2. Functional composition of MSCs and interface for handover
3. Initiation
4. General description of the procedures for handover to another MSC
5. Detailed procedures in MSC-A
6. Detailed procedures in MSC-B
7. Authentication
8. Handling of supplementary services
9. Location updating after handover
Recommendation Q.1005
HANDOVER PROCEDURES
1. Introduction
This Recommendation contains a detailed description of handover
procedures to be used in PLMNs.
The following cases are considered:
i) handover between radio channels of the same base station;
Note: This capability is mandatory and could be used in the
following situations:
- when the radio channel carrying the call is subject to
interference or other disturbances, and/or
- when a radio channel or channel equipment carrying a call
has to be taken out of service for maintenance or other
reasons;
ii) handover between base stations of the same MSC in order to ensure
continuity of the connection when an MS moves from
one BS area to another;
iii) handover between base stations of different MSCs of the same
PLMN; and
iv) handover between base stations of MSCs in different PLMNs.
The same procedures can be used on the radio path for all four cases.
Cases i) and ii) involve only one MSC.
Note - Depending on the handover criteria, case ii) may involve measurements in
other MSCs.
Cases iii) and iv) involve more than one MSC. For these cases, two
procedures are defined requiring the use of the mobile application part:
a) basic handover procedure where the call is handed over from the
controlling MSC (MSC-A) to another MSC (MSC-B); and
b) subsequent handover procedure where the call is handed over from
MSC-B to MSC-A or from MSC-B to a third MSC (MSC-B').
In most respects case iv) is similar to case iii). However, any
additional aspects of case iv) not covered by the specification of case iii) will not be
included in this Recommendation for the time being.
The procedures in the mobile application part for supporting handover
are specified in Recommendation Q.1051.
In the following, the controlling MSC will be referred to as MSC-A also
when the handover only involves this MSC (cases i) and ii) above). For
cases iii) and iv), the controlling MSC (MSC-A) is the MSC on which the call was
originally established.
All MSCs should be capable of acting as MSC-A and MSC-B.
2. Functional composition of MSCs and interfaces for handover
2.1 MSC-A
For handover the controlling MSC can be regarded as being composed of
functional units as shown in Figure 1/Q.1005.
Signalling functions
1. The BS/MSC (MS/BS) procedures MSC-A for signalling between the
MSC and the BS and between the MSC and the MS. The
functional unit interfaces the BSs through interfaces A'
(to the previous BS) and, for case ii), also through
interface A'' (to the new BS). Interworking with
other functional units takes place through the internal
interface x.
2. The call control procedures MSC-A for normal call control
functions (interface B') and for signalling and call
control of connections to other MSCs (interfaces B''
and B'''). Interfaces B'' and B''' apply only to
handover cases iii) and iv) where interface B''' is
required for subsequent handover.
3. The handover control procedures MSC-A for overall control of the
handover including interworking with other functional
units (interfaces x, y and z).
(recoup)
4. The MAP procedures MSC-A for information exchange with other MSCs
and location registers. This function is required for
handover cases iii) and iv). The external interface
is interface c and the internal interface to the handover control
functions is interface z. Interface C represents the
interface to all entities with which MSC-A is
communicating during handover (other MSCs, location
registers).
Note - This functional unit may also be required for cases i) and ii) if
measurements have to be performed in other MSCs for determining the new BS
(see below).
Switching functions
5. The switch and handover device MSC-A for connecting the new path.
This function is additional to normal switching
functions in the MSC. The handover device has interfaces
to the previous BS (interface A') and the new BS
(interface A'') for handover case ii). Interface B'
represents the original connection with the fixed network
and interface B'' represents the new connection to and
MSC-B for handover between MSCs (cases iii) and iv)).
Interface B''' represents the connection to a third
MSC (MSC-B') for subsequent handover from MSC-B to MSC-B'. The connections which can exist in the handover
device are shown in Figure 2/Q.1005.
The connection via interface A' is released after completion of a
successful handover (Figures 2a and 2b/Q.1005).
For MS to MS calls in the same MSC the configuration in
Figure 2b/Q.1005 applies. Then interface B'' is not to another MSC but internal to MSC-A.
