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What is SDSL?

R. Kayne
R. Kayne

SDSL (Symmetric Digital Subscriber Line) is high-speed Internet access service with matching upstream and downstream data rates. That is, data can be sent to the Internet from the client machine or received from the Internet with equal bandwidth availability in both directions. Normally, DSL service is asymmetrical (ADSL), with the bulk of the bandwidth reserved for receiving data, not sending it.

Normally, SDSL is used by businesses with a Web presence, VPN, extranet or intranet needs. In these cases, the client server may be required to upload large streams of data to the Internet on a regular basis. ADSL would be slow and inadequate for this purpose, as the bandwidth available for uploading is normally less than 1 megabit per second (mbps). SDSL bandwidth can be as high as 7 mbps in both directions.

An Internet service provider offering SDSL may offer different grades for varying prices. The faster the data rate, the more expensive the service. Usually, long-term contracts are required for service, regardless of the grade chosen.

Man holding computer
Man holding computer

SDSL utilizes a digital frequency traveling across existing copper telephones lines to send and receive data. When using the telephone line for this service, phone and fax services on that line must be suspended. Therefore a dedicated second, or additional, line is typically needed. This differs from ADSL, which "leaves room" for both standard analog telephone equipment and the digital signal, so that one can talk on the phone or use a fax machine while online.

This is an "always on" service, meaning that the computer is actively connected to the Internet whenever powered up. If the computer always remains on, the Internet connection will be continuously active.

SDSL service requires a special modem, normally supplied by the Internet service provider, and the equipment is often proprietary. The SDSL modem will likely require same-vendor equipment in the LAN, or common DSL chipsets.

Apart from businesses, SDSL can also serve individuals that require high upload speeds. Network sharing, for example, has become very popular, and with it the need for uploading programs and files — often extremely large files. SDSL is a good choice for heavy network sharing, as long as the user has a second telephone line to dedicate to the service or chooses to suspend telephone services while online.

SDSL is not available in all areas and speeds might vary depending upon the user's physical distance from local hubs. The service is also more expensive than ADSL, but well worth the difference for those with demanding upstream needs.

Discussion Comments


@nony - I think that mobile Internet is where it’s at for businesses.

Right now the 4G network boasts that it can get up to 21 Megabits per second for downstream and almost 6 Megabits per second for upstream. That’s not SDSL, because the numbers are different in both directions, but it’s plenty fast for most business applications, and sure beats a lot of household Internet connection speeds.

We use mobile Internet a lot when we travel, and if the boss calls and asks us to upload a file, he usually needs it pretty quick. The 4G network does the job.


@NathanG - I think that business DSL using SDSL technology would be appropriate if you do a lot of meetings over the Internet.

At our business, for example, we do a lot of video conferencing using VOIP technology and we often upload files and images and stuff.

I don’t know for sure, but I think that we probably use SDSL. Some of our clients are in remote locations like Asia and Australia, so we can’t afford a slow Internet connection. It has to be fast going both ways.


@allenJo - Yeah, I have DSL too. I guess it’s all right. I’ve never tried cable and was thinking of switching because everyone tells me it’s faster.

You’re one of the few people I’ve heard singing the praises of DSL over cable. However, one thing I’ve noticed is that my DSL upstream speed is far lower than my downstream Internet speed. I guess that’s where an SDSL Internet connection would be much better.

The difference between upstream and downstream is significant. I took one of those speed tests on the Internet and it reported a downstream of about 7 megabits per second, and an upstream speed of less than 1 megabit per second. That’s a big difference in my opinion.

Fortunately I don’t spend all day uploading videos to YouTube, otherwise it wouldn’t be worth my time to stay with DSL and I’d be forced to switch to cable.


I recently switched to a DSL Internet provider after having been with cable for several years.

That may come as a shock to some, because cable is supposedly faster than DSL. In theory, it is. But cable suffers from a major problem – congestion.

With cable, everyone in your neighborhood shares the same connection. As a result, I’ve had too many times where there seemed to be a pause between the time I clicked on the link and the time it took me to a web page. That proved to be too unacceptable so I switched to DSL.

DSL has congestion issues too, but so far it hasn’t been as bad as cable. I have my own dedicated line running through my phone line. I realize that the Central Office has to divvy up the signal among its subscribers, so it’s not as dedicated as having your own T1. So far, however, it’s proved to be acceptable.

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