134 Personal Content Experience: Managing Digital Life in the Mobile Age This removes one of the key challenges in synchronization. In fact, a 5 large number of unconnected devices will have their clocks set wrong. Lately, with the advent of Internet connected devices, this problem has lessened but new problems have arisen. Many new devices are capable of synchronizing their time to some known reference time, by 6 mostly using Network Time Protocol (NTP) or Simple NTP (SNTP). One manufacturer of a popular router used the NTP server at the University of Wisconsin to set its time. Since all routers from this manu- facturer, from all over the world, connected to the same Wisconsin server, the server became quickly overwhelmed and could no longer serve anyone. For more details about the incident, see Mills et al. 7 (2004). Consequently, timestamp-based synchronization is prone to error. It is diffi cult to ensure synchronized clocks in a distributed environment. 5.2.3 Version Control “Version control” is a term often associated with software develop- ment. It allows keeping track of changes made to the source fi les, probably by multiple software engineers, and making different builds of the whole project by choosing which versions are to be included. In addition to software projects, version control is also an essential component of personal content management. Often, there are many versions, or instances, of a piece of personal content. For example, the user may have several versions of the same photo: the original, one with adjusted colour levels, and one scaled down for web use. Likewise, a song digitized from an original analogue recording may be stored on a CD as an uncompressed version, while a compressed version may exist for music collections. Furthermore, a new version is implicitly created when an image is sent to a friend. What is important to consider with the different versions is to deter- mine whether the subsequent changes made to the derived content are also refl ected back to the original. For example, when the photo album software in a PC keeps track of how many times a certain photo has been viewed full-screen, should the viewing count for a version of the same image in a mobile device be included as well? 5 This is the mobile version of the infamous blinking “00:00” clock problem commonly seen in VCRs (Video Cassette Recorders) and microwave ovens. Setting the VCR is too complicated for many people, so they just leave the clock unset. 6 http://www.ntp.org 7 http://www.cs.wisc.edu/~plonka/netgear-sntp/, checked 15.12.2006