Summarizing Client/Server IP-PBX Design Issues

It is apparent that client/server IP-PBX designs differ across models. There are even major design differences within one supplier’s product family. For example, the Cisco Systems 7750 ICS all-in-one design is radically different from the supplier’s larger MCS 7825/7835 multiple server cluster design; Siemens offers client/server models based on a closed, embedded Windows NT server or a customer-provided server option. Client/server IP-PBX designs may be far simpler than traditional circuit switched PBXs and converged IP-PBX platforms, but there is a sufficient number of design variables to create major differences between system models.

Figure 1 shows how three layers (call telephony server, gateways, and applications) of a client/server IP-PBX can be designed and integrated into the system architecture. The choices available to a designer include a proprietary or a nonproprietary call telephony server, integrated or external advanced applications support, integrated or external gateways. Gateways and application servers may be a mix of proprietary or third-party solutions. Designers may select proprietary components for quality control and development of feature/function options capabilities not supported by third-party solutions. Third-party components may reduce system costs and provide customers with more design flexibility in their purchase decisions.

Figure 1: Client/server IP-PBX design.

Several IP-PBXs originally designed using third-party call telephony servers were redesigned with customized servers because distributors and customers often failed to use a third-party server with the technical specifications as recommended by the supplier. Several manufacturers that currently offer a proprietary call telephony server model plan to migrate their systems to a less expensive third-party server solution, but not until all the system design bugs and problems are solved after one or two system generations.

If issues related to proprietary or third-party components are not part of the customer-buying equation, there are other important design and performance criteria to be evaluated before a purchase decision is made. System reliability and survivability are as important when evaluating a client/server IP-PBX as they are in a circuit switched PBX design—port capacity, traffic handling, and call processing power. The following is a summary checklist of design performance issues for evaluating a client/server IP-PBX (many unique to an IP telephony communications system):

• System redundancy—Fully duplicated or shared back-up components for call processing, memory, and power functions

• System port capacity—Stations (IP, non-IP) and trunk interfaces (analog, T1/E1, IP)

• System traffic handling capacity—Distribution of gateway channels and conference circuits

• System call processing—BHCC

• Supported call control protocols and interfaces—H.323, SIP, MGCP, SGCP, MEGACO, etc.

• Voice codec support—G.711, G.723.1, G.729(A,B), GSM, etc.

• QoS support—DiffServ, 802.1p/Q, COS, TOS, IP precedence, RSVP, dynamic jitter buffer, packet loss replacement

• Standard messaging system interfaces—AMIS-A, VPIM, LDAP, IMAP

The first client/server IP-PBX systems were shipped less than 5 years ago, and in that time the design and performance capabilities have changed significantly. It will take several more years until the product design stabilizes, and its reliability is comparable to current circuit switched PBXs, which have been shipping for more than 25 years. Although a new generation of data communications systems seems to appear every year or two, new voice communications system platforms take many years to evolve.

Client/Server IP-PBX System Design

Overview

The first IP-PBX systems were based on a client/server design. When most telecommunications managers hear the term IP-PBX, they usually envision a client/server design, although the converged system design is gaining in popularity and likely will dominate the market for the next few years. An IP-PBX system based on client/server design fully uses and depends on a LAN/WAN switching network infrastructure for call control and communications signaling. Like converged IP-PBX systems, client/server designs are not standard or uniform across manufacturers, although the competing models share some common design elements.

The term client/server is borrowed from the world of data communications. It describes an IP-PBX system that does not use a traditional PBX common control complex and integrated circuit switched network or traditional common equipment hardware (port cabinets and port interface circuit cards). A client/server data communications design specifies a data processing topology in which a personal computer (client) depends on a centralized computer (server) for applications software and database management functions. For many years the traditional PBX system design was compared with a mainframe computer because all call control and switching functions were centralized and desktop terminals (teleprinters, CRTs) lacked processing functions of their own. As enterprise voice communications systems evolved toward distributed and dispersed modular design topologies, similar to the concurrent evolution of minicomputer and personal computer networks, the term client/server was used more and more to describe the improved PBX system design. The first IP-PBX systems more closely conformed to data communications and processing client/server design topology; hence, the adoption of the term.

