Note: Descriptions are shown in the official language in which they were submitted.
2~ ~76~2
097/~s88 ~ rl,.
Communication system and call establishment methods
Field of the Invention
The present invention relates to a communication
system comprising a cellular mobile communication network,
which includes a mobile services switching center, a base
station controller and base stations, as well as a private
branch exchange (PB~) system having at least one base
station.
sackground of the Invention
The conventional access technique to a fixed
telephone network is a subscriber line (2-wire line)
extending from a local exchange all the way to a
subscriber's telephone. Presently, however, wireless
access techniques are the fastest growing area within data
communication. As a result, an ever increasing portion of
new subscriber connections to the communication network
will be wireless. The first generation of wireless
subscriber connections consisted of analogue cordless
phones (the CT1 system, for example), which consisted of a
base station connected to a wired subscriber line of a
public switched telephone network (PSTN), and a hand-held
handset. The succeeding generation consisted of a digital
cordless phone, known as the CT2 system. In addition to
the conventional cordless phone, the CT2. system offered a
telepoint service and a wireless Private Automatic sranch
exchange (P~B~). Furthermore, a standard is being
developed for a European digital wireless telephone
system, knowr. by the abbreviation DECT. To a large extent,
DECT is similar to the CT2, but utilizes TDMA technique.
Additionally, cordless systems based on radio interfaces
of mobile commur.ication systems, such as the NMT system,
have been realized wherein a subscriber's mobility related
features are limited or completely non-existent.
One form of wireless communication is represented
~ ~ 97~4~
W097l00~88 l~llrl._ 4
by mobile communication systems which do not provide the
subscribers with a fixed access point in the network at
all. Instead, the subscribers may communicate with the
network via an~ of the base stations of the network. In
order to route mobile termln~ing calls to a correct point
at any one time, the network r~;nt~;nq, at the subscriber
database, the location information of the subscriber. As a
mobile terminating call is received by the networ~., the
first MSC in the network carries out an enquiry to the
subscriber database, and is provided with the routing
addre~s in the network.
An interesting area within wireless communication
is represented by wireless PABXs. PABXs (or PBXs) are, as
known, a type of office-specific private telephone
exchanges that have a plurality of extensions and a
smaller number of trunk lines to the locaI exchange. At
the PABX, in~r~ncive ~free) internal calls can be set up
from an extension to another, and external calls can be
set up between an extension and a PSTN trunk line. A
2C wireless PABX is typically, as tO its basic structure, a
conventional "wired" PAsx, but the cabling within the
building is partly or entirely replaced with radio
connections. The radio standards potentially applicable to
PAs~ applications are represent for example by the
aforementioned CT2 and D~CT, or solutions evolving from
the Pan-European digital mobile communication system GSM.
The advantages of wireless PABXs are claimed to include:
- Mobilit~ of terminal e~uipments. The user no
longer needs to be at a specific place in the building.
- Location of the personnel. It will be easier to
contact people who are outside their offLce buildings.
- Reduced investments in cabling inside the
building.
- No costs regarding re-arrangins or maintaining
the cabling at the extension end of the PARX.
7 6 4 2
W097/00588 .~I/r~ '~ 4
As noted in the above, a wireless PABX is typically
implemented by using the CT2 or ~ECT radio interface at
the extension end of the exchange. The trunk line of the
PABX is in the conventional way connected to a PSTN local
exchange. Consequently, the freer mobility provided by a
wireless PABX is iimited to the area covered by the base
stations of the PA3X itself, i.e. the office environment
Using phones outside the office is confined to telepoint
services.
