Getting there from here

Looking to implement voice over IP? Better look before you leap — the market is exploding with new vendors, products and services. Some are ready for prime time, while others could benefit from a little more experience. Here are some tips to get you started:

• Tap your existing resources. In many cases, voice and data network responsibilities are handled by separate departments. Each may have a separate budget and an empire to match. There are likely two areas of expertise: circuit switching and traffic analysis on the voice side, and packet switching and IP knowledge on the data network side. Both skill sets are required for a successful voice-over-IP (VoIP) implementation, so make sure these two groups collaborate.

• Know your traffic. Make sure your data network can handle the traffic increase before you add voice to the mix. With steady growth of only a few per cent per month, that DS-3 circuit you put in just last year may be getting close to its capacity during peak traffic periods. Verify that excess capacity exists before adding more traffic. In addition, get a handle on the number of hops between selected destinations and the resulting network latency, as these delays will dramatically affect the quality of the packetized voice and video service.

• Measure performance. End users will likely judge your VoIP implementation based on current perceptions of their telephone service, which is generally high-quality and reliable. The figure 99.999 per cent, or “five nines” of reliability, is an oft-quoted statistic that reflects how infrequently phone service is unavailable. Thus, the net architect must manage client expectations of service and reliability. To do this effectively, the various VoIP network elements must be able to provide packet delay and packet loss information, signal-to-noise ratios and other statistics that affect voice and data transmission quality. Support for SNMP- and RMON-compatible network management systems is another important factor. The ability to baseline the network and identify trends as they develop are also key ingredients for optimum performance tuning.

• Examine existing carrier contracts. Many net managers have carrier service contracts that should be reviewed prior to jumping into VoIP service. You may need to divert a significant number of minutes from existing contracts to new IP services in order for the economics of the new hardware investments to be favourable. And when that diversion occurs, you may end up paying more for your existing voice services.

For example, assume your existing service agreement specifies a rate of five cents per minute if you use one million minutes per month, and three cents per minute if you use two million minutes per month. You now use more than two million minutes per month (at three cents), but you estimate usage will be cut by more than half when you divert some voice traffic to the IP net. In this new scenario, the existing (non-IP) voice traffic will cost you five cents per minute because you have dropped below the three-cent price point. In other words, in order to reduce costs with new IP services, you increase costs for the remaining voice services. A word to the wise: look at the interrelated economics before you commit.

• Weigh international vs. domestic. One of the early driving factors in the VoIP market place has been the promise of “free” or very low-cost long-distance service. But before you take this promise at face value, get a good handle on your calling patterns and determine what percentage of your traffic is international vs. domestic. With international rates in excess of $1 per minute to some destinations, VoIP rates that are only a few cents per minute look very favourable and may be worth some compromises in quality. The domestic story may prove to be completely different, as noted above.

• Make sure it’s compatible. Your VoIP gateway may need to interoperate with a number of existing and future voice processing systems, such as your PBX, automatic call distributor, interactive voice response system and others. Do the trunk circuit port types on your PBX match those available from your gateway vendors? Are you planning any future expansion or applications, such as a migration to ISDN or the installation of a Web-enabled call centre? Make sure the new VoIP hardware is compatible with all other voice systems.

• Be careful about codecs. Analogue voice must be converted to a digital pulse stream before it can be placed in packets and sent over an IP net. A coder/decoder is the device that performs this voice processing function. A variety of standards are available, including the ITU-T G.711 (64Kbps voice), G.729 (8Kbps) and G.723 (5.2Kbps to 6.3Kbps). In addition, a number of vendors have developed proprietary schemes. Each alternative has unique characteristics, including the quality and delay associated with the coding algorithm, which vary with the amount of voice traffic being crammed into the packet. So it’s important to understand the characteristics of the voice you’re transmitting. Do the net requirements include the ability to pass fax traffic or music-on-hold over IP, or is voice traffic the sole need? Does the gateway product allow for multiple codec options, or is it locked in to one standard or algorithm? Ask some questions about the codecs to get the right match for your network requirements.

• Manage your bandwidth. Real-time traffic, such as voice and video, should be given priority over more routine transmissions such as file transfers and e-mail. Several methods are possible, including setting priority by IP address, setting priority by protocol or using a reservation mechanism such as the Resource Reservation Protocol. However, not all routers are configurable to support such schemes. Check your existing routing infrastructure to see if prioritization capabilities exist.

• Keep ease of use in mind. If voice gateways are to be accepted, they must be easy for end users to operate, and they must work within existing dialling plans. As you research various products, ask for a demonstration of the dialling sequence required to access the VoIP network, and verify that it’s compatible technically and procedurally with your existing methods of establishing, transferring and otherwise managing voice calls.

Mark Miller is president of DigiNet, a Denver-based data communications engineering firm. He has written 13 books on internetwork design and analysis, including Troubleshooting TCP/IP and Implementing IPv6.

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