Friday, April 29, 2005

What's The Difference Between DS1 and T1?

T1 lines are becoming ubiquitous for business digital connections. T1 phone lines replace up to 24 standard telephone lines in one digital trunk line. T1 Internet service gives you 1.5 Mbps bandwidth both upload and download. You can even combine the two with T1 Integrated line service that allocates part of your T1 line for telephone voice circuits and part for Internet data. So, then, what should you order? Is it a T1 line or DS1 service?

DS1? Where did that come from? Is DS1 a better or worse choice than T1? What's the difference between DS1 and T1 anyway?

Relax. In colloquial terms, DS1 and T1 mean the same thing. There is a very fine point of technical difference between the two. You can get DS1 service without a T1 line but if you have a T1 line, you've got DS1.

DS1 is a digital signal level that is part of a standardized hierarchy or building blocks of digital services for TDM (Time Division Multiplexing) digital lines. A DS1 is a DS1 regardless of whether it is delivered on a T1 line, a microwave relay, a T3 line or a fiber optic carrier.

Actually it all starts at the bottom with DS0. DS0 is the bandwidth you need to transmit one digitized telephone call using the legacy telephone standard for PCM or Pulse Code Modulation. It's an 8 bit channel transmitted at 8 Kbps or a total of 64 Kbps. That's the gold standard for "toll quality" voice. That same channel can also be used to carry 64 Kbps of data rather than a telephone call. The data bits might represent a dial-up modem connection, a portion of a broadband Internet service, a portion of an Ethernet signal, a small portion of a video transmission, or signaling and Caller ID information to support a call center.

A T1 line multiplexes or aggregates 24 of these DS0 channels into a DS1 with a total bandwidth of 1.536 Mbps. But a T1 line runs at 1.544 Mbps. What's missing? The extra 8 Kbps represents the framing bit that T1 adds to each group of 24 DS0 channels so that the CSU or channel service unit at the other end of the line knows where to demultiplex the frame of 24 DS0s. With synchronous lines such as T1, framing bits are needed to keep the sending and receiving ends tightly synchronized.

Remember that most people will associate DS1 with T1 and that's normally how you would get DS1 service. If you needed a lot more bandwidth, you might order DS3 service that would be delivered on a T3 line. DS3 is the equivalent of 28 DS1s. A DS1 service might be separated from a higher speed service using an add/drop multiplexer. For instance, an OC3 fiber optic carrier can deliver 3 DS3 services or 84 DS1 services or 2,016 DS0 channels. Because they are all constructed according to the rules of the same digital signal hierarchy, you can multiplex and demultiplex these signals at will.

Get the best DS1 and T1 line prices from T1 Rex. We can also get you excellent T3 line pricing through T3 Rex and low OC3 prices from Gigapackets. Our team of technical experts will respond to your inquiry on any of these services.

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

Follow Telexplainer on Twitter