Case Initial connection Resulting connection
Figure 2a)/Q.1005 A' to B' A'' to B'
Figure 2b)/Q.1005 A' to B' B' to B''
Figure 2c)/Q.1005 B' to B' B' to B'''
2.2 MSC-B
The functional composition of an MSC acting as MSC-B is essentially the
same as that of MSC-A. However, there are some differences. The functional units
are as follows (see Figure 3/Q.1005).
Signalling functions
1. The BS/MSC (MS/BS) procedures MSC-B for signalling between the
MSC and the new BS and between the MSC and the MS
(interface A'').
(recoup)
(recoup)
2. The call control procedures MSC-B for normal call control
functions and for signalling between MSC-A and MSC-B.
3. The handover control procedures MSC-B for control of the handover
in MSC-B.
4. The MAP procedures MSC-B for information exchange with MSC-A and
the VLR of MSC-B.
Switching functions
5. The switch MSC-B for connecting the circuit from MSC-A
(interface B'') to the circuit to the BS
(interface A'').
MSC-B will also require a handover device for subsequent handovers to
BSs (or to another channel of the same BS) in the MSC area of MSC-B.
Subsequent handovers to other MSCs will not require switching in MSC-B (see below).
3. Initiation
The decision that a handover shall take place can be made by both the
MS and the BS by monitoring the channel quality. If the decision is made by the
MS, a handover request message should be provided to the BS.
Depending on the radio sub-system arrangement the new BS, to which the
call is to be handed over, can either be determined by the MS or the MSC. If
determined by the MS, the indication of the BS candidates will be providing to
the MSC by signalling on the radio path.
If the new BS is to be determined by the MSC, this may require
measurements on its own BSs and BSs in other MSCs. Procedures defined in
Recommendation Q.1051 are used for initiating measurements on BSs in other MSCs.
The initiation procedures are for further study.
4. General description of the procedures for handover to another MSC
4.1 Basic handover procedure
The procedure which takes place after initiation, i.e. after the
identity of the new BS has been determined, is shown in Figure 4/Q.1005 for a
successful handover. The procedure makes use of messages of the Mobile Application
Part (MAP) of Recommendation Q.1051.
Firstly, MSC-A sends a radio channel request message to MSC-B. The
message will contain all parameters needed by MSC-B for allocating a radio channel
(see Recommendation Q.1051). The message will also identify the BS to which the
call is to be handed over. MSC-B will return the radio channel acknowledge
message after having received the mobile station roaming number from its VLR
(exchange of the messages request for handover number and handover number
acknowledge). The roaming number is to be used for routing the call from MSC-A to MSC-B.
If a traffic channel is available in MSC-B the radio channel acknowledge message
will contain the identity of the new radio channel and the mobile station
roaming number. Other parameters may also be included (see Recommendation Q.1051).
If there is no free traffic channel in MSC-B, this will be indicated to
MSC-A and MSC-A will terminate the handover procedure. The existing connection
to the MS will not be cleared.
At this point, MSC-A establishes a connection between MSC-A and MSC-B
by signalling procedures supported by the network to which MSC-A is connected.
In Figure 4/Q.1005 this is illustrated by the messages IAM (Initial Address
Message) and the ACM (Address Complete Message) of Signalling System No. 7. MSC-B initiates the handover procedure on the radio path when the ACM is sent and
MSC-B initiates the procedure when the ACM is received (illustrated by HB-
and HA-INDICATION, respectively).
The connection is through-connected in MSC-A by use of a handover
device. The through-connection is done and the old radio channel is released when
MSC-A receives an acknowledgement from the MS (HA-CONFIRM) or when the successful
handover message is received from MSC-B. MSC-B sends this message when it
receives an acknowledgement from the MS (HB-CONFIRM).
In order not to conflict with the PSTN/ISDN signalling system(s) used
between MSC-A and MSC-B, MSC-B must generate an answer signal when HB-CONFIRM is
received.
If the connection between MSC-A and MSC-B cannot be established
(e.g. an unsuccessful backward message is received instead of the ACM), MSC-A
terminates the procedure without clearing the radio path.
