Wednesday, May 04, 2005

What's The Difference Between DSL and T1?

Digital is digital, right? If so, then there really isn't any big difference between DSL and T1 lines, is there?

Actually there is quite a bit of difference, but there are also similarities. You can get always-on Internet access on both a DSL line and a T1 line. T1 lines download at 1.5 Mbps. DSL may also be that fast. You can use either with your VoIP telephone adaptor. So why would you spend ten times as much for a T1 line as you would to get DSL?

The answer is that there's digital and then there's digital. Digital line services come in many flavors and the each have their advantages and disadvantages.

Take DSL for instance. When you think about getting a DSL line for under $50 a month, what you are thinking of is residential DSL service that is technically called ADSL or asymmetrical Digital Subscriber Line. The asymmetrical part means that upload and download speeds are different. The reasoning is that most Internet users need fast download speeds for web browsing, but can live with lower upload speeds for sending email, typing in web addresses, filling out forms, etc. You might get 1.5 Mbps download but 128, 256 or 384 Kbps upload speed. There is a symmetrical type of DSL service with the same upload and download speed, but that is usually sold to businesses at a higher price.

T1 is based on a different technology, although both T1 and DSL are digital lines. T1 is a synchronous time division multiplexed system, meaning that it is not only on all the time, but it is sending something all the time and expecting to receive something all the time. That something might be no more than framing bits to keep both ends of the connection synchronized to define the individual time slots. That is not really important for Internet access, but does allow T1 lines to carry standard switched telephone calls as well as VoIP phone. T1 can carry 24 telephone lines to support a business PBX system. Each phone call occupies a well defined time slot in the T1 transmission.

T1 is also symmetrical. That means that the download speed is 1.5 Mbps and the upload speed is 1.5 Mbps. That synchronized line speed never varies and it doesn't matter how far you are from the phone company office. It's always 1.5 Mbps. DSL on the other hand, uses a Discrete Multitone Modulation (DMT) technology that adapts itself to the condition of the telephone line it shares. The speed you get depends on how far away you are from the equipment providing the DSL service, and the amount of noise and interference on the line. The closer you are, the higher your line speed. If you are more than a few miles away, you may not be able to get DSL at all. Just about everyone can get T1 service installed, but the cost goes up as you get more and more remote.

The other difference between DSL and T1 is not technical. It is in the commitment to availability of the service. T1 lines come with a Service Level Agreement or SLA that provides compensation for outages. DSL is a "best effort" service. That doesn't mean DSL is inherently unreliable, but if there is an outage then T1 with its SLA gets the highest level of attention.

So, why would you buy one service over the other? For residential use, DSL provided on your phone line or cable Internet service provided by the local Cable TV company give you very good Internet broadband service at a reasonable price. Businesses might choose business grade DSL if their need is fairly light Internet access or running a free WiFi hotspot.

But if full-time Internet access is key to your business or if you plan on running VoIP telephones or standard switched telephones using a PBX system, T1 is really what you need. T1 lines can also be set up as dedicated point to point connections between business locations. Granted, T1 prices are 8X to 10X what you'll pay for DSL. But that's still reasonable as a dependable digital lifeline for most medium and even small businesses.

If you need broadband voice, data, or Internet access for business applications, let our T1 consultants help you get the right service at the lowest price.

Click to check pricing and features or get support from a Telarus product specialist.

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