In order to extend the mobility of wireless
subscriber stations several solutions have been proposed
in which the wireless PABX is integrated as a part of a
mobile commur.ication system. In such a case, the trunk
line end of the wireless PABX is connected to the BSC or
the MSC in the place of a conventional base station. The
extension end of the PABX is implemented with CT2, DECT or
similar radio interfaces similarly as in wireless P~3Xs
connected to the PSTN network. As far as location
management is concerned, wireless subscribers are managed
like ordinary mobile subscribers in the mobile
communication network. In principal, this enables a
wireless subscriber to move from a wireless P~3X to
another if both PABXs are connected to the mobile
communication network. In addition, prior art phones may
be dual-mode devices so that they can operate both via the
base stations of the wireless PABX and via the base
stations of the mobile communication network.
Consequently, one subscriber equipment provides mobility
within the entire mobile communication network and
inexpensive PABX internal calls in the office. These
solutions, however, utilize procedures of the mobile
~ communication network for establishing externals calls,
making them more expensive. In addition, special solutions
are typically required ir, the signalling between the
elements of the mobile communication network and the
7642
WC)97~0~X8 r~llr~
signalling used in the radio interface of the PAsx.
Furthermore, dual-mode subscriber equipments are requ1red
in order to be able to utilize all the features o~ the
mobile communication network, increasing the costs of the
subscriber equipments.
Disclosure Or the Invention
An object of the present invention is a
communication system in which a wireless pri~ate exchange
can be implemented in association with a mobile
1~ communication network in a way that requires less changes
than before in the PAsX or the mobile communication
network.
It is also an object of the invention to pro~iide
inexpensive internal and external calls at a wireless PAsX
connected to a mobile communication network.
Yet another object of the invention is a
communication system having similar radio interfaces and
similar subscriber e~i c at the mobile communication
network and the wireless PAsX.
2~ These objects are achie~ed by means of a
communication system comprising a cellular mobile
communication network, which includes a mobi~e services
switching center, a base station controller and base
stations, as well as a private exchange system having at
least one base station. According to the invention the
system is characterized in that the subscriber side of the
PABX system is connected to the BS~ and the trunk side is
connected to another communication networki the PAsX base
station is connected to the base station controller; t h e
base station controller is arranged to route subscriber
calls originating from the PAsX base station to the PABX
system, and calls coming in via the PA~X system to the
P~3X base station.
The invention also relates to a method for
establishing an outgoing call in a communication system
Zl i~7~2
O971OOsa8 I.l/rl:.t 5~
comprising a cellular mobile communication network which
contains a mobile services switching center, a base
station controller and base stations, as well as a private
branch (PABX) system having at least one base station, the
method comprising the steps of transmitting a connection
request from the subscriber to the base station controller
via the PABX base station; activating call set-up.
According to the invention, the method is characterized by
checking at the base station controller whether the
transmitting party of the connection request is a PABX
subscriber or a standard subscriber of the mobile
communication network; routing the call of the PABX
subscriber from the base station controller to the PABX
system, whose subscriber side is connected to the base
lS station controller and the trunk side to another
communication network; connecting the call to the mobile
services switching center if the subscriber is not a PA~X
subscriber.
The invention also relates to a method for
establishing a terminating call in a wireless
communication system comprising a cellular mobile
communication network which contains a mobile services
switching center, a base station controller and base
stations, as well as a private branch (PABXI system having
at least one base station, in which method a call
terminating to PABX subscriber is received in the PABX
system. According to the invention, the method is
characterized by the steps of initiating, from the private
branch exchange system, call establishment at the base
station controller to which the subscriber side of the
PABX system and said PA3X base station are coupled;
transmitting from the base station controller a paging
message to said PABX subscriber via the PAF3X base station;
receiving an acknowledgement message from the PABX
subscriber; establishing a connection according to the
W097~ 8 P~l/r~ Y ~ ~
mobile communication network between the base station
controller and the PABX subscriber; and connecting a
speech connection between the PABX subscriber and said
PABX system via the base station controller and the P~X
base station.
According to the invention, the BSC of the mobile
communication network is provided with an interface to
which the extension side of the PABX is connected. The
trunk line side of the P~BX is connected to a local
lo exchange of tne PSTN network. One or more of the base
stations under control of the BSC is placed in that space
or building which is the desired location for an offlce
cell of the wireless PABX. In this application, such a
base station will be referred to as a PABX base station.
