konsep_analogdigital

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Analog And Digital Communications Concepts

Chapter 3

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Representing Data As Analog Signals

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Converting Analog Data to Analog Signals

During the early development stages of copper-based analog telephone networks, it was discovered that voice conversations can be transmitted adequately from 300 Hz to 3330 Hz.

As a result, the telephone networks were originally designed to transmit voice conversations within a 3000 Hz range.

Voice signals that generate frequencies less than 300 Hz or greater than 3,300 Hz are discarded.

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Transmitting voice messages via an analog communications system involves recreating human speech in the form of sound waves. This is performed by the telephone set.

A telephone set converts sound into electrical signals at the sender’s end, and then reconverts these electrical signals into sound at the receiver’s end.

There are two basic ways in which analog data are represented as analog signals:• at their original frequency, called a baseband signal

• or at a different frequency

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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.

In analog modulation, an analog signal that represents the data is converted into another analog signal, which is the carrier.

Characteristics of a carrier that can be modified include the signal’s amplitude, frequency, and phase.

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Modifying the amplitude of a wave (i.e., the signal’s strength) is called amplitude modulation (AM).

Modifying a wave’s frequency (i.e., its pitch) is called frequency modulation (FM).

Modifying the phase of a wave (i.e., temporarily delaying the natural flow of the waveform) is called phase modulation (PM).

All three modulation techniques are used in conventional radio and television broadcasting; FM and PM are also used in satellite communications.

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Converting Digital Data to Analog Signals

Transmitting digital data across an analog-based communications network is done by modifying a continuous analog signal at the sender 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, (modulator and demodulator). Two modems are required— one at each end of a transmission line— and both modems must use the same modulation technique.

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When used in the context of converting digital data (or signals) into analog signals, the modulation techniques of AM, FM, and PM discussed earlier are respectively called amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK).

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Amplitude Shift KeyingASK alters the amplitude of a signal so that it

conforms to the digital data (0s and 1s).

The process of ASK involves varying the signal’s 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.

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Frequency Shift Keying

FSK alters the frequency of a signal so that it conforms to the digital data (0s and 1s). Amplitude is kept constant. One frequency is used to represent a binary 0 and a second frequency is used to represent a binary 1.

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Phase Shift Keying

PSK modifies the phase angle of the carrier wave based on the digital data being transmitted.

The changes in phase angle are what convey the data in a phase modulated signal. In its most simple implementation, one phase represents a binary 0 and a second phase represents a binary 1.

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Changing the phase of a wave at different angles enables us to encode the data with more than one bit of information at a time.

Phase shift modulation can also be combined with amplitude modulation.

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Quadrature Amplitude Modulation

Uses eight phase changes and two amplitudes to create 16 different signal changes, and can encode between four and seven bits per baud.

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Trellis-Coded Modulation

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.

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Converting Digital Data to

Analog Signals

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Representing Data As Digital Signals

As 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 signaling, which requires converting analog signals to digital signals.

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Converting Analog Data to Digital Signals

Representing analog data as digital signals requires converting the data’s corresponding analog signal, which is in the form of a sine wave, into a digital signal, which is represented as 0s and 1s. The most common approach is a process known as pulse-code modulation (PCM), and involves two steps: sampling and coding.

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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 its 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.

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Once a sample has been taken, it must then be converted into a binary digit, where 0 and 1 represent the absence and presence of voltage, respectively. Determining whether a sample gets coded 0 or 1 depends on where along the wave the sample is taken.

After the sampling and coding steps are complete, the resulting digital codes are then transmitted as a digital signal waveform.

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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.

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Converting Digital Data to Digital Signals

Transmitting digital data (e.g., output from a computer) across a digital network (e.g., an Ethernet LAN), requires representing the digital data as a digital signal. Three common coding techniques used for this task are Manchester coding, differential Manchester coding, and Non-Return to Zero, Invert on ones (NRZI).

In Manchester encoding, a 1 is sent as a half-time-period low followed by a half-time-period high, and a 0 is sent as a half-time-period high followed by a half-time-period low.

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Differential Manchester encoding is 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 between the two techniques is the interpretation of this transition. In differential Manchester encoding, interpreting these low to high and high to low mid-bit transitions is not as simple as with Manchester encoding because they are a function of the previous bit-period.

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NRZI is part of the non-return to zero (NRZ) family of codes in which positive and negative voltages are used for encoding 0s and 1s. In one form of NRZ, called NRZL (non-return to zero level) a constant positive voltage is used to represent a 0 bit and a constant negative voltage is used to represent a 1 bit.

An application of NRZI is an encoding strategy known as the 4B/5B (four bits to five bits) method. The 4B/5B encoding scheme takes data in four-bit codes and maps them to corresponding five-bit codes, which are then transmitted using NRZI.

