September 2002

BY TONY RYBCZYNSKI

• IP telephones and PC soft clients.
• Communications servers (also called call management servers or gatekeepers).
• Media gateways providing flexible network access (e.g., via traditional PBXs and the PSTN and the public wireless network and beyond).
• Application servers (e.g., unified messaging, conferencing, and SIP-enabled collaborative applications).

These functions are distributed across a telephony- or business-grade IP network that delivers the required levels of reliability, voice quality, and congestion management. Extended reach and mobility are provided over wireless LANs and public networks and the Internet. This article discusses platform choices faced by enterprises in rolling out the four logical functions of every VoIP system.

STARTING WITH THE CLIENT
The touch point for the end user is the VoIP client. This could be a wired VoIP telephone, a wireless LAN VoIP telephone, or a laptop or PC running some form of soft client. While clearly more flexible, PC-based clients are less reliable than purpose built appliances. Generally, the breadth of VoIP client solutions will grow over time in a multi-vendor interoperable environment, and be heavily influenced by end user needs and desires.

Under the covers, these generally fall into two broad categories: thin clients and thick clients. Thin clients rely primarily on network intelligence provided by the communications server to initiate communications and manage feature operation. These use so-called stimulus protocols, which in the standards arena are defined by MEGACO and H.248 (two names for the same standard initiative, the former being standardized by the IETF and the latter by the ITU). The thinnest client is in fact the nil client, one based on standard browser software. Thick IP telephony clients rely primarily on built-in intelligence for feature operation. Communications servers are still required to administer centralized numbering or naming plans, and to provide specific networking features. These rely on protocols such as H.323 and SIP.

Some vendors have implemented XML interfaces in their telephone sets to control the display and the key-press interactions. While this makes it very easy to extend the IP set to different vertical application markets, it also opens up the phone to be hacked in similar ways to Internet browsers. In the latter case, anti-virus software is generally used, but the solution for VoIP sets is less clear, with the risk being, for example, involuntary call transfers, or key-presses resulting in calls to rogue applications.

User demands are also driving the need for client-side media gateways that provide support for legacy analog and digital telephones. The former is driven by the desire to support low cost telephones (e.g., in elevators) and fax machines in a VoIP environment. The latter is driven by the need to protect the investment in digital telephones, as the enterprise moves towards VoIP. These gateways can be based on any one of the protocols referenced above.

Communications Servers � The Masters of VoIP
Communications servers are the brains behind VoIP systems, and provide the control required to allow calls to be established across the network. The amount of intelligence depends on whether thin (e.g., IP phone) or thick (e.g., PC) clients are used as end points. Communications servers coordinate address translation (between telephony and IP address spaces), and handle call signal processing, call setup and related management, resource management, and admission control in an IP network environment. For example, the communications server may make a permissioning decision to allow a particular call to proceed based upon available bandwidth. Communications servers can keep track of the state of active calls and generate required logs; state tracking is required for all calls involving thin clients, H.323 clients, and for some SIP clients. Since communications servers may support multiple protocol stacks, signaling translation may also be provided. They can instruct media gateways on how to set-up, handle, and terminate individual media flows, and provide an administration point on the network for IT managers to control traffic through these media gateways, for example, blocking off-net calls from unauthorized users. Communications servers provide the necessary application-programming interfaces to integrate application servers into the VoIP environment. Finally, they can be replicated for high availability and networked for scalability.

There are two common implementations of communications servers: standard server-based and purpose-built.

Server-Based
Standard server-based communications servers are software components implemented on industry-standard computing platforms such as Windows NT or UNIX servers, collectively providing call processing and resource management functionality for a set of media gateways and VoIP clients. They have the general advantage that these servers can run third-party applications, though this is usually only permitted subject to some form of certification procedure. They have the general disadvantage that they are vulnerable to hacking and security breeches. A variant of this approach are communications servers that are built on servers running an embedded operating systems, such as real-time UNIX variants (e.g., VxWorks). These real-time operating systems give priority to call processing over management tasks, a critical capability for telephony environments. These are considerably less vulnerable to security breaches, but make porting third-party applications more problematic. Feature-richness in either case can range from a handful of most common features to �PBX equivalence.� In all cases, these run on off-the-shelf processor platforms, use cost-effective Ethernet technology and SNMP-based management.

Purpose-Built
Purpose-built communications servers are implemented or packaged as a component of a multi-functional product. The best example of such an approach is the VoIP-enabled PBX, a solution that has a lot of appeal to enterprises wishing to evolve to VoIP while leveraging their investment in PBXs. These are often built on embedded operating systems, are closed systems, and have a proven track record of being highly scalable secure solutions. PBX equivalence and reliability are the hallmarks of this approach.

