IP-Capable PBXs

As mentioned, high-end PBX products include toll bypass features that can route calls over data networks. Until very recently, only circuit-based mechanisms (such as frame relay or ATM) have been employed, but now IP-capable PBX products are available and can send voice over the Internet or private IP networks. Such systems (depicted in Figure 1) comprise a traditional PBX, voice-over-IP gateway, and IP router. Though integrating such a system may appear simple, it is a major feat to pull it together from random components, and the communications software is the most essential glue. IP-capable PBXs from single vendors emerged in response to customer pleas. Another important advantage of procuring (rather than integrating) an IP-capable PBX is that vendors can ensure that PBXs in different locations can use IPs as tie lines: A call originating in a PBX in one location can be routed over an IP network to another PBX extension in the same enterprise with absolute transparency to both the calling and called parties.

Figure 1: IP-capable PBX. Figure 1: IP-capable PBX.

Following is a list of hardware and software components that come as part of a PBX product in general, and an IP-capable PBX product in particular. We start with hardware features:

§  Switching modules (or switches).  Can be large cabinets, or—for very small PBXs (typically, under 10 trunks and 20 extensions)—PC boards that can be inserted into desktops or servers. (In the case of the latter, current products typically allow fewer than 10 boards per server.) In the PBX nomenclature, connections to PBXs are called ports, and it is number of ports that defines the number of external lines/trunks and internal extensions. The total number of ports is less than the sum of the latter two numbers because some ports are used for connection to the console and servers.

§  Console.  In newer systems, the console is usually a PC (although it can be a dumb terminal) attached to the switch directly or to a network management server (if included in the offering) through which the system is administered.

§  Product line of specialized telephone terminals.  Includes an LED display (to present calling number or name, among other data) and specialized buttons for activating the PBX features, selecting lines, administering messages, and so on. Other displays (such as a message waiting indicator) are often also included. The terminals differ in the number of lines supported; three is a typical minimum for ISDN terminals. Additional terminal equipment includes speakerphones (with a mute button) and headsets. Often the terminals have buttons associated with special features; typically, there is a set of buttons for speed dialing preassigned numbers, auto callback, call-forwarding, and telephone directory access. The terminals operate with either two-wire or four-wire circuit packs.

§  Wiring.  Includes cables and connectors for connecting telephone terminals to switching modules; cables and connectors [such as the Network Terminating Device 1 (NT1)] for connecting to the ISDN, although in most cases this specific connector is provided by the carrier; wires for analog PSTN lines (for fax machines and analog telephone terminals); PBX connectors; and, when there are additional servers as described in the following, appropriate LAN connectors.

§  Analog line interface.  Comes in the form of circuit packs with multiple ports for analog telephone lines.

§  Voice-over-IP (VoIP) gateways.  Boards or chassis that can be colocated with the switching boards in a single compartment; alternatively, they can come in a standalone unit.

§  IP routers.  Similar to gateways, these may come on a board for smaller routers, or in a standalone unit.

§  Servers.  Standalone computers, in most cases attached to the PBX switch proper via a LAN (typically Ethernet), although direct digital links are also used (and some product offerings allow customers to chose between Ethernet and direct connections), especially when only one server is involved in the offering. It is the servers that keep and execute most of the critical software (for example, administration, network management, security, or call control); the servers also act as gateways and mediators for a mix-and-match approach, especially for building systems (described later) that combine existing and new products. One typical product example is net messaging interchange, a hub-and-spoke topology that connects currently nonnetworked messaging systems or complements the alternative point-to-point networks that exist for messaging systems. The interchange, a specialized server, acts as a post office (that is, a store-and-forward device) to which multiple messaging systems are interconnected. The interchange provides transcoding from analog to digital (and vice versa), thus reducing the cost of connecting nodes to networks. For such systems, a Point-to-Point Protocol (PPP) server acts as the base for remote user access. Specifically for the purposes of unified messaging, some products use one server for each type of messaging medium (for example, one for e-mail, one for fax, one for voice mail, and so on). Even though access is unified from the user’s point of view, the system is fault tolerant in that the machine failure of one server cannot result in a total information blackout. However, such systems are more difficult to administer because of the necessity of replicating databases on different servers; hence the single-server messaging products are often preferred by a large segment of enterprises. In most products, NT servers are used because of the rich set of software building blocks provided, although Unix-based servers are also included in offers. Overall, the messaging platforms are most versatile, and they play an especially important part in integration of PSTN and the Internet.

Typical software features include:

§  Call control.  The software for this function, which implements most of the real-time call control PBX features, is often based on computer-telephony integration (CTI) technology and typically runs on the CTI server. The presence of telephony application programmer interface (TAPI) [and particularly Java API (JTAPI)] libraries, which are available in many products, should be an important product differentiator: Experience has demonstrated that all enterprises, no matter how small, need to customize call control features or add new ones at certain points in time. To this end, application testing and debugging tools are offered with virtually all high-end products. With IP-capable PBXs, two especially important features that are pertinent to the use of IP are:

1.     Virtual tie lines. The feature by which a PBX extension in one enterprise location is connected to an extension on a PBX in another enterprise location through the IP network, but with full transparency as far as users of either extension are concerned.

2.     Toll bypass. The feature by which the call control software determines whether a particular call should be routed over the PSTN or IP network. Some factors for making such a decision are quite obvious (for example, if a terminating location cannot be dialed through an IP network, the call cannot be routed only over the PSTN), but there are factors (for example, trunk availability, the cost of a call at a given time, or current availability of the QoS in the given IP network) that are time specific, and they can produce significant cost saving if the feature is present in the product.

§  Systems administration.  In addition to the usual knobs and buttons, invariably delivered via a graphical user interface package, the software for this function also comes as a modular Windows-based suite designed to help administrators by providing the means to program, schedule, and process transactions that update user records. The switch management packages provide the means for extracting reports on switch data and changing these data. High-end products also offer software that helps optimize the performance of the switch.

§  Messaging.  The messaging software available is typically compatible with the hardware (servers) described previously. Voice and fax messaging are available with a set of networking choices. For IP-enabled PBXs, LDAP-based multilocation directory databases make up an essential part of the offered software. Messaging is one important application used not only by the PBX extensions, but also by remote users, who can dial into a server in order to retrieve the messages stored in their mailboxes. Once inside the server, such users can have access to most of the PBX features; they can, for example, dial the extension of the message’s originator.

§  Security.  Software is usually tightly coupled with hardware in order to protect the access of mobile telephone users and remote-site staff. In addition to the software for toll fraud prevention that has been part of traditional PBXs for years, IP-capable PBXs include software that enforces the registration process for the endpoints to authenticate the user.

While the IP-capable PBX products are successfully entering the market for medium to large enterprises, smaller enterprises now have a choice of either going with traditional small PBXs or going directly to purely IP-based systems (which in turn can connect to the PSTN).