Dimensionado de conductos

The terms telephony and telecommunications are frequently confused. Traditionally these words have been used to differentiate between voice telephone networks used for telephony, and other networks (including those based on telephone networks) that are used to move data between computer systems (telecommunications). Over time, the transmission facilities of the telephone network have evolved into a powerful digital communications infrastructure in which voice is but one type of data. This has led to some confusion because one can build a telephone network on top of a telecommunications network that supports telephony and one can build a telephony-specific network that supports telecommunications. Telephony has more stringent requirements, however, and its functionality is generally a superset of telecommunications functionality; you can always layer telecommunications on top of a telephony infrastructure, but you can't always do the reverse.

3.2.5 Telephone Networks: Inside the Cloud

In Figures 3-3, and 3-4, a cloud was used to represent the telephone network in which a call existed. These clouds3-5 _{represent all the telephony resources being used to establish a call.}

Telephony Resources in a Telephone Network

A telephone network is made up of a ''web" of interconnected sets of telephony resources. To establish a call between two devices, one or more sets of telephony resources are used to patch together a complete

3-5 _{Clouds — The cloud symbol has become the standard graphical representation of a switching}

fabric or network. It is a convenient abstraction because one need not be concerned with how large or small the network is, or what portion of the resources it represents are consumed in taking care of a given call. If the network is operational, all the endpoints on a call are appropriately connected. Without this ability to work with a simple abstraction of the overall network, even the simplest operations would be quite complicated to explain. It has been said that telephone companies "are in the cloud business."

end-to-end call across the network. Figure 3-8 shows a call that has been established across a telephone network between the devices labeled D1 and D2. The network "cloud" is used to abstract the network; the call between the two devices then is easily abstracted.

Figure 3-8

Call between D1 and D2

Figure 3-9 shows the same scenario, but instead of abstracting the network as a cloud, we see the actual system of individual sets of telephony resources (represented as hexagons3-6_{) that}

make up the network. The call between devices D1 and D2 is actually a series of calls. Each participating set of telephony resources establishes a different segment of the overall path that the call takes through the network. Each segment of the complete call is actually an individual call made up of connections with two devices, just as in the overall abstraction involving the entire network.

3.3 Fundamental Objects

In the telephony resource set model, the call processing and switching resources carry out the work of the telephone system by acting on three fundamental types of objects: calls,

connections and devices. (A fourth type of object, agents, is related to a specialized type of device, the ACD Group, and is covered in Chapter 4.) As we shall see, these objects are fundamental because they reflect virtually everything that is of interest to the user of a given telephone system.

3-6 _{System boundaries — The shaded hexagon is used throughout this book to symbolize the}

boundaries of a particular telephone system.

Figure 3-9

The network of telephony resources inside the cloud

3.3.1 Calls

The abstraction of telephone calls (which are referred to as simply calls from here on) consists of a media stream and associated signaling information, as shown in Figure 3-10. Keep in mind that this is just an abstraction; in Chapter 8 we'll see how this abstraction relates to different implementations.

Figure 3-10

Telephone call abstraction

Call refers to both a media stream that is established between endpoints in a telephone network and all associated control information.

In understanding telephone systems we are concerned with both the control information and with managing the media streams.

3.3.2 Devices

Another important element in the abstraction of a call are the endpoints of the media stream.

An endpoint to which a telephone network is able to connect calls is a device. In the preceding illustrations, a simple telephone is shown as the device at one end of a call. (Devices are the subject of Chapter 4 where we will look at specific types of devices and further explore the device abstraction3-7_.)

A functional telephone call involves two or more devices as endpoints to the media stream associated with the telephone call. At certain points in the life of a call, however, such as when the call is being originated or cleared, it may have only one device or no devices at all.

3.3.3 Connections

The relationship between a particular device and a particular call is referred to as a connection. If two devices are involved in a call, then in the abstract representation of that call there are two connections, and each connection corresponds to one of the devices3-8_.

As we shall see in Chapters 5 and 6, connections are the most used objects within this abstraction because they allow efficient manipulation of both a call and a device simultaneously.

3-7 _{Device configurations — Devices are further subdivided into elements that are arranged}

according to a device configuration for the individual device. There are two types of device

elements, physical elements and logical elements. Unless otherwise stated, all references to devices are to logical device elements. Logical device elements and device configurations are explained in sections 4.4 and 4.5. Physical device elements are explained in section 4.1.

3-8 _{Call appearances — Devices have call appearances that correspond to connections. A device with}

multiple call appearances can be associated with multiple calls simultaneously. Call appearances are described in section 4.4.1.

Voice Connections

By default, references to connections represent media streams that are compatible with the

voice network. This means that they carry voice or modulated data (see the sidebar

"Modulated Data"). A voice call is therefore a call made up of voice connections. A rule of thumb is that if a piece of analog telephony equipment can be added as a device on the call, the call is considered a voice call.3-9 _{A voice connection allocates, at most, a single media}

stream channel.

Digital Data Connections

Some switching implementations are able to create connections that are associated with digital data media streams. Digital data media streams support data traveling at much higher rates than are possible with modulated data because they take advantage of the digital switching capability in a digital data network. These digital data calls (calls made up of digital data connections) are treated specially by the switching implementation, however, because they cannot interoperate directly with voice calls (calls made up of voice connections) and only a subset of switching functionality applies to them. A digital data connection may be associated with any number of media stream channels.

Quality of Service

The nature of a call or connection (i.e., whether it is voice or digital data) and attributes relating to digital data rates, the number of media stream channels used, and transmission characteristics, are referred to as quality of service or QoS. The quality of service applicable to a new call or connection is limited by what a given switching implementation supports. The quality of service associated with an existing call or connection determines the switching services that may be applied to it. (Quality of service will be discussed in greater detail in Chapter 8.)

3-9 _{Voice call — Technically speaking, a voice call is one that carries media streams requiring no}

more than 3.1 kHz of bandwidth.