Telecommunications Fundamentals, Chapter 3A: Analog and Digital Transmission

Why Digital Transmission?

(Figure 3.7) Digital Transmission, demanded by our customers, has continually increased since its introduction in 1962. This is due, in large part, to the fact that more of our customers require a high degree of accuracy in the infor mation they are transmiing over our network. And with a digital transmission (as opposed to analog) system we are able to manage the quality of the signal by managing the previously discussed transmission impairments. Thus, digital systems: 1). are a better switching interface 2.) are easier to multiplex 3.)produce clearer signals

Digital Signals A digital signal is a discrete signal. It is depicted as discontinuous (Figure 3.8) -- Discretely variable (on/off) as opposed to an analog signal which is continuously variable (sine wave) A digital signal has the following characteristics:

1.) Holds a fixed value for a specific length of time

2.) Has sharp, abrupt changes

3.) A preset number of values allowed

Each pulse (on/off) is known as a bit. Bit is a contraction of the words binary and digit A binary (two-level) signal (1 or 0) is the most common digital signal in the telecommunication industry. The number of bits transmitted per second is the bit rate of the signal.

To convert analog signals to digital signals, a coding system called Pulse Code Modulation (Figure 3.9), or PCM is used. This process is also called Analog-to-Digital, or A/D, conversion. When changing a digital signal to an analog signal, the process is called Digital-to-Analog, or D/A, conversion.

The Pulse Code Modulation (PCM) Process

Pulse Code Modulation (PCM) converts analog signals to a digital format (signal). This process has four steps (Figure 3.10):

Step One: Filtering

Frequencies below 300 Hz and above 3400 Hz (Voice Frequency range) are filtered from the analog signal (Figure 3.11). The lower frequencies are filtered out to remove electrical noise induced from the power lines. The upper frequencies are filtered out because they require additional bits and add to the cost of a digital transmission system. The actual bandwidth of the filtered signal is 3100 Hz (3400 - 300). It is often referred to as 4 kHz.

Step Two: Sampling

The analog signal is sampled 8000 times per second (Figure 3.12). The rate at which the analog signal is sampled is related to the highest frequency present in the signal. This is based on the Nyquist sampling theorem. In his calculations, Nyquist used a voice frequency range of 4000 Hz (which represents the voice frequency range that contains "intelligent" speech). Thus, the standard became a sampling rate of 8000 Hz, or twice the bandwidth. The signal that is the result of the sampling process contains sufficient information to accurately represent the information contained in the original signal. The output of this sampling procedure is a Pulse Amplitude Modulated, or PAM, signal.

Step Three: Quantizing

In the third step of the A/D conversion process, we quantize the amplitude of the incoming samples to one of 255 amplitudes on a quantizing scale (Figure 3.13). Thus, in this step the sampled signal is matched to a segmented scale. The purpose of step three is to measure the amplitude (or height) of the PAM signal and assign a decimal value that defines the amplitude. Based on the quantizing scale, each sampled signal is assigned a number between 0 and +127 to define its amplitude.

Step Four: Encoding In the fourth step of the A/D conversion process, the quantized samples are encoded into a digital bit stream (series of electrical pulses).

A digital encoder: Recognizes the 255 different voltage levels of the quantized samples. Converts each into a specific string of eight bits (ls and 0s) that represent a particular voltage value. Figure 3.14 is helpful for understanding the binary code used in the encoding step. Each bit position in the 8-bit word (byte) is given a decimal weight (2 to some power), except for the first bit position. The first bit position: Using this coding scheme, we can code any number between +127 and -127 and zero.

For example, if the PAM signal measures +45 on the quantizing scale, the output of the encoding step is 10101101 (Figure 3.15). This binary number (or 8 bit word) is transmitted over the network as a series of electrical or optical pulses. This series of pulses is called a digital bit stream. The PCM process requires a 64,000 bps channel to encode a 4 kHz audio input signal because: 8,000 samples/sec. X 8 bits/words = 64,000 bps This is known as the DS0 (Digital Signal 0) or VF (Voice Frequency) in the digital hierarchy. It is the basic building block of the digital network.

Digital-to-Analog Conversion At the receive end of the transmission, the digital signal may need to be converted back to its analog form. The digital-to-analog (D/A) conversion consists of two steps (Figure 3.16): Each 8-bit word (byte) that enters the decoder results in one PAM signal value. The decoder: Reads the 8-bit binary word inputs Creates a stream of 8,000 pulses per second These pulses have an amplitude value of +127 to -127. (Our example is +45.) The filtering process smooths out the stream of 8,000 pulses per second into an analog waveform that closely resembles the waveform that was input into the A/D converter at the originating end. The filter stores a part of each pulse's energy and slowly releases it until the next pulse arrives. The filter thus reconstructs the analog signal at a rate of 8,000 times per second.

