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Call (E911) Positioning </title>
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<h1>Functionality of Support for US Emergency
Call (E911) Positioning </h1>

<p>The emergency call positioning method is based on Time
of Arrival (TOA) measurements made from a channel that the located MS uses
as an uplink. Therefore, the emergency call positioning system must get correct
uplink channel information for the located MS.</p>


<p>The main role of a BSC in emergency call positioning is
to find the uplink radio channel information for the MS and to transfer that
information to a stand-alone SMLC, which forwards it to the emergency call
positioning system. Also, the changes in the radio channel have to be noted
and forwarded to the stand-alone SMLC.</p>


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<h2>Accuracy enhancement for emergency call positioning </h2>

<p>The accuracy and processing time of Support for US Emergency Call (E911)
Positioning are related to the signal level transmitted by the MS. In some
cases, the GSM power control may degrade emergency call positioning because
the MS can be ordered by the BSC to ramp down the transmit power as soon as
it is assigned to a TCH. A handover during emergency call positioning may
also impact its performance because it may have to re-calculate the positioning
with the new TCH properties (Freq, MA, and so on).</p>


<p>It is possible to enhance Support for US Emergency Call (E911) Positioning
 by disabling MS power control and handovers temporarily for a configurable
amount of time (0 – 10 seconds) after the call has been placed on a TCH. Emergency
call positioning does not affect handovers prior to TCH placement, such as
Directed Retry and Direct Access to Desired Layer/Band. When the timer is
set (for example for 5 seconds), the BSC will keep the MS at a maximum power
for 5 seconds after it has been placed on a TCH and handovers are also prevented
during that period. This only applies to emergency calls that are detected
by the <span class="message-sem">Service Req</span> message sent from the MS. </p>


<p>For more information on accuracy enhancement for emergency call positioning,
please contact your local Sales and Marketing office.</p>
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