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Many of today's smartphones enable users to share their own internet connection with others via Wi-Fi through tethering.

However, from what I understand, current Wi-Fi has much shorter range and needs a lot more power than mobile broadband, such as 3G or 4G (or whatever tech the phone uses for internet in the first place).

What are the obstacles for using mobiles as effective mobile broadband routers?

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closed as off-topic by Ignacio Vazquez-Abrams, Majenko, pjc50, Leon Heller, Ricardo Jan 9 '15 at 18:07

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Ignacio Vazquez-Abrams, Leon Heller, Ricardo
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ It sounds like your only use case is sharing a Wi-Fi connection over 3G/4G. When would you need to do that? \$\endgroup\$ – Samuel Jan 9 '15 at 17:03
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    \$\begingroup\$ I disagree with the closing of this question; the OP was looking for the technical reason that cell modems cannot be used for tethering, which is a design issue (essentially, they're not designed for it, in addition to any regulatory issues). I was in the middle of answering the technical side when the question was closed. Oh well. \$\endgroup\$ – tcrosley Jan 9 '15 at 18:15
  • \$\begingroup\$ @tcrosley Can you maybe share in comments? I also rephrased the question to encourage re-opening. \$\endgroup\$ – Domi Jan 9 '15 at 18:30
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    \$\begingroup\$ @Domi My answer was fairly lengthy so I won't try to repeat it here, bu the gist was cell modems are controlled by a microcontroller using AT commands, like the old Hayes dial-up modems but greatly expanded. The current cell module I am using has 250+ AT commands. These are used to place calls, answer calls, and set up TCP/IP data connections. They are designed so the cell module can connect to a cell tower. To permit point-to-point communications for tethering, the cell phone doing the "broadcasting" would have to act as a cell tower. There are no provisions for this. \$\endgroup\$ – tcrosley Jan 9 '15 at 20:35
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While 3G\4G uses different 'commercial' bands, WIFI and Bluetooth uses the 2.4Ghz band which is the only band that is open for private use throughout the world (There are other bands that are open for private use but different countries have different regulations and not all of them authorize all private bands)

  1. It is easy for major RF manufacturers to build communication components that work on a single known and authorized band.
  2. This guarantees that if you do a tethered network you wont accidentally (or intentionally) mess around with the cell-phone provider network.
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  • \$\begingroup\$ Thanks for the explanation! So all possible bandwidths for 3G/4G are regulated and are not allowed to be used (except for a few exceptions you mention, which differ between countries)? Do providers pay money to use those frequencies? How is that controlled? \$\endgroup\$ – Domi Jan 9 '15 at 17:26
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    \$\begingroup\$ Yes, providers pay ALLOT of money to the government so they could use different frequencies for their business. In most countries, all frequencies are controlled by the government and if you transmit or even monitor (like police or military bands) restricted frequencies you could get in to serious trouble. Technically, you could transmit on any frequency if you have the correct equipment but try importing RF components, you will have to go though the seven sections of hell in order to authorize them and you might not succeed at all if the bands they use a restricted in your country. \$\endgroup\$ – Gilad Jan 9 '15 at 17:50
  • \$\begingroup\$ I was afraid of that. Is the frequency the reason for the long range and low energy consumption of mobile broadband though? \$\endgroup\$ – Domi Jan 9 '15 at 18:05
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    \$\begingroup\$ Not exactly. The higher the frequency the smaller the antenna but then you will need more power to go through walls. Low energy consumption is mainly because of many small antennas the provider have all around the country so you are always near one of them and don't have to broadcast far away and, of course, black magic voodoo developed by RF engineers. \$\endgroup\$ – Gilad Jan 12 '15 at 23:08

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