Tethering, or phone-as-modem (PAM), is the sharing of a mobile device's internet connection with other connected computers. Connection of a mobile device with other devices can be done over wireless LAN (Wi-Fi), over Bluetooth or by physical connection using a cable, for example through USB.
If tethering is done over WLAN, the feature may be branded as a personal or mobile hotspot, which allows the device to serve as a portable router. Mobile hotspots may be protected by a PIN or password. The Internet-connected mobile device can act as a portable wireless access point and router for devices connected to it.
Mobile device's OS supportEdit
Many mobile devices are equipped with software to offer tethered Internet access. Windows Mobile 6.5, Windows Phone 7, Android (starting from version 2.2), and iOS 3.0 (or later) offer tethering over a Bluetooth PAN or a USB connection. Tethering over Wi-Fi, also known as Personal Hotspot, is available on iOS starting with iOS 4.2.5 (or later) on iPhone 4, 4S (2010), 5, iPad (3rd generation), certain Windows Mobile 6.5 devices like the HTC HD2, Windows Phone 7, 8 and 8.1 devices (varies by manufacturer and model), and certain Android phones (varies widely depending on carrier, manufacturer, and software version).
For IPv4 networks, the tethering normally works via NAT on the handset's existing data connection, so from the network point of view, there is just one device with a single IPv4 network address, though it is technically possible to attempt to identify multiple machines.
On some mobile network operators, this feature is contractually unavailable by default, and may only be activated by paying to add a tethering package to a data plan or choosing a data plan that includes tethering, such as Lycamobile MVNO. This is done primarily because with a computer sharing the network connection, there may well be a substantial increase in the customer's mobile data use, for which the network may not have budgeted in their network design and pricing structures.
Some network-provided devices have carrier-specific software that may deny the inbuilt tethering ability normally available on the device, or only enable it if the subscriber pays an additional fee. Some operators have asked Google or any mobile device producer using Android to completely remove tethering support from the operating system on certain devices. Handsets purchased SIM-free, without a network provider subsidy, are often unhindered with regards to tethering.
There are, however, several ways to enable tethering on restricted devices without paying the carrier for it, including 3rd party USB Tethering apps such as PDAnet, rooting Android devices or jailbreaking iOS devices and installing a tethering application on the device. Tethering is also available as a downloadable third-party application on most Symbian mobile phones as well as on the MeeGo platform and on WebOS mobiles phones.
In carriers' contractsEdit
Depending on the wireless carrier, a user's cellular device may have restricted functionality. While tethering may be allowed at no extra cost, some carriers impose a one-time charge to enable tethering and others forbid tethering or impose added data charges. Contracts that advertise "unlimited" data usage often have limits detailed in a Fair usage policy.
In the UK, two tethering-permitted mobile plans offered unlimited data: The Full Monty on T-Mobile, and The One Plan on Three. Three offered tethering as a standard feature until early 2012, retaining it on selected plans. T-Mobile dropped tethering on its unlimited data plans in late 2012.
As cited in Sprint Nextel's "Terms of Service":
Except with Phone-as-Modem plans, you may not use a mobile device (including a Bluetooth device) as a modem in connection with any computer. We reserve the right to deny or terminate service without notice for any misuse or any use that adversely affects network performance.
T-Mobile USA has a similar clause in its "Terms & Conditions":
Unless explicitly permitted by your Data Plan, other uses, including for example, using your Device as a modem or tethering your Device to a personal computer or other hardware, are not permitted.
T-Mobile's Simple Family or Simple Business plans offer "Hotspot" from devices that offer that function (such as Apple iPhone) to up to 5 devices. Since 2014-03-27, 1000 MB/month is free in the USA with cellular service. The host device has unlimited slow internet for the rest of the month, and all month while roaming in 100 countries, but with no tethering. For $10 or $20/month more per host device, the amount of data available for tethering can be increased markedly. The host device cellular services can be canceled, added, or changed at any time, pro-rated, data tethering levels can be changed month-to-month, and T-Mobile no longer requires any long-term service contracts, allowing users to bring.