The common control complex of a client and server IP-PBX is based on a telephony call processing server that transmits and receives control and status signals to/from LAN-connected peripheral endpoints, known as clients. The telephony server’s primary role is as a gatekeeper to clients for call setup and teardown functions and to manage and control communications bandwidth requirements for each call. In IP-PBX terminology, a telephone terminal is also referred to as a client. The IP telephone client depends on the telephony server for dial tone, call routing, and desktop feature/function implementation, similar to the relation between an analog/digital telephone and a traditional PBX common control complex. The major distinction between a traditional circuit switched PBX system and a client/server IP-PBX is that the LAN/WAN infrastructure, not an internal network of TDM buses, is the primary switching network.

Some current and planned client/server IP-PBX models may be equipped with optional port cabinet/carrier equipment with integrated TDM bus backplanes for circuit switched connections, but only calls between ports connected to the same port cabinet/carrier are circuit switched; all other calls depend on the LAN/WAN infrastructure for transport and switching operations. This type of system also may be classified as a converged IP-PBX, because TDM/PCM circuit and IP packet switching is supported. Arguments can be made for categorizing it in either classification, but if the design primarily depends on IP packet communications, and circuit switching is secondary or optional, the system is best defined as a client/server design. Design differences between individual IP-PBX models, as illustrated by client/server designs with optional circuit switched port carrier equipment, make it difficult to definitely classify any PBX system into one category or another.

There are several hardware layer elements common to all client/server IP-PBX models:

• Call processing layer—Gatekeeper/telephony call server

• Client layer—Voice terminals and other communications devices

• Applications layer—Messaging, contact center

• LAN/WAN infrastructure—Ethernet switches, IP routers, telephony gateways

Differences between client/server IP-PBX models are based on how each design element is configured within each layer and between layers. There are also differences in how some features and functions are provisioned, and whether optional application services are integrated into the overall system design or provided through nonproprietary third-party equipment. Ethernet switches and IP routers are designed to industry standards, and products from different manufacturers are usually interchangeable at the infrastructure layer, except for a few select IP-PBX models that may require proprietary LAN/WAN solutions as part of their overall system design or integrate LAN/WAN interfaces into their hardware design.

The Case for the Client/Server IP-PBX

A client/server IP-PBX is likely to be the standard design platform of the future for enterprise communications systems. Although there are several drawbacks to current client/server IP-PBXs, there may be several benefits to customers who select it as their enterprise communications solution. Recalling the pricing discussion in the preceding paragraphs, the upfront capital investment for a client/server design is favorable for customer solutions with the following characteristics:

• Green field location

• Large percentage of IP peripherals, such as telephone instruments

• Remote location requirements with small port capacities

• Small port capacity requirements with PBX performance requirements

Green field locations without existing PBX equipment and with the newest generation of LAN switches and IP routers that can be programmed to support voice-grade QoS levels are best suited to cost effectively support a client/server design. A client/server design that supports a significant number of IP stations and minimal analog communications equipment will be more optimally priced, and support of remote IP stations likely will be less expensive than circuit switched alternatives. Customers with KTS/Hybrid port capacity requirements, but who require the performance capabilities of a larger, more complex PBX system, are likely to discover that the price of a client/server design is a more attractive alternative to a full-featured circuit switched or converged system solution.

The most common reasons given by system suppliers to promote the performance value of a client/server IP-PBX are:

1. Using a converged network for a variety of communications media: voice, data, video, text, graphics

2. Universality of IP transport

Figure 1: Basic client/server IP/PBX design.

