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Saturday, October 27, 2007

Broadband Speed bits and bytes

Some of the confusion is down to the difference between bits and bytes. 8 bits = 1 byte. You've been moved onto a line that should be good for 5Mbps (Megabits per second) or 5120Kbps. That's 0.625MBps (MegaBytes per second) or 640KBps, as 5120Kb is 640KB (5120 divided by 8 to convert bits to Bytes). Notice lower-case b for bits, and upper-case B for Bytes.

A speedtest result of 800Kbps = 100KBps. A result of 1500Kbps = 187.5KBps. The maximum figure there is close to what you'd get on a 1.5Mbit connection. As a comparison, I'm on an ADSL line rated at 1Mbit. That's 1024Kbps which is 128KBps. So in theory I should be able to download and do speed tests and get a 128KBps transfer rate, or if it gives it in bits, a 1024Kbps rate. In fact, distance from the exchange and other factors means I often get a rate of 110KBps maximum. Close enough for me.

If you were taking full advantage of a 5Mbit connection, you'd be getting around 5000Kbps tranfer rate or >600KBps. The rating of a line and what you actually get are two different things. As shown by my line being rated as a 1Mbit but in fact I don't ever get quite that high with my transfer speeds. You clearly aren't even getting close though. In fact, you're not even as close to the 2Mbit line you originally had as I'd expect you to be, and would want myself. Seems you're approaching the limit of a 1.5Mbit line at best.

The download at 300KBps (not bits) indicates a tranfer rate at that time of 2400Kbps or >2.3Mbit. Not really approaching what you'd expect from a 5Mbit connection, but better than you'd ever get on a 2Mbit rated line, of course.

Use for download and upload speed tests. Seems pretty accurate. Do the Smartest, it's the best option.

Apparently, you can expect to lose around 13% of your stated line speed due to ATM/TCP overheads. So to work out what transfer rate you should expect to get, you can deduct 13% from the stated line speed. My 1Mbit connection which is 1024Kbits (1Mbit = 1024Kbits, 1Kbit = 1024 bits) when changed to KBytes gives a line speed of 128KBps. If I deduct 13% I get 111KBps which is about the transfer rate I get under normal circumstances.

In the case of 5120Kbps = 640KBps. 640 minus 13% = 556KBps. You're getting 89.5KBps (716Kbps). That's only 16% of the maximum transfer rate you should reasonably expect on a 5Mbit line.

My modem shows connected at downstream 1024Kbps and upstream at 320Kbps. Which, taking into account the 13% loss, I do actually get. If I introduce some noise into the phoneline by connecting a telephone without a microfilter attached, my downstream speed shown in the modem drops considerably, as the error rate increases, etc., effectively slowing the line down, but I don't know how it shows a lower speed, because unfortunately I don't know how the modem detects line speed. It doesn't seem to just be a preset signal from the ISP telling it what the line speed should be, because it lowers if the noise margin on the line is bad. Maybe someone else can enlighten me on this matter?

Found this link: "This site is a speed test site run by BT Wholesale to allow new ADSL users to test their connection, and also to verify whether any slow downs in service are down to the BT network, or your ISPs network."

You can also do the tweak test below and see if your PC needs a bit of tweaking:

Follow the guidelines they give afterwards for your setup and see if that improves anything. Your speeds do seem to vary a great deal... maybe it's just the speed test sites under load or your connection has a high contention ratio and is suffering from varying load?

Test it here.

Measures Internet connection bandwidth, provides an easy to understand report of download and upload speeds, and rates the quality of the connection with NEW! MySpeed PC v2.0c
Windows 2000/XP/2003/Vista 1.1MB Download FREE! Information

# Installs and runs from your personal computer, test your Internet connection to servers in the US and UK
# Reports your connection quality - the consistency of download capacity is a critical measure for bandwidth-intensive applications such as VoIP and multimedia
# NEW! Graphical charts display the actual flow of data transfers during a connection test to help identify the cause of poor performance
# NEW! Gauge view provides additional display of your speed test performance
# Easily email your test results to your ISP

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