ToIP and IP-PBX Systems

The IP-based packet switching network is the underlying principle for the technology known as Telephony over IP (ToIP). ToIP is simply described as an IP-based LAN/WAN infrastructure that supports telephony system communications and applications. A customer’s packet switching LAN/WAN infrastructure can carry control and/or voice communications signals in IP packet format between a call processing complex and peripheral endpoints or between two peripheral endpoints. A traditional PBX common control complex can logically manage a switch connection between two endpoints, but the physical switching function between the IP peripherals may be handled external to the PBX common equipment by Ethernet switches. The internal PBX circuit switched network may have no role in the voice communications call.

A PBX system using ToIP technology in support of any or all control or voice communications signaling can be categorized as an IP-PBX system. In theory, an IP-PBX system can have any or all of the following attributes:

• LAN-connected common control complex, such as a telephony server that provides call signaling control to port cabinets and/or peripherals and stores the generic software feature program for feature/function provisioning operations.

• LAN-based control signaling of IP peripherals (stations and/or trunk circuits) with or without an integrated gateway/gatekeeper function port circuit card(s). This form of ToIP would be available with a PBX system based on a traditional common control complex or a LAN-connected telephony server.

• Ethernet switched support of IP peripheral to IP peripheral calls or of non-IP peripheral to IP peripheral calls. The former call type typically would involve no circuit switched connections. An example of the latter call type is a call connection between a traditional analog or digital telephone and an IP telephone. Voice communications signals are carried across the LAN infrastructure between IP peripherals or to gateways interfacing to non-IP communications equipment.

A circuit switched PBX system based on a traditional common control complex that supports IP peripherals using an external gateway and/or gatekeeper network element would not qualify as an IP-PBX. A PBX based on a CTI design that supports analog telephones logically integrated with a desktop computer equipped with PC telephony software would not be an IP-PBX, unless the system also supported IP telephones or had a fully integrated gateway function and interface for IP WAN trunk calls.

An IP-PBX is not necessarily based on a LAN/WAN data communications client/server design. A traditional PBX system, with an integrated gateway module board used for intersystem networking across a customer WAN, can also be categorized as an IP-PBX system based on the above listed criteria. There are some PBXs categorized as an IP-PBX that support IP telephones but lack integrated trunk support. A PBX system that supports neither IP stations nor trunk ports but does support a distributed cabinet design using a LAN/WAN infrastructure for control signaling and intercabinet communications can also be categorized as an IP-PBX even if there are no IP telephones or IP private line trunk networking capabilities. Just as the design architecture and feature capabilities of traditional circuit switched PBXs are not uniform across product models from the same supplier or competing suppliers, IP-PBXs are based on a variety of architecture designs and hardware configurations that support different feature/function capabilities.

Recognizing that there are some significant differences in PBX design and function across the spectrum of product offerings and that there is a natural tendency to label a product, a simple classification system must be defined. As a consequence, PBX systems currently can be classified into three basic categories based on call control and switching platforms:

1. TDM/PCM circuit switched—TDM/PCM circuit switched PBX system with proprietary common control complex and internal circuit switch network.

2. IP packet switched—IP packet switched PBX system based on a client/server design consisting of a telephony server that uses a LAN/WAN infrastructure for call control and communications signaling operations. A circuit switched communications network is not included as part of the standard system design.

3. Converged—Integrated circuit/packet switched PBX system with a proprietary common control complex, an internal circuit switched network, and integrated gateway interfaces to support port cabinets and/or IP peripherals connected directly to an external LAN/WAN switch network. Traditional PCM-based peripherals and new IP peripherals can be supported.

