“Signalling in GSM BSS” course focuses on signalling between GSM nodes within Base Station System (BSS). During the course all protocols and signalling procedures on all interfaces within BSS are presented in details. The organisation of channels of air interface and cell parameters is also widely covered in the course. The course also describes parts of the Signalling System No. 7 that are relevant for BSS and presents co-operation between Core Network and BSS during procedures like call set-up and location update.

1. 11 MSC in pool
279
ChapterChapterChapterChapter 11111111
MSC inMSC inMSC inMSC in PPPPoolooloolool
TopicTopicTopicTopic PagePagePagePage
Introduction.................................................................................................... 281
Network Resource Identification ................................................................... 283
Node Selection Function................................................................................ 284
Mobility Management.................................................................................... 288

2. Signalling in GSM BSS
280
This page is intentionally left blank

3. 11 MSC in pool
281
IntroductionIntroductionIntroductionIntroduction
The MSC in Pool, as a part of Intra Domain Connection of RAN Nodes to
Multiple CN Nodes solution, overcomes the strict hierarchy, which restricts
the connection of a BSC node to just one MSC. This restriction results from
routing mechanisms in the BSC which differentiate only between information
to be sent to the MSC (CS domain) or to the SGSN (PS domain) and which do
not differentiate between multiple CN nodes in each domain. The MSC in
Pool solution introduces a routing mechanism and other related functionality,
which enables the BSC to route information to different CN nodes within the
CS or PS domain, respectively.
MSC2MSC1
BSC3BSC2BSC1 BSC5BSC4
MSC3
BSC6
Figure 11-1 MSCs in Pool (logical view)
The MSC in Pool solution introduces further the concept of ‘pool-areas’
which is enabled by the routing mechanism in the BSC. An MSC pool-area is
comparable to an MSC service area as a collection of one or more BSC
service areas. In difference to an MSC service area a pool-area is served by
multiple MSCs in parallel which share the traffic of this area between each
other. Furthermore, pool-areas may overlap which is not possible for the MSC
service areas. From a BSS perspective a pool-area comprises all LAs of one
or more BSC that are served by a certain group of MSC nodes in parallel. One
or more of the MSCs in this group may in addition serve LAs outside this
pool-area or may also serve other pool-areas. This group of MSCs is also
referred to as MSC pool.
The MSC in Pool enables a few different application scenarios with certain
characteristics. The service provision by multiple MSCs within a pool-area
enlarges the served area compared to the service area of one MSC. This
results in reduced inter MSC LA updates and it reduces the HLR update
traffic. The configuration of overlapping pool-areas allows to separate the

4. Signalling in GSM BSS
282
overall traffic into different MS moving pattern, e.g. pool-areas where each
covers a separate residential area and all the same city centre. Other
advantages of multiple MSCs in a pool-area are the possibility of capacity
upgrades by additional MSCs in the pool-area or the increased service
availability as other MSCs may provide services in case one MSC in the pool-
area fails.
MSC7
MSC6
MSC4MSC2
MSC1
BSC3
BSC2
BSC1
BSC5BSC4
BSC7
BSC6
BSC8
MSC3
MSC5
Pool-area 1 Pool-area 2 Pool-area 3
Figure 11-2 Pool area configuration example
An MS is served by one dedicated MSC node of a pool-area as long as it is in
radio coverage of the pool-area. Fig. 11-2 shows most of the possible pool-
area configurations. It contains Pool-area 1 (BSC area 1, 2 and 3 served by
MSCs 1 and 2), Pool-area 2 (BSC area 4 and 5 served by MSCs 3 and 4) and
Pool-area 3 (BSC area 5, 6 and 7 served by MSCs 5, 6 and 7). In addition the
BSC areas 8 is served by MSC 7 without any usage of the MSCs in Pool
feature. The possibility to configure overlapping pool-areas is shown by the
Pool-areas 2 and 3. The Pool-area 1 is configured non-overlapping with any
other Pool-area. The number or capacity of MSCs is configured independently
for each pool-area. The usage of MSCs in Pool may be configured in parts of
the network only and can co-exists with other areas not using this feature.

