Civil Rights
Movements, leaders, victories and the continuing fight for equality.
q2K BHP
Black History Portal
THE BHP WIRE —
HIDDEN TRUTHS
What's New!
Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
Mansa Musa was the ruler of the Mali Empire in West Africa. Details recorded here should be sourced; unknown information is left blank.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
In engineering and computing, clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized. In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication. Clock drift is most commonly expressed in parts-per million (ppm), in which each unit equates to 86.4 milliseconds of drift per day. Common market watches usually range from ± 1 to ± 100 ppm.[1] More advanced clocks may even use parts-per billion, and the most accurate atomic clocks have reached an accuracy that is expressed in parts-per sextillion.[2] Computer clock drift can be utilized to build random number generators. These can however be exploited by timing attacks.
Everyday clocks such as wristwatches have finite precision. Eventually they require correction to remain accurate. The rate of drift depends on the clock's quality, sometimes the stability of the power source, the ambient temperature, and other subtle environmental variables. Thus the same clock can have different drift rates at different occasions.
More advanced clocks and old mechanical clocks often have some kind of speed trimmer where one can adjust the speed of the clock and thus correct for clock drift. For instance, in pendulum clocks the clock drift can be manipulated by slightly changing the length of the pendulum.
A quartz oscillator is less subject to drift due to manufacturing variances than the pendulum in a mechanical clock. Hence most everyday quartz clocks do not have an adjustable drift correction.
Atomic clocks are very precise and have nearly no clock drift. Even the Earth's rotation rate has more drift and variation in drift than an atomic clock due to tidal acceleration and other effects. The principle behind the atomic clock has enabled scientists to redefine the SI unit second in terms of exactly 9192631770 oscillations of the caesium-133 atom. The precision of these oscillations allows atomic clocks to drift roughly only one second in a hundred million years; as of 2015, the most accurate atomic clock loses one second every 15 billion years.[3][4] The International Atomic Time (TAI) time standard and its derivatives, such as the Coordinated Universal Time (UTC), are based on weighted averages of atomic clocks worldwide.
As Einstein predicted, relativistic effects can also cause clock drift due to time dilation. This is because there is no absolute time; time is instead relative to the observer. Special relativity describes how two clocks held by observers in different inertial frames (i.e. moving with respect to each other but not accelerating or decelerating) will each appear to either observer to tick at different rates.
In addition to this, general relativity gives us gravitational time dilation. Briefly, a clock in a stronger gravitational field (e.g. closer to a planet) will appear to tick more slowly. People holding these clocks (i.e. those inside and outside the stronger field) would all agree on which clocks appear to be going faster.
It is time itself rather than the function of the clock which is affected. Both effects have been experimentally observed.[citation needed]
Time dilation is of practical importance. For instance, the clocks in GPS satellites experience this effect due to the reduced gravity they experience (making their clocks appear to run more quickly than those on Earth) and must therefore incorporate relativistically corrected calculations when reporting locations to users. If general relativity were not accounted for, a navigational fix based on the GPS satellites would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day.[5]
Computer programs often need high quality random numbers, especially for cryptography.[6] There are several similar ways clock drift can be used to build random number generators (RNGs).
One way to build a hardware random number generator is to use two independent clock crystals, one that for instance ticks 100 times per second and one that ticks 1 million times per second. On average the faster crystal will then tick 10,000 times for each time the slower one ticks. But since clock crystals are not precise, the exact number of ticks will vary. That variation can be used to create random bits. For instance, if the number of fast ticks is even, a 0 is chosen, and if the number of ticks is odd, a 1 is chosen. Thus such a 100/1000000 RNG circuit can produce 100 somewhat random bits per second. Typically such a system is biased—it might for instance produce more zeros than ones—and so hundreds of somewhat-random bits are "whitened" to produce a few unbiased bits.