MSC-A will have the overall call control until the call is cleared by
the fixed subscriber or the MS and there are no further call control functions
to be performed (e.g. servicing waiting calls). MSC-A then releases the
connection to MSC-B and also sends an end signal message which terminates the MAP
procedure. When receiving this message MSC-B will release all call control functions
and send the message remove handover number to its VLR.
MSC-A may terminate the procedure at any time by sending the MAP
message handover cancellation to MSC-B. If establishment of the physical connection
between MSC-A and MSC-B has been initiated, the physical connection must also be
cleared by procedures defined for the signalling system used between MSC-A and
the fixed network. The VLR of MSC-B is also informed by using the remove
handover number message.
The handover cancellation message is sent when MSC-A detects clearing
or interruption of the radio path before the call has been established on MSC-B.
The message is also sent in order to terminate the MAP procedure in MSC-B when
it is not possible to establish a connection between MSC-A and MSC-B.
4.2 Procedure for subsequent handover
When an MS, after the call has been handed over from MSC-A to MSC-B,
leaves the area of MSC-B during the same call, subsequent handover is necessary
in order to continue the connection.
The following cases are identified:
i) the MS moves back to the area of MSC-A, and
ii) the MS moves into the area of a third MSC (MSC-B').
In both cases the call is redirected in MSC-A using the handover
device; the connection between MSC-A and MSC-B can be released after a successful
subsequent handover has been performed.
4.2.1 Description of subsequent handover procedure i) : MSC-B to MSC-A
The procedure which takes place after the initiation procedure has
indicated that a handover has to be made back to MSC-A is shown in Figure 5/Q.1005
for the case of successful handover.
(recoup)
The procedure is as follows.
MSC-B sends the subsequent handover request message to MSC-A indicating
that the new MSC is MSC-A. Because MSC-A is the call controlling MSC, this MSC
needs no roaming number for routing purposes; MSC-A can directly search for a
free radio channel at the desired BS.
When a radio channel can be assigned in time, MSC-A will return the
subsequent handover acknowledgement message containing the new radio channel
number and possibly other information to MSC-B. If a radio channel cannot be
assigned, a no channel available indication will be given to MSC-B and MSC-B has to
maintain the connection with the MS as long as possible.
If a radio channel has been reserved in MSC-A, both MSC-A and MSC-B can
start the handover procedure on the radio path (in Figure 5/Q.1005 indicated by
the interworking messages HB-INDICATION and HA-INDICATION respectively).
After handover, MSC-A has to release the connection with MSC-B by the
procedures relevant to the PSTN/ISDN signalling system(s) used between MSC-A and
MSC-B.
MSC-A must also terminate the MAP procedure for the basic handover
between MSC-A and MSC-B. This is done by MSC-A by sending the end signal message to
MSC-B. When receiving this signal, MSC-B sends the remove handover number
message to its VLR.
4.2.2 Description of the subsequent handover procedure ii) : MSC-B to MSC-B'
The procedure which takes place after the initiation procedure has
indicated that the call has to be handed over to MSC-B' is shown in Figure 6/Q.1005
in the case of successful handover.
The procedure consists of two parts:
- subsequent handover as described in 4.2.1 between MSC-A and MSC-B, and
- a basic handover procedure as described in 4.1 between MSC-A and
MSC-B'.
MSC-B sends the subsequent handover request message to MSC-A indicating
a new MSC which is not MSC-A. The message contains the identity of MSC-B' and
of the new BS. MSC-A then starts a basic handover procedure towards MSC-B'.
When MSC-A receives the ACM from MSC-B', MSC-A informs MSC-B that
MSC-B' has started the handover procedure on the radio path by sending the
subsequent handover acknowledgement message to MSC-B containing the new radio
channel number. Now MSC-B can start the procedure on the radio path.
For MSC-A the handover is completed when it has received the successful
handover message from MSC-B'. The connection between MSC-A and MSC-B is
released by normal clearing procedures applicable for the PSTN/ISDN signalling system(s)
on the connection between MSC-A and MSC-B. MSC-A also sends the end signal message
to MSC-B in order to terminate the original handover procedure between MSC-A and
MSC-B. Receiving this message, MSC-B releases the radio path.
