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Bandwidth describes the rate at which a frequency cuts off from its highest point to its lowest point, for example, a Filter, a communication Channel, or a signal spectrum, and is typically measured in hertz. Bandwidth in hertz is a concept in many fields including electronics, information theory, radio communications, signal processing and spectroscopy.
In networking, digital bandwidth refers to data rate measured in bits. For example, channel capacity or throughput are both measured in bits and examples of digital bandwidth. The channel capacity in bits is proportional to the analog bandwidth in hertz according to Hartley's Law. According to the Shannon-Hartley theorem, the data rate of reliable communication is directly proportionate to the frequency range of the signal used for the communication. In this example, bandwidth can refer to either the data rate or frequency range of the communication system.
In web hosting, the term bandwidth is often used to describe the amount of data that can be transferred to and from the website or server. This is measured in bytes transferred over a period of time. This is more accurately described as a monthly data transfer, but the term bandwidth is used as a metaphor. Web hosting sites refer to monthly bandwidth limits for website hosting they are trying to sell. For example, 500Gb per month would mean that your website would be allowed to generate up to 500 Gigabytes worth of data transfer over a month before you'd get charged extra for overage. Data transfer can be anything from simple page impressions, to image loading, to file downloading.