CCNA Voice Summary – Part 3

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Digital Connectivity

Problems with Analog connectivity:

  • Distance limitation – regeneration of the signal also leads to regeneration of the white noise
  • Wiring requirements – Analog requires a tip and ring wire for each call – that makes it 2 wires for each call.

These problems are overcome by digital signals. Digital signals converts the signals into zeros and ones. You can therefore go over any distance and can send the digital strings over one wire

Digitizing Voice

This process follows the Nyguist Theory which states – if you can sample over twice the highest frequency, you can accurately reconstruct a signal digitally. Nyguist theory samples between 300 – 4000 Hz. Human speech ranges between 200 – 9000 Hz. The Human ear can hear between 20 – 20000 Hz.

After sampling the signal. We perform Quantization of the sample. It involves taking the value of the amplitude of the analog signal and line it up to a specific value. This process is known as the Pulse Amplitude Modulation. Using PAM we take more samples at the lower levels since most of the human voice is close to the zero scale.

After Quantization comes Coding – change the value to binary numbers. This is referred to as Pulse Code Modulation PCM. There are 2 PCM methods (a-law and mu- law). Mu-law is used in the USA, Japan and Canada.

A-law is used in the rest of the world. It uses the first code value to represent positive or negative value. 0 – negative; 1- positive. Next 3 binary digits represent the segments. More samples are taken at the lower segments.. The next 4 values is the interval between the values in a given segment.

Mu-law; 0 – positive; 1 – negative. It Is the direct opposite of a-law.

To communicate between the two laws, we need to convert between the two standards.

After coding, we can optionally compress the samples. 8000 samples. Each sample is converted into 8 bits = 64000 bits per second per voice call. Compression saves on bandwidth.

During compression, you can choose to send just the changes between the sample or build a code book for each sample. G.729 consumes 8 kbps

Digital Voice Circuits

Carries information in channels: Bearer and Data Channels. Bearer – carries the voice sample, Data Channel – call control and signaling.

Types of Digital Circuits: Channel Associated Signaling (CAS) and Common Channel Signaling (CCS). CAS – voice and data are transported in the same channel (Robbed Bit signaling). CAS example T1/E1. CCS – the only thing in the bearer channel is the actual voice. there is a dedicated channel for signaling. CCS example T1 OR E1 ISDN PRI/BRI.

TDM is used in all circuits. Voice is sampled then divided and interwoven together.

BRI – 2bearer channels + 1data



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