Internet Standards/CD2.mdf/ccitt/1992/q/q1004.asc

Text File | 1991-12-30 | 18KB | 383 lines

Recommendation Q.1004

            LOCATION REGISTER RESTORATION PROCEDURES


                            Contents


1.   Introduction

2.   Technical realizations to achieve the objectives

3.   Restoration of the location register memories

4.   Restoration of the supplementary service parameters


Recommendation Q.1004

            LOCATION REGISTER RESTORATION PROCEDURES

1.     Introduction

       The data stored in the location registers are automatically  updated  and
the main information is related to the location of the mobile station. The  data
is updated when the mobile station moves from one area to another. The  loss  of
this information would have an important impact on the service provided  to  the
relevant mobile subscribers. It is therefore necessary to define solutions to limit
the perturbations following a register failure and to restore automatically these
tables.

       This Recommendation describes some methods that could be  implemented  in
order to provide a good security of the data stored in the location registers and
procedures that could be performed to restore the location data and supplementary
services data after a location register failure.

       However, the implementation of  these  methods  and  procedures  are  not
mandatory and are open to technical innovation.

2.     Technical realizations to achieve the objectives

       To avoid a loss of all the data stored in  a  location  register  when  a
failure occurs, it is necessary to implement a periodic safeguard of the memories.
This method is normally used in the telephone exchanges where a copy of the tables
is made periodically in order to allow a  restart  if  a  control  unit  failure
occurs. This back-up can be made on either a disc device or a magnetic tape.

3.     Restoration of the location register memories

       The perturbations due to a deterioration of the location tables  and  the
restoration procedures are different if the equipment affected is a  home  or  a
Visitor Location Register.

3.1    The visitor location register

3.1.1  Status of the data after a failure

       When a visitor  location  register  failure  occurs,  some  discrepancies
between the actual location of the mobile station and the location information stored











may appear in the following cases:

       i)   since the last safeguard, the mobile moved to another location  area
            in the same MSC area; the allocated roaming number  remains  correct
            but the location area information is wrong;

       ii)  the mobile appeared in the MSC area after the last  safeguard;  this
            mobile is then unknown by the visited location register while the home
            location register stored a roaming number corresponding to this  new
            location;

       iii)      the mobile left the MSC area; a roaming number is allocated  in
            the visitor register but the updating was made in the home register;

       iv)  the mobile left the MSC area and then came  back;  for  the  visitor
            register, the mobile did not leave the MSC area and the previous roaming
            number is considered as correct by the visitor  register  while  the
            home register stored another roaming number given  during  the  last
            updating made before the failure. The location area information saved may
            not be the relevant one.

3.1.2  Restoration procedures

       When a failure occurs, the data concerning  only  a  small  part  of  the
mobiles located in the relevant area are lost. Therefore, it seems that a systematic
restoration method such as a general interrogation of the home location registers
would load the network and the equipments for so small a result.

       The restoration process is then the following:

       At the restart of the register each element of the memory is pointed  out
by an indicator. This  indicator  is  turned  out  when  the  relevant  location
information has been checked.

       a)   Outgoing calls

            When the restart occurs, each outgoing call from a  mobile  will
            initiate   the    checking    operation    of    its    location
            information.

            -    if the mobile is already registered in the MSC  area,  the
                 location area information is updated, if necessary, but
                 the location updating procedure is not initiated with the home
                 register (case i) solved);

            -    if the mobile is unknown in this MSC area, a roaming number
                 is   allocated   to   that    station    and    a    location
                 updating  procedure  is  started  with  the   home   register
                 (case ii) solved).

       b)   Incoming calls

                        Concerning the incoming calls, in the cases ii) and iv) described
            above, the roaming number received by the MSC in the IAM  does  not
            correspond to the right mobile station. In some cases,  it  is  not
            allocated or it may be allocated to another mobile; this depends on the
            method used to allocate this number. The normal solution (see  also
            note) to detect this difficulty is that the Initial Address Message
            received by the MSC  during  the  call  set-up  contains  also  the






            international ISDN number of the called subscriber. If this is the case, the
            visitor location register can check the couple to detect a possible
            mistake. If an inconsistency is noticed,  the  MSC  sends  then  an
            Unsuccessful Backward Message to inform the originating exchange that is
            is unable to complete the call. The VLR interrogates  the  relevant
            HLRs (the mobiles may be attached to two different HLRs) to correct
            its tables. Two interrogations have to be performed:

