ad hominem habituels (l'attaque personnelle est leur arme favorite, tout comme certainement dans leur société future...), puisque je tiens à ce que la neutralité reste présente dans les articles que je travaille. Ils sont donc là manque encore certains de leurs contributeurs afin de parachever leur but qui est de me faire taire (ou alors de me faire changer de pseudo). La tactique de docmactoast a été ici de considérer que je suis un imbécile/ignorant/égocentrique et que je n'ai aucune qualité pour travailler sur wikipédia. Hors en regardant la pagfe de discussion énoncé plus haut, on peut s'apercevoir que ce n'est pas de moi qu'il parle (mais de ces amis dilbert/melodious/gadrel). D'ailleurs à plusieurs reprises Mr Melodius a fait de même en m'attribuant des modifications d'articles alors que je n'en étais pas l'auteur, ça confirmerait comme quoi je suis devenu génant pour ce gens là (?). Mais c'est pas finit, ils vont pas s'arrêter là puisque je leur tiens tête. La réponse à ce commentaire devrait bientôt arriver, en lançant une autre charge d'attaques personnelles à mon encontre (ils peuvent pas laisser passer ça). Si Les Mr's Dilbert/gadrel et melodius n'avaient pas eus une attitude aussi pourrie sur la page de discussion énoncé plus haut, je n'aurais pas eus l'occasion de venir sur la page éditeurs problématiques et de voir cette section au sujet de campagne de propagande organisé... Grâce à cela, je comprend mieux maintenant la raison du fait que j'ai déjà eu affaire plus avant à certaines des personnes nommés dans ce [http://www.liberaux.org/index.php?showtopic=3985&st=0 lien de liberaux.org] au sujet de neutralité également. Ceci confirmant peut-être cela. M'enfin, je suis Libre sur la wikipédia et je neutraliserais l'article vaille que vaille, il faudra le temps qu'il faut, mais il sera neutralisé malgré ces déneutralisateurs. Libre 5 septembre 2005 à 23:34 (CEST) Comme tous les propagandistes de bas étage, "libre" (ce pseudo est un véritable oxymore !) tente de renverser la faute. Naturellement, c'est LUI qui tente de manipuler les lecteurs de l'article anarcho-capitalisme, c'est LUI qui veut faire passer sa vision tronquée, mensongère et ignorante de la réalité, c'est LUI qui fait preuve d'agressivité (combien de fois ne nous a-t-il pas insulté ?), à tel point, je le rappelle, QU'UN ADMINISTRATEUR A DÛ VENIR GELER LA PAGE QU'IL NE POUVAIT S'EMPÊCHER DE VANDALISER. INCAPABLE D'ARGUMENTER (normal, car il ne connaît rien à rien), sa "neutralisation" se limitait à mettre à répétition des bandeaux NPOV, SANS EXPLICATION NI ARGUMENTATION. Je remarque d'ailleurs qu'il répète dans son dernier message ses menaces de vandalisation de l'article. Je demande dès lors que "libre" soit rayé de wikipédia. De toute façon, je veillerai à ce qu'AUCUNE modification de sa part ne soit jamais apportée à l'article en question. --Gadrel 6 septembre 2005 à 09:57 (CEST) Ben voyons, Libre, l'article que tu as fait au sujet de l'anarcho-capitalisme sur anarchopedia est neutre peut-être ? Changer le nom d'un courant de pensée parce qu'il ne te plait pas revient à faire preuve de neutralité selon toi ? Et si je débarque sur la page "anarcho-communisme" et que je mets le terme "anarcho" entre guillemets, je suppose que tu ne me le reprocheras pas, en bon adepte de la méthode ? 82.239.241.3 6 septembre 2005 à 12:33 (CEST) ::à la vue de toutes ces insultes, menaces et de cette mauvaise foi, je pense qu'il n'y a désormais aucun doute sur le fait qu'il y a une pression sur certains articles de la part de personnes très engagés pour la défense de la doctrine à laquelle ils adhèrent. Il ne faut surtout pas se laisser manipuler par ces individus qui emploient toutes les méthodes possibles pour faire passer leurs idées ou censurer les articles. N'ayant pas de jugement à porter sur l'anarcho-capitalisme, dans la mesure où je suis amené à l'étudier dans le cadre de ma thèse, et non dans le cadre d'une recherche partisane, j'ai pourtant eu à subir des attaques diverses de la part de ces individus quand j'essayais d'améliorer la page qui traite de ce courant politique. Remarques toujours blessantes, vexantes, de mauvaise foi et stressantes. Ce genre d'ambiance est un véritable problème sur Wikipédia. Mais il est vrai que les articles politiques tendent à les favoriser. Hélas ! C'est le genre de sujet qui déchaîne les passions !! C'est pourquoi il importe à mon avis que chacun fasse des efforts sur ce genre de pages. recyclage 7 septembre 2005 à 20:04 (CEST) Je rappelle au lecteur que Recyclage est lui-même repris comme éditeur problématique. Cela devrait remettre dans leur contexte les quelques lignes de diffamation qu'il a rédigées ci-dessus. --Gadrel 7 septembre 2005 à 23:56 (CEST) Je confirme le fait qu'un certain nombre d'utilisateur de wikipedia ont formé un groupe de pression destinés à redéfinir certains termes politique à leur avantage. Il s'agit notamment de Gadrel- Dilbert - Mélodius qui sont d'authentiques ultra-libéraux. Il y a deux attitudes possibles face à cela confrontation ou conciliation. Personellement, je choisis la conciliation car c'est la seule voie permettant à wikipedia de se développer tant en quantité qu'en qualité. Ces guerres d'éditions sont peut-être parfois amusante mais empèche les contributeurs neutres de travailler correctement. Evidemment, il y aura toujours certaine confrontations mais faisont en sorte qu'elle soit constructive.Dujo 13 septembre 2005 à 22:36 (CEST) Heureusement que tu as précisé que ton intervention est conciliante, parce qu'à lire les accusations débiles dont tu nous gratifies ici, ç'aurait pu m'échapper. Il y a eu un effort concerté de la part des anarchistes de gauche pour définir tout ce qui est vaguement en rapport avec eux à leur avantage, nous on rectifie le tir, c'est tout. Arrêtez de geindre et tâchez plutôt de faire oeuvre constructive avec nous. Je m'efforce de décrire correctement en honnêtement vos idées sans faire de la polémique à deux balles (genre "ce que les anarchistes n'aiment pas: l'anarcho-capitalisme" DUH !). Tâchez de nous rendre la politesse plutôt que de vous la jouer "Protocoles des sages de Sion". --Melodius 14 septembre 2005 à 10:12 (CEST) "Protocoles des sages de Sion" c'est quoi ce délire ??? 83.79.96.191 15 septembre 2005 à 01:59 (CEST) :Les Protocoles des Sages de Sion, c'est du délire antisémite, jouant sur la confusion. Et aprés il te parle de politesse... Protocoles des Sages de Sion]] 21 septembre 2005 à 20:43 (CEST) Au fait, Libre, c'en est où ta demande d'arbitrage à propos de Dilbert ? --Gadrel 26 septembre 2005 à 17:06 (CEST) Juste pour info, le lien [http://www.liberaux.org/index.php?showtopic=3985&st=0 liberaux.org] présenté au début de cette section au sujet du doute sur une campagne organisée a été éffacé par le site, on arrive maintenant à une page en erreur. Ce retrait, est il un fait révélateur que cette page était génante par certains utilisateurs ici nommés ? est ce génant qu'ils aient étés pris sur le fait de leur propagande organisé ? Pourquoi n'assument ils pas ? Je pense que cela est une raison supplémentaire de se méfier de ce genre de personnes. Libre 10 octobre 2005 à 13:36 (CEST) Petit malin. Il y a un délestage régulier et automatique du forum afin de limiter le temps de chargement. Bravo la paranoïa ! Cela prouve toutefois que "libre" aurait agi ainsi, sinon il n'en aurait pas eu l'idée. Ou comment les vrais propagandistes se découvrent à leur insu... --Gadrel 11 octobre 2005 à 22:26 (CEST) :: La phrase précédente semble confirmer le lien de Gadrel avec le site suspecté de venir polluer Wikipedia ... Enfin, il convient de mentionner aussi un autre "auteur" référencé dans la liste des "éditeurs problématiques": Pgreenfinch http://fr.wikipedia.org/wiki/Wikip%C3%A9dia:%C3%89diteurs_probl%C3%A9matiques/Pgreenfinch.

