Private Branch Exchange Systems (PBXs)

Add a note hereTraditionally, a PBX has been a small telephone switch located on the customer (that is, enterprise) premises. Its evolution from a mere switching board operated manually by a receptionist to the versatile digital system it is today has closely followed the evolution of the PSTN. The two major aspects of the function of a PBX have remained unchanged, however: First, it is responsible for supporting telecommunications within the enterprise; and second, it is responsible for connecting the enterprise to the outside world (which has been traditionally done via the PSTN). Ultimately, a large enterprise dispersed over several geographical locations would have several PBXs, which should—in cooperation with the PSTN—behave as one virtual PBX. The perception of virtuality is created by supporting one numbering system and otherwise providing identical services throughout the enterprise so that communications across different locations appear to the user to be exactly the same as communications within any particular location.

Add a note hereMigration to IP-based voice and data delivery can be achieved in different stages by mixing and matching the existing and new technologies through a series of architectures and product types, described in this section. The enterprise manager may choose to totally replace existing PBXs with IP-based systems, or to invoke alternative solutions in which new systems are used as an add-on to the existing technology. The choice typically depends on the features of the system that are essential to the enterprise; cost is another factor. For this reason, we begin by listing generic PBX characteristics and features as a set of criteria for an enterprise manager comparing traditional and IP-based products. We then describe IP-based products and interconnection architectures. Note that it is still only the correct operation of the main features that can determine whether systems integrated from different types of products will work as expected.

Generic PBX Characteristics and Features

Add a note hereThree important PBX characteristics that are essential for the first-order comparison of existing products are:
1.     Add a note hereNumber of local extensions.
2.     Add a note hereNumber of outside connections (lines or trunks).
3.     Add a note hereNumber of busy hour call completions.

Add a note hereA traditional PBX can support from 2 to up to 100,000 telephone lines internally; the number of outside trunks is typically one order of magnitude smaller. Externally, digital PBXs have traditionally been connected to the PSTN via the ISDN PRI using digital T1 or E1 circuits. T1 lines operate at 1.544 Mbps, the equivalent of 24 analog voice lines, and E1 circuits operate at 2.048 Mbps, equivalent to 30 lines. When available, T1 or E1 lines cost much less than a set of analog lines with the same bandwidth. Note also that PBXs can be connected not only to central offices, but also directly to interexchange carrier (IXC) switches. The PSTN supports a single-numbering plan for multilocation enterprises as well as a number of important routing features, such as call distribution based on time of day or call queuing, via IN-supported virtual private network (VPN) service.

Add a note hereIf the PBX switches are located in nearby buildings, they are often interconnected directly (via so-called tie lines) rather than through a PSTN virtual private network. Traditional PBX-PSTN interconnection is depicted in Figure 1. Separation of switching fabric (switches) from call control, service control, and administration and management performed by the communications software running on servers is a feature of computer-telecom integration (CTI) that has made the current generation of PBXs very versatile. Depending on the type and size of PBX products, such servers are sometimes standalone computers connected to switching modules via a LAN or dedicated point-to-point connection. On the other end of the spectrum, the server can be a PC that has one or more switching boards inside, so the PC is the PBX.

Figure 1: Traditional PBX interconnection. Add a note hereFigure 1: Traditional PBX interconnection.

Add a note hereNote: High-end PBXs built for medium to large enterprises can provide, in addition to PSTN trunks, connections to frame relay networks to support voice over frame relay (VoFR), connections to ATM switches to support voice-over-ATM (VoATM), or both as a mechanism for toll bypass; the use of this technology and related products.

Add a note hereThe services of traditional PBX combine the delivery of voice, fax, and data with special call features of the ISDN. The provision of these specialized features is of central importance to the enterprise. Some of the most essential features are:

