Note: Descriptions are shown in the official language in which they were submitted.
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PROCEDURE AND SYSTEM FOR ENSURING EMERGENCY CALLS IN A
WIRELESS LOCAL LOOP
The present invention relates to a procedure
for ensuring emergency calls in a wireless local loop,
as defined in the preamble of claim 1.
In a wireless local loop (WLL) , a terminal de-
vice is connected via a wireless link to an access node
(AN) or a WLL controller. The access node may consist of
multiplexers, crossbar switches and various transmitting
systems. The WLL system may be based e.g. on technology
used in mobile telephone systems, such as the
GSM/DCS1800 technology (GSM, Global System for Mobile
Communications; DCS, Digital Cellular System). GSM is a
European digital mobile communication system standardi-
zed by ETSI. DCS-1800 is a mobile communication system
standardized by ETSI, which is based on the GSM specifi-
cation and aims at a more effective use of microcells
and which works in the frequency range of 1800 MHz. Bet-
ween the terminal device and the access node there is a
base station, through which call signals sent by the
terminal device over a radio channel are transmitted via
the access node to a public telephone network and vice
versa. The access node or WLL controller can be connec-
ted to the telephone exchange using e.g. the V5.2 proto-
col consistent with the ETS 300 347-2 standard.
In this context, ensuring an emergency call me-
ans that, in the event of overload on a base station in
a wireless local loop, the setup of a call initiated by
a subscriber's station is ensured while at the same time
preventing the setdown of other ongoing emergency calls
from other subscriber's stations under the same base
station when resources have to be released for the setup
of an emergency call by disconnecting other calls.
In a system consistent with mobile communicati
on specifications, such as a GSM system, if an insuffi
ciency of resources appears, a call started with an
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emergency call start message may initiate forced release
of a call of a lower priority.
When an access node is transmitting calls in
normal load conditions, the system controlling the radio
resources of the base station does not know which one of
the ongoing calls is a normal call and which one is an
emergency call. This is because only emergency calls ma-
de from a so-called non-transparent WLL subscriber's
station to a common emergency number, e.g. 112, begin
with an emergency call start message (e.g. an
EMERGENCY-SETUP message consistent with the GSM specifi-
cations). Calls made to other emergency numbers speci-
fied in the subscriber identification unit, such as the
SIM card provided in the subscriber 's station, begin
with a normal call start message, e.g. a SETUP message
consistent with the GSM specifications, if free communi-
cation channels are available. In the case of a so-
called transparent WLL subscriber's station, even calls
made to an emergency number are set up via the normal
call setup procedure and begin with a normal call start
message because a communication link to the exchange has
already been set up when the number is being dialled.
Thus, the problem is that when the base station
is overloaded and a subscriber's station in a wireless
local loop under it starts an emergency call (which be
gins with an emergency call start message, such as
EMERGENCY SETUP), forced release of resources for the
emergency call in accordance with the GSM specifications
is impossible because the other call to be disconnected
to make room for the emergency call may also be an emer
gency call although it has been started with a normal
call start message (SETUP). Hand-over of the ongoing
call to another base station is also impossible because
subscriber mobility is not allowed in the WLL access no
de.
In the original GSM system, this problem does
not appear because an emergency call to a common emer-
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gency number always begins with an emergency call start
message (EMERGENCY-SETUP message). In cases where the
cell concerned has been loaded, there have been two pos-
sible ways to cope with the situation: forced handover
to another cell or forced disconnection (pre-emption) of
an ongoing call, which is possible because the resource
management system of the wireless network is always awa-
re of ongoing emergency calls.
Forced disconnection and/or handover is allo-
wed:
- when a call ( in this case an emergency call )
waiting for a communication channel has the right to
cause disconnection of an ongoing call,
- when there is a call for which a vulnerabili-
ty indicator has been set (vulnerability allowed), and
- when a call waiting for a wireless communica-
tion channel has priority level 1.
The object of the invention is to present a
procedure that makes it possible to ensure the setup of
an emergency call in an overload situation in a wireless
local loop while at the same time preventing forced dis-
connection of other ongoing emergency calls.
A further object of the invention is to present
a procedure that allows even an emergency call initiated
with a normal call start message to be identified or
distinguished from normal calls, so that the wireless
network management of the system will know which call is
an emergency call and which call is a normal call that
can be disconnected to release resources.
The procedure of the invention is characterized
by what is presented in claim 1.