There is also a similar way to build a kind of "software random number generator". This involves comparing the timer tick of the operating system (the tick that usually is 100–1000 times per second) and the speed of the CPU. If the OS timer and the CPU run on two independent clock crystals the situation is ideal and more or less the same as the previous example. But even if they both use the same clock crystal the process/program that does the clock drift measurement is "disturbed" by many more or less unpredictable events in the CPU such as interrupts and other processes and programs that run at the same time. Thus the measurement will still produce fairly good random numbers.
Most hardware random number generators such as the ones described above are fairly slow. Therefore, most programs only use them to create a good seed that they then feed to a pseudorandom number generator or a cryptographically secure pseudorandom number generator to produce many random numbers fast.
In 2006, a side channel attack was published[7] that exploited clock skew based on CPU heating. The attacker causes heavy CPU load on a pseudonymous server (Tor hidden service), causing CPU heating. CPU heating is correlated with clock skew, which can be detected by observing timestamps (under the server's real identity).
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
In engineering and computing, clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to drift, causing eventual divergence unless resynchronized. In particular, the drift of crystal-based clocks used in computers requires some synchronization mechanism for any high-speed communication. Clock drift is most commonly expressed in parts-per million (ppm), in which each unit equates to 86.4 milliseconds of drift per day. Common market watches usually range from ± 1 to ± 100 ppm. More advanced clocks may even use parts-per billion, and the most accurate atomic clocks have reached an accuracy that is expressed in parts-per sextillion. Computer clock drift can be utilized to build random number generators. These can however be exploited by timing attacks.
In computer science and electrical engineering, clock synchronization is a technique for coordinating multiple independent electronic clock signals. All clocks are susceptible to deviation in their timing from a reference clock, a phenomenon called clock drift. Even when clocks are initially set to the same time, the effect of clock drift causes individual clocks to count time at slightly different rates, with the difference accumulating over time. There are several problems that occur as a result of clock rate differences and several solutions, some being more acceptable than others in certain contexts.
In digital transmission, bit slip is the loss or gain of a bit or bits, caused by clock drift – variations in the respective clock rates of the transmitting and receiving devices. One cause of bit slip is overflow of a receive buffer that occurs when the transmitter's clock rate exceeds that of the receiver. This causes one or more bits to be dropped for lack of storage capacity. One way to maintain timing between transmitting and receiving devices is to employ an asynchronous protocol such as start-stop. Alternatively, bit slip can be prevented by using a self-clocking signal (such as a signal modulated using OQPSK) or using a line coding such as Manchester encoding. Another cause is "losing count", as on a hard drive: if a hard drive encounters a long string of 0s, without any 1s (or a string of 1s without 0s), it may lose track of the frame between fields, and suffer bit slip. When a pulse of N consecutive zero bits are sent, clock drift may cause the hardware to apparently detect N-1 zero bits or N+1 zero bits – both kinds of errors are called bit slip. Thus one prevents long strings without change via such devices as run length limited codes. Many communication systems use linear-feedback shift register scrambling to prevent long strings of 0s (or other symbol), including VSAT, 1000BASE-T, RFC 2615, etc. While a scrambler makes the "losing count" type of bit slip error occur far less often, when bit slip errors do occur (perhaps for other reasons), scramblers have the property of expanding small errors that add or lose a single bit into a much longer burst of errors. The optimized cipher feedback mode (OCFB), the statistical self-synchronization mode, and the "one-bit CFB mode" also expand small bit-slip errors into a longer burst of errors, but eventually recover and produce the correct decrypted plaintext. A bit-slip error when using any other block cipher mode of operation generally results in complete corruption of the rest of the message.
GNSS augmentation is a method of improving the satellite navigation system's attributes, such as precision, reliability, and availability, through the integration of external information into the calculation process. There are many such systems in place, and they are generally named or described based on how the GNSS sensor receives the external information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay; "state-space representation"), others provide direct measurements of how much the signal was off in the past (differential GPS; "observation-space representation"), while a third group provides additional vehicle information to be integrated in the calculation process (sensor fusion).
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Joe Louis.