Physical Connections

Lines and phones can be connected in a variety of ways to the desktop computer and the telephone network. The following examples show a selection of configurations that could be supported by a service provider. Some of the telephone hardware required to implement some of these examples is not yet widely available.

A phone-centric connection consists of a single POTS line in which the computer is connected to the switch through the desktop phone set. Such phone sets typically connect to the computer through one of its serial ports. When an application requests an action, the corresponding service provider sends telephony commands, which are often based on the Hayes AT command set (ANSI/TIA/EIA-602), over the serial connection to the telephone. This configuration is limited because it generally provides only line control. The computer does not have access to the media stream.

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

A BRI-ISDN connection is similar to the computer-centric connection but allows for using the two B-channels in a variety of line configurations. A service provider can treat this connection in a number of ways:

As a single line device with a pool of two channels, allowing both channels to be combined for establishing 128 Kbps calls.
As two separate line devices, each with exclusive use of a single B-channel.
As two separate line devices, each drawing up to two channels from a shared pool of two B-channels.
As three line devices: one for each of the two B-channels and one for the combination.

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

In client/server networks, a pool of telephone ports attached to a server may be shared among multiple client computers using a local area network. The ports may be configured to assign a maximum number of line devices (the quota) to each client workstation. It is not unusual for the sum of all quotas to exceed the total number of lines.

Also, the assignment of lines through ports is dynamic. For example, a client computer with a quota of 2 may use ports 1 and 2 at one time and ports 7 and 11 at a later time.

The service provider for the pool may model this arrangement by giving each client workstation access to two line devices. This implies that the device identifiers (which are fixed) for each client are 0 and 1. If the application later requests information for device 0 and again for device 1, it must assume that the device capabilities for each device are constant, because that is the Windows device model. For server-based devices that are pooled as described in the example above, this constancy holds only for line devices that have identical device capabilities.

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 could model the PBX line associated with the computer (on a desktop) as a single line device with one channel. Each client computer would have 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 may be satisfactory if only a small number of lines is of interest, but may 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 is to 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 line opening operations are required. However, this modeling implies that all stations have the same line-device capabilities, although their address capabilities could be different.

A potential advantage of this model is a lowered cost per computer if the LAN is already in use, but a limitation would be 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.

In a shared telephony connection, the computer's connection may 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.