Modeling Telephone Networks

Each individual telephony product is a telephone system, that is, a set of telephony resources.

Each telephone system generally is a subsystem of a larger telephone system, and virtually all telephone systems are ultimately connected to, and thus are part of, the worldwide telephone network (the PSTN). As a result, one can model any portion of the PSTN or any private network in the same way that an individual switch or telephone station can be modeled.

In Figure 3-34 a generic telephone network, a LEC for example, is being modeled as a set of telephony resources. Though the network is actually made up of multiple switches, it can still be modeled as a single set of resources because the network operates as a single entity. The network interface devices in the network represent its connections to various IXCs and the station devices correspond to subscriber lines.

Figure 3-37

A LEC's telephone network

3.9 Review

In this chapter we have seen that a telephone system may range in scope from an individual telephone to a vast telephone network. Telephone systems represent a collection of telephony resources that also may be referred to as a telephony resource set. Types of telephony

resources include call processing, switching, interfaces, devices, and dynamic objects.

Calls are dynamic objects in a telephone system that represent a media stream and associated control information traveling between two or more points.

Devices are the endpoints that can be associated with calls. Devices are resources responsible for consuming and generating the media streams and control information associated with calls.

Connections define the relationship between a particular call and a particular device. The most important attribute of a connection is its connection state, which may be null, initiated,

alerting, connected, hold, queued, or fail.

Media stream channels are allocated for connections as needed and are responsible for conveying the associated media streams.

Switching resources are responsible for carrying out services that create, clear, and manipulate the states of connections, and for allocating and deallocating media stream channels as needed.

These switching services can be illustrated through a simple graphical notation that shows the devices, calls, connections, and connection state transitions involved.

Call processing is primarily responsible for routing or directing calls through the telephone system by managing the switching resources. It takes action based on feature settings, timers, default rules, and commands received from devices and through telephone system interfaces.

Certain types of devices exist within a telephone system in order to provide specialized routing functionality.

Now that an abstraction of the basic telephony resources has been defined along with the graphical notation for describing them, we can delve into more of the details surrounding device modeling.

Chapter 4

Telephony Devices

Devices are the endpoints of telephone call media streams. They are a principal resource in the telephony resource framework because the rest of the telephony resources are in place to serve them.

So far we have seen how switching operations revolve around associating calls with devices through connections and manipulating the states of these connections. The switching

fabric—the implementation of the switching resources used in a given telephone

system—maps these connection state manipulations into actions. Media stream channels in appropriate transmission facilities are allocated and deallocated in order to establish calls between devices.

In this chapter we'll look more closely at the variety of different types of telephone system devices and how they are modeled and used. Telephone system devices have logical elements and associated components and attributes that determine their functionality and the operations that can be performed with them. Telephone stations also have physical elements and a variety of related physical components and attributes.

4.1 Telephone Stations

Station devices are the telephony resources that correspond to the tangible telephones and telephone lines with which we are all familiar. Station devices take an almost unlimited

number of different forms. There are literally hundreds of telephone vendors around the world, each of which manufactures many different models of telephones. Each telephone is designed to appeal to the particular needs and preferences of a different type of telephone user.

Despite the fact that at the most simple level all these telephone devices provide the same basic functionality, there is an endless number of variations in the form that the physical user interface to a telephone system can take. Research has demonstrated, and the marketplace has validated, that people have very diverse and particular preferences when it comes to the form that their interface to telephony services should take.^4-1

It is important to note that from the perspective of telephony concepts, every type of telephone is a station device regardless of its functionality, interface to the telephone network, or other properties. This includes POTS (plain old telephone service) telephones (''500" and "2500"^4-2 telephone sets) and multi-line telephones. It includes wireless phones such as cordless and cellular phones, and coin and card pay phones. It includes novelty phones from "football

phones" through "shoe phones." It includes full-featured digital phones and attendant consoles, and hybrid devices such as fax phones and video phones.

4-1 Diversity of telephony user interfaces — The fact that user preferences are so diverse when it comes to a personalized interface to telephony functionality (traditionally the telephone) plays a very important role in both the motivation for CTI and the architecture of CTI implementations.

4-2 500 and 2500 telephone sets — AT&T assigned the model number 500 to the old rotary-dial deskset telephones. When touchtone was developed, the touchtone model was numbered 2500. Since then, the term 2500 set has come to mean any telephone that is functionally equivalent to the original model 2500 telephone set.