Between the BSC and the PA}3X base station there is a
normal signalling interface oE the mobile , i~ion
network, and the PABX base station has a normal radio
interface of the mobile communication network. The
interface between the BSC and the PAsX is also a normal
PAsx signalling interface towards the extensions. In a
preferred em~odiment of the invention this interface
includes t.he normal P~3X extensions, which are connected
to the BSC.
This concept makes it possible to cbtain, for a
PABX of a conventional fixed networkr a radio interface o~
the mobile communication system without changing normal
signalllng interfaces. The only element for which the
invention requires additional features is the ssc.
The capacity of the BSC is shared to serve both
PABX base stations and standard base stations of a mobile
communication network at the same time. Thus, the probably
long transmissior. connections between the BSC and the MSC
are avoided when making local calls ir. the PABX, but the
standard functions of the mobile communication network,
.such as authenticatlon and ciphering, are still available.
4 2
wos7/00s88 PCT~9h~0354
No special features are required of the subscriber
stations of a wireless PABX; they may be ordinary mobile
stations of a mobile communication network. As such, they
can also operate via normal base stations of the mobile
communication network. The mobile stations can also be
treated as normal subscriber stations of the mobile
communication network outside their home PABX base
stations.
In the preferred embodiment of the invention, a
standard subscriber of a mobile communication network will
be made a PA3X subscriber by also allocating to him a PABX
subscriber number. This subscriber number may be a PA~X
extension number or any other identifier that associates
the mobile station with a specific PABX extension. This
way, the mobile subscriber's mobile station can serve as a
cordless phone within the office building and as a normal
mobile station outside. The routing of terminating calls
is done in two ways. When a calling subscriber dials a
PABX subscriber number, the public switched telephone
network PSTN routes the call to the wireless PABX, and
connecting the cal' to the mobile station is handled via
the radio interface between the PABX and the BSC. When the
calling subscriber dials a PLMN number ~e.g. a MSISDN
number1, the PSTN routes the call directly to the PLMN
where the call is terminated with the normal call
establishment procedure.
Mobile originating calls coming from a home PABX
base station are routed by the BSC to the PABX on the
basis of the subscriber number, whereby the PABX internal
calls can be switched directly without any MSC call
control functions or connections via the MSC.
Consequently, internal calls, and PSTN calls, via a PABX
base station can be more economical than in prior art
~ solutions.
2 ' ~ 2
W097/00588 l~l/rl.~.l 5~ ~
Brief Description of the Drawing~
In the following, the invention will be descrihed
in closer detail with reference to the accompanyi.ng
drawings, in which
Figure l shows a wireleas communicatior. system
according to the invention,
Figure 2 is a block diagram illustration of an
extended base station controller,
Figure 3 is a signalling chart illustrating setting
up of a mobile terminating call, and
Figure ~ is a signalling chart illustrating setting
up of a mobile originating call.
In practice, the preser.t inventlon is applicable to
almost any mobile communication system if a wireless P~3X
or PBX is to be connected to a PAsX system.
One example is the European digital cellular mobile
communicat.ion system GSM ~Global System for Mobile
Communications). The basic elements of the GSM system are
described in the GSM R~ tions. For a closer
description of the GSM system, the GSM recommendations and
~The ~SM System for Mohile ~ommunications", by M. Mouly &
M. Pautet, Palaiseau, Frarce, 1992, IS3N: 2-9~07190-0-7 is
referred to.
The GSM and a modification thereof, DCSla00
(Digital Communication System1 operating at the 1800 Mhz
frequency range, are the primary targets for the
invention, but it is not intended that the invention be
restricted to these radio systems.
