Voice over IP (VoIP) is a proven technology that can provide cost savings and increased functionality. Companies large and small have been converting to VoIP to take advantage of these benefits. However, if the quality of the voice suffers, the benefits can be overshadowed by user dissatisfaction.

Keep it Appropriate

It is important to utilize an appropriate WAN (Wide Area Network) technology, either for main office to branch office communications or for your Internet access. Your choice of WAN technology will dictate the quality of voice services you provide to your users. An appropriate WAN technology will meet the following requirements:

  • Reliability (Uptime)
  • Adequate Bandwidth
  • Low Latency
  • Low Jitter
  • Quality of Service (QOS) for voice traffic.


Today, I want to talk about why DSL (Digital Subscriber Line) is often not a good choice as a WAN technology for VoIP. First, a disclaimer. There are some very good business-class DSL service providers out there. DSL can be an appropriate technology if managed and applied prudently.  However, most mass-marketed DSL services do not fall into this category. More on this later.


Let’s look at that list of requirements one at a time. First, reliable uptime. The problem with DSL is often what we refer to as the ‘last mile’. Most DSL services use older copper pairs which have been around for a very long time. Most carriers have not done much to maintain these copper pairs over the years and it shows.  It is a technological stretch to get high-bandwidth communications going over an unshielded twisted pair cable anyway. When cable integrity is unreliable,  you may experience periodic degradation in service or even complete disconnects.  J.D. Power notes that “DSL providers in general, often unable or unwilling to upgrade last mile connections due to financial constraints and/or limited competition, are starting to fall seriously behind“.

Get on the Bandwidth Wagon

Next, let’s look at adequate bandwidth. Adequate bandwidth can be defined as enough bandwidth to support your mission critical applications without negatively impacting the user experience. You would think DSL would meet this requirement. After all, they advertise DSL speeds in the 10’s of megabits per second. For example, Century Link advertises DSL speeds from 1.5 Mbps to 40 Mbps. You might think that would be adequate for your needs. You might be wrong.

If you read the fine print, you will realize that the quoted speed is a maximum. Not minimum, not even typical; maximum. So, what is the minimum, you might ask? Zero. Yep, zero. There is absolutely no service level guarantee on most DSL services. Google “DSL speed test” and you will find thousands of complaints of people paying for 5 Mbps of DSL and actually getting 28 kbps. That’s right, DSL speeds can actually be worse than an old dial-up modem.


The primary reason for this is that most DSL services are grossly over-subscribed. The service provider builds a network that will support a certain number of simultaneous users, say 1,000. Then they sell the service to 20,000 customers (a 20:1 over-subscription rate) . Or 50,000 (50:1) or 100,000 (100:1). Don’t get me wrong; over-subscription is a standard practice and makes perfect sense from a system design standpoint. The point is that most commercial DSL providers take this way too far.

Getting the Jitters

The next two requirements are related; latency is how long it take a data packet to reach its destination and jitter is the variability in the latency (see this post for a more detailed discussion of latency and jitter). You can test the latency and jitter of your DSL connection using a tool like PingPlotter as discussed in this post. Your results will depend on several issues:

  • Where the two endpoints are located geographically
  • Who the service provider is at each end point
  • Traffic levels within your organization
  • Traffic levels external to your organization

Latency and jitter can and will vary over time. The goal is to minimize both in order to provide the highest quality voice possible. Having the same service provider at both ends can significantly reduce latency and jitter. It is very likely that the over-subscribed nature of your DSL service will contribute to higher latency and jitter levels.

Going High-Quality

Finally, Quality of Service (QOS) is not available on most standard DSL services. In this post, I talked about the need for QOS to prioritize voice traffic. Prioritizing voice within your network and at the network edge doesn’t help if you ISP or WAN provider doesn’t do the same. Quality of Service is an essential ingredient in a successful deployment of Voice over IP.

Do We Have an Agreement?

DSL is tempting to use because of the attractive pricing. This is one of those cases where you definitely get what you pay for. If you are looking at DSL for your business VoIP project, ask the provider for a written Service Level Agreement (SLA). Ask for a guaranteed minimum bandwidth. Ask how they implement QOS. Ask for a maximum on latency and jitter (you may not get it but it doesn’t hurt to ask). As I mentioned before, there are some good business-class DSL providers out there. They charge more for their service because they don’t over-subscribe to insane levels. The price premium is worth it when voice is being carried over the circuit.

Do you use DSL for VoIP at your business? What has been your user experience? Would you make the same choice again? Share your thoughts in the comments.


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