Models

Service providers can support different models of the ways lines and phones are handled, based on how the devices are physically configured. This topic describes a selection of configurations that a service provider might support.

The computer-centric model uses a computer add-in card or an external add-on box that is connected to both the telephone network switch and the phone set. This model can easily integrate modem and fax functions, as well as the use of the telephone as an audio I/O device.

The phone-centric model connects the computer to the switch through the desktop phone set. Such phone sets typically connect to the computer through one of its serial ports. The service provider converts requests into telephony commands, such as commands based on the Hayes AT command set (ANSI/TIA/EIA-602), and sends them over the serial connection to the telephone. This configuration is limited because it generally provides only line control. The computer has no access to the media stream.

A single BRI-ISDN (Basic Rate Interface-ISDN) line could fit both the phone-centric and computer-centric models. The service provider can treat this capability in a number of ways:

• Model the BRI line as a single line device with a pool of two B channels, allowing both channels to be combined for establishing 128 Kbps calls.
• Model each B-channel as a separate line device and disallow the combination of the two channels into a single 128 Kbps channel.
• Model the BRI connection as two separate line devices, each drawing up to two channels from a shared pool of two B-channels.
• Model as three line devices: one for each of the two B-channels and one for the combination.

In the latter two models, channels can be assigned to different line devices at different times.

A LAN-based server might have multiple telephone line connections to the switch. TAPI operations invoked at any of the client computers are forwarded over the LAN to the server. The server uses third-party call control between the server and the switch to implement the client's call control requests.

This model offers a lower cost per computer for call control if the LAN is already in use, and it also offers reduced cost for media stream access if shared devices such as voice digitizers, fax and or data modems, and interactive voice response cards are installed in the server. The digitized media streams can be carried over the LAN, although real-time transfer of media may be problematic with some LAN technologies due to inconsistent throughput.

A LAN-based host can be connected to the switch using a switch-to-host link. TAPI operations invoked at any of the client computers are forwarded over the LAN to the host, which uses a third-party switch-to-host link protocol to implement the client's call control requests.

It is also possible for a private branch exchange (PBX) to be directly connected to the LAN, and for the server functions to be integrated into the PBX. Within this model, different subconfigurations are possible:

• To provide personal telephony to each desktop, the service provider can model the PBX line associated with the computer (on a desktop) as a single line device with one channel. Each client computer has one line device available.
• Each third-party station can be modeled as a separate line device to allow applications to control calls on other stations. (In a PBX, a station is anything to which a wire leads from the PBX). This enables the application to control calls on other stations. This solution requires that the application open each line it wants to manipulate or monitor, which is satisfactory if only a small number of lines is of interest, but can generate excessive overhead if a large number of lines is involved.
• Model the set of all third-party stations as a single line device with one address (one phone number) assigned to it per station. Only a single device can be opened, providing monitoring and control of all addresses (all stations) on the line. To originate a call on any of these stations, the application must only specify the station's address to the function that makes the call. No extra lineOpen operations are required. However, this modeling implies that all stations have the same line-device capabilities, although their address capabilities can be different.

A potential advantage of this model is a lowered cost per computer if the LAN is already in use, but a limitation is a possible lack of media-stream access by the computers.

The computer in use need not be a desktop computer. It can also be a laptop or other portable computer connected to the telephone network over a wireless connection.

The computer's connection can be shared by other telephony equipment. For an application to operate properly in this arrangement, neither the application nor the service provider can assume that there are no other active devices on the line.