Pulse-code modulation

Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals, which was invented by Alec Reeves in 1937. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems. A PCM stream is a digital representation of an analog signal, in which the magnitude of the analogue signal is sampled regularly at uniform intervals, with each sample being quantized to the nearest value within a range of digital steps.
PCM streams have two basic properties that determine their fidelity to the original analog signal: the sampling rate, which is the number of times per second that samples are taken; and the bit depth, which determines the number of possible digital values that each sample can take.

E-1 Carrier System:

An E1 link operates over two separate sets of wires, usually twisted pair cable. A nominal 3 Volt peak signal is encoded with pulses using a method that avoids long periods without polarity changes. The line data rate is 2.048 Mbit/s (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit sample 8000 times per second (8 x 8000 x 32 = 2,048,000). This is ideal for voice telephone calls where the voice is sampled into an 8 bit number at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31.

One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used.

One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes Channel Associated Signaling (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems used Common Channel Signaling (CCS) such as ISDN or Signalling System 7 (SS7) which send short encoded messages with more information about the call including caller ID, type of transmission required etc. ISDN is often used between the local telephone exchange and business premises, whilst SS7 is almost exclusively used between exchanges and operators. SS7 can handle up to 4096 circuits per signalling channel, thus allowing slightly more efficient use of the overall transmission bandwidth (for example: uses 31 voice channels on an E1).

History of E-carrier:

In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations, worldwide standards have been created and deployed. The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). This is now widely used in almost all countries outside the USA, Canada and Japan.

The E-carrier standards form part of the Plesiochronous Digital Hierarchy (PDH) where groups of E1 circuits may be bundled onto higher capacity E3 links between telephone exchanges or countries. This allows a network operator to provide a private end-to-end E1 circuit between customers in different countries that share single high capacity links in between.

In practice, only E1 (30 circuit) and E3 (480 circuit) versions are used. Physically E1 is transmitted as 32 timeslots and E3 512 timeslots, but one is used for framing and typically one allocated for signalling call setup and tear down. Unlike Internet data services, E-carrier systems permanently allocate capacity for a voice call for its entire duration. This ensures high call quality because the transmission arrives with the same short delay (Latency) and capacity at all times.

E1 circuits are very common in most telephone exchanges and are used to connect to medium and large companies, to remote exchanges and in many cases between exchanges. E3 lines are used between exchanges, operators and/or countries, and have a transmission speed of 34.368 Mbit/s.

What is PCM?

PCM is a Time-Domain Waveform coding method and is defined within CCITT G.711, and AT&T 43801. Basically, an analog signal is sampled at a rate of 8000 times per second. In each sample, the amplitude of the signal is assigned (quantized) a digital value. 

For a true linear system, the value of each "step" is uniform, requiring anywhere from 11 to 14 bits to transmit. However, PCM, as defined and standardized, utilizes a logarithmic scale in the weighting of each step. 

There are two PCM algorithms defined within CCITT G.711, called "A-Law" and "Mu-Law". Mu-Law PCM is used in North America and Japan, and A-Law used in most other countries. In both A-Law and Mu-Law PCM, the values used to represent the amplitude is a number between 0 and +/- 127; therefore, 8 bits are required to represent each sample (2 to the eight power = 256). 

It can be seen then that PCM operates at a rate of: 8 bits/sample * 8000 samples/sec = 64000

History of PCM Transmission:

A voice digitization technique known as Pulse Code Modulation started to be deployed in the United States Public Switched Telephone Network in 1962. 

Basically, you start with a 4 KHz analog voice channel. Then you take a "snapshot" of the voice signal's amplitude every 1/8000th of a second (you have to sample at twice the maximum frequency to avoid a problem known as "aliasing"). Then you convert the measured amplitude to a number (the "quantization" process) that is represented by 8 bits. Thus, PCM requires 64 KBPS of digital bandwidth (8 KHz * 8 bits). This basic channel represents the first level of a digital hierarchy, known as a DS0. 

A special type of Time-Division Multiplexer (TDM) called a "Channel Bank" takes 24 of these 64K DS0 channels and combines (multiplexes) them into a single aggregate rate of 1.544 MBPS. This rate is the combination of the channel data payload of 1.536 MBPS (64 KBPS * 24 Channels) + 8 KBPS of framing and synchronization bits. The 1.544 MBPS rate is known as the DS1 level in the digital hierarchy. Facilities that support this rate are usually referred to as "T-Spans" or "T1" circuits. 

International standards were developed later. Although the basic hierarchical DS0 rate of 64 KBPS was preserved, the algorithm for converting the voice signal to a digital signal is different. Also, the International standard calls for 30 voice channels + a 64 KBPS synchronization channel + a 64 KBPS signaling channel. Therefore, these systems operate at a rate of 2.048 MPBS (1.920 MBPS + 64 KBPS + 64 KBPS). Facilities that support this rate are usually referred to as "E1" circuits. 