kalorie cheap holidays lanzarote madrid accommodation Hotels Warsaw Parkiet

:: RELATED NEWS ::

Time-division multiplexing

History

Transmission using Time Division Multiplexing (TDM)

Synchronous Digital Hierarchy (SDH)

Statistical Time-division Multiplexing (STDM)

See also

Digital

Digital noise

Analog, symbolic, and digital displays; ease of reading

Analog to digital conversion

Symbol to digital conversion

Historical digital systems

See also

Frequency

Measurement

Frequency of waves

Invariance

Examples

See also

External links

Bit

History and explanation

More than one bit

See also

T-carrier

T1

Digital signal crossconnect

Bit robbing

Notes

See also

References

Packet switching

Packets

Packet routing

Packet switching in the Internet

Controversy over invention of packet switching

Work at MIT

Work at RAND

Work at National Physical Laboratory

Simultaneous Development

See also

Further reading

Reference

External links

SDH

Structure of SONET/SDH signals

SONET/SDH and relationship to 10 Gigabit Ethernet

SONET/SDH data rates

SONET/SDH system management protocols

SONET Network Architectures

SONET Synchronization

Next Generation SDH

See also

External links

Global System for Mobile Communications

History

Market situation

Radio interface

Network structure

Subscriber Identity Module

GSM security

Patent issues

Related topics

External links

Time division multiple access

TDMA features

See also:

Category:Multiplexing

Wikipédia:Éditeurs problématiques/Campagne de propagande organisée