Chapter 24: Private Branch Exchanges

Overview

Private Branch Exchanges (PBXs) are one of the most resilient telecom equipment to have been built. New improvements in their applications provision, capacity, and standardization translate into renewed demand for its services. During the next few years PBXs will undergo through an evolution that will see them migrate from proprietary standards and protocols into more open systems that support IP Trunking and Q.Sig. On the applications side, their path moves towards the Local Area Network (LAN) and the PBX needs to support it or undertake the risk of being displaced by a voice mail system that will.

PBX Technology

PBX technology can be classified into four generations:

First generation systems: used wired logic and analogue step-by-step or crossbar switching. If key features were required a separate key telephone system was necessary

Second generation: introduced stored program control processors driving reed-relay or PAM switching networks. Used either standard analog or proprietary telephones to control a limited number of key telephone features

Third generation: first to support end-to-end digital transmission, employs PCM switching technology and both digital ad analogue proprietary telephones

Fourth Generation: not clearly defined à standards, IP Trunking, Q.Sig, etc.

Conventional digital PBXs fundamental architecture is similar:
·        It has a switching fabric that connects to line and trunk interface circuits
·        A central processor operates the generic program, which is retained in memory
·        Circuitry is contained on cards that slide into slots that mount in modules or cabinets
·        The cards plug into the PBX’s back-plane, which ties the lines, trunks, and central control circuits to the switching fabric and busses over which the circuit elements communicate
·        PBXs connect to three types of external networks: local, inter-exchange, and private

Many systems support special services such as T1/E1 lines and foreign exchange trunks to local calling areas in distant cities

CTI interfaces connect PBXs to mainframes, minicomputers, or servers so the PBX can supply call information to the computer, and the computer can send routing instructions to the PBX

Line Interfaces

  PBXs have at least two types of line interface cards, analogue and digital with most systems also offering BRI ISDN cards:
·       Digital line cards support proprietary telephones that work only with that manufacturer’s system
·       Analogue and ISDN cards support telephone sets that are independent of the PBX manufacturer

PBX Trunk Interface

PBXs interface with the outside world through trunk circuits that exchange signals with other switching systems through a variety of signaling interfaces.

·       Analog trunks mount on cards that contain from 4 to 16 trunks per card.
·       T1/E1 cards support 24/32 circuits
·        Central office compatibility with line equipment is important for proper PBX operation
·        Interface standard is EIA-464: specifies technical and performance criteria for the interface between the two types of systems
·        The central office interfaces analogue PBX trunks with the central office’s local supervision
·        Supervision from a distant trunk is not transferred through the line circuit to the PBX.
·        The PBX cannot pass answer supervision through its station message detail recording (SMDR) port to an external call-accounting system
·        Require an access digit, usually 9, to connect station lines to central office trunks.

ISDN Interface

BRI capability is needed in many PBXs to support desktop videoconferencing. Important option for work-at-home services

Tie Trunks

Inter-machine trunks terminating on the trunk side of the PBX. Used to link multiple PBXs.

Special Trunks

·        Provide access to lower cost long distance service

Computer-Telephony Interface (CTI)

Links the PBX to a mainframe (or minicomputer) or to a LAN server with a two-way channel.

PBX provides call origination and call progress information to the computer, which in turn sends routing and call-handling instructions to the PBX. CTI is very useful in call centers; it is sed to "pop up" an account record on the computer screen at the same time the PBX delivers a call to the workstation.

Remote Switching Units

·        Many PBXs offer remote switching systems in which groups of lines can be contained in a module that is located away from the main switch
·        Connection is via T1/E1 lines of fiber-optic cable
·        Some of the advantages of remote switching units are:
·        Only one processor and software set is needed. This is usually less expensive than maintaining separate systems.
·       Administration is from a central site. All database changes are made on the central switch, with the remote automatically updated
·       Wiring costs are reduced. It is often less costly to install a remote than to cable from the central site
·       Total feature transparency is achieved. Users in remote location share same voice mail, numbering plan, and trunks as the central site, and have access to exactly the same features

Principal PBX Features

Direct Inward Dialing

Offers station users the ability to receive calls from outside the system without going through the PBX attendant.

