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.
Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
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
 | This article needs more citations. (September 2009) |
The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems.[1] The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronized.
Backbone transport networks replaced PDH networks with synchronous digital hierarchy (SDH) or synchronous optical networking (SONET) equipment over the ten years ending around the turn of the millennium (2000),[2] whose floating payloads relaxed the more stringent timing requirements of PDH network technology. The cost in North America was $4.5 billion in 1998 alone,[2] p. 171.
PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee that they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.
The data rate is controlled by a clock in the equipment generating the data. The rate is allowed to vary by ±50 ppm of 2048 kbit/s (according to ITU-T recommendation[3]). This means that different data streams can (and probably do) run at slightly different rates from one another.
In order to transport multiple data streams from one place to another over a common transmission medium, they are multiplexed in groups of four. Because each of the four data streams is not necessarily running at the same rate, some compensation has to be introduced. Typically, the multiplexer takes the data from the 4 incoming 2.048 Mbit/s data streams and feeds each into a 2.112 Mbit/s stream via a buffer store, leaving a series of fixed gaps in each frame.
The data rate is thus 2.112 Mbit/s × (number of bits in a frame – number of gaps)/(number of bits in a frame)
This is slightly greater than 2.048 Mbit/s + 50ppm. If an extra gap is added, this is slightly smaller than 2.048 Mbit/s – 50ppm. Thus, on average, the data rate can be made exactly equal to the incoming rate by adding a gap in some frames and not others. This extra gap is in a fixed place in the frame and is referred to as the "stuffable bit". If it does not contain data, i.e. it is a gap, it is "stuffed". The data from the 4 data streams is now contained in 4 data streams of 2.112 Mbit/s which are synchronous and can easily be multiplexed to give a single stream of 8.448 Mbit/s by taking 1 bit from stream #1, followed by 1 bit from stream #2, then #3, then #4, etc. Some of the fixed gaps accommodate a synchronisation word which allows the demultiplexer to identify the start of each frame, and others contain control bits for each stream which say whether or not the stuffable bit is stuffed or not, i.e. contains data or not. The process can then be reversed by the demultiplexer and 4 data streams produced with exactly the same bit rate as previously. The timing irregularity is ironed out using a phase locked loop.
This scheme does not allow the addition of a stuffed bit as soon as it is required, because the stuffable bit is in a fixed point in the frame. Therefore, it is necessary to wait until the stuffable bit time slot. This wait results in "waiting time jitter" which can be arbitrarily low in frequency, i.e. down to zero, and so cannot be entirely eliminated by the filtering effects of the phase lock loop. The worst possible stuffing ratio would be 1 frame in 2, as this gives a theoretical 0.5 bit of jitter. Therefore, the stuffing ratio is carefully chosen to give theoretical minimum jitter. In a practical system however, the actual decision to stuff or not may be made by comparing the read address and write address of the input buffer store. The position in the frame when the decision is made varies and adds a second variable dependent on the length of the store.
The process is sometimes called "pulse justification" because "justification" in printing is adding gaps so that each line takes up a full column width. It is believed that this term was preferred because "...... stuffing stuffable bits" and "waiting time jitter is the jitter you get while waiting to stuff a stuffable bit", though technically correct, do sound like a pleonasm.
Similar techniques are used to combine four × 8 Mbit/s together, plus bit stuffing and frame alignment, giving 34 Mbit/s. Four × 34 Mbit/s, gives 140. Four × 140 gives 565.
In telecommunications networks, independent clocks are free-running precision clocks located at the nodes which are used for synchronization.
Variable storage buffers, installed to accommodate variations in transmission delay between nodes, are made large enough to accommodate small time (phase) departures among the nodal clocks that control transmission. Traffic may occasionally be interrupted to allow the buffers to be emptied of some or all of their stored data.[4]
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous is derived from Greek plēsios, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronized. Backbone transport networks replaced PDH networks with synchronous digital hierarchy (SDH) or synchronous optical networking (SONET) equipment over the ten years ending around the turn of the millennium (2000), whose floating payloads relaxed the more stringent timing requirements of PDH network technology. The cost in North America was $4.5 billion in 1998 alone, p. 171. PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee that they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.
Digital transmission hierarchy may refer to: Plesiochronous Digital Hierarchy (PDH) Synchronous Digital Hierarchy
In telecommunications, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly synchronised. According to ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. A sender and receiver operate plesiosynchronously if they operate at the same nominal clock frequency but may have a slight clock frequency mismatch, which leads to a drifting phase. The mismatch between the two systems' clocks is known as the plesiochronous difference. In general, plesiochronous systems behave similarly to synchronous systems, except they must employ some means in order to cope with "sync slips", which will happen at intervals due to the plesiochronous nature of the system. The most common example of a plesiochronous system design is the plesiochronous digital hierarchy networking standard. The asynchronous serial communication protocol is asynchronous on the byte level, but plesiochronous on the bit level. The receiver detects the start of a byte by detecting a transition that may occur at a random time after the preceding byte. The indefinite wait and lack of external synchronization signals makes byte detection asynchronous. Then the receiver samples at predefined intervals to determine the values of the bits in the byte; this is plesiochronous since it depends on the transmitter to transmit at roughly the same rate the receiver expects, without coordination of the rate while the bits are being transmitted. The modern tendency in systems engineering is towards using systems that are either fundamentally asynchronous (such as Ethernet), or fundamentally synchronous (such as synchronous optical networking), and layering these where necessary, rather than using a mixture between the two in a single technology. The term plesiochronous comes from the Greek πλησίος plesios ("near") and χρόνος chrónos ("time").
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates, data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization. SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications, e.g. DS1, DS3, from a variety of different sources. However, they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format. The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. SONET/SDH allowed for the simultaneous transport of many different circuits of differing origin within a single framing protocol. SONET/SDH is not a complete communications protocol in itself, but a transport protocol (not a "transport" in the OSI Model sense). Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the choice for transporting the fixed-length Asynchronous Transfer Mode (ATM) frames also known as cells. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as Ethernet), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as STS-3c) into which ATM cells, IP packets, or Ethernet frames are placed. Both SDH and SONET are widely used today: SONET in the United States and Canada, and SDH in the rest of the world. Although the SONET standards were developed before SDH, it is considered a variation of SDH because of SDH's greater worldwide market penetration. SONET is subdivided into four sublayers with some factor such as the path, line, section and physical layer. The SDH standard was originally defined by the European Telecommunications Standards Institute (ETSI), and is formalised as International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803. The SONET standard was defined by Telcordia and American National Standards Institute (ANSI) standard T1.105. which define the set of transmission formats and transmission rates in the range above 51.840 Mbit/s.
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 →Shirley Chisholm, elected in 1968.