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DS3 Bandwidth vs Cable D3
Comparing costs and characteristics of DOCSIS 3.0 and DS3 bandwidth services.

By: John Shepler

Medium and larger size companies have long chosen DS3 services to obtain the bandwidth they need for broadband Internet access. Now there’s a new player in the marketplace that is looking to complement if not displace the venerable DS3 connection. If you need business bandwidth higher than you can get by bonding T1 lines, you already know about DS3. But how much do you know about D3?

What is D3?
D3 is short for DOCSIS 3.0, a cable broadband standard that gives HFC (Hybrid Fiber Cable) systems a capability that rivals fiber to the premises. It is now possible to get 50 Mbps for business use from cable systems in many cities. Some have 100 Mbps available and more will have that service offering soon. DOCSIS 3.0 can provide 152 Mbps, with even higher speeds possible. Compare that with DS3 at 45 Mbps and you can see why there is a natural competition between the services.

The raw bandwidth specs don’t tell the whole story, however. D3 and DS3 are different technologies with different characteristics. You may find that one or both of these services make sense for your business.

What is DS3?
DS3 stands for Digital Signal, level 3. It was developed as part of the T-Carrier specification by Bell Labs for use by the telephone companies to transport up to 672 simultaneous telephone calls between phone company switching centers. When provisioned over coaxial copper lines, it is called T3 line service. If you suspect that T3 is related to T1, you are right. A T3 line can transport the equivalent of 28 T1 lines on its higher bandwidth.

DOCSIS is a Cable Technology
DOCSIS 3.0 has a Cable TV rather than a telephone company heritage. The DOCSIS standard was developed by CableLabs (Cable Television Laboratories, Inc.) with contributions from other companies serving the cable industry. If you have Cable broadband, you are using a DOCSIS modem. It’s only a question of which version you are using. The original was DOCSIS 1.0, which has been supplanted by DOCSIS 1.1, DOCSIS 2.0 and now DOCSIS 3.0.

DS3 Has a Telephone Heritage
DS3 is based on multiplexing 672 channels of 64 Kbps each to create total bandwidth of 43 Mbps. Add the necessary synchronization and control bits and you have a bandwidth of 44.736 Mbps. Why so many channels? Each of those 64K channels is just the right size to carry one telephone call. That’s the telco heritage of DS3. Combine all the channels into one large pipe and you have nearly 45 Mbps of digital bandwidth.

D3 is also based on channels. In this case, each channel is 6 MHz wide. That’s the width of one television channel. Everything on a cable system is treated as a TV channel, so broadband services have to fit into those channels to get through the amplifiers and wiring on the system without interfering with any other channels. A 6 MHz channel can transport a 38 Mbps broadband signal in one direction. Cable operators bond channels together to get higher bandwidths. With 4 bonded channels, DOCSIS 3.0 can deliver a download speed of 152 Mbps.

Different Types of Service
So, are D3 and DS3 the same beyond their different technology heritages? Not really. The telco heritage of DS3 means that it is a dedicated symmetrical bandwidth service. You get 45 Mbps in both the upload and download directions. That bandwidth never varies. It is called dedicated because it is completely dedicated to your needs. Anytime you don’t use the full 45 Mbps, it just sits there and idles while waiting for more data.

Characteristics of D3
D3 is an asymmetrical shared bandwidth service. You get different speeds in the upload and download directions. Download is faster to reflect the fact that most users download more content from the Internet than they upload. Typical business bandwidth services are 50 Mbps download and 10 Mbps upload or 100 Mbps download with 10 Mbps upload. This bandwidth is shared among many users on the system. Thus, you may experience your access speed varying all over the place depending on what other users are doing. If a lot of them are downloading movies or big software updates, everyone’s speed will be slowed.

Dedicated vs Best Effort
DS3 service is generally sold with an SLA or Service Level Agreement that describes the expected performance and availability of the service, plus remedies if the provider doesn’t deliver as promised. D3 is a “best effort” service with no specific performance guarantees. Even so, it is in the provider’s best interest to provide adequate resources and service availability to keep the customers happy.

Major Price Differences
The difference in services are most dramatic when you look at the pricing. You might pay several thousand dollars a month forDS3 service with an SLA. D3 will probably cost you a tenth of that, with no guarantees. Which will work for your needs depends on if the cable system passes your business and if you can live with the variations in performance and lack of guarantees. Some companies have DS3 or even T1 lines installed for their mission critical or performance sensitive applications and D3 for general Internet access and low cost backup to their telecom services.

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