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About Bandwidth - Testes Largura de Banda


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Artigo muito interessante sobre as questões técnicas que envolvem a Largura de Banda

Artigo Completo e Utilitários para medir a largura de banda

Internet bandwidth isin simple termsthe transmission speed or throughput of your connection to the Internet. Howevermeasuring bandwidth can be trickysince the lowest bandwidth point between your computer and the site you're looking at determines the effective transmission speed at any moment.

Three factors outside of your computer control how quickly you can view Web pages:

The Internet bandwidth between your computer and the site you're viewing.

The round-trip time between your computer and the site you're viewing.

The response time of the site you're viewing.

The tests referenced on this page address the first issueand measure the Internet bandwidth between your computer and PC Pitstop's servers. We also have tests that can measure the round-trip time between your computer and seven different sites on the Internethere. Of coursethe response time of our site will always be wonderful...👍 (If notwe'll tell you on the home page.)

Tests: Download vs. Upload

The differences between our Download and Upload tests aren't as obvious as they may initially seem. Yesthe basic difference is the direction of the data transfer: Simply putthe Download test measures your connection speed for viewing Web pages; the upload test measures the speed for maintaining them--or sending data over your connection.

Howeverthe rated upload and download speeds may not be the same for your connection. Some connectionssuch as 33K and lowerare "symmetric," meaning the rated upload and download times should be the same. Other connectionssuch as cable modems and ADSLare "asymmetric" (the "A" in ADSL stands for asymmetric). This means the upload and download times won't necessarily be the same; upload times are generally not as fast as download times. For instancethe rated speeds for ADSL are 1.4Mbps downand 400Kbps up. Cable modems are typically rated at 1.5 to 3Mbps downand 400 to 600Kbps up.

Occasionallyyou may even see opposite resultsespecially on cable modems during the evening hours. If your connection has a heavy user loadthe download times may sufferwhile the upload times remain unchanged. This is because the majority of Internet users download data instead of uploading it.

In additionthe ways we actually conduct the tests aren't quite comparable. Our Download test uses the http protocol and grabs one big stream of data. Our upload test uses the ftp protocolwhich involves a lot of handshaking across Internet servers. As a resultthe test is more susceptible to interruptions because of bad ping times on a server or servers.

Bottom line: You should regularly run both tests to make sure you're getting the rated upload and download speeds from your connection.

About throughput and reproducibility

The Internet changes from one moment to the next in ways that are impossible to predict. You cannot expect to see the same bandwidth value every time you measure it. Furthermoreyou cannot expect to see the full nominal speed of your connection for your bandwidth measurement: There are always delays somewhere. As a rule of thumbif you can measure throughput that is 85% of your nominal bandwidthmore often than not your connection is performing at par. (You may need to contact your service provider or modem manufacturer to determine the rated speed of your connection and/or modem.)

This is especially true with modems. Most 56Kbps modems connect at a speed less than 46Kbpsbecause of the limitations of analog phone lines and telephone company switches.

To get the best picture of your Internet bandwidthtest several times. Also test at different times of the day: Your bandwidth measurement at 7 AM may be much better than your bandwidth measurement at 10 PM.

About bandwidth units

You will often see bandwidth and transfer speed quoted in two different units: kilobits per secondabbreviated kbps or Kb/sand kilobytes per secondabbreviated KB/s. The difference between the two units is the number of bits in a bytewhich is 8. The small 'b' stands for bitsand the big 'B' stands for bytes. Transfer speeds are often shown in KB/sand connect speeds are usually quoted in Kb/s.

Sofor instanceif a progress dialog for a modem shows you a download speed of 4.3 KB/sit is the same as 34.4 Kb/s. If a progress dialog for a cable modem shows you a transfer speed of 100 KB/sit is the same as 800 Kb/s.

We display our measured transfer speeds in Kb/sto make them easier to compare with your rated line speed.

About bandwidth and modems

Bandwidth over a modem connection can sometimes be difficult to understand. There are two connections to a modem: one from your computer to its modemand one from the computer's modem to the ISP's modem.

The connection speed between the computer and its modem (called the Maximum speed under Control Panel/Modem/General tab/Properties) should be set as high as possible without causing errors. On most computers this is 115200also written as 115.2 Kb/s.

The connection speed between your modem and the ISP'sand the compression and error checkingare negotiated between the two modems when they establish the call. In the very best possible casewhich is rarely seentwo V.90 (56 Kb/s) modems will be able to connect at 53 Kb/s with compressionand the compression on normal text transfer will average 50%giving an effective transmission rate of 106 Kb/s. Very highly compressible material could be transferred at the maximum rate of 115.2 Kb/s. Incompressible material like ZIP files could be transferred at a maximum rate of 53 Kb/s.

Our download test transmits an incompressible block of random text. The theoretical maximum transfer speed for this over a V.90 modem is 53 Kb/sif there was no latency at all on the line--that isif there was no delay between the times your computer asked for a packetour computer sent itand your computer received it. With normal latencyhowevertransfer speeds are reduced to roughly 85% of the maximumwhich for a V.90 modem would be about 45 Kb/s. If your modem connects to your ISP at the more typical 44 Kb/sthen you can expect our test to report about 37 Kb/s on a connection with normal latency.


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