In case no radio channel can be allocated in time or the connection between
MSC-A and MSC-B' cannot be established, MSC-A informs MSC-B by a congestion
message. MSC-B has then to maintain the existing connection with the MS as long as
possible. When necessary, MSC-A sends the handover cancellation message to MSC-B'.
When the MS again passes the MSC boundary, MSC-B' is considered as an MSC-B
so that the subsequent handover procedures given above are applicable for any
series of handover between MSCs.
4.3 Handover procedure using subscriber information transfer (optional
procedures)
This procedure is a handover procedure with subscriber information transfer
during handover. To realize this handover procedure, only the following additional
procedure will be needed.
4.3.1 Basic handover procedure (optional)
In addition to the basic handover procedure, described in 4.1, this
optional procedure is illustrated in Figure 7/Q.1005. MSC-A sends a handover completion
message which contains subscriber information as soon as MSC-A receives the
successful handover message. MSC-B informs its VLR to send a roaming number to the HLR to
support supplementary services (e.g., call waiting), after receiving the handover
completion message.
Insert Figure 6/Q.1005
4.3.2 Subsequent handover procedure (optional)
4.3.2.1 i) MSC-B to MSC-A
In addition to the subsequent handover procedure, described in 4.21, this
optional procedure is illustrated in Figure 8/Q.1005. When receiving the end
signal, MSC-B sends the remove handover number message to its VLR and the handover
completion message to MSC-A. MSC-A informs its VLR to send a roaming number to the HLR,
after receiving the handover completion message.
4.3.3.2 ii) MSC-B to MSC-B'
In addition to the subsequent handover procedure, described in 4.2.2,
this optional procedure is illustrated in Figure 9/Q.1005. After receiving the end
signal, MSC-B releases the radio path and sends handover completion message to MSC-B'.
MSC-B' informs its VLR to send a roaming number to the HLR, after receiving the
handover completion message.
Note - Implementation of this procedure requires careful consideration of the
handling of some supplementary services (e.g., call waiting, conference calling or call
commpletion to busy subscriber) at handover, since these aspects have not been
studied in detail. The procedure is not included in the current version of the MAP
(Recommendation Q.1051).
FIGURE 7/Q.1005
Basic handover procedure
FIGURE 8/Q.1005
Subsequent handover procedure i):
successful handover from MSC-B to MSC-A
(recoup)
5. Detailed procedures in MSC-A
5.1 BS/MSC (MS/BS) procedures MSC-A (functional unit 1)
The handover procedures in this functional unit consist of:
i) signalling between the MS and the MSC; and
ii) signalling between the BS and the MSC for
- initiation of quality measurements, and
- access management.
Signals sent to and received from functional unit 3 (handover control
procedures MSC-A) are indicated in 5.3 below.
5.2 Call control procedures MSC-A (functional unit 2)
Related to handover the call control procedures in MSC-A can be divided
into two functional entities.
The first entity is the call control procedure as part of the normal
interworking between the PSTN/ISDN and the PLMN; for an MS originating call
MSC-A is the originating exchange, for an MS terminating call MSC-A is the
destination exchange.
The second entity is the call control procedure for the connection between
MSC-A and MSC-B in case of a handover from MSC-A to MSC-B. For the latter call
control procedure the following applies.
Call set up
The connection to MSC-B is set up by procedures relevant to the signalling
system used in the PSTN/ISDN to which MSC-A is connected. The call is set up by
using the MS roaming number received from MSC-B as part of the MAP procedures.
The call set-up direction will always be from MSC-A to MSC-B, i.e. also
when the call was originally established by the MS. Functional unit 2 should therefore
keep information on call set-up direction in order to be able to correctly
interpret any clearing signals (see below).
The unit should indicate the address-complete condition to functional
unit 3 and through connect without waiting for the answer signal from MSC-B. This
applies also to signalling systems where address-complete signals are not supported. In
such cases, an artificial address complete is established by functional unit 2.
