To clarify the procedure sketched in 6.2, it was decided to specify the complete GSM system including the Mobile Station Location Centre (MSLC), and the Emergency Call Centre in SDL. Such a specification provides great insight in how the datas necessary for MSL can be obtained in a present GSM network.
The Specification and Description Language (SDL) was developed by CCITT (now ITU) to have a common language for precise and unambigous communication about telecommunication systems. SDL is specified in ITU Z.100 [6].
SDL is a language and a boxology. The system that is to be defined, is decomposed in blocks. Each block can consist of several subblocks or processes. The blocks and processes are interconnected with signalroutes, where signals can be sent. Each of these signals must be specified. The behaviour of the processes are specified in process diagrams. The definition of blocks, processes and signalling must follow the strict rules set in SDL. The reader who want to become familiar with SDL should refer to one of the many books about SDL, such as [21].
To facilitate the correct use of SDL, a number of tools exist to design SDL specifications, simulate the correct behaviour and draw Message Sequence Charts (MSC). Such tools exist for a wide range of platforms. The SDL specifications in this thesis were designed using the Cinderella SDL tool from Cinderella software.
The top-level system block is given in figure ![[*]](cross_ref_motif.gif)
.
The entities of the GSM system can be recognized on the figure. In
addition, one can see the entities external to the GSM system, but
relevant to the emergency call centre application. These are the
emergency call centre itself and the switch it is connected to. In
addition there is the MSLC block. Signalroutes with assosiated signals
connect the different blocks. It has been chosen to group the signals
into signallists that correspond to the protocols in the GSM
system. Thus, the signallist names shown on figure
show which protocols each entity speak with each other. To make the
specification possible to overcome, only signals relevant to Mobile
Station Location has been included in the specification. Each of the
interfaces between the entities has been named. These names are shown
in small letters at each signalroute. As far as possible the names are
consistent with the interface names used in GSM specifications.
Figure shows the definition of each of the signallist. The
complete SDL specification, including all subblocks and processes can
be found in appendix .
Using a simulation tool, a Message Sequence Chart (MSC) can be produced
from the SDL specification. The MSC for the MSL specification is shown
in figure .
As already mentioned, only the signalling relevant to MSL has been
included in the specification. The setup of the radio interface and
authentication procedures as seen in has therefore
been let out. With this in mind, it can be seen from the figure how
the call setup starts with the CC_SETUP from the MS to the MSC. After
finding which switch to route the call to, the MSC informs the
remote switch using ISUP_IAM. The remote switch recognizes the
receiver of the call as the emergency call centre and sends the
ISUP_SETUP message. The progress is indicated to the MSC with the
ISUP_ACM message [29]. When the ISDN phone at the call
centre receives the SETUP it starts alerting the operator, and
indicates this by returning the ISUP_ALERT message to the switch. The
switch further informs the MSC using the ISUP_CPG (Call
ProGress ), which informs the MS using the CC_ALERT message. A tone
signalling in the MS indicates that the phone is ringing in the other
end.
When the operator at the emergency call centre receives the call, the ISUP_CONN is sent to the switch, indicating the the call can be connected. The switch forwards this indication to the MSC using ISUP_CON. The MSC informs the MS using the CC_CONNECT signal. When this message arrives at the MS, the connection is established and the user at the MS can talk to the operator at the emergency call centre.
In order to perform Mobile Station Location, a number of other
messages are exchanged in the MSC in figure . The location
procedure starts when the call setup indication is received at the
remote switch. This switch is programmed to initiate Mobile Station
Location on all calls to the emergency call centre originating from a
cellular phone. Thus the switch can send a location request to the
MSLC, asking the MSLC to locate the given subscriber. The location
request and reply must be defined in a new protocol, called the Mobile
Station Location Protocol (MSLP).
When the MSLC receives the location request, the only information it has about the MS is the MSISDN number of the subscriber. In order to be able to activate trace, it must know the IMSI of the subscriber. This can be found by acting as a MSC in a mobile terminating call setup, and ask the HLR for a roaming number for the subscriber. The HLR will then return a roaming number (MSRN) and the IMSI of the subscriber.
The MSLC can now activate subscriber trace by sending the
OM_TRACE_ACTIVATION_REQ to the HLR. The HLR will look up in its
database which VLR the MS is registered with, and send this VLR a
MAP_TRACE_ACTIVATION_REQ. The VLR will in turn send the MSC a
MAP_TRACE_SUBSCRIBER_ACTIVITY. If the MS is active mode, the MSC will
send the BSC an order to trace the activity using
BSSMAP_INVOKE_TRACE. This will happen since the MS is engaged in a
call. When this order is received, the BSC will start sending trace
reports to the ``OMC'' id given in the trace order. Since the OMC-id
is set to the MSLC, the MSLC will receive trace reports including
radio and TA measurements, BTS and TRX ids and other information, as
mentioned in .
When such reports have been received, the MSLC know enough to calculate the position of the MS using either radio propagation models (6.2), combined TA and radio measurement methods (6.3.1) or measurement map methods (6.4.1). The calculated position is reported to the remote switch using MSLP_MS_POS. When the switch receives this position, it can inform the end user at the emergency call centre, for instance by using a ISDN user-message, ISUP_USR.
As mentioned, the complete SDL specification can be found in appendix
. Since the MSLC process is of special
interest, it will be presented here. On figure , the
MSLC can be seen as a block. As shown on figure , the
MSLC block consists only of a MSLC process.
This process is shown in figure . The figure shows
that the MSLC actually follows a very simple procedure:
If the position calculation is based on measurement maps, the MSLC must build and maintain such maps. Although this procedure is not included in the SDL specification, section 6.4.2 shows that the procedure is similar to locating a MS. The following outlines a procedure for measurement map building in the MSLC:
