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Bandwidth essentialsWhat is bandwidth ? Bandwidth is a measurement of how much information can be transmitted between two points in a given period of time. The greater the bandwidth, the faster the possible rate of data transmission. Bandwidth is typically measured in Bits Per Second (bps), a respectable analogue modem can send data at 28,800 bps or 28.8kbps (kilobits, or thousands of bits per second). A typical page of text contains about 15,000 bits, therefore it will take about half a second to download, which is not bad, but what if you wanted to download a much bigger document such as the Bible which would weigh in at just under 5 megabytes of text, clearly you would have a much longer wait over a 28.8k modem. Modems are not the only way to transmit data fortunately, a special line called a T-1 can move 1,544,000 bits a second (1.554Mbs), which is fast enough for most people, though it is possible to go much faster than this. At most organisations they have a Local Area Network (LAN) which can transfer data at 10Mbs (megabits or millions of bits), which is fast enough for most applications, although it may not seem it at times, when everyone else is accessing the network at the same time.
Although you might have a fast connection to the Internet, there are certain factors which affect performance. The Internet in a lot of ways is similar to our roads, if you imagine the cars as bits of data trying to get from one place to another, and what can affect the time it takes to reach a certain destination. Just like roads, the Internet has busy periods where there is a lot of traffic around, all trying to get somewhere, at the same time. Although one may have a lot of bandwidth, transfer rates diminish when sharing with other users. Lack of bandwidth can affect downloading of data, either by a drop in transfer rate or the connection being timed out. A timeout can result in a file, being chopped off before it has been successfully downloaded. Applications such as videoconferencing or live audio/video transmissions require a guaranteed amount of bandwidth, if they fail to get it, it can result in audio or video that breaks up, not good.
To demonstrate how much difference there can be between different bandwidths available, the following table shows download times of 1 Megabyte of data. In a real life scenario times can vary due to quality of the connection and also because modems provide compression of data.
At the moment the majority of people who access the Internet from home, do it via a modem connected to the telephone line. The problem with this method is that it uses analogue methods to send digital data. A modem encodes digital data into analogue sound for transmitting onto phone lines, and vice versa when receiving. ISDN which stands for Integrated Services Digital Network, is a digital telephone system that works over the current telephone network. Using ISDN, data is transmitted directly over the digital network, even voice is digitized into series of bits and transmitted at a rate of 64kbps. ISDN replaces the analogue path between the user and the exchange with a fully digital service, which offers more bandwidth and better reliability than normal modems. ISDN also offers 2 channels which can transfer data at 64kbps (making a total of 128kbps), even though there is only one line. One other advantage is the time ISDN takes to make a connection, while an analogue modem can take upto 30 secs to setup a connection, ISDN takes a fraction of a second.
Generally the way the Internet works is that information is sent from one computer to another at a time. The data being sent has one destination, for example when you access a web site, you communicate with the server which then sends the data requested to your computer, even though these servers can handle many requests per minute, the data being transferred is still one-to-one. This is fine for most applications, but imagine a situation where perhaps a radio station had a web site, and they wanted to transmit a live broadcast over the Internet. There maybe thousands of people connecting to the radio web site, all downloading the same transmission at the same time, this will quickly put a lot of strain not only on the Internet, but also on the server itself. Having so many people all downloading the same transmission will quickly eat up bandwidth on the Internet. MBone which stands for Multicast Backbone is an attempt at improving efficiency for transmission of data to multiple recipients. Multicasting is a method of sending packets to more than one site at a time. MBone programs are an implementation of IP (Internet Protocol) multicast. It works by broadcasting packets of data to anyone who is listening rather than sending data to a specific computer. For example the radio web site could have a multicast transmission, which means one copy of it's transmission is sent out to no-one in particular, and anyone who wants to listen can, a bit like a television broadcast. It does not make any difference if one person or a hundred people are listening, it will still only use the same amount of bandwidth. The main object blocking MBone from taking over the Internet, is that the majority of users do not have enough bandwidth, a multicast video stream of a few frames a second will take up about 128kbps, certainly you would need an ISDN or T1 line to cope. A audio stream is possible on a modem, due to them typically taking up between 8 and 20kbps.
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