5. 11 MSC in pool
283
Network Resource IdentificationNetwork Resource IdentificationNetwork Resource IdentificationNetwork Resource Identification
An MSC pool-area is an area within which an MS roams without a need to
change the serving MSC. A pool-area is served by one or more MSCs nodes
in parallel. The complete service area of a BSC belongs to the same one or
more pool-area(s). An BSC service area may belong to multiple pool-areas,
which is the case when multiple overlapping pool-areas include this BSC
service area. If LAs span over multiple BSC service areas then all these BSC
service areas have to belong to the same pool-area.
MSC1
MSC2
MSC3
BSC3
BSC2
BSC1
An MSC pool-area is an area within which an MS roams
without a need to change the serving MSC.
Figure 11-3 Pool-area definition
The Network Resource Identifier (NRI) identifies uniquely an individual
MSC out of all MSCs, which serve in parallel a pool-area. The length of the
NRI is the same in all MSCs in one pool-area. In areas where pool-areas
overlap the NRI identifies uniquely an MSC out of all MSCs, which serve all
these overlapping pool-areas, i.e. an NRI identifies uniquely an MSC within a
BSC. In case of overlapping pool-areas the NRI length is configured to be the
same in all the nodes serving these pool-areas. More than one NRI may be
assigned to an MSC.

6. Signalling in GSM BSS
284
The NRI is part of the TMSI, which is assigned by the serving MSC to the
MS. The TMSI allocation mechanism in the MSC generates TMSIs which
contain a configured NRI in the relevant bit positions. The NRI has a flexible
length between 10 and 0 bits (0 bits means the NRI is not used and the feature
is not applied).
The NRI is coded in bits 23 to 14 of TMSI. Regardless of the NRI length the
most significant bit of the NRI is always in bit 23 of TMSI.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
octet 1octet 2octet 3octet 4
NRI
NRI - Network Resource Identification
Figure 11-4 Structure of TMSI
The BSC node derives the NRI from any initial signalling message. The BSC
masks the significant bits out of the TMSI to determine the NRI, which
identifies the MSC. It is configured in the BSC which bits out of TMSI
provided by the MS are significant for the NRI.
The change of a pool-area is not visible to the MS. In general there is no need
to detect a pool-area change. It may be advantageous for load balancing
purposes to detect pool-area changes in the network to distribute MSs entering
a pool-area to MSCs with an appropriate load status. MSs changing a pool-
area may be detected by configuration of different NRI values for adjacent
pool-areas.
Node Selection FunctionNode Selection FunctionNode Selection FunctionNode Selection Function
This function is used in BSC and potentially in MSCs. In the BSC the
function selects the specific MSC to which initial signalling messages are
routed. The NRI identifies the specific MSC. If the Node Selection Function
has an MSC address configured for the NRI derived from the initial signalling
message is routed to this address. If no MSC address is configured for the
derived NRI or if no NRI can be derived (e.g. the MS indicated an identity
which contains no NRI) then the Node Selection Function selects an available
MSC (e.g. according to load balancing) and routes the message to that MSC.

7. 11 MSC in pool
285
MSC2
MSC1
MSC3
BSC1
BSC2
TMSI (NRI=2)
(NRI=2)
(NRI=1)
(NRI=3)
TMSI (NRI=2)
Node Selection Function (NSF)
TMSI/NRI allocation
NRI routing
Figure 11-5 Use of NRI
In case an MSC/VLR sends a paging with IMSI (i.e. the paging message does
not contain a TMSI), the node selection function in the BSC upon reception
temporarily stores the identity of the MSC that issued the paging message. If
the node selection function receives a paging response with an IMSI then it
checks the temporarily stored identities of the MSCs on entries matching this
IMSI and forward the paging response to the node identified by this identity.
MSC
MSC
poolBSC
Paging Req.
(IMSI)
Paging
Res. (IMSI)
IMSI – MSC ID
Figure 11-6 Paging with IMSI
If the MSC/VLR initiates the paging procedure via Gs-interface the SGSN has
to add the MSC/VLR-identity to the paging message.

8. Signalling in GSM BSS
286
MSC
MSC
SGSN
pool
Paging Req. (IMSI)
BSC
Paging
(IMSI + MSC/VLR-
identity)
Paging
Req. (IMSI)
Paging Res. (IMSI)
Figure 11-7 CS paging via Gs interface
Load balancingLoad balancingLoad balancingLoad balancing
The Node Selection Function in the BSC balances the load between the
available MSCs. This is performed by an appropriate selection of the MSC for
an MS:
• which was not yet assigned to an MSC, i.e. when there is no MSC
configured for the NRI indicated by the MS,
• when no NRI can be derived,
• in exceptional cases, e.g. when the MSC corresponding to an NRI
cannot be reached.
The load-balancing algorithm is implementation specific.
In case of handover into a pool-area a load balancing between all the target
MSC nodes serving this pool-area is gained by configuration. Source MSCs
which support MSC in Pool may be configured with all possible target MSCs
for each handover target. Source MSC which do not support MSC in Pool can
configure only one target MSC per handover target. In this case each of
source MSCs which handover to the same pool-area may be configured with
another target MSC out of all target MSCs serving the same handover target.
The mechanism for distribution of the traffic between the handover target
MSCs is implementation specific. This load balancing is complemented by
the Node Selection Function in the BSS, which distributes MSs between the
MSCs when these MSs enter the pool-area in idle mode.

