Telephony Gateways | IP PBX System Design

IP telephones, including PC client softphones, communicate directly with the call telephony server over a customer LAN/WAN infrastructure. Proprietary port circuit cards housed in proprietary port carriers are not required for signaling between the IP desktop and the common control controller, unlike converged IP-PBX designs. Non-IP stations and trunk circuits require telephony gateway interfaces to support server control signaling and voice communication transmissions. Telephony gateways for analog telephones and other 2500-type compatible communications devices, such as facsimile terminals, may be provided through a variety of design methods:

• Integrated call telephony server gateway interfaces

• Desktop gateway modules: proprietary, third party

• Gateway servers/interfaces: proprietary, third party

Several proprietary, closed call telephony servers have integrated gateway interfaces for PSTN digital T1/E1 trunk circuits. The gateway interfaces usually support ISDN BRI or PRI services over the T1/E1 trunk circuits. The Mitel MN 3100, 3Com NBX, and Siemens HiPath 5300 systems have integrated PSTN digital trunk gateway interfaces. For example, the 3Com NBX’s integrated analog line card connects up to four conventional (loop start) PSTN telephone lines, and the T1/PRI trunk card connects to a standard T1 circuit. The HiPath 5300 BRI gateway interface card supports four BRI ports (8 × 64-Kbps channels); the PRI gateway interface card includes a T1 carrier interface.

Mitel Networks uses a different approach to support non-IP peripherals on its MN3300 ICP. The 3300 ICP includes an analog services unit (ASU), and a network services unit (NSU), but also supports traditional Mitel SuperSet digital telephones by a link to a peripheral equipment (PE) cabi- net. An ASU supports four analog trunks and 16 stations (including MOH, paging, and PFT); an NSU supports four T1 digital trunk interfaces. Up to four ASU and four NSU carriers are supported per controller carrier. What is unique about the system is that the call server also provides control signaling to an SX-2000 Light PE cabinet. Supporting the traditional PE cabinet protects a customer’s substantial investment in the installed base of proprietary Mitel SuperSet voice terminals.

Mitel intends the MN 3300 system to be a migration vehicle for its large installed base of SX-2000 system customers and allows customers to link existing PE cabinets to the new call telephony server through one of two options: direct optical fiber cable connection or T1 trunk interface. Customers who want a centrally located call telephony server and PE cabinet can use the optical fiber link. The DTI can support remote PE cabinets. The 3300 ICP was the first client/server IP-PBX design to support common equipment originally designed for a circuit switched PBX system. All communications traffic between digital telephones is handled internally by the PE cabinet’s integrated circuit switched TDM backplane. Calls between PE endpoints and other endpoints (IP telephones and ASU and NSU ports) are handled across the integrated controller gateway channels.

Desktop gateway modules may be proprietary or industry-standard H.323 equipment. The most common desktop gateways support 2500-type communications devices, such as analog DTMF telephones and facsimile terminals. The desktop communications device links directly to the gateway module and converts analog signals to IP format for control and communications signaling. For example, 3Com NBX analog devices are available as single-port stand-alone units and four-port chassis-based cards. The single-port ATA unit also includes an additional Ethernet port that allows an analog device and an Ethernet device to share the same Ethernet LAN cabling. The multiple-port NBX analog terminal card features four analog (FXS) ports. The units connect to a wide variety of industry-standard analog devices and fax machines and provide support for door phones, paging systems, and other applications that may require analog connectivity.

The gateways may be proprietary to an IP-PBX system, like the Siemens HiPath AP 1100 (available in one- and four-port interface models), or third-party products available from a large list of suppliers. For example, Ericsson markets a downsized version of its Webswitch IP-PBX for use as an H.323 gateway module. 3Com, a major enterprise data communications equipment supplier, is another IP-PBX supplier marketing desktop gateway modules, including those that support H.323 and SIP standards.

Another type of desktop gateway module is an add-on adapter that converts a proprietary digital PBX telephone into an IP-compatible voice terminal. A few client/server IP-PBX manufacturers, including Siemens and Nortel Networks, offer this as an option to upgrade installed digital telephones originally designed for use behind their circuit switched PBXs. The same adapters can support IP desktops behind the manufacturer’s converged IP-PBX system solutions.

Gateway servers and interface modules/boards that are not fully integrated into the call telephony server or used as desktop devices are proprietary to a manufacturer’s IP-PBX or conform to industry standards, such as H.323 or MGCP, and used as OEM solutions. One example of a proprietary solution is the Mitel Networks MN 3300 ICP gateway carrier that supports traditional analog trunks (loop start) and digital trunks (DASSII, DPNSS, QSig, Euro ISDN, and BRI) for connection to the PSTN and for connecting multiple sites or systems together. This allows multiple 3300 ICPs to be clustered or networked between multiple sites over IP or traditional TDM infrastructures to support up to 40,000 users. The MN 3300’s call telephony server carrier supports the trunk gateway interface carrier.

The Cisco Systems IP Telephony system, when originally designed as the Selsius System, used desktop modules for support of non-IP communications devices and trunk circuits. The redesigned product supports analog station, analog trunk, and digital trunk interfaces with proprietary circuit boards that are housed in Cisco Catalyst 6000 Ethernet switch carriers. Three different modules are used for analog connections: 24-port analog station FXS (H.323 or MGCP), analog trunk circuit FXO (H.323 or MGCP), and analog E&M tie trunk (H.323 only). The FXS module supports fax relay, which enables compressed fax transmission over the IP WAN. An alternative to the FXS module is the standalone Cisco VG248 analog gateway module that supports 48 fully featured analog phone lines as extensions to the Cisco CallManager system. It is housed in a compact 19-inch rack-mount chassis, and its high-density gateway can be used for analog phones, fax machines, modems, and speaker phones. Digital PSTN trunk interfaces are supported by a limited-capacity stand-alone T1 adapter module or a Catalyst 6000 T1 and services module that provides eight T1 ports (192 channels) or DS0 voice trunks. The module supports voice trunk protocols such as ISDN Primary Rate Interface (PRI) and in H2 CY ‘00, channel-associated signaling (CAS). The module’s DSP resources can also be programmed for call conference bridge services and voice codec transcoding applications, instead of digital trunk gateway interfaces.

The Nortel Succession CSE 1000 MG module supports a variety of non-IP interfaces, such as analog station, analog trunk, and digital trunk. Each MG module has three IPE card slots and can support an expansion module for four additional slots. The first Succession CSE 1000 release is limited to a maximum of four MGs (28 card slots, maximum).