Sam's Notes on Mobile Phone Tethering Oct. 25, 2013
Definition: Tethering - a notebook/desktop PC using mobile phone as Internet "modem," utilizing existing (voice package) or dedicated data plan [This term is also very commonly used in completely different contexts, with different meanings: in digital photography, and in general PC networking. Here, I'm giving my definition, when used as a cell phone industry jargon.]
Software Requirements: In order to tether, the device--be it a tablet without carrier connection, a notebook PC, or a desktop PC, or a home router (which is a dumb PC acting on behalf of many PCs), must be installed with a specific version of tethering software, supporting the OS platform + phone device + the associated carrier.
Costs & Legal Issues: All subscribers are expressly prohibited from sharing of Internet bandwidth with any device other than the phone itself--not even with tablets/notebooks/PCs/routers owned and personally operated by the subscriber. Those engaging in tehtering are required to pay a monthly tethering fee ($15~$25), on top of the regular subscription fee, even if it includes "unlimited Internet." There had been a slew of high profile court cases, and the issue is now considered settled: Subscribers must pay per carrier agreement. In the same ruling, carriers are required to make available tethering add-on plans at reason costs, as opposed to out-right banning it. All carriers have some built-in technical mechanisms of foiling unauthorized tethering, and reserve their rights to retroactively sue for payments, if they didn't immediately detect and stop the subscriber from such activities.
Technical: Typically, tethering is accomplished via USB port and USB cable, after special tethering software has been installed on the client device. Some (but not all) tethering setups permit concurrent voice call function. Portable Access Point (AP): Tethering can be achieved wirelessly, without literally hooking up a cable. It is possible to turn a mobile phone into an access point, effectively providing simultaneous tethering connections to multiple devices.