1. Lowered communications bandwidth requirements and more efficient use of existing bandwidth

2. Simplified centralized management and administration

3. Rapid deployment of new technology and applications

4. Fully distributed network design

5. Scalability

Converged Network

The concept of the integrated voice/data PBX system in the early 1980s was driven by the increasing amount of data traffic generated by geographically dispersed desktop CRT terminals linked to a centralized host processing system. It was thought that the circuit switched telephony network could be used to carry voice and data enterprise communications traffic. The advent of Ethernet LAN hubs and switches in the mid-1980s effectively ended the dream of the PBX system as an all-media office systems controller. Today the pendulum has swung the other way; packet switched LANs carry telephone-generated voice communications in addition to PC client data traffic. To data communications network designers and managers, the telephone is viewed as just another client, and voice features and functions are just other applications supported by a LAN-based server. If a customer is seriously thinking about migrating to an IP-PBX system, then a client/server design using the existing LAN/WAN infrastructure is the optimal solution. As LAN bandwidth capacity continues to increase, more video communications traffic will be carried between desktops with decreasing dependence on larger, more expensive, room-based videoconferencing systems.

Figure 2: IP-PBX pricing: converged versus client/server.

Universality of IP Transport

Ten years ago the Internet was used almost exclusively by government and higher education institutions. Today the Internet is everywhere, and IP control and transmission signaling have become the standard for data communications networks. There is universal access to LANs and WANS in all medium and large enterprises across all industry sectors. The client/server enterprise data communications network design has replaced the host mainframe design that was dominant in the 1970s and most of the 1980s, and IP networking has replaced IBM’s Systems Network Architecture (SNA) as the standard. If a customer is looking at an IP-PBX system solution, the current data networking infrastructure is favorable to a client/server design.

Network Bandwidth

Using the same communications network for voice and data traffic reduces overall bandwidth requirements because the two traffic streams can be interleaved and QoS levels can be engineered and programmed to satisfy real-time voice communications requirements. There will be cost savings and increased network efficiency due to economies of scale as a customer migrates an increasing percentage of traditional circuit switched PBX traffic to the packet switched LAN/WAN. The major bandwidth cost savings and optimization will be attributed to off-prem- ises communications because circuit switched PSTN trunk carrier facility requirements can be reduced.

Simplified Centralized Management and Administration

A single communications management system is less costly to operate and more easily administered than separate systems for voice and data communications. The primary elements of an IP-PBX client/server design—desktop IP telephones and telephony call server(s)—are indistinguishable to a data network management system when compared to with PC clients and a data processing server or database manager. All voice system moves, adds, and changes are performed from the data network management workstation, as are maintenance and service operations.

Rapid Deployment of New Technology and Applications

A client/server design lends itself more to rapid deployment of new technology and applications because there are fewer hardware elements in the system architecture than in a converged IP-PBX that retains the traditional proprietary common equipment components of a circuit switched PBX. It is far easier to implement a technology upgrade for a client/server IP-PBX because there are fewer if any, proprietary switching network elements, and new advanced applications can be implemented through a software upgrade or an optional applications server. Client/server designs based on third-party servers are easily upgraded and enhanced by replacing the existing server with a newer, more powerful server. Migration between client/server IP-PBX generations will be smoother and less disruptive than between converged system generations that are based on more proprietary and costly common equipment hardware.

Fully Distributed Network Design

A client/server IP-PBX is by definition a distributed network design. A single telephony call server can support premises and off-premises IP stations. Premises stations can be distributed in a single building or across a campus. Multiple server designs can be programmed to support redundant emergency call processing in case of individual server failure. The servers can be colocated or distributed for disaster recovery situations. Customers have multiple layers of LAN/WAN switching and routing that preclude single points of failure.

Scalability

A client/server design has the potential to be highly scalable because IP telephones are easily added to the system without the need for specialized port interface circuit cards, and port capacity can be expanded through the addition of another server. Converged IP-PBXs require port interface cards, if only for control signaling, and, except for the rare system model based on a distributed processing design, there are call processing and port capacity limitations when using a single common control complex. The switching and transport limits of a client/server design are virtually boundless because a customer can continually install switches and routers to the LAN/WAN infrastructure to support increased traffic or more stringent QoS levels.

There are many customers for whom a converged or client/server IP-PBX system design will satisfy financial, feature performance, and applications requirements. Many enterprise communications system customers may also decide that their circuit switched PBX system will continue to be a satisfactory solution for current and near-term needs. A customer may not be ready to immediately replace his legacy PBX system with an IP-PBX system, but should seriously consider testing ToIP technology very soon.