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The Digital Private Branch Exchange

Evolution of the Digital PBX

The digital PBX is a marriage of two technologies: digital switching and telephone exchange systems. The fore-runner of the digital PBX is the private branch exchange (PBX). A PBX is an on-premise facility, owned or leased by an organization, which interconnects the telephones within the facility and provides access to the public telephone system. Typically, a telephone user on the premises dials a three or four-digit number to call another telephone on the premises, and dials one digit (usually 8 or 9) to get a dial tone for an outside line, which allows the caller to dial a number in the same fashion as a residential user.

The original private exchanges were manual, with one or more operators at a switchboard required to make all connections.

In the 1920s these began to be replaced by automatic systems, called private automatic branch exchanges (PABXs), which did not require attendant intervention to place a call. These first-generation systems used electro-mechanical technology and analog signaling. Data connections could be made via modems. That is, a user with a terminal, a telephone, and a modem or acoustic coupler in the office could dial up an on-site or remote number that reached another modem and exchange data.

The second-generation PBXs were introduced in the mid 1970s. These systems use electronic rather than electro-mechanical technology and the internal switching is digital. Such a system is referred to as digital PBX, or computerized branch exchange (CBX). These systems were designed primarily to handle analog voice traffic, with the codec function built into the switch so that digital switching could be used internally. The systems were also capable of handling digital data connections without the need of a modem.

The third-generation systems are known as integrated voice/ data systems. Perhaps a better term is improved digital PBX. Some of the characteristics of these systems that differ from those of earlier systems include.

  • The use of digital phones - This permits integrated voice/ data workstations.
  • Distributed architecture - Multiple switches in a hierarchical or meshed configuration with distributed intelligence provides enhanced reliability.
  • Non blocking configuration - Typically, dedicated port assignments are used for all attached phones and devices.

As new features and technologies are employed, incremental improvements make difficult the continuing classification of PBXs into generations. Nevertheless, it is worth noting the subsequent advances in PBX products that, together, might be considered to constitute a fourth generation.

  • Integrated LAN link - This capability provides a direct high-speed link to a local area network.
  • Dynamic bandwidth allocation - Typically, a PBX offers one or only a small number of different data rate services. The increased sophistication of capacity allocation within the PBX allows it to offer virtually any data rate to an attached device. This allows the system to grow as user requirements grow. For example, full-motion color video at 448 kbps or advanced codecs at 32 kbps could be accommodated.
  • Integrated packet channel - This allows the PBX to provide access to an X.25 packet-switched network.

It is worthwhile to summarize the main reasons why the evolution described above has taken place. To the untrained eye, analog and digital PBXs would seem to offer about the same level of convenience. The analog PBX can handle telephone sets directly and uses modems to accommodate digital data devices; the digital PBX can handle digital data devices directly and uses codecs to accommodate telephone sets. Some of the advantages of the digital approach are

  • Digital technology: By handling all internal signals digitally, the digital PBX can take advantage of low-cost LSI and VLSI components. Digital technology also lends itself more readily to software and firmware control.
  • Time-division multiplexing: Digital signals lend themselves readily to TDM techniques, which provide efficient use of internal data paths, access to public TDM carriers, and TDM switching techniques, which are very cost effective.
  • Digital control signals: Control signals are inherently digital and can easily be integrated into a digital transmissions path via TDM. The signaling equipment is independent of the transmission medium.
  • Encryption: This is more easily accommodated with digital signals.

Telephone Call Processing Requirements

The characteristic that distinguishes the digital PBX from a digital data switch is its ability to handle telephone connections. There are eight functions required for telephone call processing.

  • Interconnection
  • Control
  • Attending
  • Busy testing
  • Alerting
  • Information receiving
  • Information transmitting
  • Supervisory

The interconnection function can be for 3 types of situations:

  1. A call originated by a station bound for another station on the digital PBX.

  2. A call originated by a digital PBX station bound for an external recipient. For this, the PBX must not only have access to an external trunk, but must perform internal switching to route the call from the user station to the trunk interface. The PBX also performs a line to trunk concentration function to avoid the expense of one external line per station.

  3. A call originated externally bound for a PBX station. Referred to as direct inward dialing, this allows an external caller to use the unique phone number of a PBX station to establish a call without going through an operator. This requires trunk-to-line expansion plus internal switching.