Figure l very briefly describes the basic elements
of the GSM system. A mobile services switching centre MSC
handles the connecting of incoming and outgoing calls. It
performs functions similar to those of an exchange of a
public switched telephone network (PSTN~. In addition to
these, it also performs functions characteristic of mobile
~5 communications only, such as subscriber location
~ ~ ~7642
~ W097~0s8~ r~l/r~
management. The mobile stations MS are connected to the
center MSC by base statior. systems. The base station
system consists of a base station controller BSC and base
stations BTS. An A interface is determined for the BSC
towards to MSC, and an Abis interface towards the BTS.
These interfaces are specified in the GSM recc. -n~tions~
As subscriber registers, the system includes the
home location register HLR and the visitor location
register VLR. The ~LR permanently stores the subscriber
information as well as information on which VLR area the
MS is located in. The visitor location register VLR, in
turn, temporarily stores the subscriber information of the
subscribers visiting the VLR area. In addition, the VLR
stores the subscriber~s more detailed location
information, i.e. the so-called location area
identification.
Figure 1 also shows an office system implemented as
a wireless PABX. The office system lO comprises a
conventional fixed network PABX whose extension cabling
has been replaced with a radio interface. Accordingly, the
office system 10 comprises at least one base station
PABX_BTS under control of the PABX, the PABX_BTS creating
an internal cell for the office (e.g. a building).
Cordless handsets can freely move within this cell and
utilize the normal PABX functions, such as internal calls,
calls to the public switched telephone network PSTN, etc.
The trunk line side of the PABX is connected in the
conventional way to the trunk lines from a local exchange
or an ISDN exchange (or any other type of a communication
exchange) of the PSTN. As noted in the above, in the prior
art solutions the extension side of the PABX is directly
connected to the base station PABX_BTS. In accordance with
the invention, Figure l shows the extension side of the
PABX being connected to the BSC of the mobile
communication network PLMN. A special interface module IF
21 ~4~
W~ 9'7~0058~!1 PCI'IF196/01~354
is provided in the BSC for this purpose. In this
application, such a base station controller in accordance
with the invention is referred to as an ~to~ base
station contrDller XBSC. The PABX BTS, in turn, is
connected to the extended base station controller by Abis,
the normal BSC-BTS interface in the GSM system. Since the
PABX_BTS iB a normal GSM base station, also the radio
interface of the wireless PAsX in the office cell 10 ir; a
GSM/DCS interface antirely in accordance with the
lo rec~ ~tions. As a result, a conventional GS~DCS
mobile station can be used in the office cell lG as a
cordless handset 11. However, the invention does not
prevent the base station PABX_BTS of the PABX system. frow
having a different radio interface (e.g. DBCT~ than
standard base stations of the mobile communication
network. This results in that the subscriber stations of
the PABX sy3tem must be dual-mode devices 80 as to be able
to operate on both radio interfaces, which is a
signiEicant deterioration in comparison to the preferred
~C ~mh~S; ~~~t.
The A interface between the MSC and the extended
base station controller xssc can be maintained as a normal
GSM interface without any modifications. This way, by
means of the present invention~ the features of the
2~ ~SM~DCS interface can be can be issued to a standard PABX
without changes in the standard signalling interfaces. The
only element requiring alterations in the PLMN network is
the extended base station controller XBSC. The capacity of
the XBSC is shared to serve both PABX_BTS base stations
and normal P~M~_BTS base stations at the same time. Thus,
the probably long tr~ns~;Cc;on connections between the BSC
and the MSC are avoided when making local ~internalJ calls
in the PABX. In the ofiice cell 10, the standard functions
of the mo~ile communication network, such as
authentication and ciphering, are still available.