Using a transmission line code known as Bipolar-Alternate Mark Inversion (AMI), a 1.544 MBPS T1 circuit requires 772 KHz of analog bandwidth. So, why go digital? I could use Frequency Division Multiplexing (FDM) and combine those same 24 channels into a 96 KHz (4 KHz * 24) analog pipe, right? While FDM saves bandwidth, noise is added as the signal travels through every amplifier and modulator. In a digital system, "ones" and "zeroes" go in, and "ones" and "zeroes" go out. Since major sources of analog noise are removed in digital systems, circuit lengths can be extended, and network topologies simplified through the reduction of the number of circuits required between any two telephone exchanges. Quality improves, operating costs decrease!

History of Telecommunications

‘Telecommunication’ is a term coming from Greek and meaning ‘communication at distance’ through signals of varied nature coming from a transmitter to a receiver. In order to achieve effective communication, the choice of a proper mean of transport for the signal has played (and still plays) a fundamental role.

In ancient times, the most common way of producing a signal would be through light (fires) and sound (drums and horns). However, those kinds communications were insecure and certainly left room to improvement as they did not permit message encryption nor a fast transmission of information on a large scale.

The true ‘jump’ in terms of quality came with the advent of electricity. Electromagnetic energy, in fact, is able to transport information in an extremely fast way (ideally to the speed of light), in a way that previously had no equals in terms of costs reliability. Therefore, we may say that the starting point of all modern telecommunications was the invention of the electric cell by Alessandro Volta (1800).

It was shortly thereafter that the first experiments on more advanced communication system begun. In 1809, Thomas S. Sommering proposed a telegraphic system composed of a battery, 35 wires (one for each letter and number) and a group of sensors made of gold, which were submerged in a water tank: when a signal was passing from one of those wires, electrical current would split water molecules, and small oxygen bubbles would be visible near that sensor. Many other experiments were soon to follow: Wheatstone, Weber and Karl Friedrich Gauss tried to further develop Sommering’s idea in a product that could be mass-distributed, but their efforts were without success.

For the next step we would have to wait until 1843, the year in which Samuel Morse proposed a way to assign each letter and number to a ternary code (point, line, and space). This way turned out to be extremely convenient and more affordable than Sommering’s idea, especially in terms of reduced circuitry (you wouldn’t need anymore a wire for each symbol). Meanwhile, technology became advanced enough to find a way to convert those signals in audible (or sometimes graphic) signals. The combination of these two factors quickly determined the success of Morse’s symbol code, which we can still find used today.

The system was further developed and improved in the following years by Hughes, Baudot, and Gray (1879), who theorized other possible codes (Gray’s code has still applications today in the ICT industry and in barcodes technology).

However, the telegraph could still be used just by trained personal and in certain buildings like offices, so it could only be used by a limited amount of people. Research of the time therefore took another direction and aimed at producing a machine that could transmit sounds, rather than just signals. The first big step in this direction was the invention of transducers which could transform an acoustic signal into an electric one and vice versa (microphone and receiver) with acceptable information loss, in 1850.

Seven years later, Antonio Meucci and Graham Bell independently managed to build a prototype of an early telephone (’sound at distance’) machine. Since Meucci didn’t have the money to patent his invention (the cost was $250 at the time), Bell managed to register it first.

Both with telegraphs and telephones, the need for a distributed and reliable communication network soon became evident. Routing issues were first solved by means of human operators and circuit commutation: the PSTN (Public Switched Telephone Network) was born. However, this system didn’t guarantee the privacy and secrecy of conversations, and efforts towards the development of an automatic circuit commutation were made.

In 1899, Almon Strowger invented an electro-mechanic device simply known as ’selector’, which was directed by the electrical signals coming from the calling telephone device, achieved through selection based on geographical prefixes.

Many other innovations were soon to come:

• In 1985, Guglielmo Marconi invented the ‘wireless telegraph’ (radio);
• In 1920, valve amplifiers made their first appearance;
• In 1923, the television was invented;
• In 1947, the invention of transistors gave birth to the field of electronics;
• In 1958, the first integrated circuit was built;
• In 1969, the first microprocessor was invented.

With the last step, electronics becomes more than ever a fundamental part in the telecommunication world, at first in the transmission, and soon also in the field of circuit commutation.

Moreover, in 1946 the invention of ENIAC (Electronic Numerical Integrator and Computer) starts the era of informatics. Informatics and telecommunications inevitably begun to interact, as it was to be expected: the first made fast data processing possible, while thanks to second the data could then be sent to a distant location.


The development of microelectronics and informatics radically revolutionized techniques both in telecommunication networks and performance requirements for the networks. Starting from 1938, an innovative technology called PCM (Pulse Code Modulation) started to grow more and more popular. This technology could achieve the digital transmission of a voice signal by digitally encoding and decoding, rather than by means of transducers: however, PCM was first used on a large scale only in 1962 in the United States (the so-called ‘T1′).

