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Telecom Questions Answered: T1 vs DSL

by Sean Rivers on August 31st, 2011

Q: I’m interested in VoIP service, so what’s the “right” Internet for my business. I don’t know what kind to get. Can you explain the difference between a T1 and a DSL data connection?

A: As you research broadband Internet access for your company, you will realize that there are a lot of options out there. In order to get crystal clear call quality with VoIP, your business must have the appropriate bandwidth.

The two most popular choices for a business-class Internet connections are DSL and T-1, otherwise known as a T1. On the surface, the main difference is cost. A DSL connection will cost around $45 per month, whereas a T1 connection will start at $300 month and go up from there. Cost is but an indication of greater differences between the two data connections. But what could possibly make a T1 line cost three to ten times as much as a DSL connection? It’s all the same Internet, right?

The main factors to consider in your purchase decision are speed and reliability. Fundamentally speaking, the difference comes down to reliability. If you want reliable, 99.9%+ guaranteed Internet speeds at all times, then a T1 is probably a good bet. If you don’t require reliability, then DSL may be fine option for you.

A T1 is a dedicated, high-speed Internet connection that reliably delivers 1.5 Mbps broadband speeds. It is guaranteed to be that fast by the T1 connection provider through a service level agreement, or SLA. Virtually all T1s from Tier 1 providers are under a SLA, as well as a mean time to repair (MTTR) agreement. This means if the provider doesn’t fix your problem, they will be faced with a loss of revenue, which is a motivator to keep you up and running!

Typical SLAs provide for 99.9% up time, or approximately 2 hours of downtime per month, and some carriers go to 99.99% or 99.999% up time on their T1. Your $300+ per month is buying you peace of mind and a guaranteed speed. These factors could be a high priority if you host your own website, download/upload a lot of large files, or use your Internet connection for VoIP.

As an alternative, DSL can give you speed (some providers claim up to 20 Mbps), but you never really know what speed you are going to get because you essentially share your bandwidth with other customers who are on the same “line.” As a general rule, the cheaper the DSL connection, the more people sharing the bandwidth. It is common practice for DSL providers drive cost down by putting more and more subscribers on a single “line” connection – this is called “over subscription” and is widespread among low-cost Internet service providers advertising cheap and fast DSL.

Furthermore, your variable connection speed could be affected by the local demand for bandwidth. When you use VoIP on a DSL connection, you will probably see things like voice quality issues in the afternoon when the kids get home from school. As long as you can accept that your broadband speed will be variable, low-cost DSL may be a solid choice for your business, especially if you primarily use the Internet for email and Web browsing.

There are a few other differences to be aware of as you contemplate your decision. Upload speed and the location of your office related to your carrier’s network are also important considerations.

One of the main issues that causes confusion with DSL connections is that the upload speed is markedly slower than the download speed. Most of our customers do not even consider this fact when getting ready for VoIP. Businesses often believe they have enough bandwidth to support their office with a 6 Mb connection – not realizing that they may only have a 128k or 256k upload speed.

VoIP technology demands both fast upload and fast download speeds, since calls go in both directions. The most common call quality issue for VoIP customers is that the call sounds clear to them, but the caller reports that their voice is breaking up on the other end. The explanation for this is simple enough. There just is not enough bandwidth to support the outgoing audio for the call.

In the end, it comes down to the reliability of your broadband Internet connection. At Phonebooth, we want you to have the best phone experience possible. We recommend that businesses use a wired broadband connection like Cable or DSL with Phonebooth VoIP phone service; however, you may need a T1, depending on the size of your company. A T1 will provide dedication and guaranteed service. It might not be as fast as some DSL, but it is far more stable.

  • Junk

    I don’t believe your information about DSL sharing a “line” is accurate. My understanding is that the last mile (DSL modem to phone office) is dedicated to a specific connection. In the case of cable I believe this can be the case. 

    If you plan to use a T1 in today’s bandwidth hungry business environment you should plan to add a DSL or cable connection to handle all your non-VoIP traffic. 

  • Sean Rivers

    You are correct that the copper line is not shared, but DSL is shared at the DSLAM (phone office) While you are not sharing the copper wire you are sharing the equipment and an Internet port. This is like your house, you may have a 100Mb network or more but your Internet connection is much, much slower. This is a bottleneck and the DSLAM is a carrier’s bottleneck. The way carriers manages this bottleneck is to optimize for the most common traffic it carries (download). The carrier reserves more bandwidth for the download of traffic across all of its customers in the process restricting upload. In addition carriers will prioritize business customers over residential helping to guarantee a level of service to businesses but delivering best-effort to consumers.

    I will agree with you that separating Internet connections for voice and data is ideal. My experience is that customers with ADSL have a lot more quality issues than T-1 customers. Maybe this is because of the lack of proper upstream sizing, I usually have to explain a few times to customers that voice is always a 2-way service and that upstream really matters.

    People get stuck on their 6Mb ADSL and forget that it is 256k upload. They think they have overwhelming bandwidth and then wonder why their customers are complaining that their voice is breaking up.

    I would love to have our users discuss this in the Community. 

  • Jonathon

    I don’t want you to think that I’m recommending DSL for voice but I still feel your missing why I feel the blog post is misleading.

    I couldn’t agree more that you pay for what you get and that cheap internet connections will tend to be over subscribed. This can (and does) apply for cheap T1s, cable, wireless point-to-point connections and increasingly even 5/10/100mbit fibre connections where consumers are using “cheap” as their only purchasing requirement.

    Find an internet provider who knows you want to use VoIP, have them tell you it will work with their service (in writing!). Even better, don’t sign a contract so they are on the hook to provide quality service.

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