Distributed PBX Common Equipment Design

Numerous customer configurations require a distributed PBX common equipment design that supports a single system image for all features, functions, and operations. These configurations include a campus environment covering a large geographic area, with loop length runs exceeding supported parameters, and customers with communications requirements across two or more locations with right-of-way cabling options. A distributed common equipment design may consist of any combination of the following design elements:

1. Centralized, dispersed, or distributed call processing

2. Centralized, distributed, or dispersed switch network design

3. Distributed port equipment cabinets/carriers (main and remote equipment rooms)

A distributed call processing design is preferable because local call processing reduces the probability of down time for each port cabinet. Distributed port cabinets linked to a centrally located common control complex (centralized or dispersed design) depend on intercabinet links outside a secure equipment room for all call processing functions, and link failure or problems increase as distances between cabinets increase.

For switching functions a distributed or dispersed design is preferable to a centralized stage because local switching requirements do not depend on intercabinet links for all switched call connections, as the centralized design does. Localized switching reduces intercabinet link communications channel requirements.

The first fully distributed common equipment design was the Intecom IBX S/40, introduced in 1980, which was an immediate success with universities with large campuses. The first IBX installation was at the University of Chicago. Another PBX system based on a distributed cabinet design, which has been popular on university campuses, is the Ericsson MD-110. University systems have been the largest single customer market sector for the Intecom and Ericsson systems during the past 20 years. Intecom PBX systems can support distributed cabinets several miles from the main equipment room housing the central control complex. Since its introduction, the Intecom PBXs have used a fiber optic cabling infrastructure in support of call processing signaling and switch network transmission links. The MD-110 can support more than 200 distributed port cabinets, each with a local common control complex, across virtually unlimited distances over PCM-based transmission links, with the use of copper, broadband fiber, or microwave transmission media.

Several new IP-PBX systems designs using a LAN/WAN infrastructure to support dispersed station users are ideal single system solutions in support of large coverage communications requirements. Systems from recent market entrants such as Cisco Systems and Sphere Communications fall into this design category. The traditional Siemens 300H architecture design is not based on an IP-based LAN/WAN cabling infrastructure, but a recently introduced IP-based remote carrier option is supported over a dark optical fiber cable between the main equipment room and the remote building location. The Alcatel OmniPCX 4400 can use a variety of intercabinet transmission links between distributed cabinet clusters, including PCM, IP, or ATM formats and copper or optical fiber cabling. Avaya, too, can use an ATM LAN/WAN infrastructure to support its EPN equipment cabinets for on-site or off-site requirements. Each of these systems also supports redundant common control capabilities.

Among the new IP-PBX system designs, the Cisco offering stands out. The Cisco AVVID IP telephony system can be configured with up to five active call servers. The call servers can be in the same equipment room, dispersed across the same customer premises, or across multiple customer premises. IP telephones can be supported by one of three servers for redundant call processing control. In case of WAN link failure, Cisco was the first IP-PBX supplier to announce support of basic telephony functions at remote locations with the use of a call processor blade embedded in one of its IP routers. The remote survivable telephony system option is an emergency backup system with limited call processing and feature management capabilities when desktop telephones/soft-phones do not have access to a centralized call telephony server.

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

Distributed and Dispersed Switch Network Designs

A distributed topology is defined simply as a switch network design comprised of multiple, independent local switching networks that are connected with direct communications links instead of a center stage switch complex. Each local switching network operates independently of the others and supports all of the communications needs of the local port interface circuits it connects to. Communications between user ports housed in different cabinets require a direct communications path between each cabinet’s local switch network. There is no center stage switch complex (standard in centralized switch network designs with multiple local switch networks) in a PBX based on a distributed switch network design, which is a potential cost benefit to the customer. Another benefit of a distributed switch network design as opposed to a centralized design is its flexibility in supporting multiple location customer requirements. Without a center stage switch complex, the communications links between remote locations and the main customer site are minimized because most station user traffic is local to the cabinet’s switch network. Only intercabinet traffic requires communications link resources.

Figure 1: Distributed switching network topology.

A distributed switch network design is usually limited to PBX systems with a minimal number of local switching networks supporting two or three port cabinets. Once the number of local switching networks exceeds three, it usually becomes a cumbersome, and expensive, process to upgrade the system because of the necessity of having direct communications links between each cabinet, unless a cabinet can be used as a tandem switching node within the distributed cabinet configuration. The two most popular PBXs based on a distributed switch network design are the Avaya Definity G3si and the Alcatel OmniPCX 4400. A Definity G3si can be installed with up to three port network cabinets (a PPN control cabinet and two EPN expansion port cabinets). Each cabinet has a local switching network based on a 32-Mbps TDM bus and can be equipped with expansion interface circuit boards to connect to an EAL (see above) for intercabinet communications. There is no center stage switch complex, and each port network cabinet TDM bus functions independently.