§  Add a note hereCaller ID.  Displays the telephone number (and often even the name) of the calling party. With integrated computer-telephony applications, this feature can come in many flavors, including opening and presentation to the called party of a file associated with the calling party (for example, a customer file). Incidentally, even in its simplest form this feature can be provided in different ways that benefit customers. The most straightforward way of implementing caller ID is to support the ISDN automatic number identification (ANI) [or calling line identification (CLI) in Europe]; however, caller ID can also use the in-band ANI and dialed number identification service (DNIS) information delivery via DTMF signaling over incoming trunks. Some enterprise managers find that the latter option is less expensive than paying for ISDN ANI. The availability and pricing of the in-band ANI, however, depends on the telephone company to which the PBX is connected.
§  Add a note hereDial-by-name.  A call can be placed by spelling the name of a called party. (A more sophisticated version of the same feature allows a user to place a call by typing in or simply clicking on a name in a directory accessible through a computer terminal or monitor.)
§  Add a note hereCall-forwarding.   A call is automatically forwarded to an internal extension or external telephone number. (To prevent potential abuse, control of this feature is needed to restrict it only to internal forwarding for most users.)
§  Add a note hereCall transfer.  A called party may transfer a call to an internal extension or external telephone number. (As with call-forwarding, this feature must often be restricted to use within the enterprise.)
§  Add a note herePaging.  A call from an extension (usually that of a manager) rings all the telephones in a pre-configured group. Often, the same name is also used to refer to a much more elaborate feature by which enterprise individual employees or their entire groups can be simultaneously reached (through speakerphones or computer speakers) for announcements.
§  Add a note hereGroup listen.  Enabled through special terminals that are often a part of the overall PBX system offer, group listen allows an enterprise employee to communicate with a distant party using the terminal handset while the terminal speakerphone operates in listen mode only, thereby permitting other employees in the room to hear the conversation but not participate in it.
§  Add a note hereConference calling.  A party to the call can merge several existing calls into one conference call. In the advanced implementations of this feature, so-called legs of conference calls can be muted or deleted and new ones can be added as the conference progresses.
§  Add a note hereCall pick-up.  An authorized party can pick up calls destined to another party. One widespread use of this feature is allowing a secretary to screen the originating caller ID and then make a decision on whether to pick up the boss’s call.
§  Add a note hereCall distribution.  A family of features that allow calls to be routed by a PBX in order to balance the load of all the enterprise employees within a particular group. For example, a PBX can route an incoming call to the first nonbusy extension in the group; if none of the extensions is busy, the PBX can chose the terminating extension based on a round-robin scheme or select the extension that has handled the least number of calls during the given day. Calls can also be routed based on time of day. For example, night calls could be routed to a guard desk or another PBX within the enterprise.
§  Add a note hereCall-waiting.  When the called party is occupied with another call, he or she is informed (by a distinctive sound or a message on the computer terminal screen) about an incoming call, at which point the called party may respond by putting the active call on hold. Toggling between calls or even switching between several waiting calls are other functional capabilities associated with call-waiting.
§  Add a note hereWhisper page.  A sophisticated variation on call-waiting that can be used to permit a secretary to interrupt a manager’s call for an announcement. The manager is alerted by an audible signal that is heard by all parties to the call, but the subsequent message from the secretary is private.
§  Add a note hereToll restriction/outside call blocking.  Certain enterprise extensions are restricted from making toll calls, receiving calls from outside of the enterprise, or both.
§  Add a note hereMessage or music on hold.  Music or messages (usually promoting the enterprise’s products) are played for the caller on hold.
§  Add a note hereDistinctive ring.  Incoming calls from predefined addresses can be introduced with unique ringing patterns.
§  Add a note hereVoice messaging.  Each extension has a voice-mail box where incoming voice messages are stored. The owner of the extension can typically administer his or her outgoing announcements and even schedule different ones to be played to a caller depending on the time of day or whether the line is busy. Traditional features include the ability to broadcast messages to a list of extensions, forward messages, and respond to messages (the latter can also be achieved by either dialing the extension from which the message came or recording and sending a response message—possibly with an attachment—to the originating extension). If voice messaging is already supported by the existing system at the time the decision is made about purchasing hardware and software for this set of features, an important criterion to apply is whether the add-on really interworks with the existing system. Confirmation of compatibility may require thorough checking of interworking of all features.
§  Add a note hereUnified messaging.  Integrates voice messaging with e-mail and fax and provides a straightforward, uniform way of handling all aspects of messages arriving from different media.
§  Add a note hereMessage waiting indicator.  The telephone terminal’s message waiting light is lit when a voice-mail message is waiting. (A message can appear when a call is redirected to voice mail because the called party is busy; alternatively, it can simply be sent via voice mail.) After the message has been retrieved, the light is deactivated.

