Bandwidth is a term used to describe how much information can be transmitted over a connection. It is usually given as bits per second, or as some larger denomination of bits, such as Megabits per second, expressed as kbit/s or Mbit/s. Bandwidth is a gross measurement, taking the total amount of data transferred in a given period of time as a rate, without taking into consideration the quality of the signal itself.
Throughput can be looked at as a subset of bandwidth that takes into account whether data was successfully transmitted or not. While the bandwidth of a connection might be quite high, if the signal loss is also high, then the throughput of the connection will remain somewhat low. Conversely, even a relatively low-bandwidth connection can have a moderately high throughput if the signal quality is also high.
Bandwidth is most familiar to consumers because of its use by hosting companies or Internet service providers. The sense in which it is used by most web hosting companies, that is, as a measure of total data transferred in a month, is not strictly correct. This measurement is more rightly 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 caps 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, the service is usually either suspended or else additional bandwidth is billed separately, often at a much higher cost than the base cost included with the hosting plan.
Some hosts offer so-called unlimited plans, which in theory have an unlimited amount of data transfer per month. Usually the actual bandwidth, that is the per-second transfer of a connection, is somewhat limited on these services, ensuring data transfer for the site never becomes too large. If the limit is met, speeds for users may be throttled down substantially, or service may even be interrupted.
Different technologies for connecting to the Internet have different bandwidth limits associated with them, as well. These act as an upper limit to how much data may be transferred each second by a user. At the low end of the spectrum, a simple dial-up connection, using a modem and a normal phone line, has a maximum of around 56 kbit/s. In comparison, a DSL connection can reach nearly 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 given their dedicated nature the actual bandwidth they reach is often higher than cable or DSL. Larger 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, more than three-hundred times faster than a cable connection as its maximum speed.
Bandwidth is also a limiting factor for the technology that connects the computer itself to the modem or device interacting with the direct Internet line. Basic Ethernet, for example, has a bandwidth of 10 Mbit/s, so that 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 featuring 11 Mbit/s, Wireless-G 802.11g having a 54 Mbit/s cap, and Wireless-N 802.11n a blazing 300 Mbit/s.