2~ ~6~2
WO97/00588 1
11
The subscribers of the mobile communication retwork
have each been allocated unique identities in the mobile
communication network, such as IMSI ~International Mobile
Subscriber Identity) in the GSM system. According to the
preferred embodiment, a mobile subscriber entitled to use
the office cell is in addition allocated a PABX subscriber
number, such a PA'3X subscriber number beins for example a
PAsX extension number or some other suitable
identification. A subscriber's mobile station MS provided
with an IMSI and a PABX subscriber number can be used as a
cordless handset within the office cell and as a standard
mobile station of a mobile communication network outside
the office cell. Routing mobile terminating calls is done
in two ways. When a calling PSTN subscriber dials a PABX
subscriber number, the PSTN routes the call to the
wireless PABX, and forwarding the call to the mobile
station will be handled via the interface IF between the
PABX and the XBSC. When the calling subscriber dials a
subscriber number (such as a MSISDN number) allocated to
the subscriber in the mobile commur.ication network, the
PSTN routes the call directly to the gateway MSC o~ the
mobile communication network. Calls made from an MS in the
office cell are routed by the XBSC to the PABX. This way
the PABX internal calls can be switched directly without
any MSC call control functions or connections via the MSC.
Below, various call routing procedures will be
described in more detail. At first, however, the structure
and operation of the extended XBSC and particularly its
interface module IF will be disclosed with reference to
Figure 2.
The block diagram of Figure 2 is but one exemplary
embodiment of the base station controller XBSC. ~owever,
it should be understood that the invention is by no means
restricted thereto, but there exists several
implementations depending in particular on the type and
W097/00~Y
12
properties of the PP~3X used. Generally spea~ing, it can be
said that the interface module IF is implemente.d depending
on the PABX features so that a standard PA}3X can be
coupled to an extended XBSC without alterations to the
signalling and subscriber connections of the PA3X. In some
cases the PA;3X may be tailored as far as its signalling
interface and extensions are concerned so that it i8
particularly well suitable to be connected to a base
station controller. A11 the variations, however, are
characterized in that the signalling and speech
connections associated with the PP~3X subscriber side, i.e.
extensions, are connected to the X.5SC a~d not directly to
the PAsX base station as in prior art systems.
The interface module IP of Figure 2 is implemented
so that the extensions o~ a standard analog PAl3X can be
coupled as such to the interface module IF. It is assumed
that the PA~3X has n extensions. If that is the case, the
interface module IF has N line modules, one per each PABX
extension. Each line module comprises a D~A and A~D
converter 21, a transcoder 22 a ring signal detector 23
a DTMF generator 30N and an OFF/ON-HOOK switch 24. The D~A
converter 21 converts the analog speech signal received
from a PABX extension into a digital signal to be encoded
in the transcoder 22 by a speech encoding method of the
mobile communication system prior to sending it to the
mobile communication network. In the same manner, the
encoded speech signal recei~ed from the mobile
communication network is decoded i~. the transcoder 22, and
converted in the D/A converter 21 into an analog speech
3~ signal, which is fed to a PABX extension. The ring signal
detector 23 is used in case of an incoming call to detect
the ringing tone fed by the PA~3X to the extension in
cluestion. Vpon detecting the ringing tone, the detector 23
provides a signal DET as an output. The switch 7.4,
controlled by a control signal ~OOK, is in turn used to
2t ~7~2
~ W097/00S88 I~/rl:.l 5
13
close the subscriber loop of the PABX extension similarly
as in a conventional phone with a subscriber answering the
call by lifting the earpiece (OFF-HOOK) or making a call,
and to open the subscriber loop similarly as in a
conventional phone with the subscriber replacing (ON HOOK)
the earpiece onto the hook at the end of the call. A dual-
tone multi frequency (DTMF) generator 30 is used for
generating the tone-frequency dialling digits to be sent
via the PABX extension to the wireless extension PABX, in
a similar manner as dialling is carried out in an
extension with a conventional phone. The generator 30 is
controlled by a control signal DIA~, which in turn is
generated on the basis of GSM type of dialling signalling
of the mobile stations MS.
The interface module IF in addition comprises a
controller 25, which is common to all the line modules.