Another example of purpose-built communications servers is the small office system supporting a range of communications functions including VoIP telephony, messaging, automatic call distribution, media gateway, and data networking functionality. This converged �office-in-a-box� solution addresses the need for dramatic simplification of branch and remote office environments.

Media Gateways To The Legacy World
Media gateways come in two forms: client-side and trunk-side gateways, which translate between VoIP packet data and analog or digital T1 voice trunks, and analog and digital telephone set interfaces, respectively. VoIP-based interworking between enterprises and service providers will ultimately result in the collapsing out of the trunk gateways resulting in QoS improvement, cost reduction, and management simplification. More generally, a media gateway provides the media mapping and/or transcoding functions between the IP network and circuit-based networks. This includes compression, silence suppression, and echo cancellation mechanisms as necessary. They also have to handle H.323 or SIP on the VoIP side and whatever signaling is required on the client or trunk-side. In addition, media gateways reserve and release resources, and track the state of calls, all under the control of the communications server. They also provide maintenance functions such as loopbacks and continuity tests.

There are three common implementations of media gateways, with variations primarily in terms of number and types of interfaces supported: standalone, network-based, and integrated.

Standalone
Standalone media gateways are based on purpose-built devices of fixed or modular design. The sole purpose of these devices is to provide the client and or trunk-side media gateway functionality. The advantage of standalone media gateways is that they can be deployed wherever they are required, and can evolve independently of the underlying networking infrastructure. Reliability can be achieved through resilient gateway design and by having redundant gateways. On the flip side, they are another device to be managed in the network. Standalone media gateways are well suited for high-density applications typically found in service-provider networks and very large enterprises.

Network-Based
Network device-based media gateways are integrated with networking devices such as routers and access devices (e.g., FRADs). The key advantage is fewer devices to manage. There may also be advantages from having the gateways coupled with the network routing, bandwidth, and traffic management characteristics of the host network device. However, they are less reliable than standalone gateways, since they are part to the networking software environment with its not infrequent fixes and upgrades. Router hardware upgrades and replacements (as often as every couple of years compared to two or three times that for voice products) result in a total cost of ownership hit for VoIP media gateways. Security may also be a concern with more popular routers.

Integrated
The most common integrated media gateways are those tightly integrated with PBXs. These are closely coupled with all the PBX features, including the call routing, trunk selection, and telephony class-of-service capabilities inherent in the PBX. They also inherit the reliability and scalability of the host voice switching system. The key disadvantage is that their deployment is tied to the modularity of the PBX, which may not be as granular as standalone or network-based solutions. Media gateways are also being offered as integrated functionality of aforementioned �office-in-a-box� solutions.

In all three cases, interworking with existing circuit switched environments is provided with vendor-specific degrees of feature/functionality. The same is true of analog phone/fax support on the client side. Support for media gateways for digital telephones is currently the domain of the incumbent PBX vendors.

Application Servers For Both VoIP and Legacy Worlds
Application servers support a range of services and applications including unified messaging, customer contact centers, conferencing, and collaborative multimedia services. These are made available to all capable users, whether they are running VoIP or legacy voice systems. Capable users are those that have a need to access an application and are recognized by the application, the richness of the communications being dictated by the user�s choice of appliance and access method. Application servers can be located anywhere in the network, balancing application performance and network optimization. In smaller implementations, application servers can be co-resident on a single server with communications server functionality.

A special form of application services provides media processing: These provide services such as conference bridging, and interactive voice recognition units. These are often implemented using specialized hardware, packaged as cards in general purpose servers or as modular multislot units.

VoIP Platforms � A Means To An End
Every VoIP system consists of four functional building blocks: hard and soft clients, communications servers, client and trunk media gateways, and application servers. The best solution for a particular customer needs to meet the price/performance, management, reliability, and scalability requirements of the enterprise. Options include specialized servers dedicated per function to office-in-a-box solutions, well suited for small office environments. Major factors impacting the enterprise business case include the business trigger (e.g., end of Centrex contract, new building construction), the choice of targeted environment (e.g., branch, telecommuters) and the ability to leverage the installed investment.

These platform options are but a means to an end. VoIP is less about recreating PBX functionality in an IP environment, and more about creating a new multimedia communications paradigm. The end user wants to be able to communicate whenever and wherever he desires, with the richness of communications dictated by the capabilities of the chosen device and access speed. In the longer term, the end is primarily end user productivity enhancements and improved customer engagement and secondarily operational cost effectiveness through convergence.

Tony Rybczynski is director of strategic enterprise technologies for Nortel Networks with 30 years experience in networking. For more information, visit the company�s Web site at www.nortelnetworks.com.

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