Call clearing
The call clearing consists of two parts after inter MSC handover, clearing
of the BS-MS connection and clearing of the inter MSC connection.
The MAP is used to transfer information between MSC-B and MSC-A in order to
maintain full control with MSC-A. MSC-A determines, based on information received
from MSC-B, the appropriate signals to be sent to the MS, and sends this information
to MSC-B.
MSC-A shall initiate inter MSC connection release and send the end signal
to release any resource attached to the call.
The clearing of the connection is by procedures relevant to the signalling
system in the PSTN/ISDN to which MSC-A is connected.
When the signalling system No. 7 - ISDN User Part is used, the normal
symmetric release procedures apply on both the connection to the fixed network and to
MSC-B.
When a signalling system is used with a symmetric release possibility, some
notice should be given to the clear-forward and clear-back procedures.
For MS terminating calls the following conditions apply on clear- forward
and clear-back:
- when a clear-forward signal is received on interface B' (see
Figure 1/Q.1005), MSC-A clears the circuit to MSC-B by normal clear-forward
procedures; and
- when a clear-back signal is received from MSC-B, MSC-A starts normal
clear-back procedures towards the fixed network (interface B') and sends
the clear-forward signal on interface B'' in order to clear the
connection with MSC-B.
Note: This case corresponds to a fault situation. O&M actions are for
further study.
For MS originated calls the following applies:
- when an MSC-A receives a clear-back signal from MSC-B, this signal
must be interpreted as indicating clear-forward condition. MSC-A then
clears both the connection on interface B' (see Figure 1/Q.1005) and to
MSC-B by normal clear-forward procedures; and
Note: This case corresponds to a fault situation. O&M actions are for
further study.
- when MSC-A receives a clear-back signal on interface B',
MSC-A should distinguish between national and international
connections:
- for international connections MSC-A sends a clear-forward
signal on both interface B' to the fixed network and
interface B'' to MSC-B; and
- for national connections a timer is started according to
national practice for clear-back supervision and MSC-A
proceeds as follows:
i) if a clear-back signal is received from MSC-B, MSC-A
interprets this as indicating a clear-forward
condition and proceeds by clearing the
connections on interface B' and to MSC-B by normal
clear-forward procedures, or
ii) if the timer expires, MSC-A proceeds by normal clear-
forward of the connections on interface B' to MSC-B.
5.3 Handover control procedures MSC-A (functional unit 3)
The procedures of functional unit 3 are given in terms of SDL diagrams
in Figure 10/Q.1005. For all signals sent to or received from another
functional unit the source or sink of the signal is indicated (e.g. from 4, to 2, etc.).
The procedures of functional unit 3 include the following.
i) Initiation (states 1, 2 and 3). The initiation condition is shown
by the signal HA-REQUEST. This may either be generated by the MS or
the BS depending on the initiation condition (see 3). The diagram
includes all possibilities described in 3, i.e. the MS identifies
the new BS, or the new BS is identified by the MSC by measurements in
adjacent BSs. These may include BSs in other MSCs.
The diagram also includes queuing when there is no channel
available. Calls for which handover had been initiated should be queued with
priority higher than normal calls. They should have lower priority
than emergency calls.
ii) Handover of calls within the area of MSC-A, i.e. handover cases i)
and ii) (states 1, 2, 3 and 4). MSC-A controls the procedures on
both the previous and the new radio channel. Both signals HA-INDICATION
and HB-INDICATION are required. The handover procedure is completed
when HB-CONFIRM is received. If this signal is not received, the
radio path and the connection on interface B' are either released or
the original connection is maintained depending on national choice.
The handover device is first set up so that all interfaces A', A''
and B' are connected (illustrated by the signal set up handover
device). This is done when HA-INDICATION is sent. The device is
connected in its final position (i.e. A'' to B' for case ii)) (illustrated
by the signal connect handover device) either when HA-CONFIRM is
received or when HB-CONFIRM is received.
iii) Handover to MSC-B (states 1, 2, 5, 6 and 7). This procedure is
the one described in 4.1. The handover device is set up when MSC-A
send the HA-INDICATION, i.e. the interfaces A', B' and B'' are
connected. The device is connected in its final position (i.e. B' to B'')
when either the HA-CONFIRM signal is received from the MS or the
successful procedure indication is received from functional unit 4.
iv) Subsequent handover to MSC-A (states 7 and 9). This procedure is
described in 4.2. When a handover to MSC-A indication is received
from functional unit 4, the handover device is set up so that
interfaces B', B'' and A' are connected. When HB-CONFIRM is received, the
device is connected in its final position (i.e. B' to A').
