Bandwidth

Bandwidth – is an expression of data transfer resources,expressed in bits per second for digital bandwidth and Hertz for analog bandwidth.

In 1928 an electronics researcher named Ralph Hartleydetermined a method of quantifying the rate information transfers over acommunications channel.  Hartley measuredthis rate of error free data communication in Hertz.

For practical use in networked server applications such asweb site hosting, bandwidth refers to the amount of data transferred to andfrom the server over a particular amount of time.  Engineers also call this monthly datatransfer.

Engineers also describe bandwidth as the maximum amount ofsuccessful data transfer possible over a network path.  When an Internet Service Provider offersplans with different bandwidth, they refer to the maximum average rate.  Users (customers) in the network find theirreal bandwidth, or transfer rates, vary above or below this maximum numberdepending on network quality and network traffic.

Different common Internet connection types have differentmaximum bandwidth.

• 56 kbit/s – Dialup
• 1.5 Mbit/s – ADSL
• 1.544 Mbit/s – T1
• 10 Mbit/s – Ethernet
• 11 Mbit/s – Wireless B
• 44.736 Mbit/s – T3
• 54 Mbit/s – Wireless G
• 100 Mbit/s – Fast Ethernet
• 155 Mbit/s – OC3
• 300 Mbit/s – Wireless n
• 10 Gbit/s – 10 Gigabit Ethernet

kbit/s (kb/s)  = 1,000bit/s
Mbit/s (Mb/s)  =1,000,000 bit/s
Gbit/s (Gb/s)  = 1,000Mbit/s

Different applications require different bandwidth levels.

Audio

• 4 kb/s – minimum bandwidth needed for encoding clear speech using speech codecs
• 8 kb/s – telephone quality
• 192kb/s – near CD quality for MP3 encoded audio
• 1,411 kb/s – CD quality audio, uncompressed

Video

• 2.0 Mb/s – VHS quality
• 8 Mb/s – DVD quality
• 27 Mb/s – HDTV quality

Digital cable streams use varying bandwidth.  Digital television channels use up to 19Mb/s.  For example, Comcast digitalchannels use the following bandwidth.

• 2-3 Mb/s – very clean low definition channel
• 5-6 Mb/s – a digitized analog signal channel
• 8-12 Mb/s – medium to high definition channel at DVD quality such as HBO-HD
• 18-20 Mb/s – a high definition channel

In computer use network cards and cables support differinglevels of bandwidth.  Older cards andcables supported 10 Mb/s, while newer cards and cables support 100 and even1000 Mb/s.  As shown above, wireless nachieves greater bandwidth than all but the best wired networks.

When people measure bandwidth in a network they transfer a large file, thenmeasure the time it took for the transfer to complete.  The file size divided by time results inthroughput measured in b/s, kb/s, Mb/s or Gb/s. This is an accurate practical measurement of goodput, the amount of datasuccessfully transferred over a network. But a true measurement of maximum bandwidth or throughput includesoverhead such as error checking and takes into account machine limitations suchas physical transfer delays to a memory device (hard drive.)

Some networks and service providers practice bandwidth throttling.  In other words the cap theamount of data transferred over a network, or to and from a particular networkdevice, over a period of time.

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