About Download Accelerators and Issues Using Them

Tom · Guest

[b:001b96d010]About Download Accelerators and Issues Using Them[/b:001b96d010]

Generally speaking this is a function that allows a download to be resumed in spite of a "time-out" or connection drop between the file server and the user's system. In my opinion, the resumable function is much more important than the "accelerator" function. A DA "accelerates" a download by opening up multiple ports, or connections, to the download library. How many ports it opens can be configured by you. DA's also typically provide a resume function.

Now some details on [b:001b96d010]AVSIM[/b:001b96d010] Library server and how we connect to you when you request a file. When you download a file, what you are doing is asking the server to open up a port or ports and start the feed of the file to your system via the net. We commence sending you the file in "packets". Each packet is sent with a check sum suffix that your computer then receives. When your computer receives the packet and the checksum, it then sends back to the server an acknowledgement that its calculation of the checksum and that received from server match. If they do not match, the server resends the packet. The server/local computer continues this process until the entire file is transmitted.

Packets can get corrupted for a number of reasons. The primary cause of corrupt packets is noisy connections to the net. This can happen with all types of connections, but is most serious with modem connections. The more corruption that takes place, the longer the download will take. At some point, depending upon your system, download software, etc., your system may stop accepting the file. That's when having the resume function of a DA comes into play as it allows you to return and complete the download without having to start all over again.

AVSIM's server is currently set to allow NO MORE THAN FOUR connections at one time from any single IP address.

If you allow your DA to use more than four download ports, two alternatives could result. Your accelerator doesn't start, or it only connects to the four ports and no more. Depending upon the accelerator program, it may not initiate the download, leaving you to believe that AVSIM's server has not started the download process. Ensure that you have no more than four ports open at any one time.

The further implication to this is that if you are running four ports in your DA program, and you attempt to download a second file concurrently, again depending on the specific DA program, the DA could go into a "send file later" mode, or it may not initiate the download all - again appearing to you that the AVSIM server was not providing you the file.

So, if you are using a download accelerator, make sure you know how many ports it is attempting to open for any one download!

Back to acceleration; if you are a modem user connecting to the library, opening multiple ports does you absolutely no good. If you have a 56Kb connection to the Internet, the best data rate you are going to get is about 5.2 Kbytes per second. More typically, you will get 4.9. In any case, you are never going to get data faster than that, irrespective of how many ports your DA opens. If you are a modem user, ONLY open one port. Your file is not going to be fed to you any faster than 5.2 kb.

If you are on cable, ADSL/DSL, T1 or fractional, you may see a benefit to multiple port openings, however, your file transfer speed is always going to be limited by the maximum feed rate of the server, the number of "hops" between your system and that server, and the degree of corruption of packets that occurs along the path. All of these can vary day to day and hour by hour. We attempt to deal with the "hops" issue by having our connection to the net in as close proximity to major trunks as we can get. Our distribution of files is very fast for users in Europe, Asia and South America as a result.

Okay, now to the final issue with DA's. If you set your DA to be your default download method, then it will "trigger" on certain file extensions; zip, exe, etc. It will open up when you click on a URL that includes one of these extensions. If your DA is not being triggered when you click on a file download button, make sure that the DA is indeed your default method and that you have all the proper extensions in your DA setup. If the browser's process opens up instead of your DA, then you can be sure the DA is not set as your default process.