Telecommunications Fundamentals, Chapter 3: Analog and Digital Transmission

ANALOG AND DIGITAL TRANSMISSION

Objectives

Describe the major characteristics and transmission quality measurements of analog signals.

Describe a carrier system.

Describe the major differences between analog and digital signals.

Describe the analog to digital and digital to analog conversion process.

Compare and contrast Frequency Division Multiplexing and Time Division Multiplexing.

Describe the components required for a Digital Loop Carrier system.

Introduction

The telecommunications network can transmit a variety of information, in two basic forms, analog and digital. In this lesson we will examine both. This information may be transmitted over a circuit/channel or over a carriersystem.

Where: A circuit/channel is a transmission path for a single type of transmission service (voice or data) and is generally referred to as the smallest subdivision of the network. A carrier, on the other hand, is a transmission path in which one ormore channels of information are processed, converted to a suitable format and transported to the proper destination.

The two types of carrier systems we will be discussing in this lesson are:

1. FDM (Frequency Division Multiplexing) -- analog

2. TDM (Time Division Multiplexing) - digital

There are various ways to look at signaling.

Analog versus Digital (Figure 3.1)

Electrical versus Optical

Signals can also be distinguished by their function:

• Voice Transmission - By far the largest number of signals found on the network are voice frequency conversations.
• Programming - These signals are from radio or television. They may be voice or music or video. The laer two requiring special circuits and facilities because or their wide frequency bandwidth requirements.

Network Control - These are the signals used for network housekeeping and control. There are four basic types:

• Alerting
• Addressing
• Information
• Supervision

Digital Transmission

The single fastest growing family of signals. Discrete and well defined, digital signals vary from dial tone pulses to complex computer and data signals.

Analog Signals

An analog signal, or sine wave, is a continuously varying signal (Figure 3.1 top portion).

Analog Carrier: (Figure 3.2) Is a transmission path that accepts a band of frequency and is compatible with analog signals. With the introduction of carrier systems the concept of multiplexing was created.

Multiplexing

Multiplexing is the process of transmiitng two or more individual signals over a common path. In effect, it increases the amount of information transmitted, while decreasing the requirement for the physical media (no longer a 1:1 ratio).

Frequency Division Multiplexing

The first type of multiplexing was an analog multiplexing technique. Frequency Division Multiplexing (FDM). In FDM, the bandwidth of the transmission path serves as the frame of reference for all of the information being transmitted. The total bandwidth is divided into subchannels consisting of smaller segments of the available bandwidth (Figure 3.4). Each subchannel is capable of carrying a separate signal. Signals are transmitted simultaneously. Thus, with FDM each channel is:

• Assigned a different frequency
• Separated into channels 4000 Hz. wide.

The different channels are then stacked and transported over a common path. In other words, each channel occupies a portion of the total frequency bandwidth.

Transmission Impairments

With analog transmission systems using copper cable there are three major categories of impairments:, attentuation, noise, and distortion.

(1) Attenuation: (Figure 3.5)

There are two commonly used processes to compensate (overcome) for attenuation or loss:

(a)

(b)

Repeaters are the most commonly used device to compensate for "loss." However, repeaters amplify the noise along with the signal resulting in a poor signal to noise ratio.

Signal to Noise Ratio: The ratio of the average signal power (strength) to the average noise power (strength) at any point in a transmission path.

(2) Noise: Any random disturbance or unwanted signal on a transmission facility that obscures the original signal (Figure 3.6). Noise is generally caused by the environrnent in which the system is operating.

(3) Distortion: Inaccurate reproduction of a signal caused by changes in the signal's waveform, either amplitude or frequency, to compensate for distortion equalizers may be used. One type of equalizer used in the analog environment is the load coil. Load coils are used to flatten the frequency response. Note: Generally the higher the frequency the greater the distortion. That is: the higher voice frequencies aenuate at a higher rate than the lower voice frequencies.

Noise and distortion on a carrier facility can be separated into two types:

(a) Predictable impairments that are almost always present on our facilities.

(b) Unpredictable impairments that are transient in nature and difficult to overcome.

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