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Chapter 1

What is Switching?

1.1 Introduction

1.2 About Telephony Data

: 1.2.1 Voice Data
: 1.2.2 Signaling Data

1.3 Making a Switch Connection

1.4 The Switch Block

1.5 Switch Connections

1.6 Switching Example

1.1 Introduction

Switching is an integral part of telecommunications. Telecommunications is the transport of voice and data in the network environment. Voice and data is routed throughout the network by switching. Switching provides the ability to take a call and connect it through a network to a voice system or to a live operator, and then connect it somewhere else again.
Computer Telephony (CT) is the merger of computer technology with the telephone services offered by public and private networks. Early telephony applications required extensive low-level programming in order to interface with proprietary hardware devices. All switching was accomplished with PBXs (Private Branch Exchanges) and central office switches.
In computer telephony systems, Multi-Vendor Integration Protocol (MVIP) minimizes software maintenance and development by providing a flexible and uniform way of integrating telephony components into a standard computer chassis. MVIP applications include voice processing, fax, speech recognition, telephone call switching and call processing, audio conferencing, data communications, and numerous telephony interfaces. Figure 1. Computer Telephony Integration and MVIP

1.2 About Telephony Data

In a typical telephone call, two types of information are exchanged: voice data and signaling data.

1.2.1 Voice Data

Voice is an analog signal. It is a continuous signal that varies in amplitude and frequency. Intelligible speech ranges in frequency from 200 Hz to 4000 Hz.
In computer telephony systems, analog voice signals are converted into digital signals. Nyquist's theorem in signal processing states that no information is lost if a signal is sampled at twice the maximum frequency. By making a digital measurement at 8000 (2 x 4000) times a second, all the information in an analog voice signal can be digitally represented.
A number with the range -4095 to 4095 (or 13 bits) can adequately represent a single speech sample. Since the ear cannot detect the softest sounds in the presence of the loudest, the 13 bits can be compressed logarithmically to eight bits. Each sample of voice data is represented in eight bits. Figure 2. Voice Data
Although a single wire can be used for each digital signal, it is more common to place a number of signals on one line using time division multiplexing (TDM).
Time division multiplexing is a method of combining data on one physical wire. This allows more conversations on fewer phone wires. Using TDM, a stream of digital data is created by sampling each conversation and interleaving the samples.
Each of the samples occupies a timeslot. One complete cycle of timeslots is called a frame. Figure 3. Time Division Multiplexing
As shown in Figure 3, each conversation is sampled, and the 8 bit sample is placed in a particular timeslot. For example, phone call 0 is placed in timeslot 0 in every frame.
In TDM, there is a start-of-frame signal. Each device connected to the data stream counts from the start-of-frame signal to determine when to place a sample in a particular timeslot or when to read data from a particular timeslot.
On a digital phone path, each second of data is divided into 8000 (2 x 4000) frames. Each frame is 125 microseconds long and is divided into 8-bit timeslots, one timeslot for each connection.
The rate at which the frame is sampled determines the number of timeslots in each frame. The sampling rate (or bus speed) is determined by the system standard. For example, a T1 system has 24 timeslots per frame; a CEPT E1 system has 32 timeslots per frame.

1.2.2 Signaling Data

Along with voice information, streams of data carry signaling information. Signaling is used to control switch connections or to indicate the status of a call.
For example, the network informs the local device when an incoming call is arriving (ringing), and the device informs the network that the call is accepted (answered or goes off-hook).
In common channel signaling (CCS), signaling information is carried on a single, separate communications path. In channel associated signaling (CAS), the signaling is carried on, or along with, the voice information.
Some examples of signaling methods are: An analog line uses high voltage ringing signals and current flow for channel associated signaling. A T1 system uses the least significant bit of every sixth sample in the voice path for channel associated signaling. A CEPT E1 system has channel associated signaling bits packed into certain timeslots. An ISDN primary rate systems uses a timeslot to carry packets of common channel signaling messages.

1.3 Making a Switch Connection


	In the earliest telephone systems, the wires from each customer's phone ended at a jack on a switchboard. A human operator would have a set of cables with plugs at each end. A connection between two customers was made by plugging the cable into each customer's jack. Connections between two switchboards were made by the operator plugging into special lines called trunks that ran between the switchboards. The first operator would ask the operator at the second switchboard to complete the call, or, perhaps, to plug into yet another trunk. When automatic switching was invented, the phone customer put a finger into a numbered hole on a dial and turned it to a stop. As the dial spun back, the phone circuit was interrupted from 1 to 10 times. These current interruption pulses drove banks of electromechanical relays that made the required connections between pairs of lines. Later more sophisticated switching systems were introduced that worked on pairs of audio tones instead of dial pulses.