What Is the Difference Between Downloading & Uploading?by Milton Kazmeyer
Whenever you connect to the Internet, you both upload and download data. Uploading involves any data packets that travel from your system to another server on the Internet, while downloading covers those packets that flow from outside servers to your PC. For most users, this process is largely asymmetrical, with downloading data much easier and faster than the uploading process.
During your normal activities on the Internet, your computer is constantly uploading and downloading data. When you click on a link on a website, for instance, your computer sends a small packet of data to the server to request the new page. The server responds by sending a series of packets containing data for the page, and your computer sends back a small acknowledgment for each packet received. Under normal circumstances, you will download considerably more data from the Internet than you upload.
Since most users require more download capacity than upload capacity, most consumer Internet accounts are asymmetrical. According to PCMag.com, in 2013, Verizon offered plans with an average download speed of 34.5 Mbps while only allowing an average of 20.1 Mbps upload. Typically, the discrepancy is even larger, with many Internet service providers limiting upload speeds to around 10-15 percent of maximum download speeds. These companies use their excess outgoing bandwidth to serve business customers and for hosted content on their servers.
One side effect of these asymmetric Internet connections is that it is easy to overwhelm your available upload allotment, which can slow your downloads and Internet browsing to a crawl. Any time a server sends your computer data, your computer must acknowledge its receipt before more data arrives. If you are uploading a file at maximum speed, the packets you are sending can crowd out these acknowledgments, slowing them down and reducing the speed of incoming packets. BitTorrent is a notorious source of congestion, since the client will upload at the maximum available speed and grind all other Internet activity to a halt. Whenever you upload a file, set the maximum upload rate to a few kilobytes per second below your maximum speed to allow enough overhead to prevent congestion.
When you purchase Internet service, your ISP will quote upload and download speeds, but these are best-case estimates. Latency and delay anywhere in the global network can affect how fast any given data stream arrives at your PC and affect your upload speeds. Your speeds can fluctuate significantly, especially during prime time when many users are downloading data at once. Speed test sites transfer test data back and forth to your PC, and give you an up-to-the-minute snapshot of your connection's upload and download speeds. Be sure to shut down any file transfers or programs that may access the Internet before testing in order to get an accurate picture of your transfer rates.