The control function includes the logic for setting up and tearing down a connection path. In addition, the control function serves to activate and control all other functions and to provide various management and utility services, such as logging, accounting, and configuration control.

The PBX must recognize a request for a connection; this is the attending function. The PBX then determines if the called party is available (busy testing) and, if so, alerts that party (alerting). The process of setting up the connection involves an exchange of information (i.e., information receiving and information transmitting) between the PBX and the called and calling parties.

Finally, a supervisory function is needed to determine when a call is completed and the connection may be released, freeing the switching capacity and the two parties for future connections.

Let us look more closely at the sequence of events required to complete a call successfully. Consider an internal call from extension 226 to extension 280. The following steps occur:

  1. 226 goes off-hook (picks up the receiver). The control unit recognizes this condition.
  2. The control unit finds an available digit receiver and sets up a circuit from 226 to the digit receiver. The control unit also sets up a circuit from a dial-tone generator to 226.
  3. When the first digit is dialed, the dial-tone connection is released. The digit receiver accumulates dialed digits (280).
  4. After the last digit is dialed, the connection to the digit receiver is released. The control unit examines the number for legitimacy. If it is not, the caller is informed by means such as connection to a rapid busy signal generator. Otherwise, the control unit then determines if 280 is busy. If so, 226 is connected to a busy-signal generator.
  5. If 280 is free, the control unit sets up a connection between 226 and a ring-back-tone generator and a connection between 280 and a ringer. A connection path between 226 and 280 is reserved.
  6. When 280 answers by going off-hook, the ringing and ring-back connections are dropped and a connection is set up between 226 and 280.
  7. When either 280 and 226 goes on-hook, the connection between them is dropped

In a similar way outgoing and incoming calls can be handled.

Digital PBX Architecture

A variety of architecture have been developed by digital PBX manufacturers. Since these are proprietary, the details are not generally known in most cases. Here, we attempt to look at the general architectural features common to all PBX systems.

The figure below presents a generic PBX architecture.

The heart of the system is some kind of digital switching network. The switch is responsible for the manipulation and switching of digital signal streams. Many of the PBXs are not sufficiently large, in terms of lines or capacity, to require complex switching networks. Indeed, some have no network as such, but simply use a TDM bus switch.

Attached to the switch are a set of interface units, which provide access to/ from the outside world. Typically, an interface unit will perform a synchronous TDM function in order to accommodate multiple incoming lines.

Other boxes in the figure can be explained briefly. The control unit operates the digital switch and exchanges control signals with attached devices. For this purpose a separate bus or other data path may be used or the control signals may propagate through the switch itself. Service units would include such things as tone and busy-signal generators and dialed-digit registers. The PBX systems provide protocol converters for connecting dissimilar lines. A connection is made from each line to the protocol converter.

It should be noted that this generic architecture lends itself to a high degree of reliability. The failure of any interface unit means the loss of only a small number of lines. Key elements such as the control unit can be installed with sufficient redundancy for achieving reliability.

Distributed architecture

For reasons of efficiency and reliability many PBX manufacturers offer distributed architectures for their larger systems. The PBX is organized into a central switch and one or more distributed modules, with twisted pair, co-axial cable, or fiber optic cable between the central switch and the modules, in a two level hierarchical star topology.

The distributed module off-loads at least some of the central-switch processors real time work load (such as off-hook detection).

There are several advantages to a distributed architecture;

  • It permits growth beyond the practical size of a single digital switch.
  • It provides better performance by off-loading of functions.
  • It provides higher reliability: the loss of a single module need not disable the entire system.
  • It reduces twisted-pair wiring distances.

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About the Author
Rajeev Kumar
CEO, Computer Solutions
Jamshedpur, India

Rajeev Kumar is the primary author of How2Lab. He is a B.Tech. from IIT Kanpur with several years of experience in IT education and Software development. He has taught a wide spectrum of people including fresh young talents, students of premier engineering colleges & management institutes, and IT professionals.

Rajeev has founded Computer Solutions & Web Services Worldwide. He has hands-on experience of building variety of websites and business applications, that include - SaaS based erp & e-commerce systems, and cloud deployed operations management software for health-care, manufacturing and other industries.

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