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Analog Transmission 101

A human voice creates a sound wave. This sound wave is captured by a telephone device that converts this sound wave into electric current wave as its analog equivalent so it can be transmitted into telephone copper wire. Then another device at the other end of telephone line converts again the electric current wave to its equivalent sound wave to hear the voice of the person speaking. That is the basis for voice or telephone communication.

Digital Transmission 101 and Beyond

The human voice sound can be represented by a sine wave of its analog equivalent electric current sine wave. This electric current sine wave can travel into telephone copper wire at about 18,000 to 20,000 feet after that distance the voice conversion will be distorted. To solve this distance limitation, telephone company installed analog amplifier every 18,000 feet to regain the correct voice sine wave. But the analog amplifier cannot identify the voice sine wave against the noise injected to the copper wire. So it amplifies both the voice signal and the noise signal.

With advancement in digital signal processing, the analog equivalent electric current sine wave is now possible to be converted to digital pulses, represented by number 1 if there is a pulse and 0 if none. To duplicate the voice sound wave, the equivalent electric current sine wave must be converted to equivalent digital pulses which also takes the shape of a sine wave for discussion purposes.

Nyquist discovered a rule , that the analog sine wave can be converted to a digital pulse sine wave with less distortion from its original analog signal if the sampling rate of the analog sine wave is twice of its highest frequency range. The human voice sound wave was observed at oscilloscope, a graphical instrument that display the analog sine wave and the digital square wave at about 3000 Hz. Adding another 1000 Hz to compensate for any losses of signal, the highest frequency range of human voice for estimation purposes is 4000 Hz (3000 + 1000). Following the discovery of Nyquist to convert this analog human voice sine wave to digital pulse square wave signal with less distortion, it must be sampled at 8000 Hz ( 2 x 4000 Hz). Remember Hz means cycle per second.

The analog sine wave will now become a square wave as it is converted into a digital pulse of 1s and 0s.

The common microprocessor bus is 8 bus. How many possible pulses of 1s and 0s you can have in one cycle that will duplicate closely the analog sine wave? Answer is 28 = 256 pulses. 128 pulses on the positive part of sine wave and another 128 pulses on the negative part of sine wave.

Pulse Code Modulation (PCM) is one of the technologies that do the conversion of analog signal to equivalent digital signal. Again going back to voice transmission, the PCM must convert the analog sine wave to digital square wave at 8000 Hz ( cycle per second ) in order to have a minimum signal distortion according to Nyquist rule. With 8 parallel bus of microprocessor, each bus can process or sample 8000 pulses of 1s or 0s . This 1s or 0s are binary digits abbreviated as bits . So 8000 pulses is the same as saying 8000 bits. Therefore for 8 bus microprocessor, its processing capability is 8 bus x 8000 bits per bus equals to 64,000 bits/sec.



Voltage Sine Wave Frequency Addition

Voltage Signal Wave 1 = 1 Hz

Freq: 1 Phase: 0 Click the animation button to start


Voltage Signal Wave 2 = 30 Hz

Freq: 2 Phase: 0


Voltage Signal Wave 3 = 8000 Hz our eyes can't see it.

Freq: 3 Phase: 0


Addition of Waves 1, 2, and 3

New waveforms can be made by adding three voltage sine waves. Increase or decrease the frequencies and increment the phase of the component sine waves to see how they add together.


Remember this 64,000 bits per second (64 kbps ) history, because it becomes the reference standard . In North America the digital signal are classified as: [ Based in ANSI T1.107 Rates ] DS0, meaning Digital Signal level 0, or reference digital signal. It is equal to 64 kbps. Now we understand the reason behind why ANSI standard committees decided to use 64 kbps as digital signal reference or DS0. The use of number 0 is a common practice to indicate the point of reference. Hence we have DS0.

Also keep in mind when you see the word bits it means binary digits ( 1 or 0 ) and it is derived from the pulses of 1 ( also means Voltage is detected in the microprocessor output voltage) and 0 (It means Voltage is not detected in microprocessor output voltage). As we can see if the data rate of transmission is less than 64kbps ( kilo bits per second ) there is still possibility of not properly recreating the voice sound wave to the other end of the telephone line. So the knowledge of 64 kpbs also helps in troubleshooting why the voice is distorted.

Telephone voice is the beginning of all these Analog to Digital conversion ( remember DS0). Now we need the internet services, streaming video services, data services, digital telemetry services, SCADA services, teleprotection services and much more in the future. All of these services can be transported using the telephone copper wires, microwaves, sattelites, and fiber optics technologies. In order to properly design all these services , remembering the history of 64,000 bits per second (64 kbps ) or DS0 will be a good starting point.

In copper twisted wire the digital signal can travel only around 2000 ft to 6000 ft. After that distance signal repeater is needed to transmit the digital signal at a longer distance.

In copper twisted wire the analog signal can travel around 18,000 ft to 20,000 ft. After that distance signal repeater is needed to transmit the analog signal at a longer distance.

