ADSL versus SDSL – The Reflex Solution

At Reflex we pride ourselves on our IT infrastructure. We want the best for our clients and believe we have put in place facilities to rival those of a standard managed office service.

One of the questions we are constantly asked about the IT we provide is why we don’t use ADSL. I therefore thought I would put together a little explanation as to why we think SDSL is a better solution for our customers and our office space.

What is ‘Upstream’ and ‘Downstream’? Anything you send from your PC to the Internet goes ‘upstream’, while anything you receive to your machine from the Internet comes ‘downstream’. So, when you click on a hyperlink, your browser sends a few characters upstream to request the page that the hyperlink points to, and the page comes downstream to your PC so you can see it. For general browsing of the internet you send very little information upstream, but get a lot back downstream. With this in mind ADSL broadband was designed and intended primarily as a home user service with a high download and slow upload speeds.

Asymmetrical DSL (aka ADSL) The incoming speed (download) from the Internet on an ADSL connection is faster than the outgoing speed (upload) to the Internet. ADSL is best used for light web surfing and downloading email. It is not designed for sending large files, large Email attachments or uploading files to the Internet. It is also not designed for downloading very large files because each packet of information in a large downloaded file must be acknowledged as it is received. This can use over half of the available upload bandwidth of an ADSL circuit. When this happens, a high speed ADSL circuit can choke on the outbound traffic and become very slow.

Symmetrical DSL (aka SDSL) While the download speed of ADSL is usually faster, SDSL service is usually faster overall than ADSL because the incoming speed from the Internet and the outgoing speed to the Internet are exactly the same. SDSL is preferred when you want remote users to have access to a shared database, file or email server on your network. It is also preferred when you are downloading extremely large files. These transactions take advantage of the maximum speed available in both directions, which is why a symmetrical line is needed. SDSL is also much better for remote screen sharing applications, such as Timbuktu, VNC, Remote Desktop, Citrix and so on.

Contention Ratio “Contention ratio” means ‘sharing ratio’.
The “contention ratio” is the maximum number of other connections you will have to share the infrastructure with at the local telephone exchange. So a contention ratio of 50:1 would mean that the maximum number of connections at anytime is 50 and if all 50 connections were downloading at the same time then your download speed could drop significantly, hence sporadic speeds and peak usage slowdowns. Also bear in mind that the other 49 connections could be companies with many people sharing that connection. In reality you could end up sharing a connection with potentially 100’s of individual users.
A second issue with contention is the un-published contention ratio. The 50:1 ratio is simply the rate at which BT divide it up. Low cost ISPs are also renowned for not having the full capacity required to service the capacity they publish and have all sorts of small print regarding fair usage. Fair usage gives them the right to slow your connection down even further.

Actual speeds Every ADSL provider states their broadband speeds as ‘maximum download’ and ‘maximum upload’ and for good reason. The actual speed that a user may achieve varies considerably depending on a host of reasons, distance from the exchange, quality of the copper, contention ratios and peak demands are just a few. SDSL lines typically have a lower contention and are not as affected by distance from the exchange etc as ADSL. Ultimately SDSL lines deliver a consistent more resilient connection.

Service Level Agreements ADSL agreements typically have a ‘best endeavors’ fix clause. In other words they don’t want the connection to fail but if it does they are not responsible but will use best endeavors to fix the fault as soon as possible. After 24 hours you will probably received daily compensation — not much use if you need to send / receive e-mails!!. SDSL agreements provide short fix times.

Conclusion Although ADSL is cheaper than SDSL, SDSL is overall more reliable and consistently quicker than ADSL.

Reflex Solutions offer managed offices in Central London in period style character filled buildings. Please contact us with your specific requirements if you can not find what you are looking for.

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