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Integrated Voice/Data (IVD) Terminals

These terminals combine a highly functional feature phone with a computer workstation. For the most part, such terminals have been stunning failures, because they are expensive and impractical, and as generally are behind the curve of computer technology. More commonly, digital telephone sets offer an EIA-232-E standard, 25-pin computer connection port in order to allow a computer workstation and telephone set to share a common, twisted-pair link to the PBX. (EIA-232E also is known as RS-232 referring to Recommended Standard 232, which is identical in function to ITU-T standard V.24/V.28)

ISDN Sets

These sets provide a highly-functional ISDN connection to the desktop, through an ISDN-compatible PBX, connected to an ISDN circuit.

Attendant Consoles

These provide attendants with the ability to answer and extend incoming calls, provide operator assistance to outgoing callers, and establish conference calls. They often provide alarm indications in the event of a performance failure. When associated with a computer workstation, they often provide electronic directory information and other functionality through an enhanced user interface.

Administrative and Maintenance Terminals

These terminals are connected directly to a maintenance port on the system. Authorized users can access the system software for purposes including Moves, Adds, and Changes (MACs); Class of Service (COS) changes; ARS/LCR programming; requests for traffic and usage statistics; requests for status reports; and diagnostic testing and analysis.

System Configuration and Capacity

The capacity of the PBX must be considered carefully in order to ensure that the organization does not outgrow the system over a reasonable period of time. The system initially should be configured and installed to serve current and near-term requirements, as subsequent software and hardware additions are more costly. Additionally, the system should have the ability to grow to meet long-term needs, in terms of hardware, software and feature content. The following describes the capacity and configuration of PBXs.

Centralized or Distributed

PBXs generally are configured as a centralized system, with all cabinets and peripheral devices collocated for ease of administration. However, a large campus environment may best be served by a decentralized approach, with multiple intelligent cabinets located in campus quadrants and connected with high-capacity circuits. Very large organizations with multiple, distant locations may require the capability to network multiple PBXs over either a private, Tie line network or over a Virtual Private Network (VPN) [4-7].

Physical Capacity

Physical Capacity is the measure of the number of lines and trunks that can be supported through additional line and trunk cards and additional cabinets. Every system has a finite limitation to the number of cabinets and ports that it will support.

Traffic Capacity

Traffic Capacity is the measure of the number of simultaneous conversations that can be supported. This measurement is critical, especially when both voice and data are switched. The capacities of both the processors and the busses must be considered.

Blocking switches (see Figure 4.5), which are typical in a voice-only application, are engineered is such a way as to support a reasonable level of traffic, with some call attempts being blocked during periods of high activity. As the traffic characteristics of voice traffic are well understood and as voice is not particularly bandwidth-intensive, system capacity can be optimized to provide a high level of availability (grade of service) during even the peak busy hour, and at a reasonable cost.


Figure 4.5  Blocking PBX.

Nonblocking switches (Figure 4.6) provide a nonblocked (guaranteed) talk path for every terminal. There exists a 1:1 relationship between (line) ports and time slots. Especially critical in voice/data applications, this admittedly expensive approach ensures access to critical host computing resources within the organization. Access to trunks rarely is provided on a nonblocking basis, as such a high level of trunking adds significant cost in the form of trunk interfaces and the trunks, themselves.


Figure 4.6  Nonblocking switch.

Application Processors

Application Processors often are interfaced with PBXs over high-speed links in order to provide specialized functions such as voice processing, computer host access, e-mail, and facsimile. Generally speaking, the PBX manufacturer supports interfaces to either proprietary application processors or to those developed by third-parties with which there is a strong, strategic relationship. While this approach is limiting, it ensures that the systems and the interfaces between them is highly functional and remains so. The ability of the PBX to support such processors is of critical importance, as they add great value to the performance of the organization.


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