Creating a secure and reliable VoIP solution

• Tags:
• Authority,
• Delays,
• Routers,
• Circuit

Deb Shinder

Published: 10 Aug 2007 14:09 BST

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…can cause transmission errors that may interrupt your phone calls. Of course, viruses, worms, and hack attacks that bring down the network can also bring down your phone system when it's IP-based.

VoIP services that run client software on PCs ("softphones", such as Skype) are also dependent on the resources of the computer. Call quality may be much better when you run the software on a higher-powered system (faster processor, more RAM) or when lots of other applications aren't competing for the system resources.

Increasing VoIP reliability
Despite these problems, there are steps you can take to make your VoIP deployment more reliable:

• Power backup (UPS and/or generator): this will continue supplying electrical power to your VoIP equipment and internet router if there's a power failure.
• Redundant internet connections: two broadband or T-carrier connections from different providers can be aggregated with some routers to provide more bandwidth and also to automatically failover when one connection goes down.
• Dedicated internet connection for VoIP: keeping your VoIP line on its own internet connection, separate from your data network, allows you to isolate it from any viruses or attacks that threaten your data network and to protect it with its own firewall, which can be configured to block everything but the specific protocols needed for the VoIP communications.
• Redundant VoIP lines: if you need multiple voice phone lines, you don't have to get them all from the same VoIP provider. Although that may be more convenient and you may get a better price, having different lines from different providers can keep you talking if there are problems at the provider's end that cause your voice services to be unavailable.

The packet-switching problem
Circuit-switching technology used by the PSTN establishes a dedicated connection between two endpoints (the caller and the person called). During the call, all of the signals that make up the voice transmissions travel across that same link, in much the same way as trains travel from one city to another over a dedicated track.

In a packet-switching network (the internet and other TCP/IP networks), the transmissions are broken up into small chunks (packets) and are routed over multiple routes from caller to person called; the packets eventually arrive at the same destination, but take different routes to get there. This is a more efficient means of transmission because it doesn't tie up an entire route for the duration of the call. The packets can go across the least congested and least expensive lines. The same amount of bandwidth used by one PSTN phone call can be shared by several VoIP calls.

The problem with packet switching is that latency, jitter, and dropped packets are fairly common. Latency refers to the amount of time it takes for a packet to reach its destination. Delays result in high latency. Packets can be delayed at a router or other gateway that they pass through, or travel more slowly along a low-bandwidth link or one that is crowded with a large amount of traffic. Jitter refers to uneven transmissions, with data flowing in quickly at times and being delayed at other times.

Delays are also caused by errors and packet loss. If a packet is lost, it must be resent, which causes a delay. Propagation delays are caused by the distance between the two points. The type of link used can affect delay. For instance, satellite transmissions are always subject to high latency because of the long distance travelled from earth to the satellite in orbit and back down again (satellites in geostationary orbit are a little over 22,000 miles above the earth).

Packet-switching networks were originally designed to transmit data, and some delay in most data transmissions is acceptable and usually not even noticeable. Voice transmissions, however, are not nearly as forgiving.

Transmission errors and delays can cause VoIP transmissions to be distorted or lost entirely. Voice may sound garbled on one or both ends, there may be an echo effect, or calls may be dropped entirely. This is annoying and is unacceptable for organisations that depend on phone calls to conduct their business.

Hardware issues
VoIP hardware also affects performance, and thus call quality. Network hardware that's unable to handle the volume of VoIP traffic can cause degradation in performance. Endpoint performance is another issue. Softphones, in which VoIP software is installed on a PC with the PC serving as the phone, can be subject to poor performance and loss of call quality (or even an entire system crash) if the PC doesn't have sufficient resources (processor, memory) to handle the VoIP application plus any other applications that the user is running at the same time.

Bandwidth and latency issues
VoIP call quality is also affected by network bandwidth. A low-bandwidth connection such as dial-up…

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