Theory question on SDSL

I heard some say that SDSL is exactly like a leased line (compared to ADSL).

My poor knoledge says that a leased line ensures that there is no contention (up until a certain point, or #of hops).

Logically I expect SDSL connections to fall under the same ISP policies about contention, as ADSL conn's do.

Is that how things are?

piouAsked:
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grbladesCommented:
Hi piou,
Yes SDSL is basically the same as ADSL with regard to contention ratios and the reliance on the equipment at the local exchange. You just get syncronous bandwidth (the same for upload as download) which is ideal for companies running VPN etc... who use a lot of outbound bandwidth.
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piouAuthor Commented:
Can we make this a bit authoritative, with some reference maybe? On what network layer a leased line is different than an SDSL for example? Both have symmetric up/down, but where *exactly* lies the difference?

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grbladesCommented:
I don't have any references to hand.

With leased line you have a fixed telephone line connection from one end to the other.

With ADSL and SDSL you have a fixed connection to the telephone exchange where the termination equipment resides. Then the data goes on an effectivly leased line circuit back to their data center or internet connectivity point. This is where the cost saving is made because from the exchange onwards there is a single high bandwidth link compared to hundreds of low bandwidth links which is oviously much cheaper. However if the internet connection from the exchange is 100meg and the ISP has 1meg customers they will put more than 100 customers on the line so if everyone used it you wont get anything like full bandwidth. A home ADSL line normally has a contention ratio of 20:1 so for a 100 meg line they would put up to 2000 people on it.

Standard telephone lines are half-duplex where you cannot transmit and receive at the same time. Most home users do far more downloading than uploading therefore given the total bandwidth capability of the line it made sense to split the available frequency band such that the download bandwidth is far higher than the upload.

IWhen ADSL first came out it was 512K only but now you can get 2M and I think even higher. It now makes sense to use this new technology and increased bandwidth it offers to give faster upload bandwidth as an option which is basically what SDSL is.

At the end of the day you can normally choose the contention ratio that you want. When I had a 1Meg ADSL line installed at work I went for the standard 20:1 ratio as it is mainly a residential area so the peak usage will be in the evening so it wont be heavily used during the day when we need it. I did have the option for opting for a 5:1 or even a 1:1 ratio at an additional monthly cost.
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