Analog and Digital Communications Concepts
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- Converting Digital Data-to-Analog signal
- Representing Data as Digital Signals
- Converting Analog-Data-to-Digital signals
- Converting Digital data-to-Digital Signals
Analog and Digital Communications ConceptsRepresenting data as Analog SignalsConverting Analog Data-to-Analog SignalsDuring the early development stages of copper-based analog telephone networks it was discovered that human voice could be carried adequately from 300Hz to 3330 Hz. As a result telephone networks were originally designed to transmit voice conversations within a range of 3000 Hz. Voice signals that generate a signal less than 300 Hz or greater than 3330 Hz are discarded. There are 2 basic ways in which analog data are represented as analog signals:
When we pick up a telephone and speak into it, the telephone network transmits our analog voice signal at face value (somewhere between 300 and 3330 Hz). This face value is an example of a baseband signal. Alternatively, the telephone network can combine our signal into another signal (called a carrier), and then transmit these combined signals at a different frequency. A carrier is a continuous signal that operates at a predefined frequency. Changing a carrier so that it can represent data in a form suitable for transmission is called modulation. Characteristics of a carrier that can be modified are Amplitude, Frequency and Phase. Modifying the Amplitude of a wave is called Amplitude Modulation (Changing the signal’s strength) Modifying the Frequency of a wave is called Frequency Modulation (Changing the signal’s pitch) Modifying the Phase of a wave is called Phase Modulation (Temporarily delaying the natural flow of the waveform) Converting Digital Data-to-Analog signalTransmitting digital data (output from a computer) across an analog based communication network, is done by modifying (modulating) a continuous signal (Carrier) at the sender end so that the signal conforms to the digital data being transmitted, and then converting the signal back into digital form at the receiver. The device that performs these functions is called a modem, which is a contraction of the words modulator and demodulator. Two modems are required- one at each end of the transmission line- and both modems must use the same modulation technique. A sending modem first produces a continuous carrier signal and then modified the signal using a specific modulation technique. When the modified signal is received by the receiving modem, the signal is then converted back into digital form. When used in the context of converting digital data into analog signals the modulations techniques are called:
ASK this process involves varying the signal voltage while keeping the frequency constant. One amplitude is used to represent a binary 0 and a second amplitude is used to represent a binary 1. A sending ASK modem, generates a continuous carrier and used this unmodulated signal to represent a binary 0; the modem also modulates this signal (using ASK) to represent a binary 1. FSK Alters the frequency (cycles per second) of a signal so that it conforms to the digital data. Amplitude is kept constant. One frequency is used to represent a binary 0 and a second frequency is used to represent a binary 1. A sending FSK modem generates a continuous carrier and let’s the signals standard frequency represent a binary 0. Whenever a 1 needs to be converted, the modem can either reduce or increase the frequency. PSK modifies the phase angle of a carrier wave based on the digital data being transmitted. The changes in phase angle are what convey the data in a phase-modulated signal. Changing the phase of a wave enables data encoding with more than one bit of data at a time. If a phase shift occurs at 0, 90, 180 and 270 0 then 2 bits of information (called dibits) can be transmitted for each signal change. These dibits pair 00, 01, 10, and 11 respectively. Similarly if phase shift occur at 8 different angles (0, 45, 90, 135, 180, 225, 270 and 3150 then these signal represent three bits of information (tribits). These tribits are respectively 000, 001, 010, 011, 100, 101, 110, and 111. As a result PSK can encode up to three bits per baud. QAM Phase shift can also be combined with Amplitude modulation. One common strategy employed is called Quadrature Amplitude Modulation, which uses eight phase changes and two amplitudes to create 16 different signal changes. A QAM can encode between four and seven bits per baud. A modified version of QAM called trellis-coded modulation (TCM) incorporates extra bits for error-correction. Both QAM and TCM provide high data transfer rates because they are able to incorporate several bits per signal change. Modem Demo Representing Data as Digital SignalsAs digital technology and computer data applications emerged, analog technology was unable to separate data from noise in a satisfactory manner. This led to the introduction of digital signalling, which requires converting analog signals to digital signals. Converting Analog-Data-to-Digital signalsRepresenting Analog data as digital signals requires converting the data’s corresponding analog signal, which is in the form of a sine wave, into digital signal, which is represented by 0’s and 1’s The most common approach is a process known as pulse-code modulation (PCM), and involves two steps: Sampling and coding. Digitizing an analog signal requires taking regular samples of the amplitude of the signal’s waveform over time so that the generated digital signal matches the corresponding analog signal. According to a sampling theorem known as Nyquist’s Rule, if an analog signal is sampled at regular intervals and at twice the highest frequency on the line, then the sample will be an exact representation of the original signal. Even though voice transmission requires 3000 Hz bandwidth, phone companies allocate 4000 Hz and install filters at the 300 Hz and 3300 Hz. Therefore actual sampling is 8000 per second when converting voice to digital form. Once a sample has been taken, it must be converted into binary digit where 0 and 1 represent the absence or presence of voltage, respectively. Determining whether a sample gets coded 0 or 1 depends on where the sample was taken. If an eight-bit sample is used, then the sound wave can be partitioned into 256 (28) possible points. The first 128 points (0 to 127) get coded 00000000, and the last 128 (128 to 255) get coded 00000001. After the sampling and coding steps are complete, the resulting digital codes are then transmitted as a digital signal waveform. Digitizing an analog signal via PCM is done using a device called a codec (coder-decoder), which can be thought of as the opposite of a modem. A codec converts analog data into a digital signal; a modem on the other hand converts digital data into an analog signal. 