9. 11 MSC in pool
287
Load ReLoad ReLoad ReLoad Re----DistributionDistributionDistributionDistribution
There are situations where a network operator wishes to remove load from
one MSC in an orderly manner (e.g. to perform scheduled maintenance, or, to
perform load re-distribution to avoid overload) with minimal impact to end
users and/or additional load on other entities. The re-distribution procedure
does not require any new functionality in the terminal, that is, all terminals
can be moved.
Re-distribution of MSs is initiated via an O&M command in the MSC, which
needs to be off-loaded.
In a first phase (a couple of Periodic LA Update periods long), MSs doing LA
Update are moved to other MSCs in the pool. When the MSC receives the,
LA Update request, it returns a new TMSI with a null-NRI, and a non-
broadcast LAI in the accept message. The non-broadcast LAI causes the MS
to immediately send a new Location Update, which the BSC node then routes
to a new MSC due to the null-NRI.
MSC
MSC
MSC
(NRI=2)
(NRI=1)
(NRI=3)
BSC
LA Upd. Accept ( )
LA Upd. (periodic)
Null-NRI,
non-broadcast LAI
LA Upd. (periodic)
O&M
Figure 11-8 Load Re-Distribution (phase 1)
A second phase includes scanning through remaining MSs and initiating a
move of them to other MSCs by allocating to these MS a new TMSI using the
TMSI re-allocation procedure (with null-NRI and non-broadcast LAI) so that
a Location Update is triggered when the ongoing CM transaction ends, which
will cause them to be moved.

10. Signalling in GSM BSS
288
MSC
MSC
(NRI=2)
(NRI=3)
TMSI re-allocation ( )
BSC
LA Upd.
Null-NRI,
non-broadcast LAI
Figure 11-9 Load Re-Distribution (phase 2)
The MSs being moved from one MSC are stopped from registering to the
same MSC again by an O&M command in BSCs connected to the pool. The
MSs moving into a pool area are also stopped from registering into an MSC
being off-loaded in the same manner.
Mobility ManagementMobility ManagementMobility ManagementMobility Management
An MS performs LA Updates, which may result in a change of the serving
MSC. In these procedures the new MSC requests from the old MSC MS
specific parameters. If multiple MSCs are configured in the new MSC for the
old LA indicated by the MS then the new MSC derives the NRI from the old
TMSI indicated by the MS. The new MSC uses the old LA together with the
NRI to derive the signalling address of the old MSC from its configuration
data.
MSC
MSC
MSC
(NRI=2)
(NRI=1)
old pool
LA Update Request
(TMSI, old LAI)
NRI & old LAI ► MSC/VLR number
Send Identification /
Send Parameters
Figure 11-10 MSC change (new MSC outside old pool)

11. 11 MSC in pool
289
If the network contains MSCs that cannot derive the old MSC from LAI and
NRI a default MSC for each LA is used to resolve the ambiguity of the
multiple MSCs serving the same area.
Default MSCDefault MSCDefault MSCDefault MSC and backwards compatibilityand backwards compatibilityand backwards compatibilityand backwards compatibility
MSCs that can only derive one MSC from the LAI (e.g. because they do not
support the MSC in Pool feature, or no detailed knowledge of the NRIs is
configured) are not aware, that multiple MSCs may serve a LA. These nodes
can therefore contact only one MSC (a default MSC) per LA.
A default MSC resolves the ambiguity of the multiple MSCs per LA by
deriving the NRI from the TMSI. The default MSC relays the signalling
between the new MSC and the old MSC.
Note that the default MSC is configured per LA. So different MSCs in a
network might have configured different default MSCs for a LA. With this
approach more than one of the MSCs that serve a pool-area can be used as
default MSC, so load concentration on one MSC and a single point of failure
can be avoided.
MSC
MSC
MSC
old pool
LA Update Request
(TMSI, old RAI)
LAI ► default MSC number
(default)
Send Identification
/ Send Parameters
Figure 11-12 Default MSC

12. Signalling in GSM BSS
290
This page is intentionally left blank