Networks Analyzed @ Netralized

Nowadays a lot of people talk about DSL but what exactly is the DSL technology?

In this article I will try to give you a very brief overview about DSL and how it works.

DSL (Digital Subscriber Line) is an OSI layer 1 technology that allows you to deliver extra bandwidth and high speed data connections over a copper wire pair that is typically used for phone connections. This technological breakthrough is based on the research of Bell Labs that several years ago identified that a typical conversation on the telephone only uses a small frequency range of the available frequency spectrum (from 300HZ up to 3KHZ). That leaves all the other frequencies range up to 1MHZ, free to be used for other services.
Therefore, DSL uses our existing phone lines with very few changes from the phone company side, giving us extra services.

There are two major categories of DSL: Asymmetric (gives you different Upload and Download speeds) and Symmetric (gives you equal Upload and Download speeds).

There are many different types of DSL that can be used to reach the different needs of the customer. The most common types of DSL are:

• ADSL (Asymmetric): 8Mbps / 1Mbps
• ADSL2 (Asymmetric): 12Mbps / 1Mbps
• ADSL2+ (Asymmetric): 24Mbps / 1Mbps
• G.SHDSL (Symmetric): 2,3 Mbps
• HDSL (Symmetric): 2 Mbps
• IDSL (Symmetric): 144 Kbps
• SDSL (Symmetric): 768 Kbps
• VDSL (Asymmetric / Symmetric): 53Mbps / 13Mbps

With all this speed comes a limitation and that is the distance from the CO (Central Office). The farther the CO is, the lesser types of DSL are available and lesser the speed of them. Nevertheless, have in mind that with DSL you don’t share your bandwidth with anyone else (like cable technology).

ADSL is the most common flavor of DSL. It has a distance limit of 5.46 km, gives you 8Mbps download rate and 1Mbps for upload and uses the full spectrum of frequencies on a POTS line, even the voice frequencies. This means that if you install ADSL without phone filters every phone in your house will be useless. But phone filters are not the only solution; the telecommunications company (TELCO) can filter non-voice frequencies range by installing a filter in the box outside your house (demarcation point)..

But how does it really work?
DSL splits the frequency range of 0Hz to 1MHZ to 3 different channels.

• The first one (from 0 to 4 KHZ) is the voice channel.
• The second one (from 25 to 160 KHZ) is the Upload channel.
• The third one (from 240 to 1.1 MHZ) is the download channel.

Each channel is widely separated from the others to minimize the possibility of interference between them.

All the DSL types need something to communicate at OSI layer 2. For layer 2 DSL uses one of these three methods:

• RFC1483 bridged mode
• PPPoE (PPP Over Ethernet)
• PPPoA (PPP Over ATM)

That’s a brief introduction to DSL. In the future I will write an article about the Layer 2 standards and protocols of DSL.

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This entry was posted on Friday, September 14th, 2007 at 7:20 pm, by Jim Skenderlis and is filed under General Networking. You can leave a response, or trackback from your own site.