Bandwidth measures the amount of data that can be transmitted over a connection, while throughput considers successful transmission. Hosting companies often limit bandwidth and charge extra for exceeding it. Different internet connection technologies have varying bandwidth limits, and ethernet and wireless technologies are also limited by bandwidth.
Bandwidth is a term used to describe the amount of information that can be transmitted over a connection. It is usually given as bits per second or as a larger denomination of bits, such as Megabits per second, expressed as kbit/s or Mbit/s. Bandwidth is a gross measure, which takes as speed the total amount of data transferred in a given period of time, without considering the quality of the signal itself.
Throughput can be thought of as a subset of bandwidth that takes into account whether the data was transmitted successfully or not. While the bandwidth of a connection can be quite high, if the signal loss is also high, the throughput of the connection will remain quite low. Conversely, even a relatively low bandwidth connection can have moderately high throughput if the signal quality is also high.
Bandwidth is more familiar to consumers due to its use by hosting companies or Internet service providers. The sense in which it is used by most web hosting companies – as a measure of total data transferred in a month – is not strictly correct. This measure is more aptly referred to as data transfer, but the use of bandwidth by hosting companies is so pervasive that it has become accepted by the general public.
Many hosting providers place limits on the amount of bandwidth a site can transfer in a given period of time, usually a month, but sometimes 24 hours or a week. If the site exceeds its allotment, service is usually suspended or billed separately for the additional bandwidth, often at a much higher cost than the base cost included in the hosting plan.
Some hosts offer so-called unlimited plans, which in theory have an unlimited amount of data transfer per month. Typically the effective bandwidth, i.e. the transfers per second of a connection, is somewhat limited on these services, ensuring that the data transfer for the site never gets too large. If the limit is reached, the speed for users could be significantly reduced or the service could even be interrupted.
Different internet connection technologies also have different bandwidth limits associated with them. These act as an upper limit on the amount of data that can be transferred each second by a user. At the low end of the spectrum, a simple dial-up connection, using a modem and a regular telephone line, has a maximum of about 56 kbit/s. By comparison, a DSL connection can reach almost 10 Mbit/s, or two hundred times that of a dialup connection, while a cable connection can theoretically reach around 30 Mbit/s. Connections such as a T1 line can reach 1,544 Mbit/s, but due to their dedicated nature the effective bandwidth they achieve is often higher than that of cable or DSL. The largest connections include T3 at around 43 Mbit/s, OC3 at 155 Mbit/s, OC12 at 622 Mbit/s and the monumental OC192 at 9.6 Gbit/s, over three hundred times faster than a cable connection in maximum speed.
Bandwidth is also a limiting factor for the technology that connects the computer itself to the modem or device that interacts with the direct Internet line. Basic ethernet, for example, has a bandwidth of 10 Mbit/s, so using an internet connection faster than that would be largely wasted speed. Fast Ethernet reaches 100 Mbit/s, more than fast enough for all consumer uses, while Gigabit Ethernet can reach 1 Gbit/s and 10 Gigabit Ethernet is 10 Gbit/s. Wireless technologies are also limited by bandwidth, with Wireless 802.11b having 11 Mbit/s, Wireless-G 802.11g having a limit of 54 Mbit/s, and Wireless-N 802.11na having 300 Mbit/s.
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