            -    one about the mobile station to which the VLR  allocated  this
                 wrong roaming number (MS 1);

            -    the other about the station to which  the  call  was  destined
                 (MS 2).

                 i)  The MS 1 left its MSC area; the VLR erases it from its
                     table  and  updates   it   by   allocating   the   roaming
                     number  to  MS  2  which  is   introduced   in   the   VLR
                 tables.      The   data   attached   to   that   station   are
                 requested from     its HLR;

                 ii) The MS 1 is still in the MSC area:

                     - the VLR allocates a new roaming number to that
                       station and then updates the relevant HLR;
                     - the MS 2 is introduced in the VLR table and the
                       parameters  attached  to  that  station   are
                 requested       from its HLR.
            If the mobile station left its location area since  the  last
            safeguard,   the   paging   message    sent    will    remain
            unanswered and the mobile will be considered as lost  or  out
            of  service.  To  improve  the  service,  the  call   message
            may be sent in all  the  location  areas  controlled  by  the
            MSC.  If  the  mobile  answers,  the   location   information
            is then  updated.  If  not,  the  mobile  is  considered  out
            of reach and the appropriate unsuccessful end-of-selection message is sent backwards.

            If the mobile is switched off when it is called, the  result  is
            the same as the above.

Note - As a national option, the HLR may use the "send parameter  from  VLR"
operation of MAP to obtain the MSRN from the VLR on a per call  basis.  This
is normally allowed only within a PLMN.

       c)   Particular cases

                        In case iii), as the mobile leaves the area, no traffic is
            related to that mobile; restoration is then impossible and a roaming
            number is frozen for nothing.  To solve  this  problem,  if  the
            validation of the location information does not occur after a certain
            delay (in the order of one  day  or  more),  the  VLR  may  then
            interrogate the HLR to know if this station is still located in its
            area. This method can also  solve  cases  ii)  and  iv)  if  the
            corresponding mobiles have a very low traffic.

3.2    The home location register

       The deterioration of the data contained in the home location register
is of concern not only for the PLMN but also for the whole service. The home
location register needs the help of all the visitor registers in  charge  of











the MSC areas where its mobiles are located.

       When a restart of the home location register occurs, a specific reset
message is sent to all the visitor location registers to inform them about
the failure. As the home register is unable to know the addresses of all the visitor
registers in service, the only  solution  is  to  send  the  message  only  to  the
registers known. The list is extracted from the tables saved previously; of course some
modifications occurring since the last back-up are lost and therefore some  visitor
registers involved in the control of mobiles managed by this home register will not be
contacted. But the number of registers forgotten will be very low. Another solution
could be that the reset message is sent only to the "neighbour" VLRs; a specific table
giving the addresses of these VLRs is then  contained  in  the  HLR  memories.  The
content of that table is defined by the operating people according to the roaming traffic
of the mobile managed by this HLR. In  that  case  too,  the  number  of  forgotten
registers will be very low.

       After receiving this reset message, when a mobile concerned by  the  failure
sends a radio message, to update its location, to set up an outgoing call, to answer
an incoming call or a request coming from the MSC  or  to  activate  or  request  a
supplementary service, the relevant visitor location register will initiate a location
updating procedure with the home location register. The latter then updates its tables
and validates the relevant data.

       If, after a certain delay, the location of some mobiles  is  not  confirmed,
the home register interrogates the relevant visitor registers. If a positive answer
can be obtained, the location information is validated. If not, because the  mobile
left the MSC area between the back-up and the failure, an alarm message may be given to
the technical staff in order to inform them about the loss of the location of  this
subscriber.

3.3    Periodic registration

       The delay to confirm the location of a subscriber after a failure depends on
the traffic of this station. If a station is silent for a long time,  it  would  be
difficult to know if the location information stored is correct or not during  this
period.

       A solution to reduce this delay is to force the mobile  to  send  a  message
when it remains still during a long time. For that purpose, a time-out is reinitiated
at each message sent by the mobile. When this time-out expires, the station sends a
location updating message to the base station. A rough estimate  of  this  time-out
value may be a few hours (this value is to be fixed according to traffic simulations and
it seems that it could be comprised between 12 and 24 hours); if  the  IMSI  detach
procedure when switched off is not used, to avoid an overload of the control channel in
the morning, this time-out runs only when the station is  switched  on.  With  this
method, the delay during which the mobile can be lost is less than the duration of this
time-out. The interruption of the time-out when the station is switched off is not a
problem because it is then unable to  receive  any  call;  therefore,  the  service
provided to that subscriber is not degraded. If the IMSI detach procedure is used, the
first message sent by the mobile when it is switched on is the IMSI attach; in that
case the interruption of the time-out may or may not be implemented.

