Signaling and Switching

Chapter 6

Objectives

In this chapter, you will learn to: Define modulation and explain its four basic versions Explain the different types of multiplexing techniques,

their benefits, and hardware requirements Discuss the fundamental types of switching used in voice

and data communications Identify the differences between local and tandem

switching Describe modern signaling techniques used to route calls

to their destination

Modulation

A technique for processing signals in which two waves are combined to produce a wave that possesses characteristics of both and can be decoded to separate these characteristics.

Information wave - the wave containing the signal you want to transmit.

Carrier wave - the wave that is modulated, and whose properties are constant and known to both the sender and receiver.

Guided waves - waves that are bound to a carrier wave’s frequency.

Modulation

Modulator - the device that imposes the information signal on the carrier signal at the transmission end.

Demodulator - separates the information from the carrier signal at the receiving end.

Modem - a device used to convert digital into analog signals and analog into digital signals.

Modulation

Amplitude Modulation

Amplitude Modulation

Frequency Modulation

A method of modulation in which the frequency of the carrier signal is modified by the addition of the information signal.

Frequency shift keying (FSK) - when FM signals are converted into digital signals, the differing frequencies are conveyed as either 0s or 1s.

Frequency Modulation

Frequency Modulation

Phase Modulation

Phase Modulation

Phase Modulation

Pulse Code Modulation (PCM)

The process of converting analog signals into digital signals

Sampling - the first step in converting analog signals to digital signals is to measure the amplitude of an analog signal at multiple instants.

The higher the sampling rate, the more closely the digital signal resembles the original analog signal.

Pulse Code Modulation (PCM)

Quantizing

To create a replica of the audio wave in digital format, each sample is converted into its binary equivalent in a process called quantizing.

One problem with quantizing is that it introduces noise. This occurs because a signal composed of quantized values is not as precise as the original analog signal, which is made of one continuous wave.

Companding - This process of compressing and expanding a signal.

Quantizing

Quantizing

Frequency Division Multiplexing (FDM)

Frequency division multiplexing (FDM) - a method of sending multiple analog signals simultaneously over one channel by separating the channel into subchannels, or bands.

Guardband - a narrow range of unused frequency between each band’s frequency. It ensures that signals do not interfere with each other.

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Time Division Multiplexing (TDM)

Statistical Multiplexing

Wavelength Division Multiplexing

Switching

A method of establishing connections and sending information between nodes on a network.

Circuit switching - establishing a connection between two devices on a network, before they begin transmitting data.

The advantage of circuit switching is that it guarantees two devices exclusive use of a communications channel.

Switching

Message Switching

Packet Switching

Switching Systems

The combined collection of hardware and software that establishes connections between lines and trunks in order to complete calls.

Although makes and models differ between manufacturers, all modern switches share four essential elements:

• Switching matrix • Line or trunk circuits • Central control computer • Common equipment

Local Switching Systems

A local switch performs the following functions:

Provides dial tone to a local subscriber Accepts and interprets signals (including off-hook

notification, dial tones, and so on) from the local subscriber Receives signals from the destination’s local switch about

when to terminate the call Records local subscriber billing information Stores information about subscribers (such as what type of

service they have chosen) in a subscriber database Tests and maintains the subscriber’s local loop

Switching an Intra-Office Call

Tandem Switching

A tandem switch may perform the following functions: Provide termination for trunks at Class 1, 2, 3, and 4 central

offices Gather and transmit information about telephone network

traffic and congestion Determine the fastest path over the PSTN for long-distance

calls Carry data and voice signals between central offices Test and maintain trunks Assist in trunk configuration

Tandem Switching

Lucent’s ESS Switch

Nortel’s DMS Switch

Nortel’s DMS Switch

Switch Signaling

The exchange of information between the components of a telephone network or system for the purposes of establishing, monitoring, or releasing phone circuits as well as controlling system operations.

Switch-signaling functions include:• Transmitting Address Information • Supervising• Transmitting Information

Subscriber Loop Signaling

The exchange of information about a telephone circuit over the local loop.

The primary types of supervisory signals include: Idle circuit Busy circuit Seizure Disconnect

Common Channel Signaling (CCS)

Signaling System No. Seven (SS7)

Summary

Modulation is a signal-processing technique in which an information wave is imposed on (combined with) a carrier wave to create a unique wave pattern.

Common methods of multiplexing include frequency division multiplexing (FDM), time division multiplexing (TDM), statistical multiplexing, and wave division multiplexing (WDM).

Switching is a method of establishing connections and sending information between nodes on a network.