Add a note hereThe preceding features are normally required by enterprises of all sizes. Large enterprises, however, may need additional sophisticated features for private networking enhancements:
§  Add a note hereTrunk optimization.  Because of feature interactions that arise from dialing out or transferring calls while using voice mail, the path for the new call may be selected inefficiently. Highly sophisticated PBX products optimize trunk use on rerouting.
§  Add a note hereCall-independent signaling.  Supports passing of supplementary service information over signaling links, independent of any active call.
§  Add a note hereCall completion  Ensures the completion of a call to a busy called party or an unanswered call by retrying after the calling party hangs up, and subsequently establishes the call the moment the called party is available.

Add a note hereIn addition to features for PBX users, there are a number of features designed for people in the enterprise who maintain and administer the PBX systems. By far, the most important feature is application builders (that is, software tools for building new PBX service applications as well as customizing existing ones). Others include the presence and ease of use of the configuration manager and diagnostics software, real-time status monitoring (including the capability of monitoring remote PBX switches), administration and security mechanisms, and the capability of installing hot-swappable hardware.

Add a note hereWhile on the one hand a rich set of features is essential to the efficiency and productivity of the enterprise, on the other hand, unrestricted access to such features may result in abuse of company resources. For example, an unscrupulous employee may use the conference calling or call transfer feature (described later) to arrange a two-party long-distance call between his or her friends at the enterprise’s expense. For this reason, PBX products are equipped with restrictive control of the features they provide. The existing systems may differ in their restrictive capabilities as much as they differ in their main features, for which reason the restrictive capabilities are indeed an essential factor in product evaluation. The following is a list of the most important restrictive features and associated tasks used by PBX administrators:

§  Add a note hereConfiguring an extension so that it cannot be used to dial out. Such extensions are called local-use-only extensions. Note, however, that the definition of dialing out varies from enterprise to enterprise. For example, when the enterprise is spread over several geographic locations so that the PBX function itself becomes distributed (as depicted in Figure 1), calls within the enterprise are in many cases considered local even though they may leave an enterprise PBX and go through the PSTN before they return to another enterprise PBX. The problem of distinguishing intraenterprise calls that involve dialing through the PSTN from other nonlocal calls is easily solved by establishing an enterprise-wide numbering plan so that the PBX recognizes all enterprise-specific numbers as local (for restriction purposes) even though it will access the PSTN for all nonlocal numbers. Subsequently, the PSTN (typically by means of the IN-supported VPN) translates the enterprise-specific numbers and routes calls to their respective destinations. Alternatively, the involved PBXs can provide the translation of the enterprise extensions to the PSTN numbers and perform the following functions.
§  Add a note hereDisabling the use of pay-for-access area codes (such as 900 in the United States) from an extension.
§  Add a note hereDisabling call pickup to certain extensions (usually by configuring call pickup groups in which only the extensions in the group can pick up calls from other extensions).
§  Add a note hereDisabling conference calling for local-use-only extensions.
§  Add a note hereConfiguring certain restrictions on call transfer (for example, transfer of an outside call to an outside number).
§  Add a note hereDisabling paging in all but authorized extensions, and configuring paging groups.

Add a note hereThe need for and significance of the restrictive capabilities grows with the size of a PBX. In addition, large PBXs require network management capabilities. To this end, the most important products address fault management and performance management:

§  Add a note hereFault management systems. Give customers tools to monitor all elements of the enterprise network and produce a variety of robust traffic and alarm reports. Typically, the systems provide a real-time graphical map of the enterprise network configuration and fault status. The administrator can see an on-screen hierarchical view of the layers of the network.
§  Add a note herePerformance management systems. Enable customers to poll traffic data and generate a variety of performance reports that cover the switch traffic and statistics of activities (such as automatic call distribution).

Add a note hereThere are products on the market that offer unified network management software to enable customers to centralize the management of voice, data, and mixed-media environments for the enterprise. These products automate consolidated alarm handling as well as management of network topology configurations spanning multiple locations and are capable of working with the products of multiple vendors. Unified network management products are uniformly based on the Simple Network Management Protocol (SNMP) standards.