Voice and Data Integration in the PBX

·        Mostly implemented by organizations large enough to justify a T1/E1 line to a distant PBX data communications network that can also ride on the same facilities
·        Most PBXs provide a data adapter of some kind that connects data through a digital PBX port
·        Method is effective for connecting a single channel application through the PBX

Automated Call Distribution (ACD)

An ACD allows PBXs to route incoming calls to a group of service positions. Calls can be routed by the 800 number that was dialed using a device celled dialed number identification system (DNIS). When agent’s positions are idle, the call routes to an agent immediately. If all positions are occupied, the ACD holds calls in queue and notifies the caller by recorded announcement that the call is being delayed.

Automated Route Selection (ARS)

This feature is also known as least-cost routing (LCR). Users dial the number and the system determines the preferred route and dials the digits to complete the call over the appropriate trunk group. The most sophisticated ARS systems can manage calls based on the entire dialed number.

Networking Options

·        Allows PBXs to operate as a single channel
·        Call-processing information, such as station’s identification and class of service, and in most systems the calling name, travel across the network to permit features to operate in a distant PBX the same as they do in the local system
·        Objective is to provide complete feature transparency, which is the ability of users to have the same calling features across the network as they have at their PBX

Q.Sig

·        Protocol that allows networking and transparency between PBXs of different manufacturers
·        More widely used in Europe than in North America

Single-Button Feature Access

·        Users can access a PBX’s principal features by code dialing.
·        The solution is to assign the features to single buttons on proprietary telephone sets

Follow-Me Forwarding

·        Allows the user to receive telephone calls at home, on a cell phone, or in a remote location
·        User keeps the PBX informed of his or her location, and the PBX forwards calls accordingly

Call Detail Recording (CDR)

·        Also known as station message detail recording (SMDR)
·        In combination with a call-accounting system provides the equivalent of a detailed toll statement for PBX users
·        Required by most businesses to control long distance usage and to spread costs among the user departments

Voice Mail

·        Available as an optional integrated feature in most PBXs
·        Can be added as a nonintegrated service to any PBX
·        When a station is busy or unattended, the caller can leave a message, which is stored digitally on a hard disk. The station user can dial an access and identification code to retrieve the message

Dialed Number Identification System (DNIS)

·        Provides the equivalent of DID for 800 calls  
·        Call is accompanied with a number that the PBX can convert to the number the caller dialed so the PBX can route the call to the appropriate station number.

Direct Inward System Access (DISA)

·        Enables callers to dial a telephone number and a password to gain access to PBX features
·        Restricted: callers can dial extension numbers or tie lines to on-net locations
·        Unrestricted: callers can gain access to long-distance services

N x 64 Capability

Enables the user to dial as many channels of contiguous bandwidth as required

Centralized Attendant Service (CAS)

Enables attendants at one location to perform complete attendant functions for remote networked PBXs

Power-Failure Transfer

Connects central office trunks to standard DTFM telephones

Trunk Queuing

Enables the user to camp on a busy trunk group.
·        Callback queuing: the user activates the feature by dialing a code or pressing a feature button and hangs up. The system calls back when available
·        Hang-on queuing: the user activates the feature but remains off-hook until the call completes

Restriction Features

·        Ability to restrict the calling privileges of certain stations

Uniform Call Distribution (UCD)

·        Distributes calls evenly among a group of calling stations
·        One or more active stations idle: incoming calls are directed to the station that is next in line to receive the call
·        All stations in the UCD group are busy: incoming calls are answered with a recording and held in queue
·        When a UCD station becomes idle, the call that has been in the queue the longest is directed to the station
·        Compared to ACD, UCD is unsophisticated lacking the supervisory, management, and reporting features that an ACD offers

Unified Messaging

Combines the PBX with voice mail, fax, and email so that messages can be viewed and handled on a PC screen.

Data Switching Capability

The PBX could also serve as a data switch, providing port selection and port concentration

Modem Pooling

A modem pool has circuit cards or external data adapters in the PBX that interface with a bank of modems.