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Converting

Digital Data to Digital

Signals

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Data Rate and Baud Rate Redux

Both data rate and baud rate can be used to express the capacity of a communications channel.

At low speeds (under 300 bps) data rate and baud rate are the same because signaling methods are relatively simple.

As speed increases, however, signaling methods become more complex because several bits are frequently encoded per baud, which enables each signal to represent more than one bit of information.

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Digital Carrier Systems

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T1 and DS Circuits

Digital signaling and the T-carrier system was introduced (circa 1962) to resolve attenuation and noise amplification.

In digital transmission facilities, attenuation is resolved by repeaters that regenerate digital signals to their exact form. This enables the new, regenerated signal to be compared to the previous signal, thus reducing the chances of propagating errors during a transmission.

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T1 and DS CircuitsThe T-carrier system, which uses time division multiplexing to

support multiple channels in a single digital signal, was the first system designed to implement digitized voice transmissions.

The T1 terminology, which is a product of the T-carrier system, was originally defined by AT&T and describes the multiplexing of 24 separate voice channels into a single, wideband digital signal.

A T1 frame consists of 193 bits - eight bits per channel plus one bit for framing. Bits 1 through 8 are dedicated to channel 1, bits 9 through 16 are dedicated to channel 2, and so forth.

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T1 and DS CircuitsEach voice channel is digitized using pulse code

modulation and has a data rate of 64 kbps.When multiplexed into a digital signal, a voice

channel is referred to as a digital signal at level 0 (DS-0). Thus, DS-0 represents a single, digital voice channel rated at 64 kbps.

A T1 circuit carries a DS-1 signal, which consists of 24 DS-0 channels plus one 8 kbps channel reserved for framing. This results in an aggregate bandwidth of 1.544 Mbps.

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T1 and DS CircuitsThe T1 concept eventually evolved to what is now known as

the North American Digital Hierarchy (NADH), which consists of multiplexed T1 lines.

• Two T1 lines are combined to form a T1C circuit rated at 3.152 Mbps.

• A T2 circuit (DS-2) consists of four multiplexed T1 circuits and has an aggregate bandwidth of 6.312 Mbps.

• A T3 link (DS-3) consists of 28 multiplexed T1 circuits with an aggregate bandwidth of 44.736 Mbps.

• A T4 channel (DS-4), rated at 274.176 Mbps, consists of 168 multiplexed T1 circuits.

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T1 and DS Circuits

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T1 and DS Circuits64 kbps was selected as the basic building block of T1/DS-1

circuits because telephone companies partitioned their circuits into channels of 4000 Hz, and hence 8000 samples per second are used for the conversions.

The construction of a T1’s line rate is as follows: Data: 56,000 bps per channel at 24 channels = 1,344,000 bps Control: 8,000 bps per channel at 24 channels = 192,000 bps Framing: 8,000 bps for frame synchronization = 8,000 bps

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T1 and DS CircuitsA T1 circuit requires special termination equipment

called a CSU/DSU.A channel service unit regenerates the signal,

monitors the line for electrical anomalies, provides proper electrical termination, performs framing, and provides remote loopback testing for diagnosing line problems.

A data (or digital) service unit (DSU) provides the interface for connecting a remote bridge, router, or switch to a T1 circuit.

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T1 and DS Circuits

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Fractional T1Fractional T1 service (FT1), provides a fraction of a

T1’s capacity. This is achieved by combining multiple DS-0 (i.e., 64 kbps) channels.

When ordering FT1 service from a telecommunications (telco) provider, you actually receive a full T1 channel but only pay for the number of DS-0 channels you order.

FT1 service is attractive to customers who do not require full T1 service but need more capacity than an ISDN 64/128 kbps line.

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SONET and OC Circuits SONET was developed by the telcos to address the need for

a fiber-optic based standard for broadband transmissions within the telecommunications industry.

The basic building block of the SONET signal hierarchy is STS-1 (51.84 Mbps).

Synchronous Optical Network (SONET) and the Synchronous Digital Hierarchy (SDH) are international physical layer standards that provide a specification for high-speed digital transmission via optical fiber.

SONET is an ANSI standard; SDH is an ITU-T standard.

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SONET and OC Circuits

SONET offers several advantages over the copper-based T1 hierarchy.

• As a fiber-based medium

— hundreds of thousands of simultaneous voice and data transmissions are possible using fiber

— fiber is immune to EMI

— fiber is available in either single or multimode and thus can be used for LAN connections or as the backbone of a WAN

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SONET and OC Circuits

As a synchronous transmission facility, bandwidth can be allocated on an as-needed basis and routes can be dynamically reconfigured.

As a carrier service, SONET can serve as the transport facility for any type network technology or service, including ATM, FDDI, SMDS, and ISDN.

SONET can support various topologies including point to point, star, and ring.

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Summary of Encoding Strategies

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Summary of Conversion Schemes

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