Interconnection Architectures

Add a note hereOne successful implementation of voice over IP for small enterprises (that is, one-location businesses with fewer than 100 extensions) has been demonstrated by local area network (LAN)-based PBXs. Figure 1 depicts a simple LAN-based PBX, where the LAN performs the physical switching function among H.323 terminals while the calls are administered by a control unit. The latter contains a gatekeeper at a minimum, and—in some products—a T.120 MCU. Other products postulate a more modular approach by which multimedia features are offered on a separate server. Such multimedia servers are separate products that can be integrated with LAN-based PBXs offered by different vendors. These servers support conferencing with multipoint collaboration, application sharing, and other conferencing services. Still other servers—for messaging, for example—can be integrated into the LAN-based PBX.

Figure 1: LAN-based PBX.

Add a note hereFigure 1: LAN-based PBX.
Add a note hereConnections to the PSTN (as well as to traditional PBXs and non-IP voice messaging systems in a LAN-based PBX) are achieved by integrating a reverse VoIP gateway. There, the role of the gateway was to connect a traditional PBX to the IP network; here, it is to connect what is effectively an IP network to the PSTN. This is an important example of duality manifested in the old PSTN and new IP converging products.
Add a note hereExcept for a few especially sophisticated features (for example, whisper paging), the leading LAN-based PBX products support all traditional PBX features. The minimal list of such features includes call-forwarding, call transfer, call-waiting, calling party ID, conferencing, and support of multiple lines on one extension.

Add a note hereA LAN-based PBX can be distributed over several locations or even the whole enterprise network by adding routers, as depicted in Figure 2, thus creating a full-blown IP PBX.

Figure 2: IP PBX. Add a note hereFigure 2: IP PBX.

Add a note hereThe hardware of a typical IP PBX offer includes the following components:
§  Add a note hereH.323 or SIP terminals, which may look and feel (with their many buttons, the number of which determines their pricing) like traditional PBX multiple-line telephones
§  Add a note hereVoIP gateways to access the PSTN and other (non-IP) PBXs (number of ports varies)
§  Add a note hereAnalog station gateways for connecting fax devices and traditional (POTS) telephones
§  Add a note hereControl unit (includes the gatekeeper at a minimum)
§  Add a note hereAll the necessary connectors, LAN hubs, and cables
§  Add a note hereServers for multimedia conferencing, unified messaging, administration, and other functions

Add a note hereSoftware is, of course, the most versatile part of the IP PBX. Its key enabling component, almost invariably called a call manager, resides in the control unit. In addition to provision and control of the PBX features, the call manager is responsible for gatekeeper-like functions, including registration of the H.323 or SIP terminals, admission control, guaranteeing QoS to participants (a significant function if the IP PBX spans several LANs), and, when necessary, cooperation with the software on the servers. Again, the presence of telephony application programmer interface (TAPI) and Java TAPI (JTAPI) libraries should be an important product differentiator, because these libraries allow you both to modify many service applications already provided by the vendor and to develop your own services. Theoretically, with a single control unit, the whole IP network can be turned into a PBX of sorts.

Add a note hereIf different LAN-connected parts of an enterprise each have their own control units, as depicted in Figure 3 (such a configuration can often speed up and otherwise improve both communications and network management), the PBX is called distributed. Note that what actually is distributed (compared to the configuration of Figure 4) is the function of call manager software. In this case, support of a form of signaling among the control units that aids in negotiating QoS and optimizing the use of servers is also required.

Figure 4: Distributed IP PBX. Add a note hereFigure 4: Distributed IP PBX.

Add a note hereFinally, we should demonstrate what may already be obvious to you: Despite the introduction of IP PBXs, legacy non-IP PBXs can still be used. Figures 6 and 7 depict two ways of integrating IP and non-IP PBXs: (1) through the IP network and (2) through the PSTN. Both cases demand tailoring of the original software (one more reason to emphasize the importance of software tool kits!), but in the latter case the PSTN (through IN, with the SCPs as servers) will cooperate in or even do the whole job of supporting features such as call distribution and single numbering plans.

Figure 5: Hybrid PBX (IP connection).

Figure 6: Hybrid PBX (PSTN connection).

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