Emergency Service Interface

Special dialing code for universal access to emergency services à 911

Multitenant Service

A PBX that provides service to users from different organizations

Property Management Interface

Organizations that resell service often connect the PBX to a computer to provide features such as checking room status information, disabling the telephone set from the attendant console, and determining check-in or check-out status

Uniform Dialing Plan (UDP)

·        Software in a network of PBXs that enables the caller to dial an extension number and have the call completed over a tie-line network without the callers being concerned about where the extension is located

Universal Card Slots

·        Allow PBXs to accept any type of line or trunk card in a vacant slot

Wireless Capability

Wireless systems are new to the market and can be classified as follows:
·        Type that plus into the analogue port on the PBX giving the user analogue phone capabilities
·        Proprietary wireless systems which provide the features of digital telephones

Attendant Features

Most PBXs have attendant consoles for incoming call answer and supervision. Consoles’ features include but are not limited to:
·        Camp-on: allows an attendant to queue an incoming call to a busy station
·        Direct station selection: allows the attendant to call any station by pressing
·        Automatic timed reminders: alerts the attendant when a called line has not answered within a prescribed
time
·        Attendant-controlled conferencing: available for multiport conference calls
·        Computer console: integrates the attendant functions into a PC

System Administration Features

System administration is a costly part of the PBX so any features that ease the administrator’s job is extremely valuable:
·        Automatic set relocation: allows users to move their telephones from one location to another without
the need to retranslate
·        Network move: similar to automatic set relocation with the exception that it works across a network
·        Ethernet connectivity to the management terminal: allows the manager to go into any computer and access the system management port without dialing in form a modem
·        Integrated PBX and voice mail update: enables the voice mail and PBX databases to be updated from a single source

Call-Accounting Systems

·        Call detail recorder (CDR) port that receives call details at the conclusion of each call
·        Call details can be printed or passed to a call-accounting system for further processing
·        Most PBXs can output any combination of long-distance, local, outgoing, and incoming calls to the CDR port
·        Call-accounting systems add details to create management reports, a complete long-distance statement for each user, and departmental summaries
·        Most call-accounting systems on the market are software programs for personal computers
·        In multi-PBX environments, a networked call-accounting system may be required
·        These systems use buffers or computers to collect information at remote sites and upload it to a central processor at the end of the collection interval

Applications

·        Nearly every business that has more than 30 to 50 workstations is in the market for a PBX or its central office counterpart à Centrex

PBX Standards

Few standards exist for PBXs. The interface between a PBX and its serving local central office is standardized by EIA, and trunk interfaces follow accepted industry practices for signaling and electrical interface. Standards include:
·        Q.Sig
·        H.323

Evaluation Considerations

·        Uniqueness of every organization makes universal PBX specification impractical
·        Different entities will have different needs that would require different application solutions
·        Considerations to be taken into account before purchasing a PBX include but are not limited to:

• Universal shelf architecture: permits various types of lines and trunk cards to be installed in any slot
• Switch Network
• Voice and Data Integration Issues
• ISDN compatibility issues
• Administrative Interface
• Redundancy
• Environmental considerations: operating temperature must be evaluated as some system require an air-conditioned environment
• Database updates
• Diagnostic capability: the degree to which a PBX can diagnose its own trouble
• Station wiring limits: station loop range of both proprietary and ordinary telephones
• Bandwidth requirements
• Wireless capability

Acronyms Glossary

·        ACD:         Automated Call Distribution
·        ARS:         Automated Route Selection
·        BRI:           Basic Rate Interface
·        CAS:         Centralized Attendant Service
·        CDR:         Call Detail Recorder
·        CTI:            Computer-Telephone Interface
·        DID:           Direct Inward Dialing
·        DISA:         Direct Inward System Access
·        DNIS:         Dialed Number Identification System
·        DTMF:       Dual-Tone Multifrequency
·        EIA:            Electronics Industry Association
·        IP:              Internet Protocol
·        ISDN:         Integrated Systems Digital Networks
·        LAN:          Local Area Network
·        LCR:          Least-Called Routing
·        PBX:          Private Branch Exchange
·        PC:            Personal Computer
·        PCM:         Pulse Code Modulation
·        SMDR:      Station Message Detail Recording
·        UCD:         Uniform Call Distribution
·        UDP:         Uniform Dialing Plan