This is an example of what happens when a computer using a modem communicates with another computer equipped with a modem over the telephones lines. The telephone company maintains analog lines from clients to the local switching stations. In between switching stations lines are digital. The signal from the computer at the sending end has to be converted from digital to analog, when the signal arrives at the switching station it is converted again from analog to digital. Again when the signal arrives at the second switching station is has to be converted again to analog…. Finally at the destination computer the signal is converted again from analog to digital… In total it required 4 conversions for these two computers to effectively communicate. This is the role of a modem (modulator / demodulator) to convert a digital signal into an analog one at the sending end and to convert it from analog to digital at the receiving end. Converting Digital data-to-Digital SignalsTransmitting digital data across a digital network (Ethernet LAN) requires representing the digital data as a digital signal. Three common techniques are used for this task are:
Manchester encoding Used on Ethernet/802.3 networks. Its main benefit is error recovery. A 1 is sent as a half-time-period low followed by a half-time-period high, and a 0 is sent at half-time-period high followed by a half-time-period low. Consequently, the end of the last transmitted bit is easily determined immediately following the transmission of the last bit. Differential Manchester Encoding used on token ring networks. Similar to Manchester encoding: each bit-period is partitioned into two intervals and a transition between high and low occurs during each bit period. The difference in between the two methods is in the interpretation of this transition. In Differential Manchester Encoding the interpretation of low-to-high or high to low is a function of the previous bit-period. More specifically, the presence of a transition at the beginning of a bit-period is coded 0, and the absence of a transition at the beginning of a bit-period is coded 1. NRZI used on Fast Ethernet and FDDI (Fiber Distributed Data Interface). Instead of user level voltages to encode the data, encoding is based n transitions from one voltage state to another. Data are coded 0 if no transition occur, but are coded 1 at the beginning of a transition. An application of NRZI is an encoding strategy known as 4B/5B (four-bits to five bits) method. The 4B/5B-encoding scheme takes data in four-bit codes and maps it to corresponding five-bits codes. By transmitting five-bits codes using NZRI, a logical 1-bit is transmitted at least once every five sequential data bits resulting in a signal transmission. This 4B/5B scheme makes it possible for a LAN to operate at 125MHz and provides a data rate of 100Mb/sec. Note that the use of one extra bit for every five translate to 20% overhead for every clock encoding. In contrast, Manchester Encoding requires 50% bandwidth overhead for clock encoding because it guarantees at least one signal transition for every bit transmitted. Digital Carrier SystemsT1 and DS circuitsDigital signalling and the T-carrier system were introduced to resolve attenuation and noise amplification.
T1, a product of T-carrier, describes the multiplexing of 24 separate voice channels into one single wideband digital signal.
NADH Lines have the same meaning in Australia, Japan as it does in North America. However in Europe, South America, Africa, part of Asia and Mexico, an analogous service called E-1 is used in these locations. An E-1 carrier supports thirty 64 kbps channels plus 2 signalling and control channels. E-1 links can be multiplexed into higher capacity lines. 
A T1 circuit requires special termination equipment called CSU/DSU
 Document on CSU/DSU Fractional T1 (FT1)
SONET and OC CircuitsSONET (Synchronous Optical NETworks) and SDH (Synchronous Digital Hierarchy) are both International standards that provide specification for high-speed digital transmission via optical fibre. This involves converting signals in electrical to optical form at the source and an optical-to-electrical at the destination.
Advantages over copper based (T1) hierarchy
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