There seems to be a shroud of mystery surrounding corporate connectivity. Much of this confusion has been created by the tele-communications industry themselves by the creation of a multitude of acronyms and standards.

This special report is designed to help you quickly understand the newer standards in leased lines, the benefits that are provided by each one and so provide you with a quick guide to working out which line would be correct for your business.

A Byte out of Bits

Before we begin lets get you up to scratch on bits and bytes. There is a common misconception that if for example you download a 10MB file over a 2Mb/s line you should get it in five seconds.

This is actually incorrect as the file is measured in mega bytes but you actual line is measure in megabits.

There are eight bits to a byte so when you purchase a 2Mb/s line you are actually subscribing to a 250,000 bytes per second line (ie 2000000/8).

Therefore a 10MB file would actually take forty seconds to download if we assume that 10MB is equivalent to 10,000,000 bytes. (In fact it’s actually 10485760 bytes but let’s try to keep things simple!) The table below gives you the actual calculations.

DSL Speed

Maximum Transfer Speed in bytes/sec

Transfer Speed In Kilobytes/sec

256Kb/s

32000 bytes/sec

31 KB/s

512Kb/s

64000 bytes/sec

63 KB/s

2Mb/s

250000 bytes/sec

244 KB/s

4Mb/s

500000 bytes/sec

488 KB/s

8Mb/s

1000000 bytes/sec

977 KB/

So as you can see from the above table a 2Mb/sec line is actually equivalent to roughly a 250KB/sec. Note how bits is written compared to bytes ie:

• two megabits per second is written 2Mb/s and
• two megabytes per second is written 2MB/s

ADSL - Asymmetric Digital Subscriber Line

First, let’s deal with something many of us already know about – ADSL or Asymmetric Digital Subscriber Line. The traditional ADSL line has or ADSL1 has a maximum speed of 2Mb/s. However if you read the tiny print you would see that this only applies to the downstream speed (ie files you receive).

• Must be enabled in local exchange
• Can only be used over copper,
• at time of writing it has a maximum speed of 8Mb/s
• No SLA

ADSL prices fluctuate but typically cost £180 per annum.

SDSL – Symmetric Synchronous Digital Subscriber Line

Current limitations of SDSL are that

• Must be enabled in local exchange
• can only be used over copper,
• at time of writing most ISP only offer speeds of up to 2Mb/s

SDSL typically costs around £3580 per annum.

A Bone of Contention

An important factor when purchasing ADSL and SDSL lines are their contention ratios. Contention ratios are the number of people who share your line. So if you have a 10:1 contention ratio, it means that ten other businesses or people are using the connection with you.

Now you can quickly see that the ideal contention ratio is 1:1 or zero contention which is essentially, your very own leased line.

Otherwise your traffic speed could be affected by the amount of traffic that other businesses are sending.

If you have ever regularly experienced slow internet speeds at specific times of the day this is due to other users on your line. The higher the contention ratio, the greater the number of businesses sharing the same line and potentially the slower your line can be.

Leased Line Technology

A leased line or dedicated line is a permanent telephone connection set up between two points for private use.

Leased lines are dedicated lines (ie have 1:1 contention ratios) and are typically used to connect two geographically separate branch offices together. As they work over fibre optics, they have none of the copper limitations of SDSL and so can have speeds up to 100Mb/s.

Leased lines typically cost three to five times more than SDSL lines.

In Summary

If you are looking at purchasing internet or branch office connectivity, your three main options are:-

• ADSL which is only good if you have very minimal uploads (ie you don’t send many emails or the files you send are quite small).

Ben Chai is a renowned international IT expert and trainer having been editor of over ten IT magazines and taught a wide variety of courses in security and Microsoft products. He is co-owner of incomingthought.com and the content director for www.itproportal.com