Distributed Modular Design

A category of converged IP-PBXs was originally designed to leverage a customer’s existing data communications LAN/WAN infrastructure. These are systems based on distributed, modular design architecture for call processing, switching, and port interfaces. Traditional analog telephones were used as the primary voice terminals (at least in the early system releases, until IP telephones were supported), and advanced system features and functions were available through CTI-based PC telephony. IP-PBX systems that best illustrate this IP-PBX system category are from Sphere Communications and Shoreline.

Sphere’s Sphericall Enterprise Softswitch solution consists of several major system components:

• Sphericall Manager—Host platform for Sphericall Softswitch call control software; includes remote management and monitoring

• PhoneHub—MG carrier for up to 24 analog or CLASS stations from IP or ATM networks

• COHub—MG carrier for T1/E1 connections to PSTN or PBXs from IP or ATM networks; Q.sig, ISDN, CAS, and international protocols are supported

• BranchHub—Remote or small office (6 × 12) analog trunk and station MG carrier to IP or ATM networks; six lines of power failure transfer

• VIM—Remote office or campus extension carrier, ATM IAD for T1/E1 or fiber connections to the MAN/WAN; downlinks for voice/video/data

A Sphericall system requires a single centralized Sphericall Manager to support customer premises PhoneHub and COHub carriers and remotely located BranchHub and VIM carriers. IP telephones are supported by the manager through direct control signaling over the LAN or across a WAN. Redundant managers can be configured for purposes of survivability. Individual manager, PhoneHub, and COHub carriers are interfaced to each other using an Ethernet LAN. The manager uses TCP/IP transmission to support call processing signaling to dispersed port interface carrier equipment. The BranchHub is remotely linked over a customer WAN. The PhoneHub and BranchHub carriers have TDM switching backplanes to handle local intercom calls; calls between dispersed station hubs (local, remote), IP telephones, and trunk hubs are handled over the LAN/WAN via integrated MGs in each port carrier. A Web browser systems management interface tool allows system administration support from a centralized management workstation or from multiple dispersed workstations.

The Shoreline system is based on a similar design concept, with one distinct difference. The Shoreline3 system is a completely distributed, modular voice communication solution, with no single point of failure, which is layered on top of the IP network. At the heart of the system is the standards-based Distributed Internet Voice Architecture (DIVA) software, which uniquely distributes call control intelligence to voice switches connected anywhere on the IP network. In addition, DIVA distributes voice applications, including voice mail and automated attendant, to servers across locations rather than centralizing applications at the network core. There are four types of ShoreGear voice switch:

• ShoreGear–24—A 24-port (16 telephone ports and 8 universal analog telephone or trunk ports) stackable or rack-mountable nonblocking voice switch with an integrated IP media gateway

• ShoreGear–12—Twelve universal port stackable or rack-mountable nonblocking voice switch with integrated IP media gateway

• ShoreGear–T1/E1—Digital trunk interfaces to the central office; it can also be used as a VoIP gateway to other PBXs; alternatively, the ShoreGear-E1 can be used as a VoIP gateway to an existing PBX, thereby bridging legacy systems to the Shoreline system

• ShoreGear–Teleworker—Supports remote station users while maintaining full communications functionality

As the names imply, each port carrier has specific interface capabilities. Each ShoreGear voice switch has a local switching TDM backplane and a call processor that runs ShoreWare software to support fully distributed call control, voice applications, desktop applications (via CTI-based PC telephony), and management tools. ShoreGear voice switch carriers, equipped with Ethernet connectors, can be dispersed across a customer LAN/WAN infrastructure to support single- or multiple-location requirements. Voice QoS is monitored and maintained with dynamic jitter buffering and packet loss replacement. Voice codecs can be administered to support linear, G.711, ADPCM, and G.729/A compression formats, echo cancellation, and silence suppression. The distributed call control design provides a high level of local survivability in case of LAN/WAN link failure.

The primary design difference between the Sphere and Shoreline systems is that the Sphere is based on dedicated telephony call server (with optional redundant servers) and the Shoreline embeds call processing functionality and software in each port carrier. Both systems use circuit switched connections for intercom calls between stations interfaced to the same port carrier/hub unit. IP signaling format is used only for intercabinet communications, although the limited port capacity of each carrier/hub increases the likelihood that a significant percentage of calls will be handled across the LAN. Each system is based on an incremental modular expansion design and is ideally suited for a network of numer- ous small locations. A single-location customer configuration with significant port requirements will require a large number of port carrier/hubs. The interface capacity of each port carrier/hub is comparable to a single port interface circuit card in a traditional PBX system. In a large customer configuration, the limited port capacity of each carrier/hub increases the complexity of the network design necessary to support basic port-to-port communications because the QoS level of the customer LAN/WAN is a factor for most premises calls.

There are several important benefits to a dispersed LAN/WAN infrastructure design, including ease of expansion; single-system image across multiple customer locations, including unified dialing plan and feature-transparent operation; toll bypass using private WAN facilities; dynamic bandwidth use of network transmission resources; and centralized administration.

There is often confusion regarding the classification of IP-PBXs into different system design categories. The Sphere and Shoreline systems are sometimes categorized as client/server IP-PBXs because they lack a traditional common control and switching network complex and are heavily dependent on a LAN/WAN infrastructure communications signaling. Both designs are based on circuit switched networking within each port carrier and retain many of the characteristics of a traditional PBX. The Shoreline system can be viewed as a network of mini-PBX systems that uses an IP-based infrastructure to link the multiple systems. The Sphere system is based on a LAN-connected common control complex that supports a cluster of circuit switched port cabinets interconnected over an IP network. Instead of a multicarrier port cabinet capable of supporting dozens of port circuit cards, these systems have substituted a network configuration of port interface carriers/hubs, each one the equivalent of a single port interface card. There are two disadvantages to this approach: more complex hardware equipment is required to support large single-location port requirements and there are limited shared equipment resources. For example, an integrated center stage switching complex is sometimes more advantageous than a complex network of LAN switches. A centralized power supply to support a large system configuration can also be advantageous. As usual, there are advantages and disadvantages associated with every PBX system design