The second and third PBX categories are classified as IP-PBX systems because each uses ToIP technology for some or all system processing and/or switching functions. Just as there are many design variations of a traditional circuit switched PBX system, there are design variations of IP packet switched and converged IP-PBXs. For example, the Cisco System AVVID CallManager solution has a radically different design than the 3Com NBX 100. Both systems are clearly categorized as IP packet switched IP-PBXs, but the telephony server and peripheral support options differ greatly. The Cisco solution requires a variety of gateway options to support analog stations and PSTN trunk circuit interfaces, many of which are housed in Ethernet switches exclusive to Cisco and require a dedicated messaging server in addition to the primary telephony server. The 3Com solution is based on a telephony server cabinet with an integrated digital T1 trunk gateway and fully integrated unified messaging support and supports analog stations using desktop adapter modules. Even within the 3Com IP-PBX family of switches there are distinct differences between model designs. The NBX 100 is based on a server cabinet design, and the SuperStack 3 (SS3) model uses a modified SuperStack Ethernet switch chassis. The functions and applications of the two 3Com models are essentially identical, although the common control hardware equipment is different.

A client/server IP-PBX design does not necessarily preclude support of traditional circuit switched PBX equipment hardware. The Mitel Networks 3300 (MN 3300) is based on a closed server cabinet that can support a traditional Mitel SX-2000 Light port equipment cabinet with a dedicated optical fiber cable or digital T1 trunk connection. The MN 3300 supports direct call control signaling to LAN-connected IP telephones and depends on the Ethernet switch network for voice communications connections and transmission, but also provides the call control signaling for analog and digital telephones supported by traditional port circuit interface cards. Switch connections for calls between analog and digital telephones are circuit switched over the SX-2000 TDM backplane. The MN 3300 T1 gateway interface is used for calls between LAN-connected IP telephones and non-IP peripherals housed in the port equipment cabinet.

Support of TDM/PCM port cabinet equipment may not be unusual for IP-PBXs based primarily on a client/server design architecture. As customers continue to migrate their PBX systems from a circuit to packet switched platform, a telephony server directly supporting LAN-connected IP telephones will become more popular, but traditional desktop instruments will need to be supported until the migration is complete. If not making use of modified existing port cabinet or carrier equipment with a TDM backplane, the telephony server will support newly designed LAN-connected port carriers with integrated gateways for cus- tomers who wish to continue to using analog or proprietary digital telephones originally supported on traditional TDM/PCM port interface equipment. For example, several manufacturers of traditional circuit switched PBX systems plan to make available a telephony server to optionally replace the traditional common control cabinet but continue support of existing TDM/PCM port equipment cabinets. Direct signaling support over the LAN of IP telephones eventually will be supported, as will the legacy port equipment cabinets. The port equipment cabinets will have integrated gateway interfaces for call control signaling and intercabinet communications. Avaya and Alcatel have indicated that they plan to use a LAN-connected telephony server to support traditional port equipment cabinets in near-term future releases.

The converged IP-PBX system design is based on a traditional circuit switched design platform with proprietary common control, internal TDM buses for local switch network access, and a center stage switch complex, if required. Station and/or trunk interface circuit cards with integrated gateways are used for IP peripheral support. The gateway function provides channel connections to the internal local TDM for calls between IP peripherals and non-IP peripherals. The interface card may also support gatekeeper call control signaling over the LAN to the IP peripherals. The gateway and gatekeeper functions sometimes can be divided between two port circuit cards. For example, the Nortel Networks Meridian 1 ITG card supports control signaling and TDM bus channel connections for IP peripherals; the Avaya Definity requires an IP Media Processing Board to support gateway channel connections and a CLAN card for call control signaling.

A converged IP-PBX system may also support multicarrier TDM/PCM multicarrier or single-carrier port cabinets over a LAN/WAN infrastructure similar to that described above with a telephony server, except the converged system is based on a traditional common control complex. This design arrangement would require a gateway/gatekeeper interface card in the centralized common equipment cabinet and an integrated gateway interface function in the distributed LAN-connected port cabinet/carrier equipment. All call processing functions would originate in the common control complex and be transported over a LAN/WAN infrastructure in support of a remotely located port equipment cabinet. A distributed or dispersed switch network design lends itself better to a converged IP-PBX system as opposed to a centralized switch network design because there is less dependency on the LAN/WAN for call switching requirements. Only intercabinet calls would require use of the gateway and LAN switches in this converged design.