During the mid Sixties Paul Baran, a RAND Corporation employee working on communication problems concerning the US Air Force, first gave birth to the concept of ‘packet switching network’ rather than the conventional idea of circuit commutation network. According to this model, there should be no hierarchy in the nodes of a network, but each node should rather be connected to many others and be able to decide (and, in case of need, modify) the packet routing. Each packet is a bulk of data which consist of two main parts, a ‘header’ containing routing information and a ‘body’ containing the actual data.

In this context Vincent Cerf, Bob Kahn and others developed, starting from the 70s, the TCP/IP protocol suite, which made possible communication of computers and heterogeneous machines through a series of physical and logical layers. Packet switching network and TCP/IP were later chosen by the military project ARPANET. The rest of the story is widely known: in 1983, ARPANET became available to universities and research centers, among which NSFNET (National Science Foundation + NET), which finally gave birth to the Internet.

In the latest years, the importance of the Internet has been constantly growing. The high flexibility given by the TCP/IP suite and the ISO/OSI protocols provide a strong foundation on which communication among devices of different kind — be it a laptop or a cell phone, an iPod or a GPS navigator — has finally been made simple and easy to achieve.

Electronics - History

The history of electronics is a story of the twentieth century and three key components—the vacuum tube, the transistor, and the integrated circuit. In 1883, Thomas Alva Edison discovered that electrons will flow from one metal conductor to another through a vacuum. This discovery of conduction became known as the Edison effect. In 1904, John Fleming applied the Edison effect in inventing a two-element electron tube called a diode, and Lee De Forest followed in 1906 with the three-element tube, the triode. These vacuum tubes were the devices that made manipulation of electrical energy possible so it could be amplified and transmitted.

The first applications of electron tubes were in radio communications. Guglielmo Marconi pioneered the development of the wireless telegraph in 1896 and long-distance radio communication in 1901. Early radio consisted of either radio telegraphy (the transmission of Morse code signals) or radio telephony (voice messages). Both relied on the triode and made rapid advances thanks to armed forces communications during World War I. Early radio transmitters, telephones, and telegraph used high-voltage sparks to make waves and sound. Vacuum tubes strengthened weak audio signals and allowed these signals to be superimposed on radio waves. In 1918, Edwin Armstrong invented the "super-heterodyne receiver" that could select among radio signals or stations and could receive distant signals. Radio broadcasting grew astronomically in the 1920s as a direct result. Armstrong also invented wide-band frequency modulation (FM) in 1935; only AM or amplitude modulation had been used from 1920 to 1935.

 

Communications technology was able to make huge advances before World War II as more specialized tubes were made for many applications. Radio as the primary form of education and entertainment was soon challenged by television, which was invented in the 1920s but didn't become widely available until 1947. Bell Laboratories publicly unveiled the television in 1927, and its first forms were electromechanical. When an electronic system was proved superior, Bell Labs engineers introduced the cathode ray picture tube and color television. But Vladimir Zworykin, an engineer with the Radio Corporation of America (RCA), is considered the "father of the television" because of his inventions, the picture tube and the iconoscope camera tube.

Development of the television as an electronic device benefitted from many improvements made to radar during World War II. Radar was the product of studies by a number of scientists in Britain of the reflection of radio waves. An acronym for RAdio Detection And Ranging, radar measures the distance and direction to an object using echoes of radio microwaves. It is used for aircraft and ship detection, control of weapons firing, navigation, and other forms of surveillance. Circuitry, video, pulse technology, and microwave transmission improved in the wartime effort and were adopted immediately by the television industry. By the mid-1950s, television had surpassed radio for home use and entertainment.

 

After the war, electron tubes were used to develop the first computers, but they were impractical because of the sizes of the electronic components. In 1947, the transistor was invented by a team of engineers from Bell Laboratories. John Bardeen, Walter Brattain, and William Shockley received a Nobel prize for their creation, but few could envision how quickly and dramatically the transistor would change the world. The transistor functions like the vacuum tube, but it is tiny by comparison, weighs less, consumes less power, is much more reliable, and is cheaper to manufacture with its combination of metal contacts and semiconductor materials.

The concept of the integrated circuit was proposed in 1952 by Geoffrey W. A. Dummer, a British electronics expert with the Royal Radar Establishment. Throughout the 1950s, transistors were mass produced on single wafers and cut apart. The total semiconductor circuit was a simple step away from this; it combined transistors and diodes (active devices) and capacitors and resistors (passive devices) on a planar unit or chip. The semiconductor industry and the silicon integrated circuit (SIC) evolved simultaneously at Texas Instruments and Fairchild Semiconductor Company. By 1961, integrated circuits were in full production at a number of firms, and designs of equipment changed rapidly and in several directions to adapt to the technology. Bipolar transistors and digital integrated circuits were made first, but analog ICs, large-scale integration (LSI), and very-large-scale integration (VLSI) followed by the mid-1970s. VLSI consists of thousands of circuits with on-and-off switches or gates between them on a single chip. Microcomputers, medical equipment, video cameras, and communication satellites are only examples of devices made possible by integrated circuits.