The controller 25 comprises inputs for detection signals
DETN generated by the ringing tone detectors 231-23N, and
control outputs HOOK1-HOOKN for the switches 24~-24N and
DIAL~-DIALN for the generators 30. The controller 25 in
addition has a signalling link with a control unit 28 of
the BSC. The controller 28 controls a BSC internal switch
29 and handles all the call control and signalling duties
assigned to the sSC.
In the extended XBSC according to the invention,
the interface module IF is additionally provided with a
signalling interface 28, by means of which a signalling
link is created between the BSC and the visitor location
register V~R. In the preferred embodiment of the
invention, the interface in question is similar to a
signalling interface (B interface) defined between an MSC
and a V~R in the GSM system. The B interface is specified
in the GSM recl -n~tion 0902 Chapter 2.4.3, and the
signalling used therein is referred to as MAP_B
signalling. As far as the present invention is concerned,
I q 7
~097~00~ r~l~r. c~ ;4
1~
it is only significant that some kind of a VLR-BSC
signalling lin~ exists for database en~uiries c~n~rning a
different kind of call set-up, such as ciphering and
authenticatiom, and possibly also for location updating.
For this purpose, the MAP_B signalling interface 28 is
used by the controller 28.
Furthermore, the controller 28 of the XBSC has
access to a PABX/IMSI database 26 in the interface module
IF. The database 26 contains the identities IMSI of the
a mobile subscribers entitled to use the office cell ~0 and
the PABX. In addition, the database 26 informs of mapping
between each IMSI and the correspon~ing PABX subscriber
number, in this case the physical PABX extension. The
database 26 is also used by the controller 2S of the
inter~ace module IF itself, as will be disclosed below.
The switch of the base station controller BSC may
be an~ MSC e~ployed digital switch to whose ports the
connections botween the BSC and the MSC are coupled:i.e.
those between the sSC and the PAsX_sTS, those between the
2~ ssc and conventional base statlons, afi well as connections
between the s5c and the transcoders 22. Controlled by the
controller 28, the switch 2g switches these connections to
one another in such a manner that the calls can be routed
according tc standard pr~cedures of the m~bile
communication network and procedures of the present
invention, to be disclosed below.
In the following, a more detalled description will
be given of establishing a mobile terminating call and a
mobile originating call with reference to the ~o~ n~
3~ of Figures 1 and 2, and the signalling charts of Figures 3
and 4.
Mobile terminating call
It should be noted that the signalling chart in
Figure 4 by no means i5 a comprehensive description oE
signalling associated with call establishment. The purpose
2 1 97642
097l~s88 r~llr
of Figure 3 is only to give a picture of information
exchange related to call set-up to the extent necessary
for describing the invention. Hence, in practical call
set-up other signalling than the signalling described in
Figure 3 is also present, but this is not essential to the
invention.
It is assumed that a call is made to an extension l
of the PABX from the PSTN or another PABX extension. As a
result, the PABX must somehow inform the BSC of the
incoming call in order to set up the call to the mobile
station MS of the extension in question. Activating this
call set-up is illustrated in the signalling chart of
Figure 3 by a Set_up message transmitted by the PABX to
the BSC. In some embodiments, this may be an actual
message. In the preferred embodiment of the invention, in
which the PABX has not been altered on account of the
present invention and in which the extensions are directly
connected to the interface module IE, the PABX establishes
the call in a similar manner as to a conventional
extension. In other words, the PABX generates a ring
signal to the extension l, the ring signal being detected
by the ringing tone detector 231 of the interface module
IF. The detector 231 indicates to the controller 25 of
having received the ring signal by activating a signal
DET1. The active state of the signal DET1 indicates to the
controller 25 that there is an incoming call to the
extension l. The controller 25 searches from the database
26 the IMSI identifier corresponding with the PABX
extension l and identifying the mobile subscriber of the
extension l. Eollowing this, the controller 25 gives the
controller 28 a call set-up command, which contains the
retrieved IMSI. As a result, the controller 25 transmits a
Paging_command paging message via the switch 28 to the
base station PABX_BTS of the office cell. The base station
PABX BTS forwards the message to the mobile station MS in
~ l ~ 7
W~:~9~/~5~8 ~ r
16
the office cell. Via the PABX_BTS, the mobile station
transmits a response message, Paging Response, to the BSC.