4.     Restoration of the supplementary service parameters

       As well as the location data, the supplementary service  parameters  may  be
disturbed when a register failure occurs. Therefore, it is necessary to define methods
to restore them.

4.1    VLR fault recovery







a)     When the VLR fails, the HLR is able to retrieve the activation status of the
supplementary services. However, if the visitor location register does not  require
any information from the home location register  in  order  to  comply  with  a  MS
supplementary services activation request, the involved data are not available in the HLR
when the VLR fails. This situation cannot appear if the location area is  the  only
information in the VLR which is unknown from the HLR. Otherwise, it would be necessary
to include in the deregistration request  and  in  the  location  cancellation  ack
messages sent by the VLR to HLR the parameters of activations which would be only known
from the VLR.

b)     After the restart of a VLR, risks of inconsistency appear between the tables
of the VLR and of the HLR:

       -    relating to incoming calls,  the  mobile  may  have  recently  modified
            activation status of supplementary services; reverse charging acceptance,
            diversion call on no reply, connect when free...;

       -    relating to outgoing  calls,  this  method  allows  checking  of  other
            parameters; conditional barring of outgoing calls, preferential closer used
            group ... .

       Two few mobiles are involved in this situation  to  justify  the  systematic
interrogation of the HLR by the VLR so it  is  suggested  that  the  VLR  sends  an
information request message to the HLR if, and only if, one SS at least was registered in the
saved tables of the VLR. This message must request from the HLR all  parameters  of
supplementary services that are related to the mobile.  Moreover, as soon as the data
of supplementary services are validated in the tables of the VLR, an indicator  has
to be turned out.

       The retrieval procedures are not influenced by handover.

4.2    HLR fault recovery

       When the restart of a home  location  register  occurs,  the  loading  of  a
previously saved state is useful. However, the mobile may have changed its parameters of
registration or activation since the last back-up  of  the  HLR;  these  cases  are
presented here.

4.2.1  Retrieval of SS-registration status

       If the mobile station changed recently, by administrative means, the list of
the supplementary services for which it contracts a subscription, the operation can
be lost by the system when the HLR fails. It seems important to avoid this situation
with a high security.

       When the MS requests, by signalling means, the HLR to provide a registration
for a specific supplementary service, this capability being additional to  that  of
providing subscriptions by administrative means, the HLR has to save this command with
a high level of security, against an eventual HLR failure. After that, the HLR  can
send back a category/supplementary services information acknowledge message to  the
VLR.

4.2.2  Retrieval of SS-activation status

       After the HLR failure, the information which is related to  the  activations
of supplementary services by a not-registered station are available in no VLR.

       Therefore, the reset message which is sent by the reinitialized HLR  to  all











VLRs should contain implicitly an information request about the current  activation
status of the supplementary services. Since in some cases the VLR may not know these
data, the relevant parameters should be held in the mobile  equipment.  To  recover
them, two possibilities are available:

       -    to include this request into a "search" message, from the  VLR  towards
            the MSC, and then to send a category/supplementary services information
            message to the HLR; however, the HLR cannot recover by this method  the
            data associated with the non- registered mobiles;

       -    to wait for the next mobile originating message and to indicate to  the
            mobile the loss of supplementary services status  in  the  system;  the
            simplest solution is that the information is only given after a  status
            request message from the mobile; but the quality of the service would be
            improved if the information was introduced into a field of any originating
            mobile message acknowledgement. It may be envisaged, too, that the mobile
            station equipment or the subscriber card contain the description of all
            supplementary service parameters.

4.3    MSC fault recovery

       No information is stored in the home or visitor location  register  for  the
following services:
       -    charging information (different forms of facilities)

       -    credit card call

       -    debit card call

       -    reverse charging, MS originating call

       -    completion of calls to busy subscriber, MS orig. and term. calls.

       All these services are invoked on a call per call basis; if the VMSC  fails,
the location registers  cannot  help  the  MSC  to  recover  the  contexts  of  the
established calls. There is no difference with a normal fixed exchange.