Digital Channel Banks or D-Banks A device that does the analog-to-digital and digital-to-analog conversion process is called D-Bank (Figure 3.17), this is commonly used with analog switches. On the sending side: Input to the D-Bank is an analog signal. Output from a D-Bank is a digital signal in the form of a bit stream. On the receive side: Input to the D-Bank is the PCM signal. Output from the D-Bank is an analog signal.

The D-Bank has two primary functions: A/D conversion - D/A Multiplexing - Demultiplexing Time Division Multiplexing

Time division multiplexing (TDM) is a digital multiplexing technique. In TDM, a number of low rate channels are fed into a multiplexer (e.g., D Bank), which combines them into one high rate digital signal (Figure 3.18). Each of the 24 VF(voice frequency)/DS0 channels is assigned a specific time slot by the TDM(Time Division Multiplexer). Thus, TDM is a process by which several digital signals are combined onto a single path and sent sequentially. Relating this back to the PAM process: The analog signal is sampled 8000 times a second. There will be 8,000 eight-bit words transmitted per second. These words will be 1/8000 second (or 125 microseconds) apart.

In the standard North American PCM system, 24 channels are time division multiplexed together and transmitted over a common path. This common path is known as a digital carrier system and operates at the DS1 rate (1.544 Mbps). Using TDM, the 24 channels are sampled sequentially. First, channel one is sampled Then channel two is sampled, and so on. These samples are then passed on to a common quantizer/encoder where they are converted into a single bit stream. In comparison: (Figure 3.19) FDM is simultaneous. TDM is sequential. The multiplexing equipment counts the bits. 24 channels X 8 bit words = 192 bits When it reaches 192, it will add one framing bit for timing, and start over. Additionally, throughout the digital network regenerative repeaters (Figure 3.21) are used to "recreate" the original signal, thus reducing transmission impairments found in the analog network. The "Regen" also filters out noise.

Digital Hierarchy

The digital hierarchy (Figure 3.22) represents the standard rates by which digital communications are sent in North America. The basic building block of the digital hierarchy is the DS0 rate at 64 Kbps. Remember that multiplying 8-bit words by the sampling rate of 8000 times/second produces the 64,000 bps rate. With Time Division Multiplexing, multiplexing by an additional 24 time slots and including 8000 framing bits for timing information produces the 1,544,000 bps or 1.544 Mbs. DS1 is considered the beginning of high capacity digital transmission rates.

Digital Loop Carrier (DLC) DLC is the generic terminology for a pair gain system in the local loop. Like multiplexing in the interoffice environment, DLC uses a system of 24 channels tied together using time division multiplexing.

(Click here for an informative chart.) DLCs bring electronic technology to the traditional costly local loop con nections. In suburban areas local loop technology is effected by rapid growth and movement, which requires costly cable installation to serve new customers. In rural areas, these lines extend over many miles to serve relatively few people. By substituting electronics for cable, DLCs offer an economical way to serve these areas. Carrier systems increase the number of customers served by existing facilities (e.g., wire pairs).

Digital Loop Carrier Systems

(Figures 3.24 & 3.25) The four main components of a Non-Integrated Digital Loop Carrier System are: 1. Central Office Terminal (COT) Performs PCM, TDM, Protection Switching, Alarms, System Test, Tests Trunk Access, Line Concentration. Not required for an IDLC (Integrated DLC). 2. Remote Terminal (RT) Provides Baery, Ringing, Signaling, and Supervision functions to the subscriber line. Performs PCM, TDM, Protection Switching, Alarms, System Test, Subscriber Line Test Access, Line Concentration. 3.Digital Facilities (Active and Protection) 4.Support Facilities Order Wire (For maintenance talking). Subscriber Line Test System.

Match Up Review

1. The name of a path for a single type of transmission, generally referred to as the smallest subdivision of the network.

2. The fastest growing family of signals.

3. The process of transmiing two or more individual signals over a common path.

4. A decrease in signal strength during transmission from one point to another.

5. A type of multiplexing where all signals are transmitted simultaneously.

6. A gain device placed in the transmission path.

7.A signal that is discretely variable.

8. He developed a sampling theorem.

9. This is known as the basic building block of the digital network.

10. Device that does A/D and D/A conversion.

11. Aprocess by which several digital signals are combined onto a single path and sent sequentially.

12. Device used to recreate the original digital signal.

13. DS3 rate.

14. DS1 rate.

15. Brings electronic technology to the local loop.

16. The central office end of a DLC.

17. Number of channels on a T1 carrier system.

18. Type of signal used for network housekeeping and control.

19. The process that converts analog signals to a digital format.

20. The third step of the A/D conversion process.

Answers: Quantize B. PCM C. 24 D. DLC E. 1.544 Mb/s F. 44.736 Mb/s G. COT H. Channel I. Digital J. Aenuation K. Repeater L. Nyquist M. DSO N. D-Bank O. TDM P. Regenerator Q. Multiplexing R.FDM S. Supervision

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