Having received the response from the mobile
station, the BSC transmits an authentication request
MAP_pro-Cess_access_request (MAP_authentication) to the
V~R, to which the VBR responds with a M~P pro-
cess_re~uest_ack message. These messages convey the same
information as normally in the GSM system between the MSC
and the V~R. In the preferred embodiment of the invention,
the controller 2~ carries out this signalling via MAP
signalling interface 28.
Having received the answer from the V~R, the
control.ler 28 of the BSC transmits to the MS an
authenticatlon request Auth re~uest, according to the CSM
re~ tions, which is responded to by the MS with an
authentication message Auth_response according to the GSM
rec. ~tions. If authenticatlon has been successful,
and the mobile station MS is allowed access to the
network, ciphering mode will be set. For this purpose, the
ssc transmits, to the V~R, a MAP_proc_acc_req (Ciph_mode)
message which is responded to by the VLR with a M~P_ proc_
acc_req ack message. These messages con~ey the same
information as normally in the GSM system between the MSC
and the V~. Then, the BSC transfers the ciphering mode ~f
~5 the MS with a Ciph_mode_cmd message, acknowledged by the
MS with a Ciph mode_com message.
When the cipherin~ mode has been set, a connection
will be set up between the switch 29 and the MS on a
traffic channel allocated to the call via the base station
3~ PABY_BTS. This is illustrated with a Set up message. The
MS will be ringing, and as the subscriber answers,
information on it will be conveyed to the controller 2~
which ~ ~ the controller 2S to set the switch 2~1 to
OFF_HOO~ state. This will close the subscriber loop of the
P~3X extension 1, and the PABX interprets this as the
2f ~42
WO 97/OOS88 1~ r ~ . .'. ~~
17
subscriber having answered the call at the extension. This
is illustrated in Figure 3 with a Call conf message.
Information on the answering by the subscriber B can
naturally also be sent by other types of messages. This
way a speech connection will be established between the MS
and the PABX.
The BSC only establishes PA~3X calls within the area
of the office cell 10. In the example above, no pre-
checking was carried out as to whether the called MS was
in the office cell 10. ~owever, it is possible to carry
out a location enquiry to the VBR prior to initiating
paging in order to avoid futile paging signalling.
Nobile originating call
In the following, establishing a mobile originating
PAsX call will be described in the system according to the
invention with reference to the signalling chart of Figure
4. The signalling chart of Figure 4 is not intended to be
a full description of call set-up signalling, either. The
figure only shows the signalling to the extent that is
essential in order to describe the present invention. In
practice, other messages can be included in the
signalling, these other messages being insignificant to
the present invention.
