How HotSpot@Home Works

By: Dave Roos
HotSpot@Home lets you make cell phone calls over your home WiFi network. Learn how this useful technology integrates seamlessly between two networks.
Image courtesy of T-Mobile

Now that you have a cell phone, why do you keep paying for that landline at home? Probably because your cell phone gets terrible reception inside your house. Cell phones communicate with the nearest cellular tower via radio signals. If your home has aluminum siding or was constructed with thick concrete and rebar, it's harder for the signal to penetrate the walls [source:].

In June 2007, T-Mobile introduced a new service, called HotSpot@Home, that improves indoor cell-phone reception by allowing users to make and receive cell-phone calls over their home wireless Internet networks (WiFi) in addition to the normal cellular network.


T-Mobile is not the only U.S. cell-phone provider to roll out a merged cellular/WiFi system -- Cincinnati Bell is another [source: GigaOM]. However, T-Mobile is the largest, so we're going to focus our article on the HotSpot@Home service.

Here's what you need to use HotSpot@Home:

  1. A home broadband Internet connection
  2. A qualifying T-Mobile calling plan
  3. A dual-mode cell phone for cellular and WiFi calling
  4. A wireless router that communicates using the 802.11b or 802.11g standard

If you already have an 802.11b/g wireless router, you don't have to buy a new one. If you don't have a router, T-Mobile sells its own wireless routers that are configured to give priority to voice transmissions over data transmissions. This could be helpful if you're downloading a large file while talking on the phone. Your call quality would remain the same and the file would simply take a little longer to download.

Setting up the HotSpot@Home service is very simple:

  1. Plug the wireless router into your broadband cable or DSL connection
  2. Plug the router into a power source
  3. Turn on your dual-mode cell phone

Instantly, the cell phone should recognize and join the home wireless network [source: Phone Scoop]. This is indicated by a symbol in the top-left corner of the cell phone screen. WiFi signal strength is indicated by a small orange ball with radio waves emanating to the right. The more waves, the stronger the signal.

Now that you're connected, let's go over how to use the HotSpot@Home service.


Using HotSpot@Home

With HotSpot@Home, users
Photo courtesy T-Mobile

Once you're connected to the home WiFi network, simply use the cell phone as you normally would. All of the typical calling features are included when talking over the WiFi network, like caller ID, three-way calling and call waiting (source: New York Times]. Web-based features such as checking e-mail or browsing the Internet can't be done over the home WiFi network. Data transfers to and from the cell phone must be made over the normal cellular network.

When using the HotSpot@Home service, any call you make or receive over your home WiFi network will remain on the home network as long as the phone receives a strong enough WiFi signal. If you walk out of your house and away from the wireless router, the signal strength will get weaker and weaker until the phone decides that it's time to switch to the cellular network. This is called a "handoff" and should happen seamlessly, without you even noticing the switch. [source: engadget mobile]


The same is true when you enter your house. As soon as the WiFi signal is strong enough, the phone will automatically handoff from the cellular network to the home wireless network.

There are many advantages to using a service like HotSpot@Home. Currently, the chief advantage is price. If you pay $9.99 extra a month for the HotSpot@Home Add-On, you can make unlimited nationwide calls from a WiFi network, any time of day. Calls initiated or received on the WiFi network are billed as a WiFi call even if you hand off mid-conversation to the cellular network. [source: New York Times]

For example, if you call the office from home, then leave the house and continue to talk as you drive an hour to work, the entire call is billed as a WiFi call. WiFi calls don't eat up minutes from your normal calling plan -- that is, if you've paid for the HotSpot@Home Add-On. If you choose not to buy the HotSpot@Home Add-On, you can still make calls from a WiFi network, but they'll be charged as part of your regular calling plan, meaning peak hours and nighttime/weekend minutes apply.

The other financial advantage of HotSpot@Home is that you can finally get rid of that landline telephone and all its additional local and long-distance charges.

With the HotSpot@Home technology you can make calls from any available WiFi network, not just your home network. The dual-mode phones allow you to easily search for available WiFi networks [source: Phone Scoop]. If the network requires a password, you can type it into the phone. The phone saves a list of the WiFi networks you have accessed -- along with passwords -- so that it can automatically connect to them in the future.

Additionally, T-Mobile has set up over 8,000 HotSpots across the United States in popular locations like Starbucks coffee shops and FedEx Kinko's offices. T-Mobile's dual-mode phones automatically access these WiFi networks without requiring a network search or a password.

Now let's look at the technology that makes merged cellular/WiFi networks tick.


HotSpot@Home Technology

Routers enable users to set up HotSpot@Home. Photo courtesy T-Mobile

HotSpot@Home takes advantage of three rapidly emerging telecommunications technologies: dual-mode cell phones, home wireless networks (WiFi) and wireless VoIP (Voice over Internet Protocol) telephony.

Wireless VoIP telephony uses an IP network, rather than a network of cellular towers and switches, to connect a cell phone to the rest of the cellular network. And, since dual-mode cell phones contain WiFi radios, they're able to access these IP networks through WiFi routers. For more detailed information on how WiFi VoIP works, check out How WiFi Phones Work.


Dual-mode cell phones communicate over home WiFi networks through a technology standard called UMA (Unlicensed Mobile Access). A UMA-enabled phone can join any unlicensed WiFi network. An "unlicensed" WiFi network simply means that the network operates over a free, unrestricted radio frequency. For example, 802.11b/g routers, communicate over the unlicensed 2.4GHz spectrum. Companies or government agencies buy licensed spectrums for exclusive use [source:].

Since dual-mode phones contain both cellular and WiFi radios, they're able to send and receive calls over two separate networks: the Cellular Radio Access Network (RAN) and the Unlicensed Mobile Access Network (UMAN) [source: UMA Technology]. The phone decides which network to use based on which one has the best signal strength: cellular or WiFi.

A Cellular Radio Access Network (RAN) is the traditional cellular network that relies on a series of cellular towers and switching stations. For a detailed explanation of how cell phones work over a Cellular Radio Access Network (RAN), see How Cell Phones Work.

Here's how a dual-mode cell phone connects to the same mobile network over an Unlicensed Mobile Access Network (UMAN):

  1. A UMA-enabled phone comes within range of a WiFi network and the network grants it access
  2. The phone sends a request through the WiFi router and over the broadband IP network to something called the UMA Network Controller (UNC).
  3. The UNC decides whether or not to give the phone authorization to access GSM voice services over the unlicensed network. GSM is the global cell phone standard.
  4. The UNC then tells the central Mobile Telephone Switching Office (MTSO) the cell phone's location. Now all calls to and from the core mobile network will be routed through the WiFi network [list adapted from source: UMA Technology].

When a dual-mode cell phone moves out of the WiFi network's range, it automatically hands off to the cellular-radio network without any service interruption [source: UMA Technology].

Routing calls over two separate mobile networks is actually a huge money-saver for cellular service providers. The only way to improve coverage and keep up with heavy traffic on a traditional cellular radio network is to build more cellular towers [source: New York Times]. But with HotSpot@Home, T-Mobile is able to re-route a significant amount of that traffic onto broadband Internet networks -- which, incidentally, T-Mobile doesn't own and therefore doesn't have to maintain.

Services like HotSpot@Home appear to be the next big thing in telecommunications. In the already booming cell phone market, dual-mode phones are the fastest growing segment worldwide [source: cellular-news]. Infonetics Research predicts a compound annual growth rate of 198 percent for dual-mode phone sales from 2006 to 2010.

For more information on HotSpot@Home and related topics, check out the links on the next page.


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