“The PBX is dead” has been heard throughout the industry for longer than most telecommunications managers can remember. The phrase first popped up more than a quarter century ago with the widespread adoption of Ethernet LANs. It made another appearance in the early 1990s when Computer Telephony Integration (CTI) standards were introduced, and yet again a few years later with the announcement of the first IP telephony system from Selsius Systems. More recently the phrase made major headlines following last year’s big Microsoft kickoff for Office Communications Server (OCS) 2007 with embedded voice call control features. Has the PBX finally come face to face with the Grim Reaper, or will it receive yet another reprieve from the big CIO in the sky?
Before we write the PBX’s obituary (again) it is important to be sure we know what we are talking about. First things first: What is a PBX? PBX is an abbreviation for Private Branch Exchange, a privately owned communications system that is physically located on a customer’s premises, provides dial tone and telephony features to subcribers, and is linked to a central office (CO) communications system (the “central exchange”) via trunk circuits for access to and egress from the Public Switched Telephone Network (PSTN). For simplicity’s sake we will consider all customer premises communications systems to be classified as PBXs, even those marketed as a Key Telephone System (KTS) or Hybrid system, because the core capabilities and functions are similar, if not identical, across the category offerings.
Until a few years ago, all voice communications traffic was handled over the circuit switched PSTN, but an increasing amount is now bypassing the traditional network in favor of the packet switched Internet. It must be noted, however, that most Voice Over Internet Protocol (VoIP) traffic across the Internet still originates/terminates at a non-IP voice terminal connected to the PSTN or a customer premises communications system. The number of pure VoIP calls will someday eclipse circuit switched-only calls, but that may not happen for awhile. Circuit switched COs are expected to remain in place, connected to customer premises PBXs, for many more years. When the circuit switched CO does fade from the scene, only then can we truly write off the PBX as we have known it.
Based on the above definition, whatever customer premises solution is used to provide dial tone, traditional telephony features, and connectivity to local/long distance exchange carrier services should continue to be known as a PBX regardless of the underlying technology. The final days of the standalone voice-centric PBX system may be on the horizon, but as long as there is demand for real time voice communications, it will continue to march on a little farther. PBXs have evolved dramatically during the past several decades, and are guaranteed to do so into future. Before analyzing the future evolution of the PBX, it may be beneficial to review the three most significant architecture and design developments of the past 40 years: computer-based call processing; digital switching fabric; and IP-based packet switched connections.
STORED PROGRAM CONTROL
The ancestor of today’s highly intelligent and feature rich software-based PBX systems was the Northern Telecom (ne Nortel) SG-1 PBX. Also known as the “Pulse,” it was first introduced in 1972. Prior to bringing the new technology to the customer premises, computer-based call control technology was first available at a large scale with the #1 Electronic Switching System (ESS) central office system. First introduced in 1965, it was designed and developed by AT&T’s Bell Laboratories. It should be noted that the first deployment of the technology on a smaller scale was available with AT&T’s 101 ESS.
Stored Program Control (SPC) system is the technical name used for telephony communications systems controlled by a computer program stored in the system memory. Earlier generations of central office exchange technologies, such as Panel, Rotary and Crossbar were classified as electromechanical switching systems without software control. SPC allowed and provided new and sophisticated calling features untenable with electromechanical technology. Computer technology was still evolving in the 1960s and early 1970s when mainframes ruled, so incorporation of microprocessors into PBXs was a major breakthrough for enterprise communications systems. By the late 1970s, SPC-based PBXs dominated the landscape, quickly replacing electromechanical systems in a few short years.
DIGITAL SWITCHING
Shortly following the arrival of SPC technology was the development of digital switching technology in the mid-1970s. Until the development of digital communications, analog switching and transmission techniques were used to establish connections between calling and called parties. The first digital PBXs, such as Rolm’s CBX, converted analog wave signals into a digital format for internal transmission and switching purposes. Voice audio signals from the desktop analog telephone to the PBX common equipment hardware were transmitted over a 4-Kilohertz (KHz) communications channel, at which point a coder/decoder (codec) embedded on the port circuit card converted the analog signal into a digital signal for transmission across the internal circuit switched network. When digital telephones were first introduced in 1980 behind the Intecom IBX S/40, the codec function was embedded in the desktop instrument itself, and transmission to the PBX common equipment was in digital format, ready for transmission across the internal circuit switched network. Digital switching, as compared to analog switching, provided improved sound quality and more reliable transmission at a lower cost. It was also ideal for supporting data communications requirements in the days of expensive, low-speed modems.
In the early days of digital PBXs, there was no official standard for digital transmission and coding followed by all system manufacturers, because transmission schemes, bit word sizes, and sampling rates varied. By the mid-1980s, however, a Time Division Multiplexing/Pulse Code Modulation (TDM/PCM) format using an 8-bit word and 8 KHz sampling rate emerged as the de facto standard. An 8-bit word and 8 KHz sampling rate translated to a 64 Kbps transmission rate now commonly referred to as the ITU-T G.711 audio standard.
PACKET SWITCHING
Lucent Technologies introduced its Multimedia Communications Exchange (MMCX) solution to work behind its Definity PBX system in 1995. MMCX was the first enterprise voice communications system option to support Ethernet-connected H.323 softphone clients with an integrated media gateway for connection to the circuit switched Definity via a PRI link. The first Ethernet-based PBX system to support Ethernet-connected desktop telephone instruments was introduced two years later by Selsius Systems (then a subsidiary of Intecom). Both the MMCX and Selsius Systems deployed packet switching technology over the Ethernet for control and communications transmission signaling. The beginning of the end of the circuit switched communications era was in sight.
There are two major differences between circuit and packet switching pertinent to to real time voice communications: encoding of communications signals and routing between endpoints. Packet switching is a communications scheme in which packets of digitized data in size-defined blocks are routed between nodes over shared transmission links. Packets are queued or buffered at each node and are usually delayed for a variable amount of time. In contrast, circuit switching is designed for a limited number of constant bit rate and constant delay connections between nodes, and provides for exclusive use of a transmission link for the duration of the communication. Packet switched networks are characterized by delay and congestion parameters; circuit switched networks by network access and blocking issues.
Computer control, digitization, and packet switching slowly transformed voice-centric communications systems over the course of three decades, causing the PBX and its telephone endpoints to eventually appear as just a another server and workstations, respectively, on the larger enterprise LAN/WAN. When voice is digitized and packetized it becomes virtually indistinguishable from other modes of communications transported and switched across the network, such as data or email-based text. When voice communications is handled like non-voice communications, it becomes relatively simple to treat all communications modes similarly for unification purposes in support of software-based applications.
