To determine how much bandwidth an ISP requires to provide users with a set bandwidth, you need to do a little division with a calculator.
To figure the appropriate number of users that an ISP can lease its bandwidth to, you must know two things: the total bandwidth available (the SUM of all of their connections) as well as the advertised speed allotted to the end user. To figure the number of users, you would divide (using the same units) the total available bandwidth by the bandwidth allocated to each user.
For example, using your figure, say a small ISP is sub-leasing a T1 line to DSL subscribers, and they advertise that their clients will have 128kbps speeds. A T1 line is rated at 1.544 megabits per second. This is equal to 1544.0Kbps. So we divide:
1544kbps / 128kbps = 12.06, which rounded down would be equal to bandwidth available for 12 (twelve) - 128kbps users.
The first number must be the SUM of the ISPs lines, because often an ISP will have a number of large lines that they use as backbone connectors; i.e., 6 (six) - OC3 lines.
The above example is for a small-scale ISP, but you get the idea how these volumes are calculated. Large ISPs need gigabits worth of available bandwidth, and have hundreds of thousands of users. The amount of money the larger lines cost can be staggering, some in excess of $100,000 a month.
ISPs could allow more users on their networks than they really have support for, which probably happens quite frequently, except as long as everyone doesn't start heavily using their connections at once, it doesn't cause too many problems.
This article isn't an all-encompassing discussion on the factors determining the amount of bandwidth an ISP needs or how many users it can support, but merely a "scratching of the surface" look at what to consider when calculating these amounts.
Generally speaking, any business looking to start an ISP or upgrade their current ISP bandwidth amounts must do their homework and not jump without a thorough analysis. For help with that assessment, I strongly suggest taking advantage of the no cost research and support available through USaveTelecom.com.
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