A traffic exchange website receives website submissions from webmasters that join traffic exchange networks. The person who submitted the website then has to browse other member sites on the exchange program to earn credits, which enable their sites to be viewed by other members through the surf system. This increases the number of visitors to all the sites involved. Exchanges enforce a certain credit ratio, which illustrates the amount of websites the surfer must view in order to receive one hit through the program for their promoted website. Many sites offer the ability to upgrade one's membership level for a more equal credit ratio. As the viewers are all website owners or affiliates, it is possible that some might find certain member sites interesting and thus make note of them on their own sites, sending more traffic their way. Most traffic programs also impose a time limit when members are browsing, ranging from 10 seconds to 60 seconds. Some incorporate the use of captcha to ensure user interaction. Almost all traffic exchange programs are free, although many of them offer special features to paid members and offer credits for purchase. Almost all traffic exchange programs encourage users to build their own referral networks, which in turn increases the referrers' amount of credits.