Listen to the Computerworld TechCast: VoIP.

Voice over IP (VoIP) has exploded in popularity since the first PC-to-PC Internet phone-calling systems were introduced in 1995, as evidenced by the millions of households now subscribed to VoIP services. Audio quality, call reliability and telephony features of VoIP systems are finally maturing to be viable on enterprise networks as well, and only a few barriers remain to widespread corporate adoption of VoIP.

The Case for VoIP

Businesses can benefit from VoIP technology in several respects. One obvious advantage VoIP brings is avoidance of long-distance toll fees by routing calls through Internet connections. VoIP phones can call to (or be called from) any number in the world just like traditional or cell phones. While VoIP will not completely eliminate public telephone charges, geographically dispersed organizations especially can save much money here.



Linking VoIP call management systems with corporate e-mail, instant messaging and database systems also expands business communication capabilities. Using the presence awareness of an application such as Microsoft Office Communicator, VoIP calls to employees can automatically be generated from instant messages as needed according to their preferences. Similarly, incoming VoIP voice mails can automatically be converted into e-mail messages for faster access and response. The benefits of voice and data integration reach even further, increasing the flexibility of IT help desks, human resources and other intranet systems that manage employee information, as well as customer call centers.

To the extent their voice and data networks converge, enterprises implementing VoIP can reduce their IT support burden through consolidation. Compared to how virtual private networks have reduced mobile workers' need for remote access servers and modem pools in some corporate environments, VoIP systems can shrink the infrastructure and support costs associated with traditional telephony systems even more dramatically.

Enterprise VoIP Building Blocks

On an end-to-end enterprise VoIP network, an IP phone replaces the traditional digital phone handset and shares a similar physical appearance. An ordinary PC can also serve as an IP phone (a "soft phone") when VoIP software is installed. Enterprise IP phones typically connect directly to the LAN via Ethernet or sometimes Wi-Fi, while Universal Serial Bus phones exist as an alternative to soft phones. VoIP phones obtain an IP address, normally via Dynamic Host Configuration Protocol (DHCP), which allows them to make and receive calls.

Analog telephone adapters (ATA) allow ordinary telephones to interface with a VoIP-enabled network as an alternative to using an IP phone. ATAs normally feature RJ-11 ports for connecting the phone and corresponding RJ-45 Ethernet ports for connecting to the LAN.

A VoIP softswitch (or "call processor") maintains the mapping of VoIP phone numbers and IP addresses. As the name implies, softswitches involve special-purpose VoIP software responsible for forwarding voice packets between IP phone endpoints or other softswitches. Call-processing software is typically installed on dedicated servers for enterprisewide deployments but some products like Cisco CallManager Express can be embedded in routers for smaller-scale needs.

Media gateways are server appliances that (among other functions) interface voice and fax communications between a VoIP network and the public switched telephone network (PSTN). PBX-IP media gateways are one such appliance that extends an organization's traditional private branch exchange with IP capability to simply the transition to VoIP.

VoIP Networks -- Bringing It All Together

Gluing all of this VoIP equipment together is the job of various network technology standards. The two leading protocols for VoIP call management (signaling for call setup and teardown) are Session Initiation Protocol (SIP) and H.323. Early VoIP systems relied on the H.323 standard, while SIP has emerged as the new front-runner thanks to a firewall-friendly, IP-centric design.

Inside a VoIP call, the individual packets of voice conversations are generated from analog voice signals by coder/decoders embedded within IP phones or terminal adapters. Many different audio codecs exist, each making different tradeoffs in sound quality, bandwidth utilization and transmission delay. The G.711 codec standard became well-established together with H.323, and most VoIP endpoints support it today. G.711, however, requires 64Kbit/sec. bandwidth per call on IP networks. Alternative codecs like G.723.1 and G.729 have been gaining in prominence and generally require only 8Kbit/sec. or less per conversation, at the cost of higher latency and potentially lesser audio quality.

For VoIP equipment to work together across the enterprise, compatible protocols and codecs must both be deployed. H.323 and SIP are incompatible with each other, and most codecs likewise do not interoperate. Fortunately, vendors today tend to support multiple VoIP standards in their gear, lessening the interoperability concern that has existed with VoIP from the outset.

VoIP Quality of Service

Implementing enterprise VoIP requires careful attention to quality-of-service (QoS) issues across the corporate WAN. Various technical challenges exist to build QoS applications on IP generally, and VoIP is no different. Without QoS in place to guarantee network bandwidth and capacity, enterprise networks can't scale to handle the demands of peak calling periods. Approaches to ensuring QoS for VoIP include bandwidth reservation via the RSVP protocol, so-called differentiated services that support traffic classification with service-level policies and Multiprotocol Label Switching (MPLS) traffic engineering.

What About VoIP Security?

The security of VoIP communications continues to be an area of strong focus. Consumer VoIP systems may choose not to encrypt calls, but enterprises generally will require encryption. With this and other security measures in place, VoIP is inherently no less secure than other forms of IP networking. Protecting IP phones from spam, preventing VoIP numbers from being spoofed for outgoing calls and guarding against call eavesdropping are all necessary on converged networks.


VoIP technology is positioned to not only replace traditional telephone networks but also to improve on their capabilities. The value proposition of VoIP includes both cost savings and worker productivity gains. While probably overhyped in the past, many organizations of all sizes have now at least begun the transition from traditional telephony to VoIP.

The architecture of VoIP systems allows for incremental migration and deployment. Traditional phones can be fitted with ATAs to preserve that investment while focusing on VoIP infrastructure. Alternatively, VoIP phones and gateways can interface to existing PBX and public-switched telephone network infrastructures. While VoIP chat applications like Skype and Google Talk also enable PCs to be used as IP phones, the support costs may outweigh their advantages.

On the other hand, compared with the relative ease with which consumers can adopt Internet phone calling systems today, greater challenges remain for enterprise VoIP adoption. Ensuring VoIP equipment compatibility and long-term viability of the various standards is imperative. Finding IT personnel with the right skill sets to deploy and manage these relatively complex technologies can be difficult. Ensuring that the corporate network can scale to handle the added load of VoIP traffic is also nontrivial. CIOs and IT managers should view VoIP technology as a long-term investment and take a conservative, phased approach to migration.

Sidebar: More on VoIP.

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Copyright © 2006 IDG Communications, Inc.

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