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Landline to Cellular Gateway

One evening during a phonecall, a friend mentioned an idea he had for creating a landline to cellular gateway. Using a cellphone and it's charging cradle, landline service could be discontinued while the landline phones themselves continue to be used in their customary manner.

I knew that audio and control signals could be routed back and forth on a cellphones system bus and that his idea was feasible. So, on nights and weekends, we began working on a cellular gateway.

The basic design consisted of a Subscriber Line Interface Card (SLIC) and an 8-bit microcontroller. The SLIC handled the analog to digital conversion between the landlines and micro-controller. The micro-controller handled AT commands to and from the cellphone as well as signals to the SLIC.

To make it work, the house had to be disconnected from the central office. This was done at the demarcation point where ownership changes between the telephone company and the homeowner. Once the house was disconnected, the gateway plugged into an ordinary wall telephone outlet. If a landline went offhook, the gateway's SLIC would detect it and generate dialtone. As the landline dialed digits, the SLIC would decode the DTMF (or pulses) and output a level understood by the microcontroller. Once the microcontroller determined the user had completed dialing digits, it would formulate an AT command to the cellphone to dial the digits. Then it would switch through the audio path from cellphone to landline.

If the called party answered, audio continued to be through connected, If either side disconnected, the SLIC and micro-controller would generate the appropriate clearing signals.

For inbound calls, when the microcontroller inside the gateway detected an incoming call, it would send a signal to the SLIC which would then generate ring current to the landlines. When a landline answered, the SLIC would signal the micro-controller which would signal the cellphone and cut through the audio path.


We named our gateway the LEK which was short for Local Exchange Killer. We thought we were going to be rich.

In our enthusiasm to develop the LEK, we didn't fully appreciate three key issues; demarcation disconnect, target customer and lack of 911 support.

Demarcation Disconnect

For two tech savvy guys, disconnecting the house from the central office at the demarcation box was less than trivial but for the non-tech savvy customer, the prospect would have been formidable. That issue wiped off the board a large percentage of potential customers.

Target Customer

The target customer was a one to two person household but such people tend to live in apartments, townhomes and condominiums where they do not have access to the demarcation point. The number of detached single family residences containing just one or tow persons was likely to be just a small fraction of the overall number of detatched single family homes.

Poor 911 Location

Even if landline service has been disconnected by the telephone company, the central office still powers the lines and it is still possible to place a 911 call and the 911 dispatcher will be able to send response to the exact adddress. Once disconnected from the central office, reliance is placed upon the cell phone to be located by the cell company which can be considerably less reliable, especially if the gateway is located away from windows where it cannot receive a GPS signal.

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