Upgraded Circuit Switched PBX

Circuit switched PBXs that are upgraded to support IP telephony can be designed to support dispersed common equipment for single or multiple customer premises configurations. It may be necessary to disperse common equipment across a single customer premises if it is a large campus environment with multiple buildings or even a very large single building complex. Multiple customer premises requirements can be satisfied by a variety of design options, but a single system solution is often the preferred choice. If an existing customer WAN is installed and can handle voice calls, there is a potential to save significant transmission service expenses that would normally be allocated to dedicated circuit switched trunk facilities.

Nortel Networks implemented the first use of IP telephony by a traditional circuit switched PBX system. It supported remote communications requirements with the use of WAN links instead of more traditional T1/E1 trunk carrier circuits. In the early 1980s, Nortel Networks also was the first PBX supplier to offer a remote port cabinet option, remote peripheral equipment (RPE). In 1999, the supplier announced a remote port carrier using IP connectivity, the 9150 Remote Office Unit. A centrally located Nortel Networks Meridian 1 equipped with an ITG port interface board (Reach Line Card) can support a remote 9150 by using available WAN trunk facilities between the two locations. The remote unit has a port capacity of 32 digital Meridian 1 telephones, a TDM backplane to support local switching between stations (reducing dependency on the Meridian 1’s center stage switch network), and an integrated IP gateway. Voice codecs supported include G.711, G.726, and G.729/A. The 19-inch carrier unit also has a 10Base-T Ethernet port connector for LAN connectivity and ISDN BRI trunk circuit interfaces for local trunk calls. Key QoS parameters on call sessions between the Meridian 1 and 9150 are monitored, included packet loss, jitter, and end-to-end packet delay (latency). If threshold parameters are exceeded during a call in progress, the call can be dynamically and transparently rerouted over ISDN BRI trunk circuits to the Meridian 1. With G.729 (8 Kbps) encoding, a single ISDN BRI B channel supports up to eight simultaneous calls back to the Meridian. Calls can be transitioned back to the IP WAN when the network can support acceptable QoS levels. If the WAN link to the Meridian 1 is lost, the remote 9150 supports limited call processing functions, including local switch connections, access to ISDB BRI trunk circuits, and basic station features (transfer, paging access).

Ironically, the 9150 unit does not directly support IP stations. IP stations remote to the Meridian 1 can be supported via an ITG line card in a centrally located IPEM port cabinet over a LAN/WAN link. Communications between a remote IP telephone and a digital telephone supported by the 9150 unit must be handled across the Meridian 1 center stage switch network at the main location. The remote IP station is totally dependent on the LAN/WAN link for all telephony services, and not even dial tone is supported when the link fails.

Other PBX suppliers soon followed Nortel’s lead by offering IP remote cabinet/carrier options on their circuit switched PBXs. For example, the Avaya R300 Remote Office Communicator is designed for branch offices that support centralized Definity/IP 600 IP Communications Server features—essentially everything available at headquarters—over the WAN, to offices with fewer than 25 employees. The R300 Remote Office Communicator was designed to lower customer networking costs by converging voice and data onto a single WAN facility. An IP port interface card in the centrally located Definity/IP 600 system transmits call processor control signaling to the remote location and functions as a gateway for calls between the central and remote locations. The R300, an Avaya-customized Ascend Communications R3000 remote office concentrator, has built-in data networking and routing capabilities including firewall, VPN, and security features, with optional remote access concentration capabilities to improve network efficiency. The R300 has integrated port circuit interfaces that support up to 24 digital and two analog stations, WAN options such as full and fractional T1/E1 and BRI, a bidirectional 10/100 Ethernet port, local analog and digital T1-carrier trunking, E911, PPP data routing to the corporate LAN, and dial tone even during WAN failure or power failure. The unit supports local switching among TDM peripherals (stations, trunks). IP telephones may be supported at the remote location when using the R300 Ethernet port connection for signaling connections over the WAN back to the IP-PBX common control complex.

The Nortel Networks and Avaya remote IP carrier options are similar in concept (remote station/trunk support, IP control signaling over a WAN link, gateway interfaces at the main location) but somewhat unique in their sum capabilities (port capacities, types of stations supported, survivability features, data networking functions). Both IP options could, in theory, be used for dispersed communications services across a campus location, instead of remote locations, like the Siemens offering originally known as the Fiber Loop Exchange (FLEX) option. First available on the supplier’s Hicom 300H platform (currently upgraded to HiPath 4000), the option consisted of a port cabinet shelf that could be installed remotely from the main system complex and a dark fiber cabling link to support IP-based control signaling and voice communications channels. Available in a redundant mode, the optical fiber cable interfaces to the Hicom 300H common equipment through an optional integrated gateway board, the HiPath HG 3800. The gateway card provides connectivity for the Hicom 300H hub location to remote shelves by using Fast Ethernet fiber over dedicated single or multimode fiber to campus locations within a 20-mile radius of the host location. The remote port cabinet shelf, designated the HiPath AP 3300, is designed with an integrated gateway interface circuit.

The newer HiPath 4000 platform can also support remote ports over a traditional LAN/WAN infrastructure. This option requires a HiPath HG 3570 gateway interface card, housed in a HiPath 4000 host system, with IP tunneling to a HiPath AP3300 remote port cabinet or an AP 3500 remote 19-inch rack mount carrier at a remote location. A HiPath HG 3575 is housed in the remote AP 3300 cabinet or AP 3500 carrier. The gateway cards are equipped with 10/100 Base-T Ethernet connectors and are used to transmit call processor control signaling from the centralized HiPath 4000 common control complex to the remote cabinet/carrier. Gateway resources convert PCM signals to IP packet format for LAN/WAN transport and provide TDM bus connectivity at the host and remote locations. The AP 3300 cabinet and AP 3300 carrier are equipped with TDM switch network backplanes to support local switching for station and trunk ports. In case of LAN/WAN failure, a dial-up modem connection over PSTN trunk circuits can provide gatekeeper control signaling support to the remote cabinet/carrier.

The Alcatel OmniPCX 4400 distributed processing, switching, and cabinet design easily lends itself to an IP LAN/WAN infrastructure for control and communications signaling. A fully configured system can support a cluster of 10 control cabinets and associated expansion port cabinets by using a variety of networking options, including TDM/PCM, IP, ATM, and frame relay. Each control cabinet/port cabinet group can be remote from the others. A control cabinet also can support a remote expansion cabinet or a single remote IP station. The IP networking option requires an INT-IP card to support gatekeeper control signaling between a centralized control cabinet (with call processor) and remote port cabinets, between control cabinets configured across a LAN/WAN, or to a remote IP station. The INT-IP card also supports gateway functions between dispersed circuit switched port cabinets configured across a LAN/WAN or between dispersed IP stations and a centralized port cabinet. The INT-IP interface card, designed with a 10/100 Base-T Ethernet connector, supports peer-to-peer LAN switching between IP endpoints, provides tone generation and signaling channel processing, and supports several voice codecs (G.711, G.723.1, and G.729/A).