Because the telephone copper wire have four wires, the two wires are now assigned to transmit the signal and the remaining two wires are assigned to receive the signal. By using the four wire telephone circuit, full duplex communication is possible for digital signal transmission.

T-1 digital signal carrier can transmit 24 channel of 64 kbps of voice signal , data signal, digital telemetry signal, SCADA signal, and teleprotection signal. T-1 is still consider as a four wire circuit. So for me when I see T-1, it reminds me that T means Telephone four wire circuit. Other people, when they see T-1, It reminds them of Twisted copper wire. So T-1 refers to the physical copper wires.

Telephone Company (Telco) normally terminates the four copper wires into a demarcation point (DEMARC) or to Network Interface Unit. So we can say that T-1 is the physical components such as the wires, plugs and jacks, repeaters, and so forth. The digital signal running into the T-1 is designated as DS1.

In North America the digital signals are classified as: [ Based in ANSI T1.107 Rates ]

DS0 - means Digital Signal Level 0, reference digital signal. DS0 can transmit and receive digital signal up to 64 kbps.

DS1- means Digital Signal Level 1. DS1 is compose of 24 DS0 and can transmit and receive digital signal up to aggregate of 1.544 Mbps. DS1 is a time division multiplexed ( TDM) pulse code modulation aggregate of 1.544 Mbps.

DS2- means Digital Signal Level 2. DS2 is compose of 4 DS1 multiplexed together yielding an aggregate rate of 6.312 Mbps. DS2 can carry 96 DS0 digital signal aggregate.

DS3 - means Digital Signal Level 3. DS3 is compose of 28 DS1 or 672 DS0 multiplexed together yielding an aggregate rate of 44.736 Mbps.

DS4 - means Digital Signal Level 4. DS4 is compose of 6 DS3 or168 DS1 or 4032 DS0s multiplexed together yielding an aggregate rate of 274.176 Mbps.

SYNCHRONOUS OPTICAL NETWORK - SONET 101

The existing telephone copper wire is no longer capable of serving the needs of growing internet, mobile, and streaming videos. It is easy to see because telephone copper wire can transport digital signal up to DS1, meaning 1.544 Mbps. Again with new technology called ADSL2+M , the plain old telephone service (POTS) copper twisted wire can transport now up to 24 Mbps downstream and 3.3 Mbps upstream.

The Data Centers are now requiring the ability to transport up to 2 Gbps of digital signal to serve their internet clients, mobile clients, and streaming videos clients. Today it is possible to transport up to Gigabits of digital signal using fiber optics technology. The network of fiber optics connection that makes the transportion of digital signal up to DS4 (274.176 Mbps) digital signal level is called SONET, which means Synchronous Optical Network. And the technology behind that makes it possible to transport up to Gigabits of digital signal is called Dense Wave Division Multiplex ( DWDM). The word wave in DWDM acronym is referring to the lightwave that travels inside the fiber optics cable. Using the DWDM technology, it now possible to add or drop the digital signal. We called this process of adding or dropping digital signal as multiplexing the digital signal. The device that do the digital signal multiplexing is called the MULTIPLEXER.

Example of vendors that supplies SONET multiplexer using DWDM are the following: Nortel Optera Metro, OM350 , Cisco 15454 DWDM, and GE JUNGLEMUX (LENTRONICS) to name a few. The DWDM device can add or aggregates the light wavelength to increase the digital signal carrying capacities to serve the requirements of streaming videos for example, requiring up to 44.736 Mbps (DS3) of video transport. It can also drop the light wavelength to decrease the digital signal carrying capacities to serve the requirements of voice telephone service or low and high speed data service up to 1.544 Mbps (DS1).

SONET, Synchronous Optical Network is the latest update to the Telephone Company copper wire network. Therefore to understand SONET, we need to have knowledge of Fiber Optic technology and all its associated components needed to transport digital signal in fiber optics network.

In 1988 ANSI and ITU-T (former CCITT) approved a standard related to Synchronous Optical Networks became known as SONET. The standard defined the SONET Hierarchy and transport rates. Below is the approved SONET Hierarchy and Transport Rates.

STS-1 , means Synchronous Transport Signal level 1, can carry OC-1, meaning Optical Carrier level 1 that can transmit and recieve up to 51.84 Mbps.

STS-3, means Synchronous Transport Signal level 3, can carry OC-3, meaning Optical Carrier level 3 that can transmit and receive up to 155.52 Mbps.

STS-12, means Synchronous Transport Signal level 12, can carry OC-12, meaning Optical Carrier level 12 that can transmit and receive up to 622.08 Mbps.

STS-48, means Synchronous Transport Signal level 48, can carry OC-48, meaning Optical Carrier level 48 that can transmit and receive up to 2488.32 Mbps.

STS-192, means Synchronous Transport Signal level 192, can carry OC-192, meaning Optical Carrier level 192 that can transmit and receive up to 9953.28 Mbps

Digital Signal Conversion Calculator