If HB-CONFIRM is not received (expiry of timer T104), the handover
device releases interface A' and returns to a position where B' and
B'' are connected. A congestion indication is returned via
functional unit 4 to MSC-B.
v) Subsequent handover to a third MSC (MSC-B') (states 7 and 8). The
procedure is described in 4.2. The handover device is set up in its
initial position, i.e. interconnection of interfaces B', B'' and
B''', when the connection to MSC-B' has been established (indicate by
the signal connection established from functional unit 2). MSC-B is
informed via functional unit 4 (send acknowledge) that the connection
has been established and that the procedure on the radio path can be
initiated. The device is connected in its final position (i.e. B' to
B''') when a successful procedure indication in received from
functional unit 4. MSC-B is informed that all procedures in MSC-B can be
terminated (illustrated by the send-end signal indication). The
device returns to the state where B' and B'' are connected if the
subsequent handover procedure fails.
Timers in MSC-A
The procedures are supervised by timers in order to avoid deadlock when
responses are not received or the procedures fail. The following timers are
defined.
T100: This timer supervises the time between sending a request for
measurements to a BS or an MSC and the receipt of the results. Results received
after time out are ignored. T100 = (FS)
T101: This timer supervises the queuing time for a free channel. If T101
expires, a no channel indication is generated. T101 = (FS)
T102: This timer supervises the time for handover completion for handover
between BSs in MSC-A. If T102 expires, the radio path and the connection on
interface B' are released. T102 = (FS)
T103: This timer supervises the time between issuing a HA-INDICATION in MSC-A
and receiving a successful procedure indication from MSC-B. If T103
expires, the handover procedure is cancelled and either the radio channel is
released (if HA-CONFIRM has been received) or it continues on the old
channel (if HA-CONFIRM has not been received). T103 = (FS)
T104: This timer supervises the time between sending an HB-INDICATION and
receiving the HB-CONFIRM for a subsequent handover from MSC-B to MSC-A. If
T104 expires, the new radio channel is released and the existing handover
device connection to MSC-B is maintained. T104 = (FS)
5.4 MAP procedures in MSC-A (functional unit 4)
The MAP procedures for handover are defined in Recommendation Q.1051.
They include:
- requesting measurements in other MSCs;
- procedures for basic handover; and
- procedures for subsequent handover.
These procedures are as outlined in 4.
6. Detailed procedures in MSC-B
6.1 BS/MSC (MS/BS) procedures MSC-B (functional unit 1)
The handover procedures in this functional unit consist of:
i) signalling between the MS and the MSC; and
ii) signalling between the BS and the MSC for
- initiation of quality measurements, and
- access management.
Signals exchanged with functional unit 3 are indicated in 6.3 below.
6.2 Call control procedures MSC-B (functional unit 2)
These procedures relate to the call control in MSC-B of the "handover"
connection with MSC-A. For these procedures the following apply.
Call set-up
The connection is set-up by MSC-A. MSC-B should provide, if possible,
the following backward signals:
- signals indicating unsuccessful call set up and, if possible, the
cause of call failure;
- address complete signal; and
- answer signal (see note).
Note - The answer signal is not related to answering by the MS and if has no
meaning in the handover procedure between MSC-A and MSC-B. But after successful
handover this signal is needed for bringing the connection in the answered state
in the intermediate PSTN/ISDN exchanges.
There will be no indication that the call applies to a handover. This
information has to be derived from the MS roaming number received during call set
up in relation to the earlier radio channel request/radio channel
acknowledgement procedure between MSC-A and MSC-B (MAP-procedure).