The Alcatel OmniPCX 4400 is an example of a PBX with a distributed switch network design that can support more than three cabinets, making it the exception that proves the rule. As part of its Alcatel Crystal Technology (ACT) system architecture, a single OmniPCX 4400 system can support up to 19 discrete cabinet clusters (control cabinet and expansion cabinets); each cabinet cluster has a local TDM bus (420 two-way channels) and can be linked to other cabinet clusters over a variety of communications paths based on PCM, ATM, or IP communications standards. A single interface board in the cluster’s control cabinet can support up to 28 communications links. The bandwidth of each PCM link is 8 Mbps; the ATM link can operate at transmission rates of up to 622 Mbps. Direct links between any two cabinets can be established, or a control cabinet can function as a tandem switching node to link two or more distributed control cabinets. The availability of very high-speed communications links between cabinet clusters can minimize the number of physical transmission circuits supporting intercabinet cluster communications requirements, and the use of hop-through connections through a tandem switch node allows Alcatel to design large and very large system configurations without a center stage switch complex. Alcatel markets a multiple system version of the OmniPCX 4400, capable of supporting a maximum of 50,000 stations, and can design the network to handle communications traffic between all cabinet clusters across all systems without a center stage switch complex.

The third type of switch network design is dispersed topology. A dispersed switch network combines the design attributes of a distributed design (functionally independent local switch networks) and centralized design (center stage switch complex connecting local switch networks). A dispersed switch network design is comprised of local switch networks that support all of the local communications requirements of its connected port interface circuits and a center stage switch complex that is used only to provide switched connections between local switch networks for calls between ports connected to different local switch networks. For example, a call between two ports in the same cabinet sharing a common switch network would be connected by using only the resources of the cabinet’s local switch network, such as a local TDM bus. If a call were placed between ports in different cabinets, the call would be connected through the center stage switch complex, and access to the center stage switch complex would be via the local switching networks.

Figure 2: Dispersed switching network topology.

For example, the Avaya Definity G3r, a larger version of the Definity G3si, can support up to 40 port network cabinets. The G3r EPN expansion port cabinets are identical to the G3si cabinets; each is designed with a local switching network capable of handling all local communications requirements—calls exclusively between ports (stations and/or trunks) in the same cabinet. Calls between ports in different EPN cabinets are handled through a center stage switching complex in the PPN cabinet (common control cabinet). The Ericsson MD-110 is another example of a dispersed switch network design; communications between LIM cabinets are handled through a centralized group switch network complex, but local communications traffic remains within the LIM. The NEC NEAX2400 IPX also can be considered a dispersed switch network design because communications between ports in the same PIM cabinet (a single carrier shelf cabinet) are supported exclusively on the local TDM bus; intercabinet and intermodule group communications are supported over a hierarchy of Highway buses.

Distributed Contact Center

Distributed Contact Center
A retail organization with outlets across the country maintains a call center with 200 agents, primarily for inbound customer service. To remain competitive, this retailer must provide extended hours of service, with a "follow-the-sun" approach, by having two call centers, one located on the Pacific and the other on the Atlantic coast. Both call centers have overlapping hours of coverage to assist during the peak call load periods during the day. The centers are supported by two networked digital PBXs with ACD and customized CTI.

In addition to supporting regular inbound calls, the retailer is looking to implement a converged IP contact center, which integrates the functionality of the call center with their Web site. Web site users are to have seamless integration when working with the site and the new contact center.

To build this capability, IP telephony technology has been defined for use. The organization has the choice of purchasing IP-PBX systems or using an IP-Centrex service from a national provider. With the IP-PBX option, the retailer will run VoIP traffic over the corporate WAN between the sites (a 3-Mbps LAN extension circuit derived from a carrier-provided MPLS network). With IP-Centrex, the carrier will be responsible for connecting the traffic between the two sites (resulting in a broadband interconnect charge).

In the estimates for IP-Centrex, we have included a figure of $80 per month for each agent, for the rental of contact center-related software (ie., ACD and CRM). Because this results in a 5-year total of $870,000, compared with an estimated $500,000 purchase cost for two copies of the software on the IP-PBXs, it is probably a higher figure than can be justified.

We should also note that the expected annual cost for the system administration/software support personnel has been set at $70,000 each, to account for the high skill level required in this complex contact center environment.

The bottom line totals show a 10% lifetime cost advantage to the IP-Centrex solution, compared with two IP-PBXs.