Switching Fabric

The foundation of any telephone systems is its switching fabric implementation. A telephony switching fabric consists of the cabling, networking technology, controllers, and protocols used to communicate the voice (and other media) plus control information associated with telephone calls.

Telephony switching fabric implementations consist of two key aspects:

• Telephony switching network

The telephony switching network is a logical network made up of all the telephone system components that transmit, receive, or interconnect media and/or signaling information and the components that manage the switching fabric.

• Transmission networks

Transmission networks are the underlying communication networks and links that provide the connectivity between components of the telephony switching network.

The switching fabric implementations in conventional telephone systems typically involve proprietary media stream switching hardware with connections for analog (POTS) and digital (T-1, ISDN, proprietary) transmission facilities. The closed nature of these implementations limits the number of transmission networks that are directly supported and limits the flexibility or functionality provided by the telephony switching network.

In the field of switching fabric development, computer telephony involves using off-the-shelf computer bus technologies as well as software protocol stacks as the basis for a new

generation of transmission networks, media stream interconnection components, open media transport, signaling, and control protocols. For example, IP Telephony and other packet-based telephony switching fabrics deliver voice over IP networking infrastructures.

1.7 CTI

While computer telephony refers to the whole field of new generation telephone system

implementations that draw on off-the-shelf computer technology, the term CTI has evolved to refer to just the call processing portion of computer telephony^1-2. Specifically, CTI refers to the technologies used to build systems where one or more software applications running on computer platforms are integrated with the call processing functionality of a telephone system.

In a telephone system that is based entirely on computer telephony technology, CTI and call processing are effectively synonymous.

CTI: Bridging Computing and Telephony

Until recently, the world of telephony technology and the world of computer technology have remained largely isolated from one another. CTI refers to the ability to combine the products, services, and systems from these different technology areas, but it does not refer to a superset of computer and telephony technologies. CTI technology is the bridge between these

technology areas (Figure 1-8). CTI permits the development of solutions that further enhance traditional applications of these individual technologies.

Defining CTI

The definition of CTI has remained vague for some time, but with the maturation of CTI technology a very concise definition of this term has emerged.

1-2 CT vs. CTI — The terms CT and CTI are often confused as the distinction is not literal. The term CTI came to refer to the use of computer technology to access the call processing functionality of a telephone system because historically telephone systems were considered closed systems to which external computer equipment was integrated. As CTI products are arguably the most functionally rich computer telephony products, this term continues to be used to distinguish call processing software and hardware from other computer telephony technologies.

Figure 1-8

CTI brings together the worlds of computer and telephony technology

CTI involves three aspects:

• Call Control

Call control is the ability to observe and control telephone calls, switching features and status, and call routing facilities, and to use switching resources that include tone

generators and detectors.

• Telephone Control

Telephone control is the ability to observe and control physical telephone devices as computer peripherals.

• Media Binding

Media binding involves binding telephone calls to other media services such as voice processing, fax processing, videoconferencing, and telecommunications.

CTI technology achieves functionality in each of these areas by interacting with telephony resources that exist within CTI-accessible telephone systems. A given CTI product may provide functionality in one, two, or all three of these areas.

1.7.1 Call Control

The most essential part of CTI is the aspect that deals with a computer observing and controlling telephone calls and the telephone system features that interact with calls.

Observing means tracking all call processing activity that takes place, and being aware of any changes to feature settings. Controlling refers to issuing instructions for the telephone system to obey with respect to telephone calls, features, and associated telephony resources. Call control functionality supported through a CTI interface includes both the telephony functionality available to users of a telephone system through telephone sets, as well as functionality that applies only to computer observation and control.

1.7.2 Telephone Control

Telephones are the most ubiquitous appliance in the world. Today they outnumber all

computer keyboards, mice, printers, and other computer peripherals combined. They come in myriad different forms, from sophisticated office sets, to home sets, to pay phones, to small wireless telephones. As such, they represent an exciting new category of computer peripheral.

This aspect of CTI involves the ability, of an appropriately enabled computer, to observe the activity of a telephone set (which buttons are being pressed, what the display says, whether the message indicator is flashing, etc.), and to control the telephone set (updating the display, simulating the press of buttons, turning on lamps, etc.).

1.7.3 Media Binding

The third aspect of CTI is media binding. This also tends to be the aspect of CTI that is subject to the most confusion. Media binding in CTI refers to the ability of a computer to get access to the media stream associated with a particular telephone call (that can be specifically observed and controlled through call control). Computer technologies that manipulate the media stream once it is accessed are part of media services (see section 1.8) and not part of CTI. For

example, many computers support fax software that can interpret a fax document and display it on the screen, or transform an electronic document into a form that can be sent using a fax modem. This capability is not a CTI capability. In this example, CTI comes into the picture when the time comes to place (or answer) a call that is to be used to send (or receive) a fax.

When the call is established, media binding functionality is used to "bind" or associate the call with available fax modem functionality so that the computer's fax software can take control of the fax modem and send (or receive) the fax.^1-3 Likewise, given the availability of appropriate media binding resources in the telephony products and the corresponding software in the computer, sound streams from the call can be recorded by the computer, sound can be played down the phone line, speech recognition and text-to-speech functionality can be applied, and digital data can be exchanged.