When the connection has been established an indication should be given
to functional unit 3 (illustrated by the signal "connection established" in
Figure 11/Q.1005).
Call clearing
The call clearing consists of two parts after inter MSC handover,
clearing of the BS-MS connection and clearing of the inter MSC connection.
The MAP is used to transfer information between MSC-B and MSC-A in order
to make it possible for MSC-B to send the appropriate signals and still leave
the call control to MSC-A.
MSC-A initiates release of the connection between MSC-A and MSC-B.
MSC-B is only allowed to initiate inter MSC connection release after the
end signal is received.
When the Signalling System No. 7 - ISDN User Part is used, the normal
symmetric release procedures apply.
When a signalling system is used without a symmetric release
possibility, the following applies.
- When MSC-B receives a clear-forward signal from MSC-A, it shall
release the radio path.
- In fault situations, e.g. machine malfunction or loss of the
connection on interface A, MSC-B may send a clear-back signal to MSC-A.
6.3 Handover control procedures MSC-B (functional unit 3)
The procedures of functional unit 3 are given in the form of SDL
diagrams in Figure 11/Q.1005. For all signals sent to or received from another
functional unit the source or sink of the signal is indicated (e.g. from 4,
to 2, etc.).
The procedures in functional unit 3 include the following.
i) Handover from MSC-A (states 1, 2, 3 and 4). This case includes
initiation by MSC-A (indicated by the allocate radio channel signal
received from functional unit 4) and allocation and establishment of the
new radio channel. The procedure is outlined in 4.1.
ii) Subsequent handover within the area controlled by MSC-B (states 4,
5, 6 and 7). This procedure is essentially the same as that of ii)
of 5.3.
iii) Subsequent handover to another MSC (MSC-A or MSC-B') (states 4, 8
and 9). The initiation procedure is essentially the same as that of i)
of 5.3. The HA-INDICATION is now generated by MSC-B after a subsequent
handover accepted indication is received from MSC-A (via functional
unit 4). The procedure is terminated in MSC-B when MSC-B receives a terminate
procedure indication from functional unit 4.
Timers in MSC-B
The procedures are supervised by timers in order to avoid deadlock when
responses are not received or the procedures fail. The following timers are defined.
T200: This timer is the same as T100 ( 5.3).
T201: This timer is the same as T101 ( 5.3).
T202: This timer is the same as T102 ( 5.3).
T204: This timer is the same as T104 ( 5.3).
T210: This timer is used to supervise the time for establishing a connection from
MSC-A to MSC-B after an allocate radio channel request has been received. When
T210 expires, the allocated channel in MSC-B is released. T210 = (FS)
T211: This timer is used to control the time between requesting a subsequent
handover and receiving the response from MSC-A. If T211 expires, the existing
connection with the MS is maintained. T211 = (FS)
6.4 MAP procedures MSC-B (functional unit 4)
The MAP procedures for handover are defined in Recommendation Q.1051. They
include:
- requesting measurements in other MSCs,
- procedures for basic handover,
- procedures for subsequent handover, and
- procedures for obtaining and releasing MS roaming number for handover
from the VLR.
These procedures are outlined in 4.
7. Authentication
Authentication will be performed after handover (for further study).
8. Handling of supplementary services
This is for further study. MAP procedures for supporting such functions are
contained in Recommendation Q.1051.
MSC-A will maintain call control until all operations, i.e. the existing call
and any supplementary service operation have been terminated. At this instant, MSC-B
is informed by the end signal message of the MAP that all functions in MSC-B can be
released.
If the call waiting service is provided for the called MS, and there are
calls waiting at the time of a handover to another MSC, these calls should be established
by MSC-A using normal call forwarding to MSC-B. If the MS requests holding of the
existing call and connection of a waiting call, the MAP is used to provide the necessary
exchange of information between MSC-A and the MS.
9. Location updating after handover
MSC-B (or VLR-B) should not initiate automatic updating of the HLR at the end
of the call. The procedures in the MS should be such that the MS should initiate
updating after the call has been completed and the MS has tuned to a common control
channel.
Automatic updating by MSC-B (or VLR-B) is for further study.