It is assumed in the following that the user of a
cordless handset MS in the office cell 10 wishes to make a
call. It is typical of mobile communication networks that
call set-up signalling on the radio path only begins after
the user has dialled the number of the called party and
pressed a specific "send" button. When the user has
pressed the ~send~ button, the MS transmits to the
PABX_BTS of the office cell 10 a Channel Request message
which i5 forwarded to the ssc. The 3SC responds by
transmitting an Immediate Assignment message. Both of
these are messages in accordance with the GSM
re~. nd~tions. The next message from the MS is a CM
2 ~ ~7~
WO 97/005A8 r ~n/rl. ~ll 54 ~--
18
Service Re~uest which includes the reason ~or the
transaction This message is in accordance with the GS~I
recommendations as well Due to the connection request
having been received via the PABX_BTS in the ofEice cell
1~, the controller 28 checks in the database 26 whether
the IMSI belongs to a PABX subscriber If the IMSI in
question is not included in the database 26, call
establishment proceeds according to the standard
procedures of the mobile communication network
If the IMSI does belong to a PABX subscriber, the
controller 28 activates the call set-up procedure of the
invention, with the call being routed to the PABX At
first, the BSC authenticates the sub~criber and sets the
ciphering mode; similar signalling between the BSC and the
VLR, as well as between the BSC and the MS, is related
thereto as that which was illustrated in Figure 3. ~hen
the ciphering mode has beer set, the mobile station
transmits, to the sSC, a call establishment message
Set_up, which also contains the dialled subscriber number
s In connection with the pre~ious en~uiry to the database
~6, the controller 28 was provided with information on
which PABX extension the IMSI of the subs riber in
questions matches. Let us assu~e that the IMSI corresponds
to the PABX extension N. Having received the Set_up
message from the MS, the controller 28 c ~ the
controller 25 to set the switch 24~ of the extensior. ~ to
OFF_HOO~ As a result, the subscriber loop of the
extension N closes The PABX reacts to this by generating
a dial tone to the extension, and awaits dialling The
3~ controller 28 now provides the controller 25 with the
number of the subscriber B The cor.troller 25 directs the
DTMF generator 30N to generate DTMF digits according to
the subscriber number B. Signalling by the subscriber B to
the PABX is illustrated by a B_number message in Figure 4
}5 Haviny received the number of the subscriber B, the PA3X
2 1 ~764~
~ W097lOOs~ r~lrrl ' ;~
19
proceeds with call set-up as if the dialling had been made
from a standard extension.
Simultaneously with the dialling, thereafter, or
prior to it the BSC establishes a connection between the
s switch 29 and the MS on a traffic channel allocated to the
call via the PABX BTS. This is illustrated in Figure 4
with a set_up message. As soon as the number of the
subscriber B has been transferred to the PABX, the
controller directs the switch 29 to connect the transcoder
22~ to said traffic channel. Thus, a connection will be
established between the MS and the PABX, which is
illustrated in Pigure 4 with an arrow ~connection~.
Call releasq
When a mobile station requests a call release, the
BSC transmits a response to the mobile station, switching
it to ON-HOOK state. Information on the ON-HOOK state of
the MS is simultaneously signalled to the PA}3X. In the
preferred embodimens of the invention, this takes place by
the controller 28 cn~~~n~1 ng the controller 25 to set the
switch 24 of the PABX extension in auestion to ON-HOOK
state. As a result, the subscriber loop opens and the PABX
interprets the connection as having been cut off.
Following this, the resources and connections reserved in
the BSC for the call are released.
When the other party decides to end the call, the
PABX generates a busy tone to the extension, also carried
over to the mobile station MS. Normally this causes the MS
user to onhook, i.e. to activate the call release
described above from the MS side. In addition, the
interface module IF may be provided with a busy tone
detector whose activation causes the controller 28 to sent
a call release message in accordance with the GSM
recommendations to the mobile station. In the latter case,
the ON-HOOK state of the MS is signalled to the PABX by
opening the subscriber loop with the switch 24.
z ) ~
w097/0~8 r~l/rl~ 4
Call~ made to the MSIS~N of the mobile station
arrive in the mobile communication network in the normal
manner Inot via PABX~, and the call set-up related
thereto is narmal regardless of whether the MS i8 in the
office cell lQ or not.
In order to save the resources of the radio
network, hando~Yers from the office cell may be restricted
to minimum. In such a case, even call release can be
accepted instead of a handover.
Furthermore, the mobile stations entitled to use
the office cell 10 may have a specific access category. If
that is the Gase, all other access categories may he
prohibited from the office cell. This is how other mobile
stations could be prevented from locking to the office
cell. In a GSM mobile station, for example, information on
the subscriber's access category is located on his SIM
card ISubscriber Identification Module), which means that
the categori~ation does not require changes to the
software of the mobile station.
Although the invention has been described with
reference to specific embodiments, it should be understood
that these are intended as examples only, and that changes
and moaifications can be made thereto without departing
from the score and spirit of the attached claims.
