ADSL overview

Speed

ADSL stands for Asymmetric Digital Subscriber Line and it is the most popular protocol among the xDSL technologies (aDSL, hDSL, vDSL and so on). Usually, home and small offices users can take advantage of ADSL whereas corporate users need hDSL or vDSL technologies. ADSL is a high-bandwidth technology and it is much faster than V90 or ISDN.

Current maximum theoretical speed follows:

- 56k modems (V90) : 56kbps
- ISDN lines : 128 kbps (two 64kbps channels)
- ADSL lines: 1Mbps upstream/8 Mbps downstream (ANSI T1.413)
- ADSL2: 1Mbps upstream/12 Mbps downstream (ITU G.992.3/4)
- ADSL2+: 1Mbps upstream/24 Mbps downstream (ITU G.992.5)

You must pay attention to 'kbps' or 'kb' or kilobits and 'KBytes' or 'KB' or 'Kilobytes' notations, in fact 'kbps' stands for kilobits per second (1 kilobits = 1,000 bits) and KB stands for Kilobyte (1 Kilobyte = 1,024 bits). Please notice that:

- Kilo = 1,000 for communication speed measurements
- Kilo = 1,024 for computer file size measurements

- Mega = 1,000,000 for communication speed measurements
- Mega = 1,048,576 for computer file size measurements

- 1 byte = 1 character
- 8 bits = 1 byte = 1 character
- 8 kbits = 1 kilobyte
- 1 bit = 0,125 bytes (1/8 char)

Just as an example, the maximum speed for a 56K dial-up modem is 56,000 bits per second, not 56 x 1,024 bits per second. But a 56K file is 56 x 1,024 = 57,344 bits actually. So you CANNOT translate from kbps to KBytes per second just by dividing by 8!

In addition, you have to consider some extra data that is not part of the file you are downloading (TCP/IP headers, ACK packets, lost packets that are sent again). You can have a rough idea of you transfer rate by means of a simple calculation:

56 kbps = 56 x 1000 / 8 = *ABOUT* 7 KB (it's much less actually)

Finally, please, remember that we are talking about theoretical speed, so you will never get that speed actually. ADSL lines are asymmetric lines because upload bandwidth is smaller than download bandwidth. Upstream bandwidth can reach about 1 Mbps whereas downstream bandwidth can reach 8 Mbps. Again, we are talking about theoretical speeds, in fact real speed is much slower. Typically you can get 512 kbps upstream and 6 Mbps downstream bandwidth.

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