Sideloading is a term used mostly on the Internet, similar to "upload" and "download", but in reference to the process of transferring data between two local devices, in particular between a computer and a mobile device such as a mobile phone, smartphone, PDA, tablet, portable media player or e-reader.
Sideloading typically refers to media file transfer to a mobile device via USB, Bluetooth, WiFi or by writing to a memory card for insertion into the mobile device. The term is also used to refer to the practice of alcohol consumption in between visits to licensed venues while on a night out. 
When referring to Android apps, "sideloading" typically means installing an application package in APK format onto an Android device. Such packages are usually downloaded from websites other than Google Play, usually through a computer. Sideloading of apps is only possible if the user has allowed "Unknown Sources" in their Security Settings.
The term "sideload" was coined in the late 1990s by online storage service xdrive.com as an alternative means of transferring and storing computer files. In 2000 idrive.com applied for a trademark on the term. Rather than initiating a traditional file "download" from a website or FTP site to their computer, a user could perform a "sideload" and have the file transferred directly into their personal storage area on the service. Usage of this feature began to decline as newer hard drives became cheaper and the space on them grew each year into the gigabytes.
The launch of Apple’s iTunes Store brought sideloading to the masses, even if the term was not widely adopted at the time. The service allowed iPod users to download content to their PCs and sideload it to their iPods. This approach was also adopted by other portable media player manufacturers.
For mobile phones, sideloading was something of a niche, particularly as the performance, storage and transfer options varied greatly between devices. Although there were some early attempts at delivery of entire movies on memory cards, the number of devices supported was small and adoption was low.
However the situation started to change in June 2007 with the launch of Clippz.com. It allowed mobile phone users to download and sideload user generated video in formats optimized for any device. In the same month, Apple launched its iPhone, which was the first mobile phone to be focused on a sideload delivery of content (again from iTunes) rather than through the wireless network.
Today, sideloading is far more widespread and virtually every mobile device is capable of sideloading in one or more ways.
Sideloading has several advantages when compared with other ways of delivering content to mobile devices:
- There are no wireless data charges. Sideloading delivery does not involve a wireless carrier.
- Content can be optimized for each mobile device. As there are no mobile network restrictions, content can be tailored for each device. This is more important for video playback, where the lowest common denominator is often a limiting factor on wireless networks.
- There are no geographic limitations on the delivery of content for sideloading as are implicit in the limited coverage of wireless networks.
- The content is not streamed, and can be permanently stored in the mobile device. It can be listened to or watched at the user’s convenience.
- Sideloading is an excellent mechanism for proximity marketing.
Sideloading also has disadvantages:
- Streaming media is generally preferred to downloading due to limited storage, thus there is limited content available to download and sideload.
- There are huge variations in performance capability for mobile devices that can make use of sideloading, from simple mobile phones with limited video playback, to high-end portable media players. Unless the audio/video file is encoded with the target device in mind, playback may not be possible. Companies like Clippz address this problem.
- Some wireless carriers (most notably Verizon Wireless) require that handset manufacturers limit the sideload capabilities of devices on their networks as a form of vendor lock-in. This usually results in the loss of USB and Bluetooth as sideload options (though memory card transfer is still available).
Sideloading over a standardized USB (Micro-USB) connection was an agreement come to by the OMTP in late 2007. Until this time, mobile phone manufacturers had tended to adopt proprietary USB transfer solutions requiring the use of bundled or third party cables and software.
Transfer performance of USB sideloading varies greatly, depending on the USB version supported, and further still by the actual engineering implementation of the USB controller. USB is available in Low-Speed, Full-Speed, and Hi-Speed levels, with High-Speed USB transferring up to 480Mbit/s (60MB/s). However, the majority of mobile phones as of the time of writing of this article are Full-Speed USB. Of the mobile products supporting USB 2.0 Hi-Speed, the actual sideloading performance usually ranges between 1-5MB/s. However, the popular BlackBerry mobile phones by RIM and the iPods by Apple distance themselves at higher performing speeds of roughly 15.7MB/s and 9.6MB/s, respectively.
Bluetooth sideloading is an option that is generally only available to mobile phones and some PDAs as it is not adopted in portable media players.
Bluetooth’s OBEX/OPP profiles allow for file transfer between a PC and a mobile device. Using this option is slightly more complicated than using a USB connection as the two devices have to be paired first. Also, unlike the familiar drag and drop that is usually available via USB, Bluetooth implementation is specific to the Bluetooth transceiver and drivers being used. Files that are sideloaded to mobile devices via Bluetooth are often received as messages, in the same way that SMS texts would be received. While these files can be saved to any storage medium, their initial location is the handset’s internal memory. As such the limitations of the internal memory have to be taken into account before beginning the sideload.
Memory card sideloading
Sideloading via a memory card requires that the user have access to a memory card writer. Audio and video files can be written directly to the memory card and then inserted into the mobile device.
This is potentially the quickest way of sideloading several files at once, as long as the user knows where to put the media files.
The practicality of this solution varies between different mobile devices according to the accessibility of the memory card. On many devices the card can be hot-swapped via a slot on the side of the devices. In other cases, the card is located under the battery and so the device must be first switched off and the battery removed before the transfer can take place.
- "How to sideload Android applications when they didn’t come from the Google Play Store [ANDROID 101]". phandroid.com.
- "Local Connectivity: Data Connectivity", OMTP, omtp.org, September 17, 2007
- "Under the Hood: Blackberry wins handset data-rate bakeoff", EETimes, eetimes.com, November 11, 2007