In the procedure of the invention,
a) a call initiated with a normal call start
message is assigned top priority and a definition is ma
de that prevents forced disconnection of the call as
well as disconnection of another call, and
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b) the dialled number information of the call
is compared with a set of predetermined emergency num-
bers, and if the call is addressed to a number not in-
cluded in the set of predetermined emergency numbers,
then the priority level of the call is lowered and for-
ced disconnection of the call is allowed.
Thanks to the invention, the system is always
able to decide which one of the ongoing calls is an
emergency call and which one is a normal call that can
be released in an overload situation.
In an embodiment of the procedure, the priority
level reduced at stage b) is updated in the interface
defining the signalling between the radio line of a wi-
reless local loop and a wired telephone network, in par-
ticular in a so-called Abis interface, and information
regarding the new lowered priority is given to the re-
source management system of the wireless network, thus
letting it know which ones of the calls initiated with a
normal call start message are subject to forced discon-
nection when an emergency call is received in an over-
load situation.
In an embodiment of the procedure, the call
start messages are messages consistent with a mobile
communication specification.
In an embodiment of the procedure, the call
start messages are messages consistent with the GSM spe-
cification.
In an embodiment of the procedure, the normal
call start message is a SETUP call start message.
In an embodiment of the procedure, the call
start message for an emergency call is an
EMERGENCY-SETUP call start message.
In an embodiment of the procedure, the highest
priority assigned at stage a) is priority level 1 accor
ding to the GSM specification.
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In an embodiment of the procedure, the lowered
priority assigned at stage b) is a priority level other
than 1 according to the GSM specification.
In an embodiment of the procedure, the compari-
5 son at stage b) is carried out in the access node.
In an embodiment of the procedure, the prede-
termined emergency number is an emergency number stored
in a subscriber identification unit, such as a SIM card,
in the subscriber's station. The emergency number prede-
fined in a subscriber identification unit, such as a SIM
card, is naturally also defined in the parameters of the
access node concerned to allow comparison.
In the following, the invention is described by
the aid of a few examples of its embodiments by refer
ring to the attached drawing, which presents a diagram
of a system in which the procedure of the invention can
be applied.
The figure shows a diagram representing a wire
less local loop system WLL. in a wireless local loop,
in which subscriber's stations MS are connected over a
radio link via a base station BS to an access node AN
and from the access node to a local exchange LE in a wi-
red network.
In conjunction with the authentication procedu-
re, all calls initiated from the WLL access node with a
normal SETUP message are given the following parameters:
top priority 1
call is not subject to forced disconnection
(vulnerability disabled)
call is not allowed to cause forced disconnec-
tion of another call.
Correspondingly, calls initiated with an emer-
gency call start message EMERGENCY-SETUP are assigned
the following parameters:
top priority 1
call is not subject to forced disconnection
(vulnerability disabled)
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call is allowed to cause forced disconnection
of another ongoing call having a lower priority.
The access node AN makes the appropriate con
nections to the local exchange LE as required for a call
started with a SETUP message. When the number dialled by
the subscriber is available to the access node AN for
interpretation, it is compared with the predefined emer-
gency numbers. If the call is addressed to the common
emergency number 112 or some other emergency number spe-
cified in the SIM, the priority of the call will not be
changed during the call, in other words, its priority
level 1 is preserved.
If a call started with a SETUP message is add
ressed to a number other than the common emergency num
ber 112 or some other emergency number specified in the
SIM, then the process comparing the dialling will send
data indicating a lowered priority of the call along the
signalling chain towards the wireless network. The para-
meters of the call are set so that
the priority of the call is lowered (to a value
other than 1)
vulnerability of the call is allowed.
When the message regarding the lowered priority
level reaches the signalling process of the Abis inter
face, which defines the signalling between the transmis
sion path of the wireless local loop and the wired te-
lephone network, said signalling process updates the lo-
wered priority for itself and also informs the wireless
network resource management of the cell in question
about the new priority level.
Subsequently, when a call initiated with an
emergency call start message, i.e. an EMERGENCY SETUP
call start message, is received in a possible overload
situation, forced call disconnection can be initiated by
the resource management system of the wireless network.
The wireless network resource management knows which
calls can be disconnected in an emergency. The procedure
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enables the wireless network resource management to de-
cide which calls can be released in an overload situati-
on to allow the connection of a possible emergency call.
The invention is not restricted to the embodi
ment example described above, but instead many varia
tions are possible within the framework of the inventive
idea defined by the claims.
