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MegaBytes versus Megabits

Views: 10321
Votes: 2
Posted: 02 Jun, 2009
There is a common misunderstanding, not just in the backup world, but in general about the difference between a MegaByte (MB ) and a Megabit (Mb).  The main cause of the problem is the fact that all data storage is quoted and referenced in Bytes (KB , MB, GB , TB , etc) and network transfer speeds are rated in bits (Kb, Mb, Gb, etc).

You will see two speeds referenced in this article, "Mbps" and "MBps", they mean:

Mbps = Megabits per second
MBps = MegaBytes per second

In order to better understand this, we already know this:

1 Byte = 8 bits

Therefore, if you have a Gigabit (or GigE) network, the top speed of GigE is 1,000 Mbps (Note "Mb" and not "MB").  That equates to 125 MBps because:

1,000 ÷ 8 = 125

Due to additional bits that are needed to actually send data out on the wire for routing purposes, the number of bits used for network traffic is closer to 10 bits for every Byte.  Therefore a GigE network would have a realistic top speed closer to 100 MBps instead of 125.

1,000 ÷ 10 = 100

Since it costs bits to send data over the wire, for any network, the same equation can be done for any network connection speed.  The reported speed of a OC-48 (Optical Carrier -- Fiber Optic Backbone) connection is 2,488.32 Mbps.  With the known bits needed for network transfer and math that done above, the equation can be layed out in the following way:

2,488.32 ÷ 10 = 248.832

Therefore, an OC-48 Fiber Optic connections speed is closer to 248.832 MBps in the real-world.

To see a listing of various network